The study was conducted under two water qualities (fresh water (FW), recycled wastewater (RWW)) and two biochar treatments (no biochar (No-B) and biochar (B)). It was determined that B reduced the actual evapotranspiration by saving irrigation water and that biomass yield increased in RWW and B; thus, RWW and B provided higher WPirrigation and WP. RWW and B increased OM, TN, P2O5, K2O, CEC, porosity, and aggregate stability, thus encouraging the development of the physical–physiological properties, ADF-NDF content, and biomass yields of the crop, but causing EC to increase. RWW and B resulted in higher macro–microelement contents and heavy metal (HM) contamination in the soil; thus, increases were observed in the macro–microelements and the HM content of the crop grown in RWW and B, but the absorption and buffering capacity of B limited the Na–Cd–Cr–Ni uptake of maize. However, the HM contents of the soil–crop did not exceed international standards in all treatments except the Cd content of maize. It was found that the use of B in irrigation with RWW can be recommended, considering the productivity-increasing contribution and the effectiveness of B in reducing the possible HM risks of RWW in maize cultivation, but monitoring the Cd content of maize and the EC of the soil.

  • Biochar saved irrigation water amounts by 3.7 and 4.1% in the experimental years, reducing actual evapotranspiration by up to 4.5%.

  • Fresh biomass yield increased by 13.1 and 17.3% in irrigation with recycled wastewater and biochar treatment compared with irrigation with fresh water and no biochar conditions.

Water scarcity is one of the biggest problems in the world. More than 25% of people on Earth experience severe water scarcity (Zhong et al. 2023). Forty percent of the world's land area consists of regions suffering from water scarcity, classified as arid, semi-arid, and dry semi-humid (Farhadkhani et al. 2018). Especially in recent years, the growing population against increasing global warming has made the severity of water scarcity even more felt. Therefore, it is inevitable that more people will face serious water scarcity in the coming years, and a greater majority of the world's land areas will experience water scarcity (Radini et al. 2023).

The agricultural sector is affected more by water scarcity, with 70% fresh water usage, compared with other sectors (Jaramillo & Restrepo 2017). Water scarcity in the agricultural sector disrupts the production balance, creates an uneconomic production table, and reduces the need for food production by limiting efficiency and quality. For this reason, searches for alternative water sources have emerged in the agricultural sector, and among these searches, the use of recycled wastewater in irrigation has been significantly adopted (Lahlou et al. 2021). Recycled wastewater not only serves as an alternative water source to fresh water but also provides increased yield and quality with its organic matter and nutrient content, and with this feature, it also enables more economical production by reducing the need for chemical fertilizers (Demir & Sahin 2017). In addition, the use of recycled wastewater improves soil properties (Cakmakci & Sahin 2021a) and reduces the risks that wastewater may pose to the environment by limiting the discharge of wastewater into the environment.

Despite these advantageous approaches to the use of recycled wastewater in irrigation, it also has some risks. The salt content of wastewater may cause the salinity level of the soil to increase, thus reducing crop productivity and quality and causing the soil to become barren in the future (Wen et al. 2018). The heavy metal content of wastewater is specifically evaluated because it poses serious risks to soils and crops (Natasha et al. 2020). Heavy metals can accumulate in the soil, make the soil infertile, pass into the crops, and cause toxic effects (Yerli & Sahin 2022). In addition, heavy metals, which can pose significant risks to the health of human and animal through soil or crops, can reduce crop productivity and directly cause the death of crops. Heavy metals that pass into the soil through recycled wastewater also cause negative effects on soil microorganisms, nitrification, ammonification, and enzyme activities (Mumivand et al. 2023). In addition to all these, wastewater changes some soil properties such as pH, cation exchange capacity (CEC), and organic matter, which may also cause an increased release of antagonist heavy metals (Nzediegwu et al. 2019).

Biochar, which is the high-carbon and black fine-grained output of biomass pyrolysis, can be defined as an environmentally friendly soil conditioner that emerges through thermochemical transformations under limited oxygen conditions. Biochar is a material that regulates soil salinity (Dahlawi et al. 2018) and reduces heavy metal pollution (Tang et al. 2020). By accelerating salt leaching from the soil, biochar can reduce the time required to reduce the salt concentration to a level suitable for crops (Yue et al. 2016). The biochar, which regulates heavy metal mobility in the soil, can limit the uptake of heavy metals by crops and enable crops to move away from the toxic effects of metals (Lashari et al. 2013). Thus, biochar can reduce the soil salinity and heavy metal risks of wastewater in irrigation with recycled wastewater (Nzediegwu et al. 2019).

Environmental benefits of biochar are not limited to this; biochar also improves the water retention capacity of the soil, causing less evaporation from the soil (Maroušek et al. 2019), regulating the air and water balance of the soil (Batista et al. 2018), accelerating germination and product development by increasing the soil temperature (Mumme et al. 2018), increasing the resistance and dynamism of soil biota and improving the crop's defense against diseases and pests and various abiotic and biotic stress conditions (Suliman et al. 2017). In addition, biochar increases many physical and chemical properties of the soil to a soil quality level that is desired as agricultural soil and more suitable for crop production (Omondi et al. 2016; Nzediegwu et al. 2019; Aon et al. 2023).

Biochar, which reduces pollutant transfers from soil to crops and regulates root zone soil salinity, is not always promising (Zhang et al. 2017). In fact, under some conditions, increased salinity and heavy metals may be encountered in the soil-applied biochar (Lucchini et al. 2014; Karimi et al. 2020; Yerli 2023). The interaction of biochar with soil is the result of an unclear complex relationship depending on the individual or combined variations in the biochar or soil (Qi et al. 2017). In addition, the raw material and pyrolysis process (temperature, method, oxygen) of biochar limit the clarity in explaining this unclear complex relationship (Yerli 2023). Zheng et al. (2010) explained the adsorption capacity of biochar with the surface charge of the biochar, hydrogen bonds, and electrostatic attraction forces, but Alburquerque et al. (2014) expressed the role of biochar in regulating the mobility of heavy metals depending on the raw material content of the biochar.

Previous studies have examined the effects of irrigation with recycled wastewater and/or biochar treatment on soil and/or crop productivity, but no study has focused on the effects of biochar on soil and crop fertility in the irrigation of silage maize fields with recycled wastewater. This two-year field study, which deals with liquid and solid waste management together by combining recycled wastewater and biochar, aimed to comprehensively examine the changes in soil and crop properties of silage maize irrigated with recycled wastewater under biochar treatment and to investigate the effect of biochar on water and irrigation efficiency by comparing it with freshwater irrigation. The hypotheses of the study were as follows: (i) Recycled wastewater would increase soil and crop productivity and this increase would increase further with biochar. (ii) Biochar would limit the crop's uptake of some heavy metals in irrigation with recycled wastewater. (iii) Biochar would improve irrigation and water productivity (WP) by reducing the irrigation water and water consumption of the crop.

Location and climate of the experimental area

The experiment was carried out in 2022 and 2023 in the experimental area of Van Yuzuncu Yil University in Türkiye, located at 38°34′38″ north latitude and 43°17′26″ east longitude, at an altitude of ∼1,670 m (Supplementary material 1). Although the experimental area is located in the continental climate zone, the partial marine effect of Lake Van causes the climate to be milder. The climate data of the vegetation period of silage maize measured by the climate station in the experimental area are given in Figure 1. Accordingly, the hottest months in both years of the experiment were July (25.4 °C for 2022 and 24.4 °C for 2023), and 26.0 and 21.6 mm of precipitation occurred during the vegetation period of silage maize in these years, respectively. The total evaporation, mean relative humidity, and wind speed values for the same years were 784.8 mm, 37.4%, and 1.9 m s−1 and 738.4 mm, 35.3%, and 2.0 m s−1, respectively.
Figure 1

Climatic data in the vegetation period of silage maize in 2022 (June 3–September 30) and 2023 (June 5–September 29).

Figure 1

Climatic data in the vegetation period of silage maize in 2022 (June 3–September 30) and 2023 (June 5–September 29).

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Properties of experimental area soil and biochar

To determine the properties of the soil before the experiment, soil samples were taken from 0.3 m layers up to a depth of 0.9 m in profiles opened at three different points to represent the experimental area. The soil samples taken were subjected to analysis and the properties of the experimental area soil are given in Table 1. Accordingly, it was determined that the texture of the soil was sandy clay loam, it contained moderate amounts of lime, there were no drainage and salinity problems, its pH was moderately alkaline and its organic matter content was insufficient.

Table 1

Properties of the experimental area soil

Property0–0.3 m0.3–0.6 m0.6–0.9 m
Soil texture Sandy clay loam Sandy clay loam Sandy clay loam 
Sand (%) 46.5 45.1 45.9 
Clay (%) 30.1 31.7 28.2 
Silt (%) 23.4 23.2 25.9 
Organic matter (%) 1.24 1.15 1.02 
Total N (%) 0.091 0.085 0.074 
P2O5 (Mg ha−188.1 84.1 80.8 
K2O (Mg ha−1786 799 816 
CEC (cmol kg−124.6 23.4 25.1 
EC (dS m−10.418 0.445 0.479 
pH 8.21 8.24 8.48 
CaCO3 (%) 10.5 12.1 14.9 
Particle density 2.70 2.71 2.69 
Bulk density (Mg m−31.29 1.36 1.32 
Porosity (%) 52.2 49.8 50.9 
Aggregate stability (%) 44.1 45.1 44.9 
Fe (Mg kg−14.08 3.75 3.51 
Cu (Mg kg−12.11 1.99 1.84 
Mn (Mg kg−17.51 6.99 6.11 
Zn (Mg kg−145.9 40.3 32.9 
Pb (Mg kg−13.31 3.15 2.91 
Cd (Mg kg−10.022 0.021 0.020 
Cr (Mg kg−10.162 0.154 0.129 
Ni (Mg kg−10.099 0.081 0.069 
Property0–0.3 m0.3–0.6 m0.6–0.9 m
Soil texture Sandy clay loam Sandy clay loam Sandy clay loam 
Sand (%) 46.5 45.1 45.9 
Clay (%) 30.1 31.7 28.2 
Silt (%) 23.4 23.2 25.9 
Organic matter (%) 1.24 1.15 1.02 
Total N (%) 0.091 0.085 0.074 
P2O5 (Mg ha−188.1 84.1 80.8 
K2O (Mg ha−1786 799 816 
CEC (cmol kg−124.6 23.4 25.1 
EC (dS m−10.418 0.445 0.479 
pH 8.21 8.24 8.48 
CaCO3 (%) 10.5 12.1 14.9 
Particle density 2.70 2.71 2.69 
Bulk density (Mg m−31.29 1.36 1.32 
Porosity (%) 52.2 49.8 50.9 
Aggregate stability (%) 44.1 45.1 44.9 
Fe (Mg kg−14.08 3.75 3.51 
Cu (Mg kg−12.11 1.99 1.84 
Mn (Mg kg−17.51 6.99 6.11 
Zn (Mg kg−145.9 40.3 32.9 
Pb (Mg kg−13.31 3.15 2.91 
Cd (Mg kg−10.022 0.021 0.020 
Cr (Mg kg−10.162 0.154 0.129 
Ni (Mg kg−10.099 0.081 0.069 

CEC, cation exchange capacity.

Oak biochar, which was brought to the experimental area, pyrolyzed at 400 °C, and subjected to sieving-homogenization processes, was commercially available. To determine the properties of the biochar before the experiment, the biochar material was subjected to analysis, and the properties of the biochar are given in Table 2. Accordingly, it was determined that the salinity of the biochar was high, its pH was significantly basic, and its organic matter content was rich.

Table 2

Properties of biochar

PropertyBiochar
Organic matter (%) 56.1 
CEC (cmol kg−130.0 
EC (dS m−14.26 
pH 9.65 
N (%) 1.80 
P (g kg−10.18 
K (g kg−13.57 
Fe (Mg kg−151.1 
Cu (Mg kg−18.61 
Mn (Mg kg−125.3 
Zn (Mg kg−115.8 
Pb (Mg kg−10.551 
Cd (Mg kg−1– 
Cr (Mg kg−1– 
Ni (Mg kg−1– 
PropertyBiochar
Organic matter (%) 56.1 
CEC (cmol kg−130.0 
EC (dS m−14.26 
pH 9.65 
N (%) 1.80 
P (g kg−10.18 
K (g kg−13.57 
Fe (Mg kg−151.1 
Cu (Mg kg−18.61 
Mn (Mg kg−125.3 
Zn (Mg kg−115.8 
Pb (Mg kg−10.551 
Cd (Mg kg−1– 
Cr (Mg kg−1– 
Ni (Mg kg−1– 

CEC, cation exchange capacity; –, not detected.

Irrigation water resources and properties

While irrigation water line of the University was used as fresh water in the experiment, recycled wastewater was taken from the Van-Tusba-Iskele Wastewater Treatment Plant, which is ∼5 km away from the experimental area. In this context, treated wastewater was transported to the experimental area by a tanker before each irrigation, transferred to the polyethylene tank, and used for irrigation.

In both years of the experiment, samples of fresh water and recycled wastewater were taken in the middle of each month representing the irrigation period, and these samples were subjected to analysis. EC and pH in water samples were measured using a conductometer and pH meter. Cations, microelements, and heavy metals were determined by inductively coupled plasma-optical emission spectrometer (ICP-OES) (Anonymous 2007). CO3 and HCO3 were obtained by the sulfuric acid titration method (Tuzuner 1990), and Cl and SO4 were determined by the potassium chromate indicator approach (Tuzuner 1990) and HACH spectrophotometer with ready-made kits (HACH 2010). Total nitrogen (TN) and total phosphorus (TP) were determined by the Kjeldahl method (APHA AWWA WEF 1989) and using phosphorus reagent (Powder Pillow) in the HACH spectrophotometer (HACH 2010). Suspended solid matter (SSM) was determined according to the residue left by filtered water samples (APHA 1995). The spectrophotometer, adsorption incubator, thermoreactor, and cuvette test of the HACH brand were used to determine chemical oxygen demand (COD) and biological oxygen demand (BOD5) (HACH 2005, 2010). While sodium adsorption rate (SAR), residual sodium carbonate (RSC), and percent sodium (Na%) values were calculated by taking into account ion concentrations (mEq l−1) with standard equations (Kanber & Unlu 2010), the Langelier saturation index (LSI) was determined by calculation according to Ayers & Westcot (1994).

The properties of irrigation waters are given in Table 3. Thus, as a result of the evaluation of fresh water and recycled wastewater according to international criteria (Ayers & Westcot 1994; WHO 2001; EPA 2004; Kanber & Unlu 2010; Cakmakci & Sahin 2019a; Yerli & Sahin 2022), it was concluded that there is no harm in using it for irrigation.

Table 3

Properties of irrigation waters

PropertyFresh waterRecycled wastewater
EC (dS m−10.335 ± 0.021 1.051 ± 0.065 
pH 8.13 ± 0.09 7.75 ± 0.10 
Ca (mEq l−10.85 ± 0.05 2.12 ± 0.15 
Mg (mEq l−11.44 ± 0.11 2.98 ± 0.25 
Na (mEq l−10.99 ± 0.05 4.39 ± 0.12 
K (mEq l−10.17 ± 0.02 1.12 ± 0.09 
CO3 (mEq l−1– – 
HCO3 (mEq l−12.19 ± 0.18 5.21 ± 0.15 
Cl (mEq l−10.37 ± 0.04 2.38 ± 0.18 
SO4 (mEq l−10.85 ± 0.09 1.58 ± 0.18 
Fe (Mg l−10.048 ± 0.005 0.438 ± 0.014 
Cu (Mg l−1– 0.009 ± 0.003 
Mn (Mg l−1– 0.081 ± 0.006 
Zn (Mg l−1– 0.010 ± 0.003 
Pb (Mg l−1– 0.002 ± 0.001 
Cd (Mg l−1– 0.001 ± 0.001 
Cr (Mg l−1– 0.001 ± 0.001 
Ni (Mg l−1– 0.025 ± 0.002 
TN (Mg l−1– 9.65 ± 0.91 
TP (Mg l−1– 1.31 ± 0.17 
SSM (Mg l−1– 28.9 ± 1.9 
COD (Mg l−1– 37.3 ± 3.1 
BOD5 (Mg l−1– 25.5 ± 2.5 
SAR 0.92 ± 0.04 2.75 ± 0.08 
RSC (mEq l−1−0.10 ± 0.06 0.11 ± 0.05 
Na% 28.7 ± 0.7 41.4 ± 0.8 
LSI 0.34 ± 0.05 0.60 ± 0.027 
PropertyFresh waterRecycled wastewater
EC (dS m−10.335 ± 0.021 1.051 ± 0.065 
pH 8.13 ± 0.09 7.75 ± 0.10 
Ca (mEq l−10.85 ± 0.05 2.12 ± 0.15 
Mg (mEq l−11.44 ± 0.11 2.98 ± 0.25 
Na (mEq l−10.99 ± 0.05 4.39 ± 0.12 
K (mEq l−10.17 ± 0.02 1.12 ± 0.09 
CO3 (mEq l−1– – 
HCO3 (mEq l−12.19 ± 0.18 5.21 ± 0.15 
Cl (mEq l−10.37 ± 0.04 2.38 ± 0.18 
SO4 (mEq l−10.85 ± 0.09 1.58 ± 0.18 
Fe (Mg l−10.048 ± 0.005 0.438 ± 0.014 
Cu (Mg l−1– 0.009 ± 0.003 
Mn (Mg l−1– 0.081 ± 0.006 
Zn (Mg l−1– 0.010 ± 0.003 
Pb (Mg l−1– 0.002 ± 0.001 
Cd (Mg l−1– 0.001 ± 0.001 
Cr (Mg l−1– 0.001 ± 0.001 
Ni (Mg l−1– 0.025 ± 0.002 
TN (Mg l−1– 9.65 ± 0.91 
TP (Mg l−1– 1.31 ± 0.17 
SSM (Mg l−1– 28.9 ± 1.9 
COD (Mg l−1– 37.3 ± 3.1 
BOD5 (Mg l−1– 25.5 ± 2.5 
SAR 0.92 ± 0.04 2.75 ± 0.08 
RSC (mEq l−1−0.10 ± 0.06 0.11 ± 0.05 
Na% 28.7 ± 0.7 41.4 ± 0.8 
LSI 0.34 ± 0.05 0.60 ± 0.027 

TN, total nitrogen; TP, total phosphorus; SSM, suspended solid matter; COD, chemical oxygen demand; BOD5, biological oxygen demand; SAR, sodium adsorption rate; RSC, residual sodium carbonate; Na%, percent sodium; LSI, Langelier saturation index; ±, standard error of the mean; –, not detected.

Irrigation system and experimental design

Irrigation was done by a drip irrigation system using lateral pipes with in-line drippers (2.3 l h−1) at 33 cm intervals, according to the infiltration value of the experimental area (∼15 mm h−1). While the operating pressure (1 atm) required to operate the irrigation system was provided by a centrifugal pump, a screen filter, pressure regulator manometer was included in the control unit for the operation of the irrigation system. In addition, a water meter was placed at the entrance of each parcel to control the irrigation water amount. In the system, the diameters of the main, manifold, and lateral pipes were 50, 32, and 16 mm, respectively, and the lateral pipes were placed in each crop row with an interval of 0.7 m.

The experiment was conducted in a total of 12 parcels with 3 replications under 2 different water qualities (fresh water (control) and recycled wastewater) and 2 different biochar treatments (no biochar: 0 t ha−1 (control) and biochar: 10 t ha−1), according to the completely randomized block factorial experimental design (Supplementary material 2). The experimental plots were planned as five rows, with 0.7 m × 0.15 m row spacing and crop spacing, each plot being 3.5 m × 7.2 m in size (25.2 m2), and a 3 m interval was left between plots and blocks to prevent side effects (Supplementary material 2).

Cultural practices and biochar treatments

The experimental field was first tillaged with a moldboard plow from a soil depth of 20–25 cm. Subsequently, the clods were broken up using a cultivator rotary harrow, and the field surface was leveled. Biochar was spread homogeneously on the soil surface of the biochar plots with a calculation of 10 t ha−1, taking into account the fertilization depth of 15 cm. A hoeing machine was used to mix the biochar into the soil. Finally, silage maize seeds (cv. DKC-6777) were sown with a pneumatic seeder. To see the cumulative effect of biochar, biochar was applied only in the first year of the experiment, and in the second year of the experiment, soil tillage was provided only with a hoeing machine, without mixing biochar into the soil.

For weed control, the first hoeing was carried out when the crop height was 15–20 cm, and the second hoeing was carried out in the form of throat filling when the crop height was 40–50 cm. During sowing, 100 kg ha−1 urea and 150 kg ha−1 triple superphosphate were manually applied to all plots, and when the crop reached a height of 40–50 cm (4–6 leaf stages), the second urea fertilization was applied by fertigation equal to the first dose during the throat filling period.

Irrigation treatments

During the period until the crop reached 30–39 stages according to the BBCH scale, irrigation was carried out with fresh water at a 30% wetting percentage, taking into account biochar treatments in the form of completing the current moisture determined at a depth of 0.3 m in the fresh water parcels to field capacity, separately for each biochar treatment (Yerli et al. 2023). After the crop reached 30–39 stages according to the BBCH scale, irrigation treatments were started with recycled wastewater, and during this period, irrigation was carried out with fresh water and recycled wastewater at a 65% wetting percentage, taking into account biochar treatments in the form of completing the current moisture determined at a depth of 0.9 m in the fresh water parcels to field capacity, separately for each biochar treatment (Cakmakci & Sahin 2021a). In determining the irrigation time, it was taken into account that the sum of the difference between crop water consumption (ETc) and precipitation (P) [Σ(ETcP)] reached a value corresponding to 40% of the available water capacity at a depth of 0.3 for the first period and at a depth of 0.9 for the second period (Allen et al. 1998).

While the moisture retained at the field capacity and wilting point required for irrigation were obtained by a pressurized membrane device before the experiment (Klute 1986), the available water capacity was calculated from the field capacity, wilting point, and the bulk density values (Kanber & Unlu 2010). In addition, the ETc value required for irrigation was obtained by multiplying the reference crop water consumption (ETo) by the crop coefficient (kc) of silage maize (ETc = ETo × kc). When the ETo value was calculated with the Penman–Monteith equation (FAO) in the CROPWAT program, the climate data required in the calculations were obtained from the climate station in the experimental area, while the kc value was taken directly from a guide valid for Türkiye (CWCG 2017).

Soil moisture measurements were done in the middle replication of the experimental plots, taking into account biochar treatments, using gravimetric sampling from 0.3 m layers to a depth of 0.9 m, before each irrigation and during the sowing and harvest periods. The moisture measurements were made at a distance of approximately 15–20 cm from the dripper, corresponding to between two crops, and moisture was also monitored by gravimetric sampling in the 0.9–1.2 m soil layer to determine deep percolation. The amount and volumes of irrigation water applied during the experimental period were calculated with Equations (1) and (2) and were also confirmed by water meter readings and time calculation approaches:
(1)
(2)
where I is the irrigation water amount (mm), FC is the field capacity (%Pweight), CM is the current moisture (%Pweight), γs is the bulk density of the soil (Mg m−3), D is the soil depth (0.3 and 0.9 m), P is the wetting ratio (30 and 65%), V is the irrigation water volume (l), and A is the plot area (25.2 m2).

Actual evapotranspiration, WPirrigation, and WP

While actual evapotranspiration was calculated with Equation (3), irrigation water productivity (WPirrigation) and WP were calculated with Equations (4) and (5):
(3)
(4)
(5)
where ETa is the actual evapotranspiration (mm), I is the irrigation water amount (mm), P is the precipitation amount (mm), ΔS is the change in soil moisture content (mm), WPirrigation and WP are the irrigation water productivity and water productivity (kg m−3), respectively, and FBY is the fresh biomass yield (t ha−1).

Considering that there is no groundwater problem in the experimental area and no water loss occurs through surface runoff since a dripper flow rate suitable for soil infiltration was selected and no deep percolation was encountered in the moisture measurements at a depth of 0.9–1.2 m in the soil, capillary rise, surface runoff, and deep percolation are not included in the water budget equation (Equation (3)) used to calculate actual evapotranspiration.

Analyses and measurements of soil

After the harvest of both experiment years, soil samples were taken from 0.3 m layers up to a depth of 0.9 m in profiles opened to represent the parcel from all parcels. While organic matter and total N in soil samples were determined according to the Walkey-Black (Nelson & Sommers 1982) and Kjeldahl methods (Bremner & Mulvaney 1982), respectively, P2O5 and K2O were obtained by an atomic absorption spectrophotometer (Olsen et al. 1982) and flame photometer readings (Knudsen et al. 1982), respectively. The ammonium acetate method was used to determine the CEC according to Kacar (1995). EC and pH were measured in the saturation extract by a conductometer and pH meter. The Scheibler Calcimeter was used to determine CaCO3 (Nelson 1982). Aggregate stability was obtained according to the wet-sieving (Kemper & Rosenau 1986). Microelements and heavy metals in soil samples were obtained by ICP-OES (Anonymous 2007). While particle and bulk densities were determined by pycnometer (Blake & Hartge 1986a) and soil-roller (Blake & Hartge 1986b), respectively, the porosity was calculated from these two values with Equation (6) (Danielson & Sutherland 1986):
(6)
where n is the porosity (%), γs is the bulk density of the soil (Mg m−3), Gs is the particle density of the soil, and γw is the bulk density of pure water at +4 °C (1 Mg m−3).

Analyses and measurements of silage maize

At the end of both experiment years, some analyses and measurements were made during and after the harvest of silage maize in all parcels, representing the parcel. In the harvested crop samples, the Kjeldahl method used for N and P was obtained colorimetrically in the solution in the combustion (Kacar 2014). ICP-OES was used to determine the macro- and microelements and heavy metal contents of the crop samples (Anonymous 2007). While crop height was determined by measuring the part of the crop from the soil surface to the top tassel, the stem diameter was obtained by measuring the thickness of the crop between the second and third nodes with a digital caliper. Fresh biomass yield was calculated by adapting the weights of the harvested crops to t ha−1, and dry biomass yield was adapted to t ha−1 after drying the same samples at 78 °C for 48 h (Kacar 2014). Acid detergent fiber (ADF) and neutral detergent fiber (NDF) contents of crop samples were determined on the ANKOM 200 Fiber Analyser by adding solutions according to the Fiber Bag System, Gerhardt (Van Soest 1963). A chlorophyll meter (SPAD-502, Konica Minolta Sensing, Inc., Osaka, Japan) was used for chlorophyll measurement. The leaf area was measured with a leaf area meter (LiCor-300C, LiCor, Lincoln, Nebraska), and the leaf area index was obtained by proportioning it to the unit area of a crop (0.7 m × 0.15 m). Electrolyte leakage was obtained by keeping a 1-cm diameter disk sample taken from crop leaves in pure water for 24 h and measuring its EC1 and by keeping the same sample in a water bath at 95 °C for 20 min and measuring its EC2, dividing the first EC value by the second EC value. Leaf relative water content (LRWC) was calculated with Equation (6):
(7)
where LRWC is the leaf relative water content (%), FW is the fresh weight of the leaf (g), DW is the dry weight of the leaf kept in an oven at 65 °C for 24 h (g), and TW is the turgor weight of the leaf kept in pure water for 4 h (g).

Analyses of data

Analyses were done in the SPSS package program. As a result of the ANOVA conducted to determine differences between the experiment years (2022 and 2023) and between the soil layers (0–0.3 m, 0.3–0.6 m, and 0.6–0.9 m), it was observed that all parameters examined in the experiment showed a general similarity between experiment years and between soil layers. In addition, considering that the surface soil layer is more critical and meaningful than deeper soil layers in agricultural production and the concept of effective root depth (Demir & Sahin 2017), statistical analyses were carried out in the 0–0.3 m soil layer and the mean of the experimental years (Cakmakci & Sahin 2021a). Thus, irrigation water quality and biochar treatment variables were evaluated as fixed factors, and the data were analyzed through the general linear model. The significant means were separated at a 5% probability level using the Duncan multiple comparison test. Furthermore, the Pearson correlation test was used to determine the correlation relationships between the data.

Irrigation water amount and actual evapotranspiration

Data on irrigation water amounts are presented in Figure 2. Equal amounts of irrigation water were used in the irrigation of biochar treatments with fresh water and recycled wastewater. In the period until the crop reached a height of 40–50 cm (4–6 leaf stages), in 2022–2023, in conditions without biochar and with biochar, 35.6–33.9 mm and 34.5–33.6 mm of irrigation water, respectively, was provided with fresh water only. After this period, irrigation was carried out with 328.2–315.8 mm and 315.7–301.7 mm of fresh water and recycled wastewater in biochar treatments, respectively, for the same years. Thus, in the mean of the experiment years, a total of 356.8 and 342.8 mm of irrigation water was applied in conditions without biochar and with biochar, respectively.
Figure 2

Irrigation water amount and actual evapotranspiration in irrigation and biochar treatments in 2022 and 2023.

Figure 2

Irrigation water amount and actual evapotranspiration in irrigation and biochar treatments in 2022 and 2023.

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In total, of 13 irrigations were carried out in both years of the experiment; the first 4 of these irrigations were done only with fresh water, and the remaining 8 were carried out in the form of planned irrigation with fresh water and recycled wastewater. In the experiment in which the mean irrigation interval was 8 days, the most frequent irrigation interval was in July and August, when air temperatures were higher (Figure 1). In the first year of the experiment, 4.0 and 4.4% more irrigation water was applied under conditions without biochar and with biochar, respectively, compared with the second year due to higher air temperatures and therefore evaporation in the first year of the experiment compared with the second year (Figure 1).

Biochar saved 3.7 and 4.1% of the irrigation water amount in the experimental years, respectively. This can be explained by the spongy and porous structure of the biochar. The spongy and porous structure of the biochar preserves soil moisture (Abel et al. 2013; Omondi et al. 2016), and the structural features of the biochar reduce the amount of irrigation water by limiting evaporation in the soil (Ahmed et al. 2019). Biochar retains soil water not only within the pores but also between micropores. For this reason, it takes a longer time for soil with biochar to lose moisture (Cakmakci & Sahin 2023). Additionally, biochar enriches the soil with organic matter (Table 4), which may support moisture retention in the soil, resulting in less irrigation water. Organic matter has a regulating and protective effect on soil moisture (Yerli et al. 2023). Soil organic matter causes more water retention by improving the pore size distribution of the soil (Minasny & McBratney 2018). A 1% increase in organic matter content for standard soil represents an increase of approximately 3 liters of soil water storage for every 0.0283 m3 of soil (Gould 2015).

Table 4

Chemical properties of the soil in irrigation and biochar treatments

TreatmentsOrganic matter (%)Total N (%)P2O5 (Mg ha−1)K2O (Mg ha−1)CEC (cmol kg−1)EC (dS m−1)pHCaCO3 (%)
FW No-B 1.18 ± 0.02N.S 0.088 ± 0.002N.S 87.0 ± 0.9N.S 740 ± 10N.S 26.6 ± 0.6N.S 0.436 ± 0.006N.S 8.15 ± 0.03N.S 10.2 ± 0.3N.S 
1.31 ± 0.02 0.107 ± 0.005 99.7 ± 0.8 811 ± 24 28.0 ± 0.1 0.672 ± 0.063 8.02 ± 0.05 9.5 ± 0.2 
Mean 1.25 ± 0.03 B** 0.098 ± 0.005 B** 93.3 ± 2.9 B** 776 ± 20 B** 27.3 ± 0.4 B** 0.554 ± 0.060 B** 8.09 ± 0.04 A** 9.9 ± 0.2 A** 
RWW No-B 1.52 ± 0.04 0.133 ± 0.007 118.3 ± 4.7 930 ± 15 28.9 ± 0.6 0.820 ± 0.043 7.92 ± 0.02 8.8 ± 0.4 
1.75 ± 0.02 0.157 ± 0.007 129.4 ± 4.0 997 ± 10 30.5 ± 0.2 0.918 ± 0.014 7.79 ± 0.04 7.9 ± 0.4 
Mean 1.63 ± 0.05 A 0.145 ± 0.007 A 123.9 ± 3.7 A 963 ± 17 A 29.7 ± 0.5 A 0.869 ± 0.030 A 7.85 ± 0.04 B 8.4 ± 0.3 B 
Mean No-B 1.35 ± 0.08 B** 0.110 ± 0.011 B** 102.6 ± 7.3 B** 835 ± 43 B** 27.8 ± 0.6 B** 0.628 ± 0.08 B** 8.04 ± 0.06 A** 9.5 ± 0.4 A* 
1.53 ± 0.11 A 0.132 ± 0.012 A 114.6 ± 6.9 A 904 ± 44 A 29.3 ± 0.7 A 0.795 ± 0.062 A 7.90 ± 0.06 B 8.7 ± 0.4 B 
TreatmentsOrganic matter (%)Total N (%)P2O5 (Mg ha−1)K2O (Mg ha−1)CEC (cmol kg−1)EC (dS m−1)pHCaCO3 (%)
FW No-B 1.18 ± 0.02N.S 0.088 ± 0.002N.S 87.0 ± 0.9N.S 740 ± 10N.S 26.6 ± 0.6N.S 0.436 ± 0.006N.S 8.15 ± 0.03N.S 10.2 ± 0.3N.S 
1.31 ± 0.02 0.107 ± 0.005 99.7 ± 0.8 811 ± 24 28.0 ± 0.1 0.672 ± 0.063 8.02 ± 0.05 9.5 ± 0.2 
Mean 1.25 ± 0.03 B** 0.098 ± 0.005 B** 93.3 ± 2.9 B** 776 ± 20 B** 27.3 ± 0.4 B** 0.554 ± 0.060 B** 8.09 ± 0.04 A** 9.9 ± 0.2 A** 
RWW No-B 1.52 ± 0.04 0.133 ± 0.007 118.3 ± 4.7 930 ± 15 28.9 ± 0.6 0.820 ± 0.043 7.92 ± 0.02 8.8 ± 0.4 
1.75 ± 0.02 0.157 ± 0.007 129.4 ± 4.0 997 ± 10 30.5 ± 0.2 0.918 ± 0.014 7.79 ± 0.04 7.9 ± 0.4 
Mean 1.63 ± 0.05 A 0.145 ± 0.007 A 123.9 ± 3.7 A 963 ± 17 A 29.7 ± 0.5 A 0.869 ± 0.030 A 7.85 ± 0.04 B 8.4 ± 0.3 B 
Mean No-B 1.35 ± 0.08 B** 0.110 ± 0.011 B** 102.6 ± 7.3 B** 835 ± 43 B** 27.8 ± 0.6 B** 0.628 ± 0.08 B** 8.04 ± 0.06 A** 9.5 ± 0.4 A* 
1.53 ± 0.11 A 0.132 ± 0.012 A 114.6 ± 6.9 A 904 ± 44 A 29.3 ± 0.7 A 0.795 ± 0.062 A 7.90 ± 0.06 B 8.7 ± 0.4 B 

FW, fresh water; RWW, recycled wastewater; No-B, no biochar; B, biochar; CEC, cation exchange capacity; ±, standard error of the mean. ** p < 0.01, * p < 0.05, N.S, not significant.

Data on actual evapotranspiration are presented in Figure 2. Irrigation water was the most important structure of actual evapotranspiration due to the low amount of precipitation in the vegetation period (Figure 1). Thus, in 2022–2023, in the biochar treatments of irrigation with fresh water and recycled wastewater, the actual evapotranspiration values were 2.9 and 4.2% to 3.5 and 4.5% lower, respectively, compared with the conditions without biochar, which can be explained by the lower amount of irrigation water (Figure 2). The actual evapotranspiration values in the irrigation of biochar treatments with fresh water and recycled wastewater resulted in actual evapotranspiration values close to each other as a result of the equal amount of irrigation water in these treatments (Figure 2). In addition, the higher amount of irrigation water in the first year of the experiment compared with the second year (Figure 2) caused more actual evapotranspiration in the first year. Ghanem et al. (2022) and El-Sayed et al. (2023) also reported that biochar treatment caused less actual evapotranspiration by reducing the amount of irrigation water.

WPirrigation and WP

The effects of irrigation and biochar treatments on WPirrigation and WP values were found to be statistically significant (Figure 3 and Supplementary material 3). WPirrigation and WP were 13.2 and 14.3% higher in irrigation with recycled wastewater than in irrigation with fresh water. This can be explained by the increased fresh biomass yield of silage maize in irrigation with recycled wastewater compared with irrigation with fresh water (Table 5). Because equal amounts of irrigation water were applied in irrigation treatments with recycled wastewater and fresh water, the actual evapotranspiration values were similar between the two treatments (Figure 2). Demir & Sahin (2017) reported that the increased yield due to the nutritious effect of recycled wastewater on the crop resulted in an increase in WPirrigation and WP. In the biochar treatment, WPirrigation and WP were 11.1 and 11.0% higher than in the conditions without biochar. This can be explained by both the increased fresh biomass yield (Table 5) and the decreased amount of irrigation water and actual evapotranspiration (Figure 2) in the biochar treatment compared with the conditions without biochar. Based on the formulaic calculation component of WPirrigation and WP (Equations (4) and (5)), the decreasing amount of irrigation water and actual evapotranspiration values against the increasing yield potential results in an increase in WPirrigation and WP. Cakmakci & Sahin (2021b) also stated that WPirrigation and WP values increased due to the increase in fresh biomass yield under irrigation conditions with recycled wastewater, while Baiamonte et al. (2020) also reported that WPirrigation and WP values increased due to increased yield and decreased amount of irrigation water and actual evapotranspiration in biochar treatment.
Table 5

Physical properties, biomass yields, ADF, and NDF contents of the silage maize in irrigation and biochar treatments

TreatmentsCrop height (cm)Stem diameter (mm)FBY (t ha−1)DBY (t ha−1)ADF (%)NDF (%)
FW No-B 229.8 ± 3.0N.S 22.3 ± 0.5N.S 68.9 ± 0.8N.S 20.3 ± 0.4N.S 28.1 ± 0.2N.S 48.5 ± 0.4N.S 
237.0 ± 4.4 23.0 ± 0.3 74.4 ± 0.3 22.5 ± 0.3 26.7 ± 1.9 47.3 ± 0.4 
Mean 233.4 ± 2.9 B** 22.7 ± 0.4 B** 71.7 ± 0.7 B** 21.4 ± 0.6 B** 27.4 ± 0.9 A** 47.9 ± 0.4 A** 
RWW No-B 246.4 ± 2.6 24.4 ± 0.2 78.6 ± 0.8 24.0 ± 0.4 23.3 ± 1.2 40.8 ± 1.1 
256.0 ± 2.8 25.0 ± 0.3 83.6 ± 0.7 26.1 ± 0.2 21.3 ± 0.4 38.6 ± 0.6 
Mean 251.2 ± 2.8 A 24.7 ± 0.5 A 81.1 ± 0.9 A 25.1 ± 0.4 A 22.3 ± 0.7 B 39.7 ± 0.7 B 
Mean No-B 238.1 ± 4.1 B* 23.3 ± 0.6N.S 73.7 ± 0.9 B** 22.2 ± 0.9 B** 25.7 ± 1.2N.S 44.6 ± 1.8 A* 
246.5 ± 4.8 A 24.0 ± 0.5 79.0 ± 0.8 A 24.3 ± 0.7 A 24.0 ± 1.5 43.0 ± 2.0 B 
TreatmentsCrop height (cm)Stem diameter (mm)FBY (t ha−1)DBY (t ha−1)ADF (%)NDF (%)
FW No-B 229.8 ± 3.0N.S 22.3 ± 0.5N.S 68.9 ± 0.8N.S 20.3 ± 0.4N.S 28.1 ± 0.2N.S 48.5 ± 0.4N.S 
237.0 ± 4.4 23.0 ± 0.3 74.4 ± 0.3 22.5 ± 0.3 26.7 ± 1.9 47.3 ± 0.4 
Mean 233.4 ± 2.9 B** 22.7 ± 0.4 B** 71.7 ± 0.7 B** 21.4 ± 0.6 B** 27.4 ± 0.9 A** 47.9 ± 0.4 A** 
RWW No-B 246.4 ± 2.6 24.4 ± 0.2 78.6 ± 0.8 24.0 ± 0.4 23.3 ± 1.2 40.8 ± 1.1 
256.0 ± 2.8 25.0 ± 0.3 83.6 ± 0.7 26.1 ± 0.2 21.3 ± 0.4 38.6 ± 0.6 
Mean 251.2 ± 2.8 A 24.7 ± 0.5 A 81.1 ± 0.9 A 25.1 ± 0.4 A 22.3 ± 0.7 B 39.7 ± 0.7 B 
Mean No-B 238.1 ± 4.1 B* 23.3 ± 0.6N.S 73.7 ± 0.9 B** 22.2 ± 0.9 B** 25.7 ± 1.2N.S 44.6 ± 1.8 A* 
246.5 ± 4.8 A 24.0 ± 0.5 79.0 ± 0.8 A 24.3 ± 0.7 A 24.0 ± 1.5 43.0 ± 2.0 B 

FW, fresh water; RWW, recycled wastewater; No-B, no biochar; B, biochar; FBY, fresh biomass yield; DBY, dry biomass yield; ADF, acid detergent fiber; NDF, neutral detergent fiber; ±, standard error of the mean. ** p < 0.01, * p < 0.05, N.S, not significant.

Figure 3

Irrigation water productivity (WPirrigation) and water productivity (WP) in irrigation and biochar treatments.

Figure 3

Irrigation water productivity (WPirrigation) and water productivity (WP) in irrigation and biochar treatments.

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Chemical properties of soil

The effects of irrigation and biochar treatments on organic matter, total N, P2O5, K2O, CEC, electrical conductivity (EC), pH, and CaCO3 values were found to be statistically significant (Table 4 and Supplementary material 3).

The organic matter, total N, P2O5, and K2O contents of the soil increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the rich content of recycled wastewater and biochar (Tables 2 and 3). While the chemical (COD) and biological oxygen demands (BOD5) of recycled wastewater improve the organic matter content of the soil, the N, P, and K contents of the recycled wastewater support an increase in the total N, P2O5, and K2O contents of the soil (Cakmakci & Sahin 2021a). Similarly, the nutrient-rich content of biochar offers soil development that can eliminate the soil's need for fertilizer (Tang et al. 2020). Both the effect of biochar on providing organic matter, N, P, and K to the soil and its ability to preserve them for a longer period of time without mineralization (Zhang et al. 2017) more clearly explain the increased organic matter, total N, P2O5, and K2O content of the soil in the biochar treatment in this study. In addition, as a result of the better development of the crop grown in the soil irrigated with recycled wastewater and applied biochar (Table 5), the fact that the crop residues contribute more organic matter to the soil can be considered a different explanation for the increased organic matter content in irrigation with recycled wastewater and biochar treatments.

The CEC content of the soil increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can also be explained by the organic matter content that irrigation with recycled wastewater and biochar provides to the soil (Table 4), as well as the cation content of recycled wastewater and biochar (Tables 2 and 3). Organic matter not only provides larger cation exchange volume due to its charged surfaces but also reduces the loss of exchangeable cations from the soil. Lu et al. (2015) also explained the increase in CEC by organic matter providing a wide cation exchange area. In this study, the positive correlation relationship of CEC with organic matter also supports this situation (Supplementary material 4). Additionally, similar to the findings of this study, Angin et al. (2005) stated that there was a two-fold increase in CEC in irrigation with recycled wastewater compared with irrigation with fresh water, and this increase was due to the effect of recycled wastewater on increasing the organic matter content of the soil, while Banik et al. (2018) reported that biochar mixed into the soil increased the CEC content of the soil and that this increase was a result of O-containing functional groups on the surface of the biochar.

The EC content of the soil increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the salt content of the recycled wastewater and biochar (Tables 2 and 3). Since wastewater contains salt even after the treatment process, the EC of the soil must be monitored, especially in long-term irrigation with recycled wastewater. Similar to the findings of this study, a different study also mentioned a two-fold increase in EC in irrigation with recycled wastewater compared with irrigation with fresh water (Cakmakci & Sahin 2021a). The various organic materials and fertilizers applied to the soil can increase soil salinity directly depending on their salt concentration (Atkinson et al. 2010) explaining the increase in soil EC in the biochar treatment in this study. Additionally, similar to the findings of this study, Karimi et al. (2020) reported that the EC of soil applied with biochar increased between 17 and 49%. In contrast, Hossain et al. (2020) found that biochar could regulate salinity rather than increase soil salinity, and this situation was explained by the characterization relationships of the biochar depending on the raw material and pyrolysis processes.

The pH content of the soil decreased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the organic matter and total N that recycled wastewater and biochar provide to the soil (Table 4), depending on their nutrient-rich content (Tables 2 and 3). During the mineralization phase of organic matter, hydrogen ions become free and acidify the soil (Adeleke et al. 2017). Similarly, free hydrogen ions resulting from the mineralization of ammonium reduce soil pH as a result of higher rates of nitrification (Singh & Agrawal 2012). In addition, decreases in soil pH may occur due to the breakdown of carboxyl-containing acids of different organic structures depending on organic matter (Zhi-An et al. 2008). In this study, the negative correlation relationships of pH with organic matter and total N also support this situation (Supplementary material 4). Additionally, similar to the findings of this study, Demir & Sahin (2017) and Yerli (2023) also reported that soil pH decreased in irrigation with recycled wastewater and biochar treatment, respectively.

The CaCO3 content of the soil decreased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the pH-reducing effect of irrigation with recycled wastewater and biochar treatment (Table 4). Decreasing pH increases the dissolution of CaCO3 (Mazen et al. 2010). In addition, organic acids produced in the mineralization of organic matter in the soil can cause further pH decreases and thus more CaCO3 dissolution in the soil (Leogrande & Vitti 2019). In this study, the positive correlation relationship of CaCO3 with pH also supports this situation (Supplementary material 4). Additionally, similar to the findings of this study, Cakmakci & Sahin (2021a) reported that CaCO3 content decreased due to the decreasing pH content of the soil in irrigation with recycled wastewater, while Chintala et al. (2014) pointed out that biochar applied to acidic soils reduced CaCO3.

Physical properties of the soil

While the effects of irrigation and biochar treatments on particle density values were not found to be statistically significant, the effects of irrigation and biochar treatments on bulk density, porosity, and aggregate stability values were found to be statistically significant (Table 6 and Supplementary material 3).

Table 6

Physical properties of the soil in irrigation and biochar treatments

TreatmentsParticle densityBulk density (Mg m−3)Porosity (%)Aggregate stability (%)
FW No-B 2.70 ± 0.003N.S 1.29 ± 0.003N.S 52.2 ± 0.2N.S 46.8 ± 0.7N.S 
2.71 ± 0.007 1.28 ± 0.003 52.6 ± 0.2 50.4 ± 1.0 
Mean 2.71 ± 0.003N.S 1.29 ± 0.003 A** 52.4 ± 0.2 B** 48.6 ± 1.0 B** 
RWW No-B 2.71 ± 0.003 1.27 ± 0.003 53.0 ± 0.1 52.2 ± 0.6 
2.71 ± 0.006 1.26 ± 0.003 53.4 ± 0.2 55.2 ± 0.6 
Mean 2.71 ± 0.003 1.27 ± 0.03 B 53.2 ± 0.1 A 53.7 ± 0.8 A 
Mean No-B 2.71 ± 0.002N.S 1.28 ± 0.005 A* 52.6 ± 0.2 B* 49.5 ± 1.3 B** 
2.71 ± 0.004 1.27 ± 0.005 B 53.0 ± 0.2 A 52.8 ± 1.2 A 
TreatmentsParticle densityBulk density (Mg m−3)Porosity (%)Aggregate stability (%)
FW No-B 2.70 ± 0.003N.S 1.29 ± 0.003N.S 52.2 ± 0.2N.S 46.8 ± 0.7N.S 
2.71 ± 0.007 1.28 ± 0.003 52.6 ± 0.2 50.4 ± 1.0 
Mean 2.71 ± 0.003N.S 1.29 ± 0.003 A** 52.4 ± 0.2 B** 48.6 ± 1.0 B** 
RWW No-B 2.71 ± 0.003 1.27 ± 0.003 53.0 ± 0.1 52.2 ± 0.6 
2.71 ± 0.006 1.26 ± 0.003 53.4 ± 0.2 55.2 ± 0.6 
Mean 2.71 ± 0.003 1.27 ± 0.03 B 53.2 ± 0.1 A 53.7 ± 0.8 A 
Mean No-B 2.71 ± 0.002N.S 1.28 ± 0.005 A* 52.6 ± 0.2 B* 49.5 ± 1.3 B** 
2.71 ± 0.004 1.27 ± 0.005 B 53.0 ± 0.2 A 52.8 ± 1.2 A 

FW, fresh water; RWW, recycled wastewater; No B, no biochar; B, biochar; ±, standard error of the mean. ** p < 0.01, * p < 0.05, N.S, not significant.

The bulk density content of the soil decreased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the organic matter that recycled wastewater and biochar provide to the soil (Table 4), depending on their nutrient-rich content (Tables 2 and 3). The lower bulk density of organic matter results in increased organic matter in the soil decreasing the bulk density of the soil. In addition, organic matter improves soil aggregation, reduces soil compaction, and reduces bulk density (Demir & Sahin 2019). Six et al. (2000) explained the lower bulk density with the formation of more stable aggregation in the soil associated with the presence of organic matter. In this study, the negative correlation relationships of bulk density with organic matter and aggregate stability also support this situation (Supplementary material 4). Additionally, similar to the findings of this study, Demir & Sahin (2019) and Verheijen et al. (2019) also reported that the bulk density of the soil decreased in irrigation with recycled wastewater and biochar treatment, respectively.

The porosity content of the soil increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the fact that irrigation with recycled wastewater and biochar treatment reduces the bulk density (Table 6). Since there was no significant change in particle density depending on the irrigation and biochar treatments, the change in porosity depended on the bulk density (Table 6). Based on the formulaic calculation component of porosity (Equation (6)), decreasing bulk density increases porosity under conditions where particle density is constant. In this study, the negative correlation relationship of porosity with bulk density also supports this situation (Supplementary material 4). In addition, increasing organic matter in the soil reduces soil compaction and provides more space in the soil, which also increases porosity (Chaudhari et al. 2013). Thus, it can be stated that increasing the organic matter content of the soil enables porosity development. In this study, the positive correlation relationship of porosity with organic matter also supports this situation (Supplementary material 4). Similar to the findings of this study, Cakmakci & Sahin (2021a) also reported that porosity increased in irrigation with recycled wastewater. Moreover, while all of these can be considered as the indirect effect of biochar increasing porosity, the porous structure of biochar can be evaluated as the direct effect of biochar increasing porosity. Omondi et al. (2016) pointed out that biochar mixed into the soil supports the development of soil porosity due to the porous structure of the biochar.

The aggregate stability content of the soil increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the organic matter that recycled wastewater and biochar provide to the soil (Table 4). In this study, the positive correlation relationship of aggregate stability with organic matter also supports this situation (Supplementary material 4). Organic matter improves aggregation by ensuring closer contact with soil colloidal particles (Aslam et al. 2014). The effect of organic matter on increasing aggregate stability arises from supporting the development of clay-humic complexes and water-repellent coatings (Heikkinen et al. 2019). In addition, the improving effect of organic matter on other physical properties of the soil also supports the development of aggregate stability. In particular, the improvement in soil porosity promotes an increase in aggregate stability (Demir & Sahin 2019). In this study, the positive correlation relationship of aggregate stability with porosity also supports this situation (Supplementary material 4). Similar to the findings of this study, Cakmakci & Sahin (2021a) also stated that increased organic matter and porosity of the soil in irrigation with recycled wastewater improved aggregate stability. In addition, Hossain et al. (2020) explained the aggregate stability-enhancing effect of biochar in relation to the pore structure of biochar, and Hua et al. (2014) reported that biochar improved soil biology and increased aggregate stability as a result of the secretion of polysaccharides by microorganisms.

Microelement and heavy metal contents of the soil

While the effects of irrigation treatments on microelement and heavy metal (Fe, Cu, Mn, Zn, Pb, Cd, Cr, and Ni) values of the soil were found to be statistically significant, the effects of biochar treatments on microelement and heavy metal (Fe, Cu, Mn, Zn, and Pb) values of the soil, except Cd, Cr, and Ni, were found to be statistically significant (Table 7 and Supplementary material 3).

Table 7

The microelement and heavy metal contents of the soil in irrigation and biochar treatments

TreatmentsFe (Mg kg−1)Cu (Mg kg−1)Mn (Mg kg−1)Zn (Mg kg−1)Pb (Mg kg−1)Cd (Mg kg−1)Cr (Mg kg−1)Ni (Mg kg−1)
FW No-B 3.45 ± 0.08N.S 2.03 ± 0.24 a* 6.50 ± 0.17N.S 40.9 ± 1.1N.S 3.09 ± 0.17N.S 0.018 ± 0.002N.S 0.158 ± 0.004N.S 0.087 ± 0.006N.S 
5.92 ± 0.49 3.15 ± 0.09 b 10.64 ± 0.53 50.8 ± 2.7 3.66 ± 0.21 0.019 ± 0.001 0.160 ± 0.003 0.093 ± 0.003 
Mean 4.69 ± 0.60 B** 2.59 ± 0.28 B** 8.57 ± 0.96 B** 45.9 ± 2.6 B** 3.38 ± 0.17 B** 0.019 ± 0.001 B** 0.159 ± 0.002 B** 0.090 ± 0.003 B** 
RWW No-B 7.81 ± 0.11 4.07 ± 0.20 c 15.70 ± 1.22 78.7 ± 3.9 4.13 ± 0.06 0.085 ± 0.002 0.212 ± 0.028 0.383 ± 0.017 
9.96 ± 0.03 4.30 ± 0.03 c 20.53 ± 0.55 90.8 ± 1.1 4.87 ± 0.05 0.089 ± 0.001 0.214 ± 0.004 0.391 ± 0.017 
Mean 8.88 ± 0.10 A 4.19 ± 0.48 A 18.12 ± 1.24 A 84.8 ± 3.3 A 4.50 ± 0.17 A 0.087 ± 0.001 A 0.213 ± 0.013 A 0.387 ± 0.011 A 
Mean No-B 5.63 ± 0.48 B** 3.05 ± 0.98 B** 11.10 ± 2.13 B** 59.8 ± 8.7 B** 3.61 ± 0.24 B** 0.052 ± 0.015N.S 0.185 ± 0.017N.S 0.235 ± 0.067N.S 
7.94 ± 0.26 A 3.72 ± 0.93 A 15.59 ± 2.24 A 70.8 ± 9.1 A 4.27 ± 0.29 A 0.054 ± 0.016 0.187 ± 0.012 0.242 ± 0.067 
TreatmentsFe (Mg kg−1)Cu (Mg kg−1)Mn (Mg kg−1)Zn (Mg kg−1)Pb (Mg kg−1)Cd (Mg kg−1)Cr (Mg kg−1)Ni (Mg kg−1)
FW No-B 3.45 ± 0.08N.S 2.03 ± 0.24 a* 6.50 ± 0.17N.S 40.9 ± 1.1N.S 3.09 ± 0.17N.S 0.018 ± 0.002N.S 0.158 ± 0.004N.S 0.087 ± 0.006N.S 
5.92 ± 0.49 3.15 ± 0.09 b 10.64 ± 0.53 50.8 ± 2.7 3.66 ± 0.21 0.019 ± 0.001 0.160 ± 0.003 0.093 ± 0.003 
Mean 4.69 ± 0.60 B** 2.59 ± 0.28 B** 8.57 ± 0.96 B** 45.9 ± 2.6 B** 3.38 ± 0.17 B** 0.019 ± 0.001 B** 0.159 ± 0.002 B** 0.090 ± 0.003 B** 
RWW No-B 7.81 ± 0.11 4.07 ± 0.20 c 15.70 ± 1.22 78.7 ± 3.9 4.13 ± 0.06 0.085 ± 0.002 0.212 ± 0.028 0.383 ± 0.017 
9.96 ± 0.03 4.30 ± 0.03 c 20.53 ± 0.55 90.8 ± 1.1 4.87 ± 0.05 0.089 ± 0.001 0.214 ± 0.004 0.391 ± 0.017 
Mean 8.88 ± 0.10 A 4.19 ± 0.48 A 18.12 ± 1.24 A 84.8 ± 3.3 A 4.50 ± 0.17 A 0.087 ± 0.001 A 0.213 ± 0.013 A 0.387 ± 0.011 A 
Mean No-B 5.63 ± 0.48 B** 3.05 ± 0.98 B** 11.10 ± 2.13 B** 59.8 ± 8.7 B** 3.61 ± 0.24 B** 0.052 ± 0.015N.S 0.185 ± 0.017N.S 0.235 ± 0.067N.S 
7.94 ± 0.26 A 3.72 ± 0.93 A 15.59 ± 2.24 A 70.8 ± 9.1 A 4.27 ± 0.29 A 0.054 ± 0.016 0.187 ± 0.012 0.242 ± 0.067 

FW, fresh water; RWW, recycled wastewater; No-B, no biochar; B, biochar; ±, standard error of the mean. ** p < 0.01, * p < 0.05, N.S, not significant.

The microelement and heavy metal contents of the soil increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar (except Cd, Cr, and Ni). This can be explained depending on the microelement and heavy metal contents of the recycled wastewater and biochar (Tables 2 and 3). Similarly, Singh & Agrawal (2012) also stated that irrigation with recycled wastewater increased the microelement and heavy metal content of the soil compared with irrigation with fresh water, while Lucchini et al. (2014) also reported that biochar treatment caused minor changes in the microelement content of the soil and the fractionation of some heavy metals. Demir & Sahin (2020) pointed out that the heavy metal content of recycled wastewater tends to accumulate in the soil and that heavy metals should be monitored, especially in long-term irrigation with recycled wastewater. In contrast to this study, Zhang et al. (2017) stated that biochar treatment did not have a significant effect on the heavy metal content of the soil, and researchers emphasized the effect of biochar's raw materials and pyrolysis processes on increasing or decreasing the heavy metal content of the soil. While the raw material characterization of the biochar determines the microelement and heavy metal content of the biochar, pyrolysis processes (temperature, method, and oxygen) are also an important criterion that determines the scale of this content (Yerli 2023).

The movement of heavy metals in the soil, their transition to crops, or their binding forms are governed by many external and internal factors. While climatic factors can be listed at the top of external factors, internal factors can be evaluated depending on soil characteristics. In this study, the positive correlation relationships of soil heavy metal content (Fe, Cu, Mn, Zn, Pb, Cd, Cr, and Ni) with organic matter, total N, P2O5, K2O, CEC, and EC and the negative correlation relationships with CaCO3 and pH were determined (Supplementary material 4). Low molecular weight organic acids in the structure of organic matter increase the accumulation of heavy metals in the soil and control their distribution (Park et al. 2016). These organic acids act as chelators, limiting the uptake of heavy metals by crops and increasing the mobilization of heavy metals away from the rhizosphere region (Cakmakci & Sahin 2021a). It is an indirect effect that total N increases the mobility of heavy metals. The decrease in pH in the mineralization of ammonium with increasing N in the soil supports the accumulation of heavy metals in the soil (Singh & Agrawal 2012). The increase in heavy metals with the increase in P2O5 and K2O is due to the antagonist effect of P and K. P and K compounds reduce the bioavailability of heavy metals, causing heavy metal contamination in the soil (Bolan et al. 2014). The increase in heavy metal accumulation in the soil with the increase in CEC depends on organic matter. The increase in organic matter supports the increase in CEC, causing heavy metal accumulation in the soil (Kizilkaya et al. 2004). In addition, the competition of cations with metals is also effective in heavy metal movement in soil (Acosta et al. 2011). Soil salinity can limit metal uptake by creating osmotic stress and increasing the accumulation of heavy metals in the soil. Bartkowiak et al. (2020) reported that heavy metal behavior changes in saline soils and metal accumulation in the soil increase with the decrease in the uptake of heavy metals by crops. Similarly, Acosta et al. (2011) also stated that increasing soil EC increased the mobilization of heavy metals. The relationship between CaCO3 and heavy metals can be evaluated depending on pH. Decreasing pH increases the solubility of CaCO3 (Mazen et al. 2010) and thus supports the accumulation of heavy metals in the soil (Cakmakci & Sahin 2021a). Soil pH is the most important factor affecting the mobility of heavy metals in soil solution. Low pH increases the mobility of heavy metals as a result of the higher ionic attraction of hydrogen to the metal (Singh et al. 2009). Decreasing pH creates competition between dissolved heavy metals and hydrogen ions, resulting in heavy metal contamination (Singh & Agrawal 2012). In general, soil pH values below 6.5–7.0 support an increase in the mobility of heavy metals in the soil (McBride et al. 1997). However, the lowest pH value in this study was found to be 7.79 (Table 4), indicating that the mobility of heavy metals did not increase in all treatments. In addition, as a result of this study, the heavy metal content of the soil in all treatments did not exceed the limit values determined by WHO (140, 300, 300, 3, and 75 Mg kg−1 for Cu, Zn, Pb, Cd, and Ni, respectively).

Macro- and microelements and heavy metal contents of silage maize

While the effects of irrigation treatments on micro- and macroelements and heavy metal (N, P, K, Ca, Mg, Na, Fe, Cu, Mn, Zn, Pb, Cd, Cr, and Ni) values of the silage maize were found to be statistically significant, the effects of biochar treatments on the micro- and macroelements and heavy metal (N, P, K, Ca, Mg, Na, Fe, Cu, Mn, Cd, Cr, and Ni) values of the silage maize, except Zn and Pb, were found to be statistically significant (Table 8 and Supplementary material 3).

Table 8

The macro- and microelements and heavy metal contents of the silage maize in irrigation and biochar treatments

TreatmentsN (%)P (Mg kg−1)K (Mg kg−1)Ca (Mg kg−1)Mg (Mg kg−1)Na (Mg kg−1)Fe (Mg kg−1)
FW No-B 1.86 ± 0.13N.S 0.41 ± 0.03N.S 1.09 ± 0.05N.S 0.56 ± 0.03N.S 0.49 ± 0.04N.S 0.069 ± 0.003N.S 119.5 ± 4.3N.S 
2.29 ± 0.16 0.54 ± 0.01 1.21 ± 0.04 0.67 ± 0.04 0.69 ± 0.03 0.026 ± 0.005 140.5 ± 2.9 
Mean 2.08 ± 0.13 B** 0.47 ± 0.03 B** 1.15 ± 0.04 B** 0.61 ± 0.03 B** 0.59 ± 0.05 B** 0.048 ± 0.010 B** 130.0 ± 5.3 B** 
RWW No-B 2.61 ± 0.19 0.91 ± 0.04 1.44 ± 0.07 0.81 ± 0.02 0.80 ± 0.03 0.206 ± 0.006 167.6 ± 3.7 
3.01 ± 0.08 1.10 ± 0.06 1.60 ± 0.03 0.96 ± 0.02 1.03 ± 0.05 0.153 ± 0.016 191.3 ± 4.3 
Mean 2.81 ± 0.13 A 1.01 ± 0.05 A 1.52 ± 0.05 A 0.89 ± 0.04 A 0.92 ± 0.06 A 0.180 ± 0.014 A 179.5 ± 5.9 A 
Mean No-B 2.24 ± 0.20 B* 0.66 ± 0.12 B** 1.26 ± 0.09 B* 0.68 ± 0.06 B** 0.64 ± 0.07 B** 0.138 ± 0.031 A** 143.5 ± 11.1 B** 
2.65 ± 0.18 A 0.82 ± 0.13 A 1.40 ± 0.09 A 0.82 ± 0.07 A 0.86 ± 0.08 A 0.090 ± 0.029 B 165.9 ± 11.6 A 
Treatments
Cu (Mg kg−1)Mn (Mg kg−1)Zn (Mg kg−1)Pb (Mg kg−1)Cd (Mg kg−1)Cr (Mg kg−1)Ni (Mg kg−1)
FW No-B 2.81 ± 0.17N.S 33.2 ± 1.6N.S 17.4 ± 1.4N.S 0.067 ± 0.004N.S 0.076 ± 0.001N.S 0.36 ± 0.06N.S 0.045 ± 0.004N.S 
3.83 ± 0.51 39.9 ± 1.8 18.9 ± 0.8 0.089 ± 0.039 0.036 ± 0.005 0.10 ± 0.03 0.025 ± 0.006 
Mean 3.32 ± 0.33 B** 36.5 ± 1.8 B** 18.2 ± 0.9 B** 0.078 ± 0.018 B** 0.056 ± 0.009 B** 0.23 ± 0.07 B** 0.035 ± 0.005 B** 
RWW No-B 6.50 ± 0.20 72.2 ± 1.1 36.4 ± 2.0 0.421 ± 0.016 0.145 ± 0.008 0.86 ± 0.07 0.184 ± 0.007 
7.94 ± 0.13 80.5 ± 1.1 37.6 ± 0.9 0.448 ± 0.007 0.098 ± 0.008 0.57 ± 0.03 0.158 ± 0.008 
Mean 7.22 ± 0.34 A 76.4 ± 2.0 A 37.0 ± 1.1 A 0.435 ± 0.010 A 0.122 ± 0.011 A 0.72 ± 0.07 A 0.171 ± 0.008 A 
Mean No-B 4.66 ± 0.83 B** 52.7 ± 8.8 B** 26.9 ± 4.4N.S 0.244 ± 0.080N.S 0.110 ± 0.016 A** 0.61 ± 0.12 A** 0.115 ± 0.031 A** 
5.88 ± 0.95 A 60.2 ± 9.1 A 28.3 ± 4.2 0.268 ± 0.082 0.067 ± 0.015 B 0.33 ± 0.11 B 0.092 ± 0.030 B 
TreatmentsN (%)P (Mg kg−1)K (Mg kg−1)Ca (Mg kg−1)Mg (Mg kg−1)Na (Mg kg−1)Fe (Mg kg−1)
FW No-B 1.86 ± 0.13N.S 0.41 ± 0.03N.S 1.09 ± 0.05N.S 0.56 ± 0.03N.S 0.49 ± 0.04N.S 0.069 ± 0.003N.S 119.5 ± 4.3N.S 
2.29 ± 0.16 0.54 ± 0.01 1.21 ± 0.04 0.67 ± 0.04 0.69 ± 0.03 0.026 ± 0.005 140.5 ± 2.9 
Mean 2.08 ± 0.13 B** 0.47 ± 0.03 B** 1.15 ± 0.04 B** 0.61 ± 0.03 B** 0.59 ± 0.05 B** 0.048 ± 0.010 B** 130.0 ± 5.3 B** 
RWW No-B 2.61 ± 0.19 0.91 ± 0.04 1.44 ± 0.07 0.81 ± 0.02 0.80 ± 0.03 0.206 ± 0.006 167.6 ± 3.7 
3.01 ± 0.08 1.10 ± 0.06 1.60 ± 0.03 0.96 ± 0.02 1.03 ± 0.05 0.153 ± 0.016 191.3 ± 4.3 
Mean 2.81 ± 0.13 A 1.01 ± 0.05 A 1.52 ± 0.05 A 0.89 ± 0.04 A 0.92 ± 0.06 A 0.180 ± 0.014 A 179.5 ± 5.9 A 
Mean No-B 2.24 ± 0.20 B* 0.66 ± 0.12 B** 1.26 ± 0.09 B* 0.68 ± 0.06 B** 0.64 ± 0.07 B** 0.138 ± 0.031 A** 143.5 ± 11.1 B** 
2.65 ± 0.18 A 0.82 ± 0.13 A 1.40 ± 0.09 A 0.82 ± 0.07 A 0.86 ± 0.08 A 0.090 ± 0.029 B 165.9 ± 11.6 A 
Treatments
Cu (Mg kg−1)Mn (Mg kg−1)Zn (Mg kg−1)Pb (Mg kg−1)Cd (Mg kg−1)Cr (Mg kg−1)Ni (Mg kg−1)
FW No-B 2.81 ± 0.17N.S 33.2 ± 1.6N.S 17.4 ± 1.4N.S 0.067 ± 0.004N.S 0.076 ± 0.001N.S 0.36 ± 0.06N.S 0.045 ± 0.004N.S 
3.83 ± 0.51 39.9 ± 1.8 18.9 ± 0.8 0.089 ± 0.039 0.036 ± 0.005 0.10 ± 0.03 0.025 ± 0.006 
Mean 3.32 ± 0.33 B** 36.5 ± 1.8 B** 18.2 ± 0.9 B** 0.078 ± 0.018 B** 0.056 ± 0.009 B** 0.23 ± 0.07 B** 0.035 ± 0.005 B** 
RWW No-B 6.50 ± 0.20 72.2 ± 1.1 36.4 ± 2.0 0.421 ± 0.016 0.145 ± 0.008 0.86 ± 0.07 0.184 ± 0.007 
7.94 ± 0.13 80.5 ± 1.1 37.6 ± 0.9 0.448 ± 0.007 0.098 ± 0.008 0.57 ± 0.03 0.158 ± 0.008 
Mean 7.22 ± 0.34 A 76.4 ± 2.0 A 37.0 ± 1.1 A 0.435 ± 0.010 A 0.122 ± 0.011 A 0.72 ± 0.07 A 0.171 ± 0.008 A 
Mean No-B 4.66 ± 0.83 B** 52.7 ± 8.8 B** 26.9 ± 4.4N.S 0.244 ± 0.080N.S 0.110 ± 0.016 A** 0.61 ± 0.12 A** 0.115 ± 0.031 A** 
5.88 ± 0.95 A 60.2 ± 9.1 A 28.3 ± 4.2 0.268 ± 0.082 0.067 ± 0.015 B 0.33 ± 0.11 B 0.092 ± 0.030 B 

FW, fresh water; RWW, recycled wastewater; No-B, no biochar; B, biochar; ±, standard error of the mean. ** p < 0.01, * p < 0.05, N.S, not significant.

The macro- and microelements and heavy metal contents of silage maize increased under irrigation with recycled wastewater compared with irrigation with fresh water. This can be explained by the effect of recycled wastewater on providing macro- and microelements and heavy metals to the soil (Tables 4 and 7), depending on its content (Table 3). With increased transpiration, the crop absorbs water from the soil with together macro- and microelements and heavy metals present in the soil. Thus, the presence of more macro- and microelements and heavy metals in the soil enables the crop to absorb more elements and metals from the soil. In addition, the fact that maize is an accumulator crop may have caused it to take in more heavy metals from the soil and accumulate them in its body. Similar to the findings of this study, Cakmakci & Sahin (2019b) also stated that the macro- and microelements and heavy metal contents of silage maize irrigated with recycled wastewater increased. Demir & Sahin (2017) also confirmed a similar statement and evaluated this increase as being related to the transition of recycled wastewater to the soil and therefore to the crop, depending on its chemical content.

The N, P, K, Ca, Mg, Fe, Cu, and Mn contents of silage maize increased in the biochar treatment compared with conditions without biochar but the Na, Cd, Cr, and Ni contents decreased, while the Zn and Pb contents were similar between the biochar and no biochar treatments. The increase in macro- and microelements and heavy metal contents of silage maize in the biochar treatment can be evaluated by the accumulation of biochar in the soil (Table 7) parallel to the crop, depending on its content (Table 2), similar to irrigation with recycled wastewater. Despite the accumulation of Na, Cd, Cr, and Ni in the soil in the biochar treatment (Table 7), it is thought that the decrease in Na, Cd, Cr, and Ni contents of silage maize compared with conditions without biochar is due to the exclusionary effect of biochar. Atkinson et al. (2010) reported that the elemental and metallic concentrations of various soil conditioners applied to the soil, depending on their contents, can be directly absorbed by crops. However, Fellet et al. (2014) pointed out that the surface capacity of biochar limits the uptake of some elements and heavy metals from the soil by crops. Park et al. (2016) explained this limiting effect of biochar with the absorption capacity of biochar. The limiting absorption capacity effect of biochar, which has a high buffering capacity, on the use of some heavy metals by crops was also emphasized by a different study (Nzediegwu et al. 2019). In addition, differences in the crop's uptake of elements and metals from the soil can be mentioned as a result of the competition of some elements and metals and their interactions with organic matter, clay, Fe, and Al2O3. There is strong competition between Na and K. Increasing K in the soil reduces the possibility of Na accumulation in crop organs (Grattan & Grieve 1998). This situation is also valid for Cd, Cr, and Ni. Increased Fe in the soil limits the crop's uptake of Cd and Cr (Kobaissi et al. 2014). In addition to its high chelation properties, Ni replaces heavy metals found in enzymes and physiologically active centers in crops (Daghan et al. 2013). In addition, as a result of this study, the heavy metal content of silage maize in all treatments, except Cd (0.05 Mg kg−1), did not exceed the limit values determined by FAO/WHO (425.5, 40, 500, 60, 0.1, 2.3, and 67.9 Mg kg−1 for Fe, Cu, Mn, Zn, Pb, Cr, and Ni, respectively).

Physical properties and acid–neutral detergent fiber contents of silage maize

While the effects of irrigation treatments on crop height, stem diameter, fresh biomass yield, dry biomass yield, ADF, and NDF values were found to be statistically significant, the effects of biochar treatments on crop height, fresh biomass yield, dry biomass yield, and ADF values, except stem diameter and NDF, were found to be statistically significant (Table 5 and Supplementary material 3).

The crop height and stem diameter contents increased in irrigation with recycled wastewater compared with irrigation with fresh water. This can be explained by the nutritional effect of recycled wastewater on the soil (Tables 4 and 7) and therefore on the crop due to its nutrient-rich content (Table 3). Similarly, the crop height content increased in the biochar treatment compared with the conditions without biochar can be evaluated as related to biochar supporting the development of the crop depending on the nutrient content (Table 2). In particular, increasing N in the soil is an important nutrient element that supports the increase in crop height and stem diameter of silage maize (Yerli et al. 2023). In this study, the positive correlation relationships of crop height and stem diameter with total N also support this situation (Supplementary material 4). Similar correlation relationships were found by Kale et al. (2018), and the researchers emphasized that the physical development of maize increased with increasing N in soil. Cakmakci & Sahin (2021b) and Khan et al. (2022) also reported an increase in the crop height and/or stem diameter of silage maize under irrigation with recycled wastewater and biochar treatment, respectively, depending on their nutrient-enhancing effects on the soil.

The fresh–dry biomass yields increased by 13.1 and 7.2% to 17.3 and 9.5%, respectively, in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. The increase in dry biomass yield was parallel to the fresh biomass yield and a positive correlation relationship was determined between dry biomass yield and fresh biomass yield (Supplementary material 4). Cakmakci & Sahin (2021b) drew attention to a similar relationship and stated that the changes in dry biomass yield of silage maize were directly related to fresh biomass yield. The yield increases can be explained by the fertilization effect of recycled wastewater and biochar due to the high nutrient content (Tables 2 and 3). Increasing the macro- and micronutrient contents of the soil under irrigation with recycled wastewater and biochar treatment (Tables 4 and 7) provides better crop growth. In particular, N is an important element that improves the yield characteristics of crops. N is involved in physical, chemical, and physiological regulations for the crop to provide higher productivity (Daghan et al. 2013). In this study, the positive correlation relationships of fresh and dry biomass yields with the total N and the N content of the crop also support this situation (Supplementary material 4). In addition, increased crop height and/or stem diameter in irrigation with recycled wastewater and biochar treatment (Table 5) are important factors that support the increase in fresh and dry biomass yields. In this study, the positive correlation relationships of fresh and dry biomass yields with crop height and stem diameter also support this situation (Supplementary material 4). In addition, chlorophyll is a good indicator of crop productivity and development. Increasing chlorophyll increases crop growth and biomass by increasing photosynthetically active radiation and radiation use efficiency (Gomaa et al. 2021). Physiological properties such as the area and relative water content of the leaf and electrolyte leakage are also helpful parameters in interpreting the biomass yield of the crop (Yerli et al. 2023). Changes in these parameters directly affect the biomass yield of the crop. In this study, the positive correlation relationships of fresh and dry biomass yields with chlorophyll, leaf area index, and LRWC also support this situation (Supplementary material 4). Similar correlation relationships were determined by Camoglu et al. (2011), and the researchers pointed out the relationship between the yield and physiological characteristics of maize. Additionally, similar to the findings of this study, many studies also stated that yield increases occur in irrigation with recycled wastewater (Demir & Sahin 2017; Nzediegwu et al. 2019; Cakmakci & Sahin 2021b; Mumivand et al. 2023) and biochar treatment (Lashari et al. 2013; Alburquerque et al. 2014; Aon et al. 2023; Cakmakci & Sahin 2023). In addition to all these, the spongy and porous structure of biochar preserves soil moisture for a longer period of time, thus protecting it from stress, and the increase in the availability of nutrients by crops by preserving moisture in the soil can be considered a different explanation for the increased fresh and dry biomass yields of the silage maize under biochar treatment. Yildirim et al. (2021) stated that crops growing in soil-applied biochar showed an increase in productivity as a result of their being able to access soil moisture more easily. Cakmakci & Sahin (2023) also stated a similar situation and pointed out that the soil moisture protective effect of biochar increases crop yield.

The ADF and NDF contents decreased in irrigation with recycled wastewater compared with irrigation with fresh water. This can be explained by the N content of silage maize (Table 8). Similarly, the NDF content decreased in the biochar treatment compared with the conditions without biochar, which can be evaluated in relation to the N content of silage maize (Table 8). The increase in the N content of the crop reduces ADF and NDF values. Similarly, Kale et al. (2018) also reported that increasing N decreased the ADF and NDF contents in silage maize. In this study, the negative correlation relationships of ADF and NDF with the N content of the crop also support this situation (Supplementary material 4). Increasing N reduces the ADF and NDF values of the crop, which has a longer maturation period, allowing the crop to remain physiologically active for a longer period of time (Safdarian et al. 2014). Thus, the relationship of the change in physical properties of the crop related to yield with ADF and NDF depending on the maturation period can be mentioned and it can be said that the increased development of silage maize (Table 5) reduces the ADF and NDF values. In this study, the negative correlation relationships of ADF and NDF with crop height and stem diameter also support this situation (Supplementary material 4). Similarly, Mut & Kose (2018) also mentioned the negative correlation relationships of ADF and NDF with the physical properties of the triticale crop. In addition, ADF and NDF, which represent cellulose, hemicellulose, lignin, fibrous carbohydrates, protein, and silicon in feed quality, provide an idea about the energy intake and digestibility of feed. Therefore, high values of ADF and NDF cause a decrease in the quality and digestibility of feed and insufficient energy intake (Khatiwada et al. 2021). As a result of this study, the ADF and NDF contents of silage maize irrigated with recycled wastewater and biochar treatment complied with the limit values (<25 and <46% for ADF and NDF) determined by the National Research Council of the National Academies, while the ADF and NDF contents in the control treatments were above these limit values.

Physiological properties of silage maize

While the effects of irrigation treatments on chlorophyll, leaf area index, and electrolyte leakage values, except LRWC, were found to be statistically significant, the effects of biochar treatments on chlorophyll, leaf area index, LRWC, and electrolyte leakage values were found to be statistically significant (Table 9 and Supplementary material 3).

Table 9

Physiological properties of the silage maize in irrigation and biochar treatments

TreatmentsChlorophyll (SPAD)Leaf area indexLRWC (%)Electrolyte leakage
FW No-B 48.1 ± 0.3N.S 5.14 ± 0.05N.S 86.0 ± 0.5N.S 14.6 ± 0.3N.S 
50.1 ± 0.6 5.28 ± 0.04 88.4 ± 0.3 13.7 ± 0.2 
Mean 49.1 ± 0.5 B** 5.21 ± 0.04 B** 87.2 ± 0.6N.S 14.2 ± 0.3 B** 
RWW No-B 51.0 ± 0.3 5.35 ± 0.06 86.2 ± 0.7 16.3 ± 0.4 
52.9 ± 0.7 5.56 ± 0.03 88.3 ± 0.9 15.5 ± 0.4 
Mean 51.9 ± 0.5 A 5.45 ± 0.05 A 87.3 ± 0.7 15.9 ± 0.3 A 
Mean No-B 49.6 ± 0.7 B** 5.24 ± 0.06 B** 86.1 ± 0.4 B** 14.6 ± 0.4 B* 
51.5 ± 0.7 A 5.42 ± 0.07 A 88.4 ± 0.4 A 15.5 ± 0.4 A 
TreatmentsChlorophyll (SPAD)Leaf area indexLRWC (%)Electrolyte leakage
FW No-B 48.1 ± 0.3N.S 5.14 ± 0.05N.S 86.0 ± 0.5N.S 14.6 ± 0.3N.S 
50.1 ± 0.6 5.28 ± 0.04 88.4 ± 0.3 13.7 ± 0.2 
Mean 49.1 ± 0.5 B** 5.21 ± 0.04 B** 87.2 ± 0.6N.S 14.2 ± 0.3 B** 
RWW No-B 51.0 ± 0.3 5.35 ± 0.06 86.2 ± 0.7 16.3 ± 0.4 
52.9 ± 0.7 5.56 ± 0.03 88.3 ± 0.9 15.5 ± 0.4 
Mean 51.9 ± 0.5 A 5.45 ± 0.05 A 87.3 ± 0.7 15.9 ± 0.3 A 
Mean No-B 49.6 ± 0.7 B** 5.24 ± 0.06 B** 86.1 ± 0.4 B** 14.6 ± 0.4 B* 
51.5 ± 0.7 A 5.42 ± 0.07 A 88.4 ± 0.4 A 15.5 ± 0.4 A 

FW, fresh water; RWW, recycled wastewater; No-B, no biochar; B, biochar; LRWC, leaf relative water content; ±, standard error of the mean. ** p < 0.01, * p < 0.05, N.S, not significant.

The chlorophyll content increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained with N and Fe contents of the recycled wastewater and biochar (Tables 2 and 3). Total N and Fe provided by recycled wastewater and biochar to the soil (Tables 4 and 7) provide better chloroplast development. N and Fe, as the building blocks of chlorophyll, take part in chloroplast and protein formation processes. In particular, N is very effective in the chlorophyll development process of maize, which is a C4 crop. Maize metabolizes N in the soil and uses it during the chlorophyll development stage (Yerli et al. 2023). In this study, the positive correlation relationships of chlorophyll with the total N and the Fe content of soil also support this situation (Supplementary material 4). In addition, the chlorophyll content of the crops growing in soil containing sufficient levels of macro- and micronutrients increases (Bolat & Kara 2017).

Thus, the chlorophyll-enhancing effect of the macro- and micronutrients provided by recycled wastewater and biochar to the soil (Tables 4 and 7) can be mentioned. Additionally, similar to the findings of this study, Khalilzadeh et al. (2020) also explained that the increase in chlorophyll in irrigation with recycled wastewater since recycled wastewater increases the biosynthesis rate of chlorophyll, while Cakmakci & Sahin (2023) also pointed out that the nutrient and water retaining effect of biochar in the soil provides better chloroplast development.

The leaf area index content increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the total N that recycled wastewater and biochar provide to the soil (Table 4), depending on their N content (Tables 2 and 3). The increased N in the leaves by the uptake of N from the soil increases the leaf area of the crop (Amanullah et al. 2007). In this study, the positive correlation relationships of the leaf area index with the total N and the N content of the crop also support this situation (Supplementary material 4). Additionally, similar to the findings of this study, Yerli et al. (2023) and Yildirim et al. (2021) also reported that the N-providing effect of irrigation with recycled wastewater and biochar treatment to the soil and crop increased the leaf area index.

The similarity of LRWC in irrigation treatments can be explained by equal irrigation water amounts and similar real evapotranspiration values (Figure 2), while the increase in LRWC in biochar treatment compared with conditions without biochar can be evaluated by the effect of biochar on providing organic matter to the soil, in addition to preserving soil moisture with the specific surface area of biochar. Increased moisture retention in the soil causes an increase in LRWC and provides crops with easier access to both nutrients and soil moisture (Cakmakci & Sahin 2021a). Organic matter contribution also helps preserve moisture for a longer period of time and thus helps crops access soil moisture more easily (Yerli et al. 2023). Yildirim et al. (2021) also reported that LRWC increased in biochar treatment as a result of the protective effect of biochar on soil moisture.

The electrolyte leakage content increased in irrigation with recycled wastewater compared with irrigation with fresh water and in biochar treatment compared with conditions without biochar. This can be explained by the fact that recycled wastewater and biochar increase soil salinity (Table 4) depending on their EC content (Tables 2 and 3). Increased EC in the soil causes damage to the membrane systems as a result of damage to the mesophyll cells in the leaves, thus increasing electrolyte leakage (Marcinska et al. 2013). In this study, the positive correlation relationship of electrolyte leakage with EC also supports this situation (Supplementary material 4). In addition, increasing EC restricts the water intake and the water content of the crop, which cannot provide sufficient water, decreases and its stomata close (Yildirim et al. 2021). The stress experienced by the crop due to increased leaf temperature when the stomata are closed results in an increase in electrolyte conductivity as a result of damage to the cellular membrane systems (Ozturk 2015). Cakmakci & Sahin (2021a) also reported that irrigation with recycled wastewater increased the electrolyte leakage of silage maize, while Mehmood et al. (2020) also stated that electrolyte leakage may increase with increasing soil EC depending on the salt concentration of biochar.

The study showed that biochar reduced the actual evapotranspiration by saving irrigation water and that the increase in the yield of silage maize with the contribution of recycled wastewater and biochar caused higher WPirrigation and WP in irrigation with recycled wastewater and biochar treatment. While irrigation with recycled wastewater and biochar treatment supported the improvement of the physical and chemical properties of the silage maize field soil, it caused a decrease in bulk density, pH, and CaCO3 and an increase in soil salinity. Improved soil properties increased the physical and physiological properties and biomass yield of silage maize and improved the ADF and NDF contents of silage maize, but irrigation with recycled wastewater and biochar treatment caused higher macro- and microelement contents and heavy metal contamination in the soil compared with irrigation with fresh water and conditions without biochar. Thus, increases were observed in the macro- and microelements and heavy metal contents of the crop grown in the soil irrigated with recycled wastewater and applied biochar, but the absorption and buffering capacity of the biochar limited the Na, Cd, Cr, and Ni uptake of silage maize. However, as a result, the heavy metal contents of the soil and crop did not exceed international standards in all treatments except the Cd content of silage maize.

The findings obtained within the scope of this study showed that the use of biochar in irrigation with recycled wastewater can be recommended, taking into account the productivity-increasing contribution and the effectiveness of biochar in reducing the possible heavy metal risks of wastewater in silage maize cultivation irrigated with recycled wastewater. However, considering that the Cd content of silage maize exceeds international standards and the increase in soil salinity in irrigation with recycled wastewater and biochar treatment, monitoring the Cd contamination in the crop and the salt content in the soil, as well as examining the effects of biochars with different raw material contents under irrigation conditions with recycled wastewater are among recommendable results of this study.

This study was financially supported by the Van Yuzuncu Yil University Scientific Research Projects Coordination Unit with the project number FYD-2022-10341.

All relevant data are included in the paper or its Supplementary Information.

The author declares there is no conflict.

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