Abstract
While knowing CO2 emissions during the seasonal period are important, determining residual effect before sowing in the following year can be an available practice in improving wastewater irrigation strategies. Therefore, this study investigated CO2 emission from the silage maize field plots irrigated with wastewater at different levels under conventional and direct sowing in the pre-sowing period after two experimental years by comparing freshwater with full irrigation, and correlated with H2O emission and, soil moisture and temperatures. The results showed that irrigation with wastewater and conventional tillage in the previous two years resulted in higher CO2 emissions in the following period also, and 27 and 11% higher emissions were determined in irrigation with wastewater at 100 and 67% levels than full freshwater irrigation. In irrigation with wastewater at 100% level and direct sowing, soil moisture was found higher, while reduced H2O emission and the soil temperatures at 5 and 10 cm depths. Considering moisture conservation effect of direct sowing, it could be concluded that to reduce on the residual CO2 emission effect of irrigation with wastewater from previous years, deficit irrigation in direct sowing can be recommended practice.
HIGHLIGHTS
In previous years, the effect of irrigation with wastewater increasing CO2 emission continues in the pre-sowing period of the following year.
Direct sowing reduces CO2 emissions and preserves soil moisture by reducing H2O emissions compared to conventional tillage.
Deficit irrigation with wastewater reduces CO2 emissions.
Irrigation with wastewater and direct sowing reduce the soil temperature.
INTRODUCTION
Due to various anthropogenic effects, freshwater resources are facing significant pollution loads day by day (Hasan et al. 2021). Rapid industrialization and increasing population are listed as the most important factors in the pollution of water bodies (Kurwadkar et al. 2022). In addition, increasing global warming causes a decrease in surface freshwater resources (Kuriqi et al. 2020). Agricultural, urban and industrial activities are at risk as a result of pollution of water bodies near areas with various land use patterns (Ambade et al. 2022). However, it is certain that agriculture, which is the largest consumer of water, will be most affected by water scarcity. For this reason, the use of wastewater in irrigation is an accepted approach for the protection of freshwater resources, the amount of which is currently decreasing and polluted (Cakmakci & Sahin 2021).
With the use of wastewater, which can be defined as water that has been polluted as a result of different uses, including domestic or industrial, and whose contents have changed partially or completely, the pressure on freshwater resources can be reduced and the discharge problems of wastewater can be eliminated. Especially in arid and semi-arid regions where water resources are scarce, it is known that wastewater, which is an alternative to freshwater resources, is used for irrigation to maintain the continuity of production in agricultural production (Abd-Elaty et al. 2022). In addition, reducing the need for synthetic fertilizers as well as increasing soil and crop productivity of micro and macronutrients in wastewater increases the possibility of using wastewater as a water source in agricultural production (Yerli et al. 2023). Since raw wastewater may contain more nutrients than synthetic fertilizer, this contributes to the realization of a more economical production process with the use of wastewater in irrigation (Qin & Horvath 2020).
Although wastewater has positive effects, it also brings some risks. The high salt concentration in wastewaters causes salinity in soils, thus negatively affecting crop productivity (Wen et al. 2018). Heavy metal and pathogen risks in wastewater can pose a danger to the health of living things through the food chain, and result in toxicity in crops. In addition, some pollutants in these waters pose a risk to public health (Ambade & Sethi 2021; Ambade et al. 2021). Moreover, special ions such as sodium entering the soil can reduce the permeability of the soil and cause the soil quality to be adversely affected (Warrington et al. 2007).
Another important source of risk is that wastewater used in irrigation triggers an increase in global warming by increasing the emission of basic greenhouse gases such as carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) from the soil, which is the most important problem of today (Yerli et al. 2022a). However, both the high increase amount and the current high level of CO2 compared to N2O and CH4 are greenhouse gases that urgently need to be taken (Tubiello et al. 2015). When the organic carbon, which is increased in the soil in irrigation with wastewater, encounters more oxygen with various interventions applied to the soil, it turns into CO2 and is released from the soil to the atmosphere (Yerli et al. 2019).
CO2 emissions from the soil can vary with soil temperature and soil moisture (Evans & Burke 2013). Increased activation of soil biology with increasing soil temperature can affect emissions (Chen et al. 2018; Du et al. 2019). The situation is similar for soil moisture and increasing moisture provides more organic carbon return to the soil from crop residues and crop roots (Entry et al. 2008). In addition, increasing moisture in the soil promotes microbial activity and increases CO2 emissions as a result of the oxidation of organic carbon (Jabro et al. 2008; Liu et al. 2008). Therefore, a deficit irrigation approach by limiting the soil moisture content can be used as an emission-reducing practice. Hou et al. (2020) reported that CO2 emissions decreased between 9.8 and 14.3% in deficit irrigation applications compared to full irrigation applications. Similarly, Yerli et al. (2022b) pointed out that deficit irrigation practice is an emission-reducing approach in wastewater irrigation conditions.
Soil tillage, which is the beginning of agricultural activities, has special importance in the management of soil carbon stocks and thus CO2 emissions from the soil. Intensive tillage operations, in which the soil is deeply cultivated, increase soil CO2 emissions as a result of creating a more suitable environment for microbial activities by increasing the level of oxygen in the soil and ensuring that plant residues come into closer contact with soil particles (Vurarak & Bilgili 2015). In addition, intensive tillage operations affect the physical properties of the soil and increase the physical distribution of organic carbon, triggering emissions (De Oliveira Silva et al. 2019). However, with the minimum soil tillage or direct sowing approach, both emissions can be reduced and productivity can be increased by supporting more moisture and carbon stocks in the soil (Kocyigit 2008). Akbolat et al. (2009) reported that the lowest emission was obtained from direct sowing among different soil tillage practices. Similarly, Liu et al. (2013) stated that emissions decreased significantly in direct sowing and more decreased as increased of residues left on the soil surface.
Since it is important to examine the effects of wastewater irrigation and/or different tillage practices on CO2 emissions from the soil during the crop production period, many studies have focused on this issue. However, it is also very important whether CO2 emissions would have a residual effect before sowing the following year from the soil treated with different tillage operations and irrigated with wastewater in the previous year, but such a study has not been found in the literature. Thus, this study examined the residual effect on CO2 emissions before sowing the following year in silage maize plots that were fully irrigated with freshwater and wastewater at different levels under different soil tillage–sowing conditions for 2 experimental years in a semi-arid region, and also CO2 emission was correlated with soil moisture–temperature and the evaporation from soil (H2O emission), which are frequently mentioned in relation to CO2 emissions. Thus, the study hypothesized that the impact of wastewater from silage maize field irrigated with wastewater for 2 years on CO2 emissions would continue in the pre-sowing period of the following year, but this would remain low in direct sowing compared to conventional tillage, and that deficit irrigation approaches would reduce CO2 emissions from silage maize field irrigated with wastewater.
MATERIALS AND METHODS
Study area and soil properties
Soil samples were collected from the top 20 cm layer of all experimental plots just before the CO2 measurements. The organic matter in the samples was determined by the Walkley–Black method (Nelson & Sommers 1982) and the organic carbon was calculated with it (Setyanto et al. 2004). Total nitrogen was determined by the Kjeldahl method (Bremner & Mulvaney 1982) (Table 1). Considering the particle size distribution determined by the Bouyoucos hydrometer method given in Gee & Bauder (1986), the soil texture was classified as sandy-clay-loam (sand: 45.6%, silt: 24.6%, clay: 29.8%).
Irrigation treatment . | Organic matter (%) . | Organic carbon (%) . | Total nitrogen (%) . | ||||||
---|---|---|---|---|---|---|---|---|---|
CT . | DS . | Mean . | CT . | DS . | Mean . | CT . | DS . | Mean . | |
FW100 | 1.83 | 1.87 | 1.85 B** | 1.06 | 1.08 | 1.07 B** | 0.088 | 0.096 | 0.092 B** |
WW100 | 2.15 | 2.20 | 2.18 A | 1.25 | 1.28 | 1.27 A | 0.130 | 0.139 | 0.135 A |
WW67 | 1.85 | 1.87 | 1.86 B | 1.07 | 1.09 | 1.08 B | 0.110 | 0.119 | 0.115 C |
WW33 | 1.55 | 1.58 | 1.57 C | 0.90 | 0.92 | 0.91 C | 0.083 | 0.086 | 0.085 D |
Mean | 1.85 B* | 1.88 A | 1.07 B | 1.09 A** | 0.103 B** | 0.110 A | |||
Variance analysis results . | Mean square . | F . | P . | Mean square . | F . | P . | Mean square . | F . | P . |
Soil tillage (S) | 0.008 | 5.762 | 0.029 | 0.003 | 5.482 | 0.032 | 0.000 | 20.541 | 0.000 |
Irrigation (I) | 0.375 | 267.722 | 0.000 | 0.125 | 263.330 | 0.000 | 0.003 | 197.238 | 0.000 |
S × I (Interaction) | 0.000 | 0.198 | 0.896 | 0.000 | 0.313 | 0.816 | 1.300E-005 | 0.828 | 0.498 |
Error | 0.001 | 0.000 | 1.571–005 |
Irrigation treatment . | Organic matter (%) . | Organic carbon (%) . | Total nitrogen (%) . | ||||||
---|---|---|---|---|---|---|---|---|---|
CT . | DS . | Mean . | CT . | DS . | Mean . | CT . | DS . | Mean . | |
FW100 | 1.83 | 1.87 | 1.85 B** | 1.06 | 1.08 | 1.07 B** | 0.088 | 0.096 | 0.092 B** |
WW100 | 2.15 | 2.20 | 2.18 A | 1.25 | 1.28 | 1.27 A | 0.130 | 0.139 | 0.135 A |
WW67 | 1.85 | 1.87 | 1.86 B | 1.07 | 1.09 | 1.08 B | 0.110 | 0.119 | 0.115 C |
WW33 | 1.55 | 1.58 | 1.57 C | 0.90 | 0.92 | 0.91 C | 0.083 | 0.086 | 0.085 D |
Mean | 1.85 B* | 1.88 A | 1.07 B | 1.09 A** | 0.103 B** | 0.110 A | |||
Variance analysis results . | Mean square . | F . | P . | Mean square . | F . | P . | Mean square . | F . | P . |
Soil tillage (S) | 0.008 | 5.762 | 0.029 | 0.003 | 5.482 | 0.032 | 0.000 | 20.541 | 0.000 |
Irrigation (I) | 0.375 | 267.722 | 0.000 | 0.125 | 263.330 | 0.000 | 0.003 | 197.238 | 0.000 |
S × I (Interaction) | 0.000 | 0.198 | 0.896 | 0.000 | 0.313 | 0.816 | 1.300E-005 | 0.828 | 0.498 |
Error | 0.001 | 0.000 | 1.571–005 |
CT, conventional tillage; DS, direct sowing; FW100, irrigation at 100% level with freshwater; WW100, irrigation at 100% level with recycled wastewater; WW67, irrigation at 67% level with recycled wastewater; WW33, irrigation at 33% level with recycled wastewater.
**p < 0.01, *p < 0.05.
Experimental design
The study was carried out with a split-plot design random block experimental plan in the 2020 and 2021 vegetation periods (from May to September) by considering two tillage–sowing practices (CT: conventional tillage, DS: direct sowing) and four irrigation treatments. Subplot factors including four different irrigation treatments were irrigation with treated wastewater at 100% (WW100), 67% (WW67), 33% (WW33) levels and irrigation with freshwater at 100% (FW100, control) level. The experimental field consisted of 24 plots with five rows for each and size of 3.5 m × 7.2 m, and the row and plant spacing in the plots were 70 and 15 cm, respectively.
Soil tillage–sowing and irrigation applications
A cultivator-rotary harrow was used in the continuation of ploughing with a mouldboard plough at a depth of 25–30 cm in conventional tillage in both years and sowing was carried out with a pneumatic seeder on the same day. In the direct sowing application, sowing was carried out with the direct sowing machine on the same day as the conventional tillage without tillage. In 2020 and 2021, sowing was performed in May and harvesting in September, and the mean vegetation period lasted for 120 days.
Silage maize was irrigated with a surface drip irrigation system in the experimental years. The recycled wastewater collected from the Biological Waste Water Treatment Plant (38°24′53″N–43°14′09″E) in the Edremit district of Van city, Turkey, was applied to wastewater plots during the irrigation periods (May–September) in 2020 and 2021. The freshwater plots were irrigated with tap water from the university network during the same period. The mean pH and EC values of freshwater and recycled wastewater throughout trials were 8.15–7.58 and 0.353–1.124 dS m−1, respectively, and the mean total nitrogen, biological oxygen demand and chemical oxygen demand of the recycled wastewater were 10.9, 23.2 and 37.5 mg l−1, respectively (Yerli & Sahin 2022).
The irrigation was carried out according to the dynamic irrigation program considering real-time weather data measured at a weather station located in the experimental area. In both years, the irrigation continued until harvest in mid-September. As the mean of 2020 and 2021, seasonal irrigation quantities of 339.3, 234.2 and 125.9 mm were applied at 100, 67 and 33% irrigation levels in conventional tillage, respectively, while the irrigation quantities were 306.4, 212.2 and 114.6 mm in the direct sowing practice.
CO2 emission measurements
CO2 emissions from soil were measured daily for 3 weeks in the pre-sowing period of the following year (April 19–May 8 in 2022) in the plots irrigated with wastewater and freshwater in the previous 2 years (2020 and 2021). Due to the completion of the spring precipitation during the measurement period, there were available field conditions for tillage–sowing practices in this period.
An EGM-5 infrared gas analyser device (CFX-2, PPSystems, Stotfold, UK) was used to measure CO2 emissions (Yerli et al. 2022a). In addition, simultaneously with CO2 measurements, the soil moisture and soil temperature values at 5, 10 and 20 cm depths and the H2O emissions, which are frequently mentioned in the literature to be related to CO2, were also measured (Yerli et al. 2022b). H2O emissions were recorded automatically by the same device at the time of CO2 measurements. While the measurement of soil temperature was performed by immersing the STP-1 temperature probe connected to the same device into the soil, soil moisture values were measured with a mobile soil moisture measuring device (TDR, Trime-Pico, IPH/T3, IMKO) (Yerli & Sahin 2021).
Data analysis
The SPSS program was used for data analysis. Considering the irrigation and tillage–sowing practices of the previous years (2020 and 2021) as fixed factors, the significant mean values of the data evaluated with the General Linear Model were classified using the Duncan multiple comparison test at the 5% level of probability. The RStudio program was used to determine the correlations of CO2 emission with H2O emissions and soil moisture and temperature.
RESULTS AND DISCUSSION
H2O emissions and soil moistures
The developing organic matter content of the soil supports the retention of moisture in the soil by placing the pores of the soil in a more suitable position in favor of water retention (Chen et al. 2010; Yang et al. 2014). Organic matter, with its low bulk density and high porosity, as well as improved aggregate stability and water absorption, supports the protection of the soil water reservoir from exposure to evaporation for a longer period of time (Rouw & Rajot 2004). Increased moisture content is due to improved aggregation provided by the organic matter contribution (Altikat et al. 2018). Considering that the moisture balance in the soil is directly dependent on pore distribution, organic matter supports moisture retention in the soil by improving the porosity in the soil as a result of its specific pore distribution (Ors et al. 2015). Similarly, Yerli & Sahin (2022) indicated that organic matter improves the pore size distribution in the soil and the specific surface area of the soil particles, supporting moisture retention in the soil and thus reducing H2O emission. Lal (2020) also stated that the increased water holding capacity of soil ensures that H2O emissions may be reduced.
In the direct sowing practice, moisture retention may have increased due to the increased bulk density as a result of the no tillage of the soil surface in addition to the organic matter contribution, and thus H2O emission may have decreased. Altikat et al. (2018) reported that aeration decreased due to the increasing bulk density of the soil in direct sowing application. Nyambo et al. (2020) stated that intensive soil tillage disrupts soil clumps and increases water loss from the soil. In a study examining the effects of direct sowing and conventional tillage applications on H2O emissions, it was determined that the H2O emission was 25% higher in conventional tillage compared to direct sowing (Akbolat et al. 2016). Gozubuyuk et al. (2014) reported that micropores increase and macropores decrease in direct sowing, as a result of which direct sowing is an application that provides moisture conservation for a longer time.
Soil temperatures
Lower soil temperatures at 5 and 10 cm depths values were obtained in irrigation at the 100% level with wastewater and direct sowing. It could be explained that higher moisture values in 100% irrigation with wastewater and direct sowing (Figures 3–5) reduce soil temperature (Figures 7–9) with the increased soil thermal diffusivity from the higher moisture content. The significant (p < 0.01) negative correlations of soil temperatures at 5 and 10 cm depths with soil moisture contents also confirmed this (Figure 6). The presence of moisture in the soil causes the thermal diffusivity of the soil and the soil temperature to decrease (Shen et al. 2018). The increased moisture in the soil and the moisture retained in the soil for a longer time cause a significant temperature decrease, especially in the surface soil (Zhang et al. 2013). In addition, the cooling effect of the water existing in the soil can be mentioned. Mancinelli et al. (2015) expressed that increasing moisture in the soil may reduce the soil temperature by creating a cooling effect. In addition, changes in soil bulk density, aggregate stability and porosity properties as a result of increased organic matter in the soil change the heat capacity and thermal conductivity of the soil (Van Wie et al. 2013). Similarly, as a result of soil tillage–sowing operations, changes in the physical properties of the soil, and differences in soil temperature are observed (De Oliveira Silva et al. 2019). Shen et al. (2018) stated that the soil temperature is up to 1.5 °C lower in direct sowing practice than in applications where the soil is intensively cultivated. In addition, Licht & Kaisi (2005) reported that direct sowing causes lower soil temperatures due to higher levels of soil moisture and also longer moisture conservation.
CO2 emissions
It could be said that higher organic matter, organic carbon and total nitrogen values in the soil irrigated at 100 and 67% levels with wastewater (Table 1) increase CO2 emission (Figure 10) with increased mineralization. The significant (p < 0.01) positive correlations of CO2 emission with organic matter, organic carbon and total nitrogen also confirmed this (Figure 6). Organic carbon, a derivative of organic matter added to the soil as a result of irrigation with wastewater, is the major agent of CO2 emission (Mahmoud et al. 2012), because when organic carbon encounters oxygen, it turns into CO2, and is emitted to the atmosphere (Yerli et al. 2019). Fér et al. (2022) explained the increased CO2 emission from the soil in irrigation with wastewater by the oxidation of the organic carbon provided by the wastewater to the soil by the microbial population. Fernández-Luqueño et al. (2010) also reported that CO2 emissions increased with the organic carbon contribution provided by wastewater irrigation to soils and that CO2 emission in soils irrigated with wastewater were 2.4 times higher than those soils irrigated with freshwater.
The moisture content is considered an important environmental factor that stimulates CO2 emissions by affecting microorganism activity and providing organic matter mineralization (Hossain et al. 2017). The results of this study showed a significant (p < 0.01) positive correlation between CO2 emission and soil moisture at 10-cm soil depth (Figure 6). Suitable moisture conditions in the soil increase the use of both organic carbon and soil nitrogen by soil microorganisms (Yu et al. 2014), causing more soil CO2 emission with more mineralization. In conditions where soil pores are filled with water, soil microorganisms work better and increase their emissions from the soil by mineralizing organic carbon and nitrogen more (Ball et al. 2008).
The presence of sufficient nitrogen in the soil is also effective in microorganism activity. Nitrogen exists in the life cycle of microbial activities. Therefore, nitrogen addition supports the activity of soil microorganisms and increases soil respiration as a result of the oxidation of organic matter (Tang et al. 2018). However, for soil microorganisms that carry out mineralization processes, nitrogen deficiency causes a delay and reduction in mineralization (Navarro-Pedreño et al. 2021). Xue et al. (2012) reported that the addition of extra nitrogen to the soil in wastewater irrigation conditions significantly increased CO2 emissions. This situation reveals the high relationship between nitrogen and organic carbon and the importance of organic carbon and the nitrogen ratio (C:N) (Zang et al. 2016). Suitable C:N supports soil biological activity and increases CO2 emissions as a result of soil respiration (Al-Kaisi & Yin 2005). However, high levels of C:N can also limit the use of nitrogen by microorganisms, resulting in reduced CO2 emissions (de Figueiredo et al. 2019). If C:N is below 20, decomposition starts and nitrogen becomes free, but if it is above 20–30, mineralization and immobilization exhibit equilibrium. Shakeel et al. (2021) stated that irrigation with wastewater increases CO2 emission, but the degree of emission increases further due to higher organic carbon and nitrogen input into the soil under irrigation conditions with raw wastewater.
The higher CO2 emission in conventional tillage compared to direct sowing (Figure 10) could be related to the increase in mineralization as a result of the acceleration of biological activity in the soil with the increase in oxygen input to the soil as a result of intensive soil tillage. Intensive soil tillage decomposes the soil and increases its aeration, causing the decomposition of carbon in the soil (Altikat et al. 2012). In addition to increasing the oxygen level of the soil, soil tillage creates a suitable environment for microbial activities by ensuring that the residues come into contact with the soil at a higher level (Vurarak & Bilgili 2015). However, minimum tillage practices provide organic carbon stability in the soil, supporting both the reduction of CO2 emissions and the preservation of soil fertility (Kocyigit 2008). In addition, the interventions applied to the soil affect emissions by changing the soil physical properties (De Oliveira Silva et al. 2019). Intensive soil tillage disrupts soil clumps and increases CO2 emissions with increased carbon oxidation as a result of more O2 entry into the soil (Nyambo et al. 2020). However, in direct sowing applications, carbon sequestration in the soil increases as a result of the decrease in aeration due to the increased bulk density of the soil, thus reducing CO2 emissions (Gozubuyuk et al. 2014). In a study examining the effects of direct sowing and conventional tillage applications on CO2 emissions, it was determined that CO2 emission was 44% higher in conventional tillage than in direct sowing (Akbolat et al. 2016). In addition, the effect of direct sowing on reducing CO2 emissions has been stated by confirming similar results in many studies (Jabro et al. 2008; Abdalla et al. 2016; Bilandžija et al. 2016; Buragiene et al. 2019; Nyambo et al. 2020).
CONCLUSION
This study examined the residual effect of the pre-sowing period in the following year on CO2 emissions from the soil where silage maize was grown by full irrigation with freshwater and wastewater at different levels under conventional and direct sowing conditions for 2 years. It was determined that irrigation at a 100% level with wastewater and direct sowing decreased soil temperature and H2O emissions from the soil and increased soil moisture, and also direct sowing practice and deficit irrigation reduced CO2 emissions from the soil, which can significantly increase the severity of global warming. Therefore, it could be concluded that a useful practice for reducing the residual CO2 emission effect of irrigation with wastewater from previous years may be deficit irrigation under direct sowing in a semi-arid region. However, to clearly evaluate the effects of wastewater on CO2 emissions in the following years, studies to be carried out in different climatic and soil conditions are needed, and it can be recommended to develop innovative studies for the development of applications such as deficit irrigation and direct sowing, that reduce the increased emission in the next periods, instead of the already proven emission-increasing effect of wastewater during the crop production period.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.