Deficit irrigation with wastewater in direct sowed silage maize reduces CO₂ emissions from soil by providing carbon savings

Agriculture is the sector that consumes the most fresh water resources compared to other sectors (FAO 2020). For this reason, it is very important to use marginal water resources instead of fresh water in agriculture to conserve fresh water resources. Wastewater, one of the marginal water resources (Cakmakci & Sahin 2021), can be defined as water that has been polluted as a result of various uses, and the properties have changed partially or completely. Wastewater increases soil and crop productivity with irrigation in terms of its rich nutrient content. In addition, the wastewater, which reduces the need for fertilizers, contributes to environmental sustainability through disposal also.

While the nutritive properties of wastewater are considered as positive advantages, the high organic matter contribution of wastewater to the soil may increase CO₂ emissions from soil (Rosso & Stenstrom 2008). It is estimated that about 10% of the amount of CO₂ in the atmosphere is released from the soil (Raich & Potter 1995). Although CH₄ and N₂O cause global warming 21 times and 310 times more than CO₂, respectively (Forster et al. 2007), the 82% share of CO₂ among greenhouse gases makes it the most important greenhouse gas. It is very worrying that the amount of CO₂, around 280 ppm in the atmosphere in the 1700s, reached 406 ppm in 2017 (WMO 2019). It is thought that if the amount of CO₂ continues to increase at this rate, it would reach 450 ppm in 2050, and as a result, agricultural, social, and economic life would be adversely affected (Aksay et al. 2005).

The agricultural sector causes 25% of global warming (Tubiello et al. 2015; Vurarak & Bilgili 2015). Many factors such as soil temperature, soil water content, and O₂ level have an effect on CO₂ emissions from soil. Soil tillage has a significant share in increasing agricultural greenhouse gases (Abdalla et al. 2016). Soil tillage provides a suitable environment for microbial activities and increases the speed of these activities by increasing the oxygen level of the soil and by ensuring that the surface residues come into closer contact with the soil particles (Vurarak & Bilgili 2015). However, direct sowing or minimum tillage contributes to both reducing emissions and increasing soil and crop productivity by providing moisture and carbon sequestration in the soil (Kocyigit 2008). In addition, considering the possibility of increased CO₂ emissions with soil moisture, CO₂ emissions can be reduced with an easy and practical approach in deficit irrigation (Hou et al. 2020).

It has been determined that no findings from previous studies about the effect of deficit wastewater irrigation in direct sowed silage maize on the CO₂ emissions during the harvesting period were reported. In this context, the effect of using domestic treated wastewater at different levels (100, 67, and 33%) in the irrigation of silage maize under conventional tillage and direct sowing on the CO₂ emission from soil at the end of the vegetation period was investigated. In addition, the relationship of CO₂ emission with H₂O emission, soil moisture, and soil temperature at this period was evaluated. Therefore, the hypothesis of this study was that (1) direct sowing would decrease CO₂ emissions from soil by providing carbon savings, (2) deficit irrigation might increase the benefit to reduce CO₂ emission, and (3) CO₂ emission would show a strong correlation with H₂O emission, soil moisture content, and soil temperature.

This study was conducted at the Experimental Field of the Faculty of Agriculture, Van Yuzuncu Yil University, for 2 years. The experimental area is a semi-arid region and located at an altitude of 1,670 m near Lake Van (38° 34′35″ N and 43° 17′26″ E). The total precipitation amount measured in the experimental area in the vegetation period of silage maize in 2020 and 2021 was 37.0 and 52.1 mm, respectively. The mean air temperature during the vegetation period was 22.4 °C in 2020 and 22.8 °C in 2021.

Each plot was planned with five rows, 70 cm×15 cm row spacing, and measuring 3.5 m × 7.2 m. The experiment was carried out according to the split plots design. The main plot factor of the experiment was conventional tillage and direct sowing, and sub-plot factors include full irrigations with fresh water and wastewater (FW100 and WW100), and 33% (WW67) and 67% (WW33) deficit irrigations with wastewater, in three replications. Therefore, 24 plots were arranged in the experimental field.

The seeds of silage maize (cv ‘OSSK 644’) in trial years were sown in May and harvested in September. The conventional tillage plots were plowed, then the clods were broken down by using a cultivator-rotary harrow combination and the plots were leveled. The seeds were sowed with a precision sowing machine on the same day after soil tillage. In the direct sowing treatment, the seeds were sowed with a direct sowing machine on the same day as conventional tillage. The hoeing in conventional tillage plots in both years was carried out twice as crop heights reached 15–20 and 40–50 cm.

Irrigation applications were carried out with a surface drip irrigation system placed one lateral to each crop row at 70 cm intervals. Laterals with in-line drippers with a flow rate of 2.3 l h⁻¹ and 33 cm spacing were used. The fresh water used in the study was obtained from tap water in the university campus. Recycled wastewater was collected from the Van city Edremit Domestic Wastewater Treatment Plant and transported before each irrigation and brought to the experimental area. The average values of soil reaction (pH), electrical conductivity (EC), biological oxygen demand (BOD), chemical oxygen demand (COD), and total N of the domestic wastewater throughout the irrigation periods in trial years were 7.58, 1.124 dS m⁻¹, 23.2 mg l⁻¹, 37.5 mg l⁻¹, and 10.9 mg l⁻¹, respectively. The pH and EC values for fresh water were 8.15 and 0.353 dS m⁻¹.

In all plots, the crops were irrigated with fresh water equally with a 30% wetting percentage considering 30 cm soil depth until they get 40–50 cm height. After this stage, irrigation with wastewater at different water deficit levels was started and continued until the harvest with a 65% wetting percentage considering 90 cm soil depth. A total of 13 and 12 irrigations were made in the first and second years of the experiment, respectively. The plots fully irrigated reached the seasonal irrigation quantities of 351.3 mm in the first year and 327.3 mm in the second year in the conventional tillage practice. In the direct sowing practice, the irrigation quantities of 318.7 and 294.1 mm were applied in the first and second trial years.

The soil texture in the experimental area was sandy clay loam. While the soil organic carbon contents in the 0–30 cm soil layer at the harvest were determined at 1.0, 1.15, 1.05, and 0.88% in the FW100, WW100, WW67, and WW33 treatments in the first year, respectively, and the second year values were 1.04, 1.24, 1.07, and 0.91%. The carbon contents in the first year were found to be 1.0% in direct sowing and 1.03% in conventional tillage practice, and 1.06 and 1.07% in the second year. Organic matter determined with the Walkley-Black method was converted to organic carbon using the standard coefficient (Nelson & Sommers 1982; Setyanto et al. 2004; Avramidis et al. 2015).

The CO₂ emissions from the soil in all plots were measured at the time of silage maize harvest in September. The measurements were made by an infrared gas analyzer device (EGM-5, PPSystems, Stotfold, UK) from three different points randomly selected from each plot with three replicates between two crops, approximately 15–20 cm from the dripper. In addition, soil temperatures and moisture contents at 5, 10, and 20 cm soil depths and the H₂O emissions were measured simultaneously with CO₂ emission measurement. The measurement of H₂O emission was provided automatically by the device, and a temperature probe connected to the CO₂ measurement device was used in soil temperature measurements. Soil moisture contents were determined using the TDR probe (Trime-Pico, IPH/T3, IMKO) calibrated to the experimental field.

The data were evaluated by variance analysis (ANOVA) using SPSS software (v23.0). Correlograms with a scatterplot, correlation coefficient, and variable distribution were built using the RStudio package to determine the relationships of CO₂ emission with H₂O emission and soil moisture and temperatures.

The higher CO₂ emissions in the WW100 treatment and the decrease in emission values under deficit irrigation can be explained by the rich nutrient content of the wastewater and by the lower entering of nutrients from the wastewater to the soil under decreased irrigation conditions. In particular, the high amount of organic carbon with wastewater earned to the soil can be considered as the main cause of CO₂ emissions. The content of organic carbon in the WW100 treatment was 16.5, 13.2, and 34.8% higher than the FW100, WW67, and WW33 treatments, respectively. The organic carbon accumulated in the soil encounters O₂, and it turns into CO₂ form and leaves the soil, and spreads to the atmosphere (Yerli et al. 2019). Similarly, Rosso & Stenstrom (2008) and Kudal & Muftuoglu (2014) reported that irrigation with wastewater increases the amount of organic carbon in the soil. Shakeel et al. (2021) determined that the highest CO₂ emission in maize cultivation was obtained from the raw wastewater plots and the lowest CO₂ emission was obtained from the plots irrigated with fresh water. Nosalewicz et al. (2013) stated that CO₂ emissions from the soil increased in wastewater irrigation conditions and this increase was related to the high organic matter contribution provided to the soils depending on the nutrient content of the wastewater.

The increased CO₂ emissions in conventional tillage can be explained with intensive soil tillage increasing the O₂ entry into the soil, thereby accelerating the oxidation of organic carbon. Therefore, in the direct sowing practice, organic carbon content was 2.0% higher than in conventional tillage, showing that the increasing carbon stock in the soil reduced CO₂ emissions. It is also thought that less emission under direct sowing practice is related to the direct sowing of the seed and thus to the more superficial intervention in the soil. Vurarak & Bilgili (2015) reported that soil tillage creates a suitable environment for microbial activities both by increasing the O₂ level of the soil and by enabling the surface residues to come into closer contact with the soil particles. Altikat et al. (2012) expressed that organic carbon decomposition increased with increased soil tillage due to physically breaking up the soil and providing better aeration. Akbolat et al. (2016) determined that the CO₂ emissions in direct sowing conditions were 30.6% lower than the conventional tillage, and pointed out that CO₂ emission from the soil in direct sowing is less due to less O₂ entry into the soil.

In the average trial years, while the WW100 and FW100 treatments provided similar H₂O emissions and soil moisture contents, H₂O emissions in the WW100 treatment (12.6 g m⁻² h⁻¹) compared to the WW67 and WW33 treatments were found to be 16.7 and 34.0% higher, respectively (Figure 2). The soil moisture at 5, 10, and 20 cm soil depths in WW100 (0.237–0.260 m³ m^−-3) also were 5.3–8.2% and 13.4–14.0% higher compared to WW67 and WW33 treatments, respectively (Table 2). It was concluded that the proportion of reduction in H₂O emission with the water deficit was greater than the proportion of reduction in soil moisture content, and this indicated that high soil moisture contents contributed more to H₂O emission. The increased H₂O emissions in high irrigation conditions due to increased soil moisture content were also determined in previous studies. Akbolat & Senyigit (2012) reported that H₂O emissions decreased due to the decrease in the occupancy rate of the pores with increasing deficit irrigation levels. Senyigit & Akbolat (2010) also, in their study examining the effects of varying amounts of irrigation water on H₂O emissions, stated that the lowest H₂O emissions were obtained from the lowest amount of irrigation quantity.

In the average trial years, while soil temperatures at 5 and 10 cm soil depths changed significantly by the tillage-sowing practices, the significant effects of irrigation treatments were determined in all measuring depths (Table 2). Soil temperatures at 5 and 10 cm soil depths in direct sowing practice (22.1–22.4 °C) were 3.9 and 2.6% less than the conventional tillage. Soil temperatures in all depths were similar in FW100 and WW100 treatments. The soil temperatures at 5, 10, and 20 cm soil depths under WW100 treatment (21.3–22.2 °C) were 3.9–6.6%, 7.1–8.6%, and 6.6–8.6% lower than the values in WW67 (22.6–23.1 °C) and WW33 (23.3–23.9 °C) treatments, respectively. The higher soil temperature in deficit irrigation can be explained by the lower moisture content in the soil considering significant negative correlations between soil moisture and temperatures (Figures 2 and 3). High moisture content in the soil can reduce the soil temperature by creating a cooling effect in the soil with increased evaporation (Mancinelli et al. 2015). The soil temperature in direct sowing is lower than in conventional tillage can be evaluated in relation to the fact that using the shading effect of the residues on the soil surface in direct sowing reduces the soil temperature. Shen et al. (2018) stated that the soil temperature was lower between 0 and 1.5 °C due to the covering effect of residues on the soil surface in direct sowing compared to intensive tillage treatments. In addition, tillage practices can significantly influence the thermal conductivity of the soil by changing soil properties such as the bulk density and aggregation (Van Wie et al. 2013). Similarly, Salem et al. (2015) reported that the soil temperature was lower under direct sowing conditions.

It has been determined that the negative linear relationships of CO₂ emission with soil temperature values at two soil depths (5 and 20 cm) were significant (p<0.05) in the conventional tillage practice (Figure 2). Similarly, Buragiene et al. (2019) reported a negative correlation between soil temperature and CO₂ emissions from soil. However, in direct sowing practice, the correlations were not significant (Figure 3). In conditions where the effect of soil temperature on CO₂ emissions is evaluated independently of moisture, increases in CO₂ emissions can be observed with the positive effect on soil biological activity of the increase in soil temperature. Therefore, many previous studies reported that soil temperature is positively correlated with CO₂ emissions (Sainju et al. 2008; Rey et al. 2011; Nosalewicz et al. 2013; Chen et al. 2018; Du et al. 2019; Zhao et al. 2020). In this study, although soil temperatures increased with water deficit (Table 1), soil moisture contents also were reduced significantly. Therefore, it was concluded that soil moisture had a dominant effect on CO₂ emissions compared to the soil temperature. It is indicated that a general result, the moisture balance of the soil and the processes of keeping it moist greatly affect the oxidation of organic carbon in the soil (Morugan-Coronado et al. 2011; Shi & Marschner 2014). Considering mentioned approaches above, it also could be said that lowering soil temperatures in the direct sowing practice, compared to the conventional tillage, made the effect of temperature insignificant on CO₂ release with better soil moisture conditions.

The harvesting time findings in silage maize cultivated conditions revealed that full irrigation with wastewater increased CO₂ emissions, and while the CO₂ emissions, H₂O emissions, and soil moisture contents in deficit irrigation conditions with wastewater were lower, soil temperatures were higher. In addition, direct sowing practice significantly contributed to the reduction of CO₂ emissions when compared to conventional tillage by providing carbon savings. It was found that direct sowing practice resulted in lower soil temperatures than the conventional tillage. While the significant positive linear relationships were determined between CO₂ emissions with H₂O emissions and the soil moisture contents measured in different soil depths (5, 10, and 20 cm), soil temperatures negatively correlated to the CO₂ emissions.

Climate change threatens populations and countries through damage to ecosystems, increases in hazardous weather events, declines in productivity, and causes greater costs to maintain current infrastructure. As a conclusion of this study, since full irrigation with wastewater and conventional tillage significantly increase CO₂ emissions from soil, it was clearly seen that deficit irrigations with wastewater in direct sowing practice reduce CO₂ emissions from soil at a considerable level. However, there is a need to increase the practicality of short-term findings with the results of long-term wastewater applications that will provide more organic carbon recovery in different geographic locations.

This study was financially supported by the Scientific and Technological Research Council of Turkey (TUBITAK Project No: 119O528).

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

The authors declare there is no conflict.