Soil CO2 Emission Linearly Increases with Organic Matter Added Using Stabilized Sewage Sludge under Recycled Wastewater Irrigation Conditions

Although adding organic matter by applying sewage sludge and recycled wastewater in agricultural lands is beneficial for most functions expected of soils, mainly by improving soil structure and fertility, it is a potential source of carbon dioxide (CO2) emissions. This study aimed to analyze the changes in CO2 emissions by season and per unit of organic matter from bare soil with the organic matter added under recycled wastewater irrigation conditions. Therefore, an experiment in containers was conducted in a greenhouse with two water types (freshwater and recycled wastewater) at five different organic matter levels (control: 1.91%, and then 2.45%, 2.99%, 3.53%, and 4.07%) by mixing in stabilized sewage sludge. The experiment was repeated three times in total. Containers were irrigated a total of ten times at 6-day intervals. CO2 emissions were directly measured at the beginning and end, and at 3 days after each irrigation cycle, with an EGM-5 infrared gas analyzer device. Water (H2O) emissions from the soil, the soil moisture, and temperatures were also recorded during the CO2 emission measurements. The results showed that higher rates of organic matter in soil resulted in higher CO2 emissions from the soil that linearly increased with organic matter content for both water types, while there were lower CO2 emissions per unit of organic matter. The mean CO2 emissions with increasing doses were found to be higher by 17.9%, 30.6%, 43.0%, and 56.4%, respectively, compared to the control. Recycled wastewater resulted in 9.5% higher emissions compared to freshwater. The decreasing amounts of CO2 emissions per unit of organic matter with increasing doses were determined to be 8.1%, 17.1%, 24.6%, and 27.2%, respectively, compared to the control. Increasing organic matter and irrigation with recycled wastewater increased soil moisture and temperature values while decreasing H2O emissions from the soil. Strong linear correlations of CO2 emissions with H2O emissions, soil moisture, and temperatures were determined. Therefore, soil carbon sequestration in irrigated conditions can be improved by managing soil moisture; thus, the contribution of the increased organic matter in the soil to improve soil properties and productivity can be increased.