Seasonal agricultural wetlands act as potential source of N2O and CH4 emissions

Seasonal wetlands and depressions in agricultural fields that accumulate nutrients from runoff water might act as hotspots of nitrous oxide (N₂O) and methane (CH₄) emissions. However, the spatiotemporal pattern of these gases from different parts of agricultural fields had not been compared. A two-year field experiment was conducted in Ontario, Canada, to test whether field locations with higher wetness act as N₂O and CH₄ hotspots. The objectives included, i) to classify agricultural field using topographical wetness index (TWI), ii) to determine the spatiotemporal patterns of N₂O, CH₄, and soil properties from TWI classes; and iii) to determine the impact of soil variables on N₂O and CH₄ emission prediction. The field was divided into four classes based on TWI, i) low TWI (C1; steep slopes), ii) medium TWI and slopes (C2), iii) Higher TWI (C3; low slopes), and iv) very high TWI (C4; seasonal wetlands and depressions). Each TWI class had six sampling points as replicates to collect soil and gas samples during corn (2019) and soybean (2020) seasons. TWI classes had a significant impact on N₂O and CH₄ emissions. Mean maximum and minimum N2O emissions were observed during corn season from C4 (153 ± 34) and C1 (69 ± 35 µg N₂O-N m⁻²h⁻¹), respectively. However, during soybean season, C1 had higher (70 ± 1), and C4 had lower average (26.5 ± 8 µg N₂O-N m⁻²h⁻¹) emissions. Higher mean CH₄ emissions were observed from C4 and C1, respectively, during corn (1264 ± 778) and soybean (60 ± 44 µg CH₄-C m⁻²h⁻¹) seasons. Regression analysis revealed that N₂O emissions were mainly controlled by nitrate, electrical conductivity, and moisture during corn and ammonium, pH, and nitrate during soybean season. Similarly, CH₄ emissions were regulated by moisture and clay during corn season and ammonium and pH during soybean season. These relationships could help upscale greenhouse gas emissions at regional levels and formulate mitigation strategies.