Tillage after Irrigation Decreases Soil N2O but Increases CO2 Emissions in a Rice-Wheat Rotation System

Increasing attention is paid to mitigate greenhouse gas (GHG) emissions through improvement in agricultural practices. Tillage after irrigation (Wet tillage; WT) can shift soil structure compared with tillage before irrigation (Dry tillage; DT), associated with GHG emissions, while information is limited on the effects of WT on nitrous oxide (N2O) and carbon dioxide (CO2). A two-year field experiment was conducted to assess the effects of the sequence between tillage and irrigation (i.e., WT and DT) on soil methane (CH4), N2O, and CO2 emissions and the underlying mechanisms in a rice-wheat rotation system. The results showed that WT significantly decreased cumulative N2O emissions from the jointing to maturity stage due to a lower abundance of nirS and nirK genes at booting stages, leading to a decrease of 35.1 %-42.6 % for cumulative N2O emissions under WT throughout the whole rice-growing stage (P < 0.05). However, WT increased cumulative CO2 emissions from the jointing to maturity stage due to disturbance for soil aggregation with lower mean weight diameter and geometric mean diameter, resulting in an increase of 18.3 %-19.1 % for cumulative CO2 emissions throughout the whole rice-growing period (P < 0.05) compared with DT. WT had a minor effect on methane (CH4) emissions and rice yield and decreased GHG intensity only in the first year. Our results highlight a trade-off between N2O and CO2 emissions under WT, and a strategic rotation between WT and DT may be a clean practice to sustain the irrespective benefits of WT and DT.