Decrease in the annual emissions of CH4 and N2O following the initial land management change from rice to vegetable production.

In recent years, rice paddies have been increasingly converted to vegetable production resulting from economic benefits and changes in demand of diets, potentially altering soil greenhouse gas (GHG) balance. Here, we implemented a parallel field experiment to simultaneously quantify the differences in emissions of CH4 and N2O among rice paddy (RP) and conventional vegetable field (CV) and greenhouse vegetable field (GV), both of which have been recently converted from rice paddy in subtropical China over a full year. The results revealed that CH4 emission was reduced dramatically by nearly 100% following the initial land management change from rice to vegetable production, with annual emissions of 720.9, 0.9, and 0.2 kg CH4-C ha-1 for RP, CV, and GV, respectively. This conversion however substantially increased N2O emissions, resulting in the transition from a minor sink of N2O in RP (-0.1 kg N ha-1 yr-1) to considerable N2O sources in CV (31.8 kg N ha-1 yr-1) and GV (52.2 kg N ha-1 yr-1). Furthermore, annual N2O emission from GV significantly exceeded that from CV due to lower soil pH and higher soil temperature facilitating N2O production in GV relative to CV. Land management change significantly decreased the annual total emissions of CH4 and N2O from CV and GV by 19-51% as compared to RP, attributing to the reduced CH4 emissions outweighing the increased N2O emissions in CV and GV. These results indicate that expansion of vegetable production at the expense of rice paddies for higher economic benefits also helps mitigate the total emissions of CH4 and N2O.