Conversion from rice to vegetable production increases N₂O emission via increased soil organic matter mineralization

The conversion from rice to vegetable production widely occurs in China. However, the effects of this conversion on N₂O emission and the underlying mechanisms are not well understood. In the present study, 12 rice paddies (R) were selected and half of them converted to vegetable fields (V) with the following treatments: rice paddies without N-fertilizer (R-CK), rice paddies with conventional N-fertilizer (R-CN), converted vegetable fields without N-fertilizer (V-CK), and converted vegetable fields with conventional N-fertilizer (V-CN) in a randomized block design with 3 replicates. N₂O emissions were measured with static chambers from December 2012 to December 2015. Within each V-CN plot, a root exclusion subplot was established to measure soil heterotrophic respiration (CO₂ effluxes), a proxy for soil organic matter mineralization. Conversion of rice paddies to vegetable production dramatically increased N₂O emissions. The three-year cumulative N₂O emissions were 0.59, 1.90, 55.50 and 160.14kg N ha^-1 for R-CK, R-CN, V-CK and V-CN, respectively. The annual N₂O emissions from vegetable fields ranged between 5.99 and 113.45kg N ha^-1yr^-1, with substantially higher emissions in the first year. N₂O fluxes from V-CN were significantly and positively related to CO₂ fluxes and inorganic N concentrations. The linear relationship between natural logarithms of N₂O and CO₂ fluxes was stronger and the regression coefficient higher in the first year, showing the dependence of N₂O on soil organic matter mineralization. These results suggest that soil organic matter and N mineralization contributes significantly to N₂O emission following conversion of rice paddies to vegetable production.