Conversion from rice to vegetable production increases NO 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 NO 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. NO 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 (CO effluxes), a proxy for soil organic matter mineralization. Conversion of rice paddies to vegetable production dramatically increased NO emissions. The three-year cumulative NO emissions were 0.59, 1.90, 55.50 and 160.14kg N ha for R-CK, R-CN, V-CK and V-CN, respectively. The annual NO emissions from vegetable fields ranged between 5.99 and 113.45kg N hayr, with substantially higher emissions in the first year. NO fluxes from V-CN were significantly and positively related to CO fluxes and inorganic N concentrations. The linear relationship between natural logarithms of NO and CO fluxes was stronger and the regression coefficient higher in the first year, showing the dependence of NO on soil organic matter mineralization. These results suggest that soil organic matter and N mineralization contributes significantly to NO emission following conversion of rice paddies to vegetable production.