Effect of combined application of organic and inorganic fertilizers on soil nitrous oxide emission in maize-cowpea systems in central China

Nitrous oxide (N₂O) is an important greenhouse gas that causes stratospheric ozone destruction. Application of chemical nitrogen fertilizers in upland cultivation systems is an important source of atmospheric N₂O. It is important to determine the effects of nitrogen fertilization on N₂O emissions in upland field in Central China where the relative researches is less conducted. In this study, soil nitrification and denitrification rates, N₂O emissions and crop yields were investigated under application of organic and inorganic fertilizers with the same amounts of N in the field. Therefore in 2017, a field experiment was conducted to investigate the effects of different nitrogen fertilizer sourcesinorganic nitrogen fertilizer (I), organic nitrogen fertilizer (O) and inorganic plus organic nitrogen fertilizers (I+O) on nitrification rate, denitrification rate, N₂O emission and crop yield under a maize-cowpea cropping system in Central China. Emission of N₂O was measured by the static chamber approach and the rates of nitrification and denitrification measured by Barometric Process Separation (BaPS). The results showed that the rate of nitrification ranged from 130.90 μg·kg^-1·h^-1 to 340.37 μg·kg^-1·h^-1 in maize season, and from 145.11 μg·kg^-1·h^-1 to 348.75 μg·kg^-1·h^-1 in cowpea season. Application of organic nitrogen significantly affected soil nitrification rate. Compared with I and O treatments, I+O treatment significantly reduced soil nitrification rate respectively by 28.74% and 13.96% in maize season, and by 24.66% and 13.28% in cowpea season. However, no significant differences were observed in the rate of denitrification among three treatments. Nitrogen fertilization significantly enhanced N₂O flux, with N₂O flux peak observed immediately after nitrogen fertilizer application. The combined application of inorganic and organic nitrogen fertilizers markedly influenced N₂O emission. The mean N₂O flux in maize season under I+O treatment was (279.54±116.58) μg·m^-2·h^-1, which was 33.44% (P < 0.01) and 32.29% (P < 0.01) lower than that under I and O treatments. In cowpea season, mean flux under I+O treatment(188.07±57.63) μg·m^-2·h^-1 decreased significantly by 27.00% and 15.14%, compared with that under I and O treatments. Moreover, compared with I and O treatments, I+O treatment significantly reduced cumulative N₂O emission respectively by 33.51% and 32.51% in maize season, and by 25.77% and 15.04% in cowpea season. However, there were no significant differences in N₂O flux and cumulative N₂O emission between I and O treatments. Linear correlation analysis showed that N₂O emission was closely related with nitrification rate. Yields of maize and cowpea varied among different treatments and I+O treatment had the highest yields of maize and cowpea. Yield of maize under I+O treatment was 1.71 and 1.23 times that under I and O treatments, and yield of cowpea under I+O treatment was 13.4 and 1.17 times that under I and O treatments, respectively. No significant difference was found in cowpea yield between I and O treatments. Our results suggested that combined application of organic and inorganic nitrogen fertilizers effectively reduced soil N₂O emissions and increased crop yield in maize-cowpea cropping systems in Central China. The study had important implications for high-yield, low-carbon crop cultivation in China.