Straw amendment with nitrate-N decreased N2O/(N2O+N2) ratio but increased soil N2O emission: A case study of direct soil-born N2 measurements

Abstract Straw application in combination with synthetic N fertilizer could increase crop yield and improve soil fertility, however, contradictory observations have been reported on the effects of straw addition on soil N 2 O emission. Straw application can affect both denitrification rate and its product stoichiometry (N 2 O/(N 2 O + N 2 ) ratio), whereas the latter remains rather unclear since the ratio is strongly regulated by other soil parameters, e.g. nitrate and oxygen concentrations at denitrifying micro-sites. In this context, we conducted an incubation experiment with a robotized continuous flow incubation system using a He/O 2 atmosphere and measured N 2 O and direct N 2 fluxes over 22 days. Soil amended with and without rice straw (2.5 g kg −1 soil) in conjunction with nitrate fertilizer (10 mM KNO 3 ) and non-amended control soil were incubated under 85% water-filled pore space. To simulate a short soil anoxic period, three different O 2 partial pressures phases were set (20%, 5% and 10%). Additionally, N 2 O site preference signatures of soil-emitted N 2 O were analyzed to identify the processes contributing to N 2 O fluxes. Addition of nitrate increased cumulative N 2 O fluxes and decreased cumulative N 2 fluxes compared with non-fertilized control, while rice straw amendment increased both N 2 O and N 2 emissions drastically compared with the nitrate only treatment. The N 2 O SP values ranged from 0.4 to 2.7‰ among all treatments, indicating denitrification/nitrifier denitrification was the dominating source. The results suggest that straw amendment can trigger high denitrification rate, whereas the effect of straw amendment on the amount of emitted N 2 O and the N 2 O/(N 2 O + N 2 ) product ratio strongly depends on soil NO 3 − concentration. As a conclusion, the present study suggests that straw amendment in conjunction with nitrate-N can increase soil N 2 O emissions under conditions favoring denitrification, even though it may decrease the overall N 2 O/(N 2 O + N 2 ) product ratio.