Straw amendment with nitrate-N decreased N2O/(N2O+N2) ratio but increased soil N2O emission: A case study of direct soil-born N2 measurements
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 N2O emission. Straw application can affect both denitrification rate and its product stoichiometry (N2O/(N2O + N2) 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/O2 atmosphere and measured N2O and direct N2 fluxes over 22 days. Soil amended with and without rice straw (2.5 g kg−1 soil) in conjunction with nitrate fertilizer (10mM KNO3) and non-amended control soil were incubated under 85% water-filled pore space. To simulate a short soil anoxic period, three different O2 partial pressures phases were set (20%, 5% and 10%). Additionally, N2O site preference signatures of soil-emitted N2O were analyzed to identify the processes contributing to N2O fluxes. Addition of nitrate increased cumulative N2O fluxes and decreased cumulative N2 fluxes compared with non-fertilized control, while rice straw amendment increased both N2O and N2 emissions drastically compared with the nitrate only treatment. The N2O 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 N2O and the N2O/(N2O + N2) product ratio strongly depends on soil NO3− concentration. As a conclusion, the present study suggests that straw amendment in conjunction with nitrate-N can increase soil N2O emissions under conditions favoring denitrification, even though it may decrease the overall N2O/(N2O + N2) product ratio.