Isotopomer analysis of nitrous oxide accumulated in soil cultivated with tea (Camellia sinensis) in Shizuoka, central Japan

Nitrous oxide (N2O) is a greenhouse gas that is destroying the stratospheric ozone to an increasing degree. Because of nitrogenous fertilizer application, agricultural soil is an important contributor of global N2O. In Japan, tea fields are amended with the highest level of N fertilizers among agricultural soils, causing soil acidification and large N2O flux. In soil, microbes play key roles in producing and consuming N2O. A previous study investigated net N2O production in tea fields using N2O flux measurement and soil incubation, which are indirect methods to analyze relevant processes of N2O production and consumption in soil. In the present study, to analyze N2O concentrations and isotopomer ratios (bulk nitrogen and oxygen isotope ratios, δ15Nbulk and δ18O, and intramolecular 15N site preference, SP) and to reveal most probable microbial production processes and consumption (N2O reduction to N2) process of N2O, soil gas was collected from a tea field (pH 3.1-4.5) at 10-50 cm depths using a silicone tube. The combination of fertilization, precipitation, and temperature rise produced significantly high N2O concentrations. During the period of high N2O concentration (above 5.7 ppmv), SP, the difference in 15N/14N ratio between central (α) and terminal (β) nitrogen position in the linear N2O molecule (βNαNO) showed low values of 1.4‰-9.8‰, suggesting that the contribution of bacterial denitrification (nitrifier-denitrification and bacterial denitrifier-denitrification) was greater than that of bacterial nitrification or fungal denitrification. High SP values of 15.0‰-20.1‰ obtained at 10, 35, and 50 cm depths on 31 May 2011 (after one of fertilizations) during which soil temperatures were 15.8 °C-17.9 °C and water-filled pore space (WFPS) was 0.73-0.89 suggest that fungal denitrification and bacterial nitrification contributed to N2O production to a degree equivalent to that of bacterial denitrification.