Effect of rice straw application on stable carbon isotopes, methanogenic pathway, and fraction of CH4 oxidized in a continuously flooded rice field in winter season

Abstract

Straw application is a common practice in rice agriculture, but its effect on stable carbon isotopes (δ13C) in each process of CH4 emission, relative contribution of acetate to CH4 production (Fac), and fraction of the CH4 that is oxidized (Fox) in the winter season is not well known. We investigated CH4 production and oxidation potentials in paddy soil, CH4 concentrations in soil pore water and floodwater, and CH4 emission as well as their δ13C from a continuously flooded paddy field with rice straw application (RSA) during the 2007 and 2008 winter seasons and quantified the Fac and Fox using the isotopic data. RSA significantly increased CH4 emission (p < 0.05) but made δ13CH4 (emission) lower by 5–8‰. RSA obviously increased CH4 concentrations in soil pore water and floodwater, but it played a slight role in porewater δ13CH4 while caused floodwater δ13CH4 4‰ lower. A relatively low contribution of acetate-dependent methanogenesis (about 50%) was observed, and RSA slightly increased Fac (about 0–10%) at a relatively low temperature whereas greatly decreased it (about 10–20%) at a high temperature. Floodwater CH4 was much more 13C-enriched than porewater CH4 (p < 0.05), suggesting that about 60–70% of the CH4 was oxidized at the soil–water interface. Fox generally decreased in the winter season and it was reduced by RSA (about 10%). Soil temperature was positively correlated with CH4 flux and CH4 flux was negatively related to δ13CH4 (emission), suggesting that temperature might be an important factor influencing Fox, thus CH4 emission and δ13CH4 (emission). The findings suggest that RSA significantly increased CH4 production while little affected CH4 oxidation, thus decreasing Fox. Moreover, the effect of RSA on methanogenic pathway might be controlled by soil temperature, and RSA generally reduced Fac and Fox was the main reason for the emitted CH4 depleted in 13C.