Effect of rice plants on methane production and rhizospheric metabolism in paddy soil

Abstract

In order to elucidate the effects of rice plants on CH4 production, we conducted experiments with soil slurries and planted rice microcosms. Methane production in anoxic paddy soil slurries was stimulated by the addition of rice straw, of unsterile or autoclaved rice roots, and of the culture fluid in which rice plants had axenically been cultivated. The addition of these compounds also increased the concentrations of acetate and H2, precursors of CH4 production, in the soil. Planted compared to unplanted paddy soil microcosms exhibited lower porewater CH4 concentrations but higher CH4 emission rates. They also exhibited higher sulfate concentrations but similar nitrate concentrations. Concentrations of acetate, lactate and H2 were not much different between planted and unplanted microcosms. Pulse labeling of rice plants with 14CO2 resulted during the next 5 days in transient accumulation of radioactive lactate, propionate and acetate, and after the second day of incubation in the emission of 14CH4. Most of the radioactivity (40–70%) was incorporated into the above-ground biomass of rice plants. However, during a total incubation of 16 days about 3–6% of the applied radioactivity was emitted as 14CH4, demonstrating that plant-derived carbon was metabolized and significantly contributed to CH4 production. The sequence of the appearance of radioactive products and their specific radioactivities indicate that CH4 was produced from root exudates by a microbial community consisting of fermenting and methanogenic bacteria.