Anaerobic mineralization of indigenous organic matters and methanogenesis in tropical wetland soils

Tropical wetlands are one of the largest natural sources in the global methane budget due to high biological activities and the anaerobiosis in soil. We studied mineralization and gas production during the early stage of anaerobic decomposition of indigenous organic matters in soils of Narathiwat, southern Thailand, to clarify the significance of the substrate quality in controlling decomposition and methanogenesis in some different tropical wetland soils. The optimal temperature of decomposition was around 35°C, while methanogenesis did not proceed at 45°C. During the first 50 days of anaerobic incubation, 5 ∼ 63% (carbon basis) of indigenous plant leaves were mineralized. The mineralization rate was strongly and negatively correlated with the lignin and/or fiber contents, but not theC/N ratio, of the substrate plant materials. Difference in δ 13C between the substrate and the produced CH 4 was generally greater (more negative in CH 4) for more recalcitrant substrates, indicating that H 2 as opposed to acetate becomes a more important metabolic intermediate in the anaerobic food web when the decomposition rate is limited by substrate recalcitrance. Thus, the CH 4 isotope signature may be used to evaluate the importance of new vs. old organic matter as CH 4 source in natural soils. The mineralization rate was higher, and the isotopic difference between the substrate and CH 4 was smaller when plant materials were incubated with sulfate-contaminated soils than with native peat soils. The isotopic difference between the substrate and CH 4 was significantly different between native peat soils. Results of a tracer experiment using 13C-labeled substrates indicated that these differences could be ascribed to difference in the mode of acetate metabolism between soils.