Department of Life Science, Chung-Ang University, Seoul, Korea,
Methane emissions, along with methanotrophs and methanogens and soil chemical properties, were investigated in a flooded rice ecosystem. Methane emission increased after rice transplantation was positively and significantly correlated with transcripts of pmoA and mcrA genes, transcript/gene ratios of mcrA, temperature, and total TOC. Methane flux was negatively correlated with sulfate concentration. Methanotrophs represented only a small proportion of total bacterial 16S rRNA reads: Methylocystis decreased quickly after rice transplantation, while Methylosinus were relatively constant throughout rice cultivation. Methanosaeta, Methanocella, and Methanosarcina were dominant methanogens, and their maximum abundances were observed at days 60_90. Finally we found normalized mcrA/pmoA transcript ratios to be a promising parameter for predicting net methane fluxes emitted from rice paddy soils. Bacteria and Archaea, along with the oxygen, methane, and TOC concentrations, were investigated along a depth gradient in a flooded rice paddy. Broad patterns in vertical profiles of oxygen, methane, TOC, and microbial abundances were similar in the bulk and rhizosphere soils, though methane and TOC concentrations and 16S rRNA gene copies were clearly higher in the rhizosphere soil than in the bulk soil. Oxygen concentrations decreased sharply to below detection limits at the 8 mm depth. Aerobic methanotrophs were maximally observed near the oxic-anoxic interface, while methane, TOC, and methanogens were highest in the rhizosphere soil at 30_200 mm depth, suggesting that methane is produced mainly from organic carbon derived from rice plants and is metabolized aerobically. The type I methanotrophs decreased more drastically than those of type II methanotrophs with increasing depth. Methanosaeta and Methanoregula were predominant methanogens at all depths, and the relative abundances of Methanosaeta, Methanoregula, and Methanosphaerula, and GOMArcIincreased with increasing depth.