Seasonal variations of stable hydrogen and carbon isotope ratios in methane from a Chinese rice paddy

Variations in the isotopic composition (delta(13)C, delta D) of methane emitted from a rice paddy in Suzhou, China (31.2 degrees N, 120.4 degrees E) were investigated during a whole rice-growing season in 1995. Isotope analysis was performed using our tunable diode laser based methane isotopomer spectrometer (MISOS). Both delta(13)C and delta D of the emitted CH4 show a pronounced seasonal variation with delta(13)C ranging between -51.6 and -70.8 parts per thousand V-PDB (mean -63.8 +/- 4.9 parts per thousand (1 sigma); n = 28) and delta D between. -352 and -311 parts per thousand V-SMOW (mean -335.7 +/- 10.1 parts per thousand; n = 19). The shapes of the seasonal delta(13)C and delta D variations were very similar between plots with two different fertilizer;treatments. Gas bubbles, collected from the stirred sediment throughout the season, show a smaller variation in their isotopic composition (delta(13)C = -64.1 +/- 3.0 parts per thousand V-PDB, n = 13; delta D = -349.2 +/- 4.7 parts per thousand V-SMOW, n = 12). In comparison, CH4 emitted into the atmosphere is enriched both in delta(13)C and delta D during the first 30 days after flooding, indicating the influence of partial oxidation by methanotrophic bacteria. Between days 60 and 90 after flooding, however, emitted CH4 becomes depleted in delta(13)C but remains enriched in delta D. Possible explanations are (1) the existence of different CH4 reservoirs, for which the partitioning of methanogenic pathways is different, or (2), a combination of isotope fractionation by CH4 oxidation and molecular diffusion during the diffusive transport through the rhizosphere. The seasonal variation in delta(13)C of emitted CH4 resembles that found in a previous study on a Japanese rice paddy [Tyler et al., 1994] suggesting (1) a typical behavior due to systematic changes related to CH4 production, transport, and oxidation during the vegetation cycle and (2) a significant impact on atmospheric delta(13)C signals.