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

Variations in the isotopic composition (δ 13 C, δD) of methane emitted from a rice paddy in Suzhou, China (31.2°N, 120.4°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 δ 13 C and δD of the emitted CH 4 show a pronounced seasonal variation with δ 13 C ranging between −51.6 and −70.8‰ V‐PDB (mean −63.8±4.9‰ (1σ); n =28) and δD between −352 and −311‰ V‐SMOW (mean −335.7±10.1‰; n =19). The shapes of the seasonal δ 13 C and δD 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 (δ 13 C=−64.1±3.0‰ V‐PBB, n =13; δD=−349.2±4.7‰ V‐SMOW, n =12). In comparison, CH 4 emitted into the atmosphere is enriched both in δ 13 C and δD 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 CH 4 becomes depleted in δ 13 C but remains enriched in δD. Possible explanations are (17) the existence of different CH 4 reservoirs, for which the partitioning of methanogenic pathways is different, or (2) a combination of isotope fractionation by CH 4 oxidation and molecular diffusion during the diffusive transport through the rhizosphere. The seasonal variation in δ 13 C of emitted CH 4 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 CH 4 production, transport, and oxidation during the vegetation cycle and (2) a significant impact on atmospheric δ 13 C signals.