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

Abstract

Variations in the isotopic composition (δ13C, 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 δ13C and δD of the emitted CH4 show a pronounced seasonal variation with δ13C 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 δ13C 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 (δ13C=−64.1±3.0‰ V‐PBB, n=13; δD=−349.2±4.7‰ V‐SMOW, n=12). In comparison, CH4 emitted into the atmosphere is enriched both in δ13C 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 CH4 becomes depleted in δ13C but remains enriched in δD. Possible explanations are (17) 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 δ13C 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 δ13C signals.