P. Bergamaschi, SEASONAL-VARIATIONS OF STABLE HYDROGEN AND CARBON-ISOTOPE RATIOS IN METHANE FROM A CHINESE RICE PADDY, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 102(D21), 1997, pp. 25383-25393
Variations in the isotopic composition (delta(13)C, delta D) of methan
e emitted from a rice paddy in Suzhou, China (31.2 degrees N, 120.4 de
grees E) were investigated during a whole rice-growing season in 1995.
Isotope analysis was performed using our tunable diode laser based me
thane isotopomer spectrometer (MISOS). Both delta(13)C and delta D of
the emitted CH4 show a pronounced seasonal variation with delta(13)C r
anging 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 tho
usand; n = 19). The shapes of the seasonal delta(13)C and delta D vari
ations were very similar between plots with two different fertilizer;t
reatments. Gas bubbles, collected from the stirred sediment throughout
the season, show a smaller variation in their isotopic composition (d
elta(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 a
fter flooding, however, emitted CH4 becomes depleted in delta(13)C but
remains enriched in delta D. Possible explanations are (1) the existe
nce of different CH4 reservoirs, for which the partitioning of methano
genic pathways is different, or (2), a combination of isotope fraction
ation by CH4 oxidation and molecular diffusion during the diffusive tr
ansport 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 oxidat
ion during the vegetation cycle and (2) a significant impact on atmosp
heric delta(13)C signals.