Seasonal variations of carbon isotopic composition of methane from Indian paddy fields

Abstract

Systematic measurements of methane (CH₄) flux and δ¹³C of methane from two paddy fields, one each from southern and western India, were carried out. There is a significant variation in δ¹³C values of methane emitted during the entire season. The estimated annual emission rates of methane for the above two paddy fields are 16 and 9 g m^-2, respectively, which are in good agreement with methane flux values reported for paddy fields in the neighboring regions within India. Methane emitted in the beginning of the growing season is relatively enriched in ¹³C, then depleted before harvesting, and finally enriched again at the end of season. Seasonal changes in δ¹³C of methane are probably due to changes in methane production, oxidation, and fractionation associated with transport. The flux-weighted annual average values of δ¹³C are calculated to be -54.3‰ and -57.2‰ for the two investigated paddy fields, which are more enriched compared to that for paddy fields in Japan, China, and Italy.