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Seasonal variation in pathways of CH 4 production and in CH 4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors
Author(s) -
Krüger Martin,
Eller Gundula,
Conrad Ralf,
Frenzel Peter
Publication year - 2002
Publication title -
global change biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.146
H-Index - 255
eISSN - 1365-2486
pISSN - 1354-1013
DOI - 10.1046/j.1365-2486.2002.00476.x
Subject(s) - methanogenesis , isotopes of carbon , isotope fractionation , methane , environmental chemistry , aerenchyma , chemistry , carbon dioxide , stable isotope ratio , isotopic signature , fractionation , isotope , carbon fibers , δ13c , total organic carbon , botany , organic chemistry , biology , physics , materials science , quantum mechanics , composite number , composite material
Abstract Flooded rice fields, which are an important source of the atmospheric methane, have become a model system for the study of interactions between various microbial processes. We used a combination of stable carbon isotope measurements and application of specific inhibitors in order to investigate the importance of various methanogenic pathways and of CH 4 oxidation for controlling CH 4 emission. The fraction of CH 4 produced from acetate and H 2 /CO 2 was calculated from the isotopic signatures of acetate, carbon dioxide (CO 2 ) and methane (CH 4 ) measured in porewater, gas bubbles, in the aerenchyma of the plants and/or in incubation experiments. The calculated ratio between both pathways reflected well the ratio determined by application of methyl fluoride (CH 3 F) as specific inhibitor of acetate‐dependent methanogenesis. Only at the end of the season, the theoretical ratio of acetate: H 2 = 2 : 1 was reached, whereas at the beginning H 2 /CO 2 ‐dependent methanogenesis dominated. The isotope discrimination was different between rooted surface soil and unrooted deep soil. Root‐associated CH 4 production was mainly driven by H 2 /CO 2 . Porewater CH 4 was found to be a poor proxy for produced CH 4 . The fraction of CH 4 oxidised was calculated from the isotopic signature of CH 4 produced in vitro compared to CH 4 emitted in situ , corrected for the fractionation during the passage from the aerenchyma to the atmosphere. Isotope mass balances and in situ inhibition experiments with difluoromethane (CH 2 F 2 ) as specific inhibitor of methanotrophic bacteria agreed that CH 4 oxidation was quantitatively important at the beginning of the season, but decreased later. The seasonal pattern was consistent with the change of potential CH 4 oxidation rates measured in vitro . At the end of the season, isotope techniques detected an increase of oxidation activity that was too small to be measured with the flux‐based inhibitor technique. If porewater CH 4 was used as a proxy of produced CH 4 , neither magnitude nor seasonal pattern of in situ CH 4 oxidation could be reproduced. An oxidation signal was also found in the isotopic signature of CH 4 from gas bubbles that were released by natural ebullition. In contrast, bubbles stirred up from the bulk soil had preserved the isotopic signature of the originally produced CH 4 .