Anaerobic oxidation of methane (AOM) in paddy soil: the alternative electron acceptors.

The anaerobic oxidation of methane (AOM) in marine ecosystems is ubiquitous and coupledlargely to sulfate reduction. In comparison, little is known about the role of AOM interrestrial environments, and the dominant mechanisms for terrestrial AOM remain elusive.Submerged agricultural fields, such as rice paddies, with intensive CH4 turnover, mayprovide a high potential for AOM; however, the AOM rate, electron acceptors andmechanisms in these environments are largely unexplored. Here, we used 13CH4isotope tracers to quantify the AOM rate in paddy soils under organic (i.e. pigmanure, biochar) and mineral fertilizers (i.e. NPK), and examine the potential ofalternative electron acceptors (AEAs) including Fe3+, NO3−, SO42− and humicacids.During 84 days of incubation, the cumulative AOM (13CH4-derived CO2) reached 0.15-1.3μg C g−1 soil depending on fertilization. There was a linear correlation (R2 = 0.55 ∼ 0.93, p< 0.05) between the amount of gross produced and net oxidized CH4. NO3− was the mostpotent AEA with an AOM rate reaching 0.80 ng C g−1 soil h−1 under pig manurefertilization. The role of Fe3+ on AOM remained unclear, whereas SO42− inhibitedAOM but strongly stimulated CO2 production indicating intensive sulfate-inducedanaerobic organic matter oxidation. Humic acids were the second most potent AEA,especially when without mineral fertilizers and after biochar addition. Humic acidsaddition increased methanogenesis for 5-6-times in all paddy soils as compared withcontrol and other AEAs. We demonstrated for the first time that organic AEAs (e.g.humic acids) are important drivers of AOM – this along with the proven nitrate(nitrite)-dependent AOM mechanisms in paddy soils, whereas Fe3+ and SO42− arepreferential electron donors for mineralization of native soil organic matter. Consequently, ina broader ecological view, the organic and mineral fertilization controls an importantmethane sink under anaerobic conditions in submerged agricultural paddy ecosystems. [ABSTRACT FROM AUTHOR]