Controls on soil methane fluxes: Tests of biophysical mechanisms using table isotope tracers

Understanding factors that control methane exchange between soils and the atmosphere remains one of the highest priorities for climate change research. Here we use a novel isotope-based technique to investigate the relative importance of three mechanisms for explaining landscape-scale variations in soil methane emissions: (1) consumption of methane by methanotrophic bacteria, (2) quantity of carbon mineralization, or (3) relative amounts of carbon flow through nonmethanogenic versus methanogenic mineralization pathways. Application of a new, nondisruptive, ¹³CH₄ isotope pool dilution technique permitted us to evaluate these mechanisms by distinguishing gross methane fluxes through both productive and consumptive pathways. We quantified each of these pathways in surface soils across broad moisture gradients in tropical montane environments in the Hawaiian Islands and temperate ecosystems in the northeastern United States. We found only limited support for the consumption control hypothesis because consumption was only important in dry soils. We also failed to find support for the carbon supply hypothesis, in that rates of carbon mineralization did not explain the observed variability in net fluxes across landscapes. Rather, dramatic differences in methane production, and thus emission, depended on surprisingly small diversions of soil carbon flow from nonmethanogenic to methanogenic pathways: on average, soils were a net source of methane to the atmosphere if more than 0.04% of total carbon mineralization passed through methanogenic pathways. We infer that fine-scale heterogeneity of soil redox status is critical for regulating soil methane fluxes.