Microbial community structure and ecosystem function are notoriously difficult to connect. Methane (CH4)-cycling is an ideal C-cycle process to study this connection because the microorganisms that produce and consume CH4 are phylogenetically constrained and relatively well studied. Wetlands in the tropics emit more than half of the global wetland CH4 flux and emit on average 400% and 50% more CH4 per area than boreal and temperate wetlands, respectively. Despite the global importance of tropical wetland CH4 emissions, there have been fewer studies on controls over tropical wetland methane flux relative to boreal and temperate wetlands. Thus understanding the biotic and abiotic controls over tropical wetland CH4 emissions is a priority if we are to improve global CH4 models. We measured CH4 emissions in eight wetlands and seven upland sites in the Central African nation of Gabon. We further measured CH4 and CO2 production and the kinetics of high- and low-affinity CH4 oxidation dynamics in soil samples in the laboratory. We surveyed the microbial community in these soils via RNA/DNA co-extraction and 16S ribotyping, in order to determine the relative contribution of the putatively active fraction of the community vs attributes of the total community to process rates.
Results/Conclusions
Wetlands displayed extreme variability of in situ CH4 fluxes, ranging from emissions of > 1,000 mg CH4 m-2 d-1 to CH4 consumption of > 100 mg CH4 m-2 d-1. Upland sites all displayed CH4 oxidation but rates varied by five orders of magnitude. The Gabon wetland soils had uniformly high CH4:CO2 production ratios. The CH4:CO2 production ratio was correlated with pH (r2 = 0.65), but all samples were acidic with pHs < 5.1. Gene copy number of methanogens explained 62% of the variance in CH4 production, and the relative abundance of individual methanogen taxa explained up to 86% of the variance, even though these taxa were not the most abundant. Moreover, phylogenetic analysis of the mcrA gene indicated a highly derived clade of methanogens found in the most methanogenic site. Wetland sites had higher maximum uptake rates but lower affinity for CH4 than drier sites. Soil moisture explained up to 20% of the variance in CH4 oxidation rates. However, high-affinity and low-affinity CH4 oxidation rates were better predicted by the abundance of alphaproteobacterial (Type II) and gammaproteobacterial (Type I) methanotrophs, respectively. Our data suggest that unique biogeochemical and microbial characteristics of tropical wetlands lead to their high CH4 emissions.