Suppression of methanogenesis by dissimilatory Fe(III)‐reducing bacteria in tropical rain forest soils: implications for ecosystem methane flux

Abstract

AbstractTropical forests are an important source of atmospheric methane (CH4), and recent work suggests that CH4 fluxes from humid tropical environments are driven by variations in CH4 production, rather than by bacterial CH4 oxidation. Competition for acetate between methanogenic archaea and Fe(III)‐reducing bacteria is one of the principal controls on CH4 flux in many Fe‐rich anoxic environments. Upland humid tropical forests are also abundant in Fe and are characterized by high organic matter inputs, steep soil oxygen (O2) gradients, and fluctuating redox conditions, yielding concomitant methanogenesis and bacterial Fe(III) reduction. However, whether Fe(III)‐reducing bacteria coexist with methanogens or competitively suppress methanogenic acetate use in wet tropical soils is uncertain. To address this question, we conducted a process‐based laboratory experiment to determine if competition for acetate between methanogens and Fe(III)‐reducing bacteria influenced CH4 production and C isotope composition in humid tropical forest soils. We collected soils from a poor to moderately drained upland rain forest and incubated them with combinations of 13C‐bicarbonate, 13C‐methyl labeled acetate (13CH3COO−), poorly crystalline Fe(III), or fluoroacetate. CH4 production showed a greater proportional increase than Fe2+ production after competition for acetate was alleviated, suggesting that Fe(III)‐reducing bacteria were suppressing methanogenesis. Methanogenesis increased by approximately 67 times while Fe2+ production only doubled after the addition of 13CH3COO−. Large increases in both CH4 and Fe2+ production also indicate that the two process were acetate limited, suggesting that acetate may be a key substrate for anoxic carbon (C) metabolism in humid tropical forest soils. C isotope analysis suggests that competition for acetate was not the only factor driving CH4 production, as 13C partitioning did not vary significantly between 13CH3COO− and 13CH3COO−+Fe(III) treatments. This suggests that dissimilatory Fe(III)‐reduction suppressed both hydrogenotrophic and aceticlastic methanogenesis. These findings have implications for understanding the CH4 biogeochemistry of highly weathered wet tropical soils, where CH4 efflux is driven largely by CH4 production.