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

Tropical 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.