Representing methane emissions from wet tropical forest soils using microbial functional groups constrained by soil diffusivity

Abstract. Tropical ecosystems contribute significantly to global emissions of methane (CH₄) and landscape topography influences the rate of CH₄ emissions from wet tropical forest soils. However, extreme events such as drought can alter normal topographic patterns of emissions. Here we explain the dynamics of CH₄ emissions during normal and drought conditions across a catena in the Luquillo Experimental Forest, Puerto Rico. Valley soils served as the major source of CH₄ emissions in a normal precipitation year (2016), but drought recovery in 2015 resulted in dramatic pulses in CH₄ emissions from all topographic positions. Geochemical parameters including dissolved organic carbon (C) (ridge ≫ slope ≫ valley), acetate (ridge ≥ slope > valley), and soil pH (valley ≫ slope ≫ ridge), and meteorological parameters like soil moisture (valley > slope = ridge) and oxygen (O₂) concentrations (slope = ridge > valley) varied across the catena. During the drought, soil moisture decreased in the slope and ridge and O₂ concentrations increased in the valley. We simulated the dynamics of CH₄ emissions with the Microbial Model for Methane Dynamics-Dual Arrhenius and Michaelis Menten (M3D-DAMM) which couples a microbial functional group CH₄ model with a diffusivity module for solute and gas transport within soil microsites. Contrasting patterns of soil moisture, O₂, acetate, and associated changes in soil pH with topography regulated simulated CH₄ emissions, but emissions were also altered by rate-limited diffusion in soil microsites. Changes in simulated available substrate for CH₄ production (acetate, CO₂, and H₂) and oxidation (O₂ and CH₄) increased the predicted biomass of methanotrophs during the drought event and methanogens during drought recovery, which in turn affected net emissions of CH₄. A variance-based sensitivity analysis suggested that parameters related to acetotrophic methanogenesis and methanotrophy were most critical to simulate net CH₄ emissions. This study enhanced the predictive capability for CH₄ emissions associated with complex topography and drought in wet tropical forest soils.