Modeling Methane Emissions in Anaerobic Microsites Along a Catena in Puerto Rico

Abstract:

Greenhouse gas emissions are important in warm, wet tropical environments. However, climatic events such as droughts can alter emissions by imparting changes in the active microbial community, affecting redox reactions, and influencing diffusional transport of substrates. Reactions in microsites in clay-rich soils can further alter the dynamics of these processes, by allowing reactions to continue to proceed, even when conditions in bulk soils become unfavorable. Here, we measured methane fluxes and soil oxygen and moisture content along a catena in the Luquillo Experimental Forest in Puerto Rico. Under normal conditions, net methane fluxes from the ridge and slope topographic positions were minimal, while methane fluxes were substantial from the valley. Following the recovery from a strong drought, net methane emissions were strong from all three topographic positions. To explain the diversity of observations, we coupled a microbial functional group model that accounted for methanotrophy and both acetoclastic and hydrogenotrophic methanogenesis, with a diffusivity model that accounted for gas and solute diffusion within soil microsites. Under non-drought conditions, the simulated oxygen diffusion and the population of methanotrophic microbes inhibited the production and release of methane in the ridge and slope positions. During drought recovery, the simulated delivery of oxygen and hydrogen to soil microsites became inhibited due to increased soil moisture, while diffusion of acetate was enhanced. Decreases in oxygen also lowered simulated methanotrophic biomass, while increases in moisture and decreases in oxygen promoted growth of simulated methanogenic biomass. Ultimately, simulated methane release from anaerobic microsites was enhanced during drought recovery, even from the ridge and slope positions, while methanotrophy was inhibited. These mechanisms help explain how soil microsites contribute to hotspots and hot moments of greenhouse gas emissions during periods of environmental change.