Researchers often think about methane emissions on big scales involving large areas over many years. But spatially isolated “hot spots” and brief “hot moments” also shape methane emissions from tropical forest soils. For example, climate shifts between drought and recovery can result in pulses of methane release from soil. Similarly, small differences in location on a landscape control the proportion of methane production versus methane consumption in soils. In this research, scientists used model simulation of a wet tropical hillslope during typical and drought conditions to understand how observed soil moisture and oxygen dynamics and different kinds of microbes contribute to the soil’s production and consumption of methane.
Drought alters the diffusion of oxygen and molecules into and out of soil microsites. This results in increased methane release from the entire hillslope, but only during drought recovery. Overall, the findings show that hot spots and hot moments of methane emissions result from a specific combination of landscape position and soil moisture status. These factors affect the activity of different soil microbes and net methane emissions. This finding is important for understanding sources of methane, an important greenhouse gas.
Methane emissions and other soil variables are vastly different along different points on the slope between a valley and a ridge. Soils and their processes also differ between drought and non-drought conditions. In particular, valley soils nearly always emit methane, in contrast to ridge and slope soils. However, during recovery from a strong drought, methane emissions were substantial from all three landscape positions. This study wanted to understand the reasons behind the complex methane dynamics, both in terms of space (hot spots) and time (hot moments). The researchers coupled a microbial functional group model that considered a range of soil variables, using capabilities to consider diffusion of solutes and gases into and out of soil microsites. The model successfully represented methane emissions under all conditions and from all landscape positions. Methanogens were dominant in the wet valley soils, while methanotrophs were dominant in the drier ridge and slope soils. When the soils undergo wetting following a drought, diffusion of oxygen becomes limiting in the ridge and slope soils, together enhancing aceticlastic methanogenesis and decreasing methanotrophy, resulting in strong methane releases from all topographic positions.
Department of Energy Office of Science, Biological and Environmental Research
Environmental System Science
Oak Ridge National Laboratory
This work was supported through an Early Career Award through the Department of Energy (DOE) Office of Science, Office of Biological and Environmental Research, by grants from DOE and the National Science Foundation (NSF), as well as funding from the NSF Luquillo Critical Zone Observatory to the University of New Hampshire and from the NSF Luquillo Long-Term Ecological Research Program to the University of Puerto Rico. Additional support was received from the U.S. Department of Agriculture National Institute of Food and Agriculture’s McIntire Stennis project. This research used resources of the Compute and Data Environment for Science at Oak Ridge National Laboratory.
Sihi, D., et al., Representing methane emissions from wet tropical forest soils using microbial functional groups constrained by soil diffusivity. Biogeosciences 18 1769–786 (2021). [DOI: 10.5194/bg-18-1769-2021]