Wetting and drying cycles reduce litter decomposition and carbon efflux in microcosm studies

Abstract:

Soil is a major source of CO₂ emission, so it is important to understand how environmental factors, such as varying moisture levels, affect C efflux. Climate change models predict changes in precipitation, with some areas receiving more water inputs and others receiving less. To investigate how moisture variation affects CO₂ efflux, we used simplified microbial communities in sealed microcosms. Simplified communities allow for a tractable level of complexity that mimics microbial communities in small unconnected soil pores.

We collected soils from 95 sites across southwestern grassland soils in the United States and inoculated them into microcosms containing sand and Ponderosa pine needle litter. These microcosms were incubated over a three-month period with two treatments: one with constantly wet moisture conditions and one treated with three wetting cycles interspersed with long periods of desiccation. We measured CO₂ efflux throughout the incubation and destructively sampled at the end of the incubation to measure dissolved organic carbon (DOC), pine needle litter decomposition, and to characterize the microbial community using 16S sequencing. From these measurements, we found that soil microbial communities subjected to wetting and drying cycles had 23% less pine needle consumption, 45% less cumulative CO₂ efflux, and 19% less DOC than communities maintained at a constantly wet moisture level. Further, wetting and drying also had less variation in CO₂ and DOC compared to the wet treatment. Overall, these findings indicate that wetting events followed by long periods of drought result in lower C degradation, sequestration, and efflux in the soil environment allowing for better modeling as water inputs change due to climate change. Our ultimate goal is to link these C fluxes with the microbial communities that drive these processes using 16S sequencing analyses.