Minor change in soil moisture induces abrupt shift in soil microbial community and methane emissions

Anthropogenic climate change affects a wide range of environmental conditions, including precipitation and soil moisture content. However, its impact on key biogeochemical processes and associated feedback mechanisms as well as the potential existence of critical thresholds in these is poorly constrained. Here we use methane flux, lipid distribution, sequencing and functional gene data along several natural grassland transects in NW China to assess the impact of changes in soil moisture content on methane cycling in grassland soils, an ecosystem that plays a crucial role in regulating the concentration of this potent greenhouse gas. We show that these soils abruptly switch from being a methane source to sink caused by a minor change in soil moisture content around a threshold of ∼30 %, which is driven by a sudden replacement of the archaeal methanogen community by methanotrophic bacteria. This abrupt switch in microbial community is driven by soil redox state with drier soils having a higher soil oxygen content. We then use the outputs of a state-of-the-art Earth system model (UKESM1) to show that the extent of wet grasslands will decline while that of dry grasslands will increase during the 21st century under a range of anthropogenic climate change scenarios. In the context of our data, these predicted changes in grassland wetness will drive a reduction in methane emissions from grasslands to the atmosphere. Our results highlight the existence of an abrupt feedback mechanism that in the context of anthropogenic climate change has the potential to alter the operation of the terrestrial methane cycle in the near future and which hold implications for carbon emissions targets and should be incorporated into projections of future emissions.