Thermal Acclimation of Soil Microbial Communities Declines with Depth

Soils store about 1,400 Pg organic carbon (C) in the top meter with half of this C storage occurring below 20 cm. While C turnover at depth is proposed to be slower than at the surface, the vulnerability of deep soil C under future climate scenarios is uncertain. Understanding of mechanisms is especially lacking for microbial temperature acclimation at depth. To determine the impact of altered resource stoichiometry (e.g., C, nitrogen, and phosphorus) on soil microbial community composition, physiology, and metabolism under a warming climate scenario, we analyzed samples from a 4.5 y whole-soil-profile warming field experiment (+4 °C above ambient) at the Blodgett Experimental Forest in the central Sierra Nevada, CA. We used amplicon (16S and ITS) and shotgun metagenomic sequencing to define changes to microbial community structure and metabolism with warming in-situ. We coupled this analysis to a series of laboratory incubations to understand resource limitations to microbial activity and the interaction with warming. Additionally, laboratory incubations with ¹³C isotopologs were used to assess the changes in microbial carbon use efficiency (CUE). We found a weak but significant shift in microbial community composition along the soil depth profile with warming. Microbial metabolism, on the other hand, changed in response to warming only in topsoils. Via metagenomics we detected increased abundance carbohydrate active enzymes in these topsoils. We evaluated the impact of resource stoichiometry on microbial respiration by incubating heated and unheated soils with C and nutrient amendments for 30 days and found that respiration in heated soils was mostly constrained by a C-limitation, while unheated soils were relatively more nutrient-limited. However, these differences in limitations were only apparent in topsoils and not in subsoils. While microbial CUE declined with depth, changes in resource stoichiometry or warming did not change CUE. Our finding that subsoil microbial communities are less responsive to warming than are their surface counterparts suggests that subsoils will lag in their acclimation to rising temperatures.