Environmental context affects microbial ecophysiological mechanisms underpinning soil carbon storage under different land use

Soil microorganisms act as gatekeepers for soil-atmosphere carbon exchange by balancing the accumulation and release of organic matter in soil. Increasing evidence now exists to suggest that microbial biomass contributes significantly to soil organic carbon formation. However, we do not fully understand the microbial mechanisms of organic matter processing and this hinders the development of effective land management strategies to enhance soil carbon storage. Here we empirically link key microbial ecophysiological traits to soil carbon storage in temperate grassland habitats ranging in land use from pristine species-rich grasslands to intensive croplands in 56 different soils across Britain. Physiological mechanisms of soil microorganisms were assessed using stable carbon isotope tracing and soil proteomics. Through spatial patterns and path analysis of structural equation modeling we discern two distinct pH-related mechanisms of soil carbon storage and highlight that the response of these mechanistic indicators is shaped by the environmental context. Land use intensification in low pH soils that increases soil pH above a threshold value ( 6.2) leads to loss of carbon due to increased microbial degradation as a result of lower acid retardation of organic matter decomposition. On the contrary, the loss of carbon through intensification in high pH (> 6.2) soils was linked to decreased microbial biomass and reduced carbon use efficiency that was linked to tradeoffs with stress alleviation and resource acquisition. We conclude that land use intensification-induced changes in soil pH can be used as a proxy to determine the effect of land management strategies on microbial soil carbon cycling processes and emphasize that more extensive land management practices at higher soil pH have greater potential for soil carbon storage through increased microbial metabolic efficiency, whereas in acidic soils abiotic factors exert a greater influence on the fate of soil carbon.