Land use effects on soil microbiome composition and traits with consequences for soil carbon cycling

The soil microbiome determines the fate of plant-fixed carbon. The shifts in
soil properties caused by land use change leads to modifications in microbiome function, resulting in either loss or gain of soil organic carbon (SOC). Soil pH is the primary factor regulating microbiome characteristics leading to distinct pathways of microbial carbon cycling, but the underlying mechanisms remain understudied. Here, the taxa-trait relationships behind the variable fate of SOC were investigated using metaproteomics, metabarcoding and a 13C labelled litter decomposition experiment
across two temperate sites with differing soil pH each with a paired land use intensity contrast. 13C incorporation into microbial biomass increased with land use intensification in low pH soil but decreased in high pH soil, with potential impact on carbon use efficiency (CUE) in opposing directions. Reduction in biosynthesis traits was due to increased abundance of proteins linked to resource acquisition and stress tolerance. These trait trade-offs were underpinned by land use intensification-induced changes in dominant taxa with distinct traits. We observed divergent pH-controlled pathways of SOC cycling. In low-pH soil, land use intensification alleviates microbial
abiotic stress resulting in increased biomass production but promotes decomposition and SOC loss. In contrast, in high-pH soil, land use intensification increases microbial physiological constraints and decreases biomass production, leading to reduced necromass build-up and SOC stabilisation. We demonstrate how microbial biomassproduction and respiration dynamics and therefore CUE can be decoupled from SOC
highlighting the need for its careful consideration in managing SOC storage for soil health and climate change mitigation.