Microbial carbon metabolism is linked to organic matter chemistry across soil systems

Soil microbial growth and respiration are critical for soil organic carbon dynamics. Yet, we lack understanding of the main controls of soil microbial carbon metabolism at large scales (regional to global). Here we investigated if and how the chemical composition of extractable organic matter affects soil microbial carbon metabolism across soil systems along a gradient of 33 temperate grassland soils. We show that biomass-normalized rates of growth and respiration are primarily positively linked to aliphatics such as carbohydrate-, proteinaceous- and amino sugar-like compounds, and secondarily to unsaturated lignin-like compounds. Biomass-normalized respiration is positively linked to compounds with carbon in a reduced oxidation state, suggesting carbon-conserving catabolism. Biomass-normalized growth appears unrelated to the oxidation state of carbon, suggesting that other mechanisms than mere energetic constraints control microbial growth in aerobic soils. Our findings demonstrate that information on the chemical composition of bioavailable organic matter can provide insights into the processes that govern the fate of carbon across different ecosystems.