Integrating microbial community composition with abiotic drivers enhances prediction of soil respiration in managed Scots pine forests

Carbon (C) turnover in forest soils is jointly controlled by abiotic factors, such as temperature, soil moisture, and soil properties, as well as microbial activity, which drive organic matter decomposition and its associated CO₂ fluxes. While microbial biomass is recognized as a predictor of soil respiration, the role of microbial community composition and diversity in shaping C dynamics under field conditions remains underrepresented in models. Here, we quantify the relative contributions of microbial biomass, taxonomic diversity, and community composition to soil heterotrophic respiration (Rh) across five managed Scots pine forests in Europe. Integrating microbial variables (fungal and bacterial PLFA, ITS and 16S sequencing) with abiotic drivers substantially improved model performance, explaining ~20% more variance in Rh than models based solely on abiotic drivers. Among microbial predictors, the fungi:bacteria ratio and bacterial community composition were consistently strong predictors of Rh. Fungal effects were modulated by soil temperature, with fungal diversity negatively and fungal biomass positively correlated with respiration at high temperatures. A structural equation model further showed that soil properties influenced Rh indirectly through their effects on microbial communities. Together, our results demonstrate that microbial community composition improves predictions of soil respiration beyond abiotic factors alone, and provide insights into how biomass, diversity, and composition interact with environmental drivers to regulate soil C cycling.