Vegetation alters how soil properties and climate influence microbial activity and functional diversity in rhizosphere and bulk soil along an elevation gradient

Microbial communities strongly influence ecological processes across space and time. However, the influence of abiotic and biotic factors on microbial functioning in a complex, heterogeneous soil environment is still poorly understood, especially in montane plant communities. Here, we ask if microbial activity and functional diversity can be influenced by plant species identity and diversity and associated root system morphological and chemical traits that also influence soil properties. We investigated microbial global catabolic activity (i.e. microbial activity) and catabolic diversity (i.e. functional diversity) in bulk and rhizospheric soil beneath three plant species (Vaccinium myrtillus, Juniperus communis and Picea abies) that shape local plant communities. To do this, we measured soil physical and chemical properties, and plant diversity along an elevational gradient (1400–2400 m a.s.l.) in the French Alps. Microbial global catabolic activity and diversity were assessed using multiple substrate-induced respiration. Morphological and chemical traits of roots in bulk soil (‘community’ level traits, where several plant species were pooled together) and of individual plants (‘species’ level, where roots of single species were excavated) were measured. Results showed that at lower elevations, global catabolic activity in the rhizosphere was higher than in bulk soil, but converged in the nutrient-poor, colder soils found at higher elevations, although changes in catabolic diversity were negligible. Variations in soil physical and chemical properties were the main drivers of global catabolic activity, especially texture, cation exchange capacity, carbon and nitrogen content and pH, but their effects on catabolic diversity were minimal. Plant root traits at the community level in bulk soil had less effect on global catabolic activity than abiotic factors, with thicker roots, high root lignin content and low cellulose content influencing microbial activity, but not altering catabolic diversity. At the species level, more dense root tissue decreased global catabolic activity, reflecting changes in chemical composition. Our results show that relationships between catabolic activity, mean annual temperature and soil properties differed between plant species.