Contrasting plant–soil–microbial feedbacks stabilize vegetation types and uncouple topsoil C and N stocks across a subarctic–alpine landscape

Abstract

Global vegetation regimes vary in belowground carbon (C) and nitrogen (N) dynamics. However, disentangling large-scale climatic controls from the effects of intrinsic plant–soil–microbial feedbacks on belowground processes is challenging. In local gradients with similar pedo-climatic conditions, effects of plant–microbial feedbacks may be isolated from large-scale drivers. Across a subarctic–alpine mosaic of historic grazing fields and surrounding heath and birch forest, we evaluated whether vegetation-specific plant–microbial feedbacks involved contrasting N cycling characteristics and C and N stocks in the organic topsoil. We sequenced soil fungi, quantified functional genes within the inorganic N cycle, and measured 15N natural abundance. In grassland soils, large N stocks and low C : N ratios associated with fungal saprotrophs, archaeal ammonia oxidizers, and bacteria capable of respiratory ammonification, indicating maintained inorganic N cycling a century after abandoned reindeer grazing. Toward forest and heath, increasing abundance of mycorrhizal fungi co-occurred with transition to organic N cycling. However, ectomycorrhizal fungal decomposers correlated with small soil N and C stocks in forest, while root-associated ascomycetes associated with small N but large C stocks in heath, uncoupling C and N storage across vegetation types. We propose that contrasting, positive plant–microbial feedbacks stabilize vegetation trajectories, resulting in diverging soil C : N ratios at the landscape scale.