Dual Controls of Shrub Encroachment in Semiarid Grasslands: Ectomycorrhizal Fungi and Soil Nitrogen Cycling

Abstract

Shrub encroachment significantly impacts biodiversity and ecosystem functions in grassland ecosystems. Soil microbial communities may play a key role in this process. Previous studies have focused on plant competition and soil abiotic factors, but the specific biological mechanisms by which soil microbiota drive shrub encroachment have remained poorly understood. Through integrated vegetation surveys and high-throughput sequencing of soil microbial communities across encroachment gradients in Inner Mongolia, we assessed the impacts of soil microbial composition and functional genes on the shrub importance value. We found that shrub importance value closely correlated with limited resources, including soil water and nitrogen (N), during shrub encroachment. Ectomycorrhizal (EcM) fungi were recruited by the semishrub Artemisia ordosica, increasing its competitiveness under conditions of resource scarcity. Changes in the microbial community affected soil N cycling by reducing the abundance of genes involved in N fixation, nitrification, and nitrate assimilation. Concurrently, shrubs preferred soil nitrate nitrogen (NO₃⁻-N) over herbaceous plants in nutrient-poor environments. The competitiveness of shrubs was facilitated by EcM fungi, enabling them to thrive in N-deficient arid environments and preferentially utilize NO₃⁻-N. Our findings establish a novel microbial-mediated pathway driving shrub encroachment, in which EcM fungal symbionts enable host plants to alleviate nitrogen limitations through modified nutrient acquisition strategies. These results suggest that targeted manipulation of EcM associations could inform restoration strategies in shrub-encroached grasslands and highlight the need for a nitrogen management approach that accounts for microbial-mediated nutrient cycling dynamics.