Root exudates of Salix cupularis orchestrate the accumulation of soil carbon and nitrogen in the rhizosphere during the restoration of a desertified alpine meadow

Salix cupularis is a key species for restoring desertified alpine meadows in the eastern Tibetan Plateau. However, the mechanisms driving soil restoration under S. cupularis remain unclear due to complex rhizosphere processes and harsh high-altitude conditions. This study investigated root exudation, soil carbon (C) and nitrogen (N) contents, enzyme activities, and microbial biomass across shoot biomass levels (high, medium, low), phenological stages (regreening, fruiting, withering), and plant gender (male, female) in S. cupularis. Root exudation was identified as a critical driver of increased rhizosphere soil C and N, unaffected by plant gender. Root exudation rates peaked at the fruiting stage, and the C exudation rate increased with shoot biomass. While S. cupularis with high shoot biomass showed greater rhizosphere and bulk soil organic C (SOC) content compared to low shoot biomass plants. Intensive root exudation enhanced the contents of dissolved organic C (DOC), NH₄+, microbial biomass C (MBC) and microbial biomass N (MBN), as well as extracellular enzyme activities in rhizosphere soil compared to bulk soil, with their highest levels observed at the fruiting stage. No significant effect of plant gender was discovered. Structural equation modeling revealed two main pathways for increasing rhizosphere C and N which are direct root exudates inputs and microbial biomass accumulation driven by root exudates. These findings highlight shrub shoot biomass as a key factor in soil restoration, emphasizing the roles of root exudates and microbial biomass formation in enhancing soil C and N during desertified alpine meadow recovery.