Plant community root traits outweigh target species in mediating soil carbon and nitrogen storage across grazing-exclusion chronosequences in semi-arid grasslands

Semi-arid grasslands serve as critical carriers for carbon and nitrogen sequestration in ecologically fragile regions, with plant root functional traits playing a pivotal regulatory role. Current restoration efforts, overly focused on target species selection strategies, may underestimate root networks’ contribution to soil carbon/nitrogen accumulation. This study examined soil organic carbon (SOC) and total nitrogen (TN) storage, and root traits of target species (Leymus chinensis) and plant community across a chronosequence of grazing-exclusion grasslands in China’s semi-arid Songnen Plain. Results demonstrated that both SOC and TN contents exerted vertical dynamics and generally increased with grazing-exclusion duration, exhibiting a strong positive linear relationship (R2 = 0.832, P < 0.001). SOC storage peaked at 27 years (79.89 t ha-1-1), while TN storage stabilized after 19 years (9.46 t ha-1-1). Root traits at target species and community levels exhibited similar temporal trends. Variance partitioning analysis revealed that community root traits had stronger independent effects on SOC/TN storage than target species. The random forest model identified community root carbon content (RCC), root weight (RW), root diameter (RD), and special root length (SRL) as key predictors. Structural equation modelling further indicated grazing exclusion directly and indirectly influenced SOC/TN accumulation. Specifically, reduced community SRL negatively affected RCC and RD, thereby promoting SOC and TN storage, respectively. This study underscores that community root traits, particularly their trade-off relationships, play a more pivotal role than target species in regulating soil functions, thereby establishing a novel trait-based framework for guiding grassland restoration in semi-arid regions.