Grassland management strategies profoundly influence ecosystem nutrient dynamics, yet how long-term practices affect carbon (C), nitrogen (N), and phosphorus (P) stoichiometry and stocks in plant-soil systems remains poorly understood, particularly in semi-arid regions. This study evaluated the effects of different management strategies-enclosing (EG), grazing (GG), and mowing with enclosure (MG)-on C, N, and P concentrations, stoichiometric ratios, and stocks in both plant and soil across a semi-arid grassland of northern China. Our results showed that GG led to higher N and P concentrations in plant tissues, while EG enhanced C concentrations and resulted in the highest ecosystem C and N stocks. The N:P ratios under EG and GG (15.3 and 15.6) suggested co-limitation by N and P, while the lower ratio under MG (11.8) indicated stronger N limitation. The concentrations of soil C, N, and P were greatest under EG and declined with depth. Soil C:N ratios remained stable across treatments and below 12. The N:P ratios ranged from 1.78 to 3.81 across all treatments and soil depths, and were significantly elevated under EG. Strong positive correlations were observed among soil C, N, and P, and between soil and plant nutrient pools. Total P stocks were unaffected by management, reflecting the geogenic origin of P. These findings highlight that enclosing is the most effective strategy for enhancing C and N stocks in semi-arid grasslands, while also revealing tight plant-soil nutrient coupling and the relative stability of P dynamics under different management regimes.