Targeted nutrient addition reduces the trade-offs between ecosystem services in degraded grasslands

Nutrient addition is widely applied in degraded grassland restoration and significantly affects ecosystem services (ESs). However, traditional nutrient addition methods mostly rely on empirical values or agricultural models, lacking precision strategies that are tailored to grassland ecosystem characteristics, and often exacerbate trade-offs between aboveground productivity (AGP), soil nutrient supply (SNS), and plant diversity (PD). Using non-degraded grassland as a reference, this study calculated the difference between plant nutrient demand and soil available nutrient supply, and for the first time constructed a targeted nutrient demand model applicable to grassland ecosystems. This study conducted experiments on degraded grasslands in Hulunbuir, Inner Mongolia, using a targeted nutrient demand model to determine nitrogen (N) and phosphorus (P) application amounts. The differences and trade-offs between AGP, SNS, and PD were quantified using the Entropy Weight Method (EWM) and Root Mean Square Deviation (RMSD), respectively. Results showed: (1) Under different nutrient addition treatments, SNS scores consistently exceeded those of AGP and PD, with the N4P3 treatment showing the highest overall ES capacity, significantly higher than the control (CK) (p < 0.05). (2) Targeted nutrient addition can shift the ES relationships from trade-offs to synergies. When the external N and P additions were insufficient (2.7–3.6 g N m⁻², 0.12–0.15 g P m⁻²) or imbalanced N:P ratios were applied (10.8 g N m⁻², 0.12 g P m⁻²), the trade-off relationships between ESs were similar to CK, predominantly showing moderate trade-offs (RMSD >0.1). Conversely, when appropriate nutrient additions were chosen (5.4–10.8 g N m⁻², 0.23–0.46 g P m⁻²), the trade-off relationships between two pairs or overall ESs were mildly synergistic (RMSD <0.1). (3) The trade-off relationships between AGP and PD, SNS, as well as soil total nitrogen, are key factors influencing the overall trade-offs of ESs. In conclusion, this study represents the first adaptation of agricultural precision fertilization models to grassland ecosystems, identifying nutrient allocation thresholds that reduce trade-offs in ESs, enhancing the overall function of grassland ecosystems, and providing a scientific basis for precision nutrient application in degraded grasslands.