Effects of Combined Application of Biochar and Nitrogen Fertilizer on Forage Growth and Water- and Nitrogen-Use Efficiency in Managed Grassland

To tackle the low nitrogen-use efficiency, water scarcity, and insufficient forage supply in improved grasslands in arid Northwest China, a two-factor field trial involving three biochar dosages (0, 10, 20 t ha−1) and two nitrogen (N) application rates (0, 225 kg ha−1) was conducted on mountain grassland in Xinjiang, with Onobrychis viciifolia and Bromus inermis as test forages. Soil physicochemical traits, soil–forage and water–nitrogen dynamics, forage productivity, and economic returns were determined, and partial least squares path modeling (PLS-PM, GoF = 0.373) was conducted to quantify the regulatory pathways. Co-treatments with biochar and N markedly lowered soil bulk density and elevated porosity, soil organic carbon, and the available N pool; application of biochar deferred the peak of soil available N and prolonged fertilizer efficacy by more than seven days. Forage total nitrogen- and water-use efficiency peaked at the first cutting, with the B1N1 treatment (10 t ha−1 biochar + 225 kg ha−1 N) boosting water-use efficiency by 45.2%. O. viciifolia exhibited optimal performance under B1N15, while B2N15 (20 t ha−1 biochar + 225 kg ha−1 N) was more favorable for B. inermis. The B1N15 regime achieved the highest net profit of CNY 12,747.99 ha−1, but high biochar input cut economic gains by 14.1%. The PLS-PM model uncovered a dual regulatory mechanism: combined amendments directly optimized soil microhabitats, and soil water–nitrogen conditions served as a central mediator to facilitate biomass accumulation and economic gains through cascading soil–vegetation transmission, without direct impacts on vegetation productivity or economic outputs. This research provides a scientific reference for the use of fertilization in the sustainable management of arid improved grasslands.