Integrated Biochar and Humic Acid Application Enhances Soil Health and Oat Forage Productivity in the Agro-Pastoral Ecotone of North China

Soil degradation in the agro-pastoral ecotone of North China severely constrains the productivity of vital forage crops like oat (Avena sativa L.). While biochar and humic acid are known soil amendments, their combined synergistic effects and optimal application ratios remain poorly understood. This study aimed to investigate the integrated effects of biochar and humic acid on soil properties and oat productivity to identify an optimal co-application strategy for this fragile ecosystem. A three-year field experiment (2023–2025) was conducted using a randomized complete block design. Treatments included a control (CK), sole biochar (B1H0), sole humic acid (B0H1), and three combined biochar-humic acid ratios (7:3, 5:5, and 3:7 by mass; B7H3, B5H5, B3H7). We measured key soil physicochemical and biological properties, as well as oat growth, forage yield, and quality parameters. Partial least squares structural equation modeling (PLS-SEM) was employed to elucidate the causal pathways linking amendments, soil quality, and crop performance. The combined application of biochar and humic acid demonstrated significant synergistic effects, outperforming sole applications in improving soil health and oat productivity. The B5H5 (1:1 ratio) treatment was the most effective, significantly reducing soil bulk density by up to 22.4% and increasing the mean weight diameter of water-stable aggregates by 40.7% compared to the CK. This treatment also led to the greatest increases in soil organic matter (68.9%), microbial biomass carbon (142.2%), and the activities of sucrase (139.8%) and urease. Consequently, the B5H5 treatment enhanced oat forage yield by 56.1% and improved its nutritional quality by reducing acid detergent fiber and neutral detergent fiber content by 22.9% and 12.7%, respectively. The PLS-SEM analysis confirmed that the positive effects of the amendments on crop performance were primarily mediated through the enhancement of soil quality (path coefficient β = 0.75, p < 0.001). The integrated application of biochar and humic acid, particularly at a 1:1 mass ratio, is a highly effective strategy for restoring degraded soils in the agro-pastoral ecotone. These findings provide a strong scientific basis for promoting this co-amendment approach as a sustainable solution to enhance agricultural resilience and productivity in fragile environments.