How biochar, horizontal ridge, and grass affect runoff phosphorus fractions and possible tradeoffs under consecutive rainstorms in loessial sloping land?

Agricultural interventions such as biochar, grassing, and different planting modes affect the migration and transformation of soil phosphorus (P). However, the effect of different management practices on the mechanism of runoff P fractions and possible tradeoffs associated with consecutive rainstorms is not well understood. This study aggregated management practices (biochar, grassing, and planting modes) to evaluate the surface pathways of P loss from loessial bare and grassed slopes with different slope angles (10°, 15°, and 20°) and consecutive rainstorms (60, 90, and 120 mm·h⁻¹). Results indicated that: (i) The highest sediment yield rate and total phosphorus loss concentrations were registered during the first rainstorm event (10° slope and 60 mm·h⁻¹). Soil erosion and P loss under different rainfall intensities could be effectively reduced by 0–3% biochar grassed slope with horizontal ridge but were aggravated under bare slope with 3–6% biochar or grassed slope with 6% biochar. (ii) Planting grass on bare slope could effectively mitigate erosion and runoff P at different rainfall intensities, and the reduction rate increased with increased rainfall intensity. The sediment reduction effect of bare slope with horizontal ridge decreased with increased rainfall intensity and was liable to fail in the 90 and 120 mm·h⁻¹ rainfall intensities. (iii) The particulate phosphorus (PP) was the primary loss fraction of soil P from loessial sloping land (> 80%), and PP increased with increased biochar content and rainfall intensity but reduced by grass and horizontal ridge interventions. Integrating grass with horizontal ridge was considered to be the most effective way to control erosion and protect soil fertility. These outcomes provide underlying insights for employing appropriate best management practices (BMPs) to decrease runoff P on sloping soils, which can be highlighted through a reasonable allocation of biochar, ridge, growing plants, and other BMPs.