Contrasting effects of rice husk and palm silk biochars on water infiltration and leakage in a phosphorus-enriched sandy-loam vegetable soil

Abstract Biochar amendment impacts soil water movement through modifications of soil properties, yet the mechanisms linking these changes to hydrological dynamics in vegetable soils with phosphorus (P) surplus remain elusive. This study systematically compared the effects of two regionally prevalent biochars—rice husk biochar (RHB) and palm silk biochar (PSB)—on water infiltration and leakage in a P-enriched sandy loam vegetable soil from southern China using a soil column experiment. Biochars were incorporated into the topsoil (0–20 cm) at rates of 0%, 3%, and 6% ( w / w ). Results demonstrated that RHB, with a broader pore size distribution and significant macropores (> 50 nm) despite an average pore diameter of 40.7 nm, inhibited water infiltration more effectively than the uniformly mesoporous PSB (average pore diameter: 4.40 nm), especially in the surface layer. At the 6% amendment rate, RHB increased saturated water content ( θ s ) by 14% and reduced saturated hydraulic conductivity ( K s ) by 52%, whereas PSB delayed water release through mesopore-dominated retention. Both biochars comparably suppressed water leakage at a given amendment rate. Structural equation modeling quantified a dual regulatory mechanism: total organic carbon (TOC) governed infiltration by enhancing θ s , while pH controlled it by reducing K s . This synergy intensified the trade-offs between water retention and infiltration suppression at higher amendment rates. Although 6% RHB amendment (theoretical scaling: 186 t ha −1 ) maximized hydrological benefits, the 3% amendment rate (theoretical scaling: 93 t ha −1 ) offers a more practical, cost-effective balance for reducing water and dissolved reactive P losses. We conclude that biochar’s feedstock-specific pore structure (macropore-dominated RHB vs. mesopore-dominated PSB) and induced physicochemical changes (TOC-driven θ s increase and pH-mediated K s reduction) synergistically dictate hydrological regulation. This mechanistic insight optimizes water-P coordination in subtropical vegetable soils (e.g., Olsen-P > 40 mg kg −1 , a threshold far exceeding the agronomic requirement and indicating a high risk of P leaching). Graphical Abstract