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).

Keywords

Biochar amendment / Sandy loam / Soil column simulation / Water movement / Infiltration / Water retention curve

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Funding

University-Industry Collaboration Project: Soil Bulk Density Test(H20250054)

Project for Innovation Alliance of Modern Agricultural Technology System of Guangdong Province(2022KJ112)

Guangdong Provincial Program of “Open Competition Mechanism to Select the Best Candidates” in “14th Five-Year Plan” for Ten Main Attack Directions of Agricultural Science and Technology Innovation(2022SDZG08)

2022–2023 Foshan Municipal Special Program for Key Technology Research in Social Sciences(2220001018511)

2023 Foshan Chancheng District Science and Technology Plan Project(2330004004905)

Guangdong Provincal Science and Technology Plan Project(2023B1212060044)

Education Institutions in Guangdong Province in 2022(2022KCXTD049)

Science and Technology Program of Guizhou Provincial Tobacco Company, China National Tobacco Corporation(2022XM11)

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