Interfacial regulation by humic acid pre-intercalation enhances structural stability and phosphorus retention of LDH-biochar under alkaline stress

Global soil salinization has been increasingly severe, resulting in severe phosphorus fixation in saline-alkali soils. Conventional layered double hydroxide (LDH) modified biochar slow-release phosphate fertilizers (LDH-BSPF) suffer from structural instability and abrupt phosphorus release due to high pH and carbonate competition when applied in saline-alkali soils. Herein, humic acid-intercalated layered double hydroxide (HALDH) was synthesized via co-precipitation and subsequently co-pyrolyzed with biomass and phosphorus source to fabricate HALDH-BSPF. XRD and FTIR spectroscopy confirmed that HA anions were successfully embedded into the LDH interlayer domain, expanding the (003) crystal plane spacing from 8.01 nm to 8.13 nm, Optimal slow-release performance was achieved at 10% HA intercalation. Under weakly alkaline conditions (pH=8.50), the 28-day cumulative phosphorus release rate was only 55.85%, which was 21.14 %age points lower than that of unmodified LDH-BSPF. Column leaching and pot experiments demonstrated that HALDH-BSPF effectively reduced phosphorus leaching and promoted pepper growth under saline-alkali stress. Specifically, compared with the unmodified treatment, HALDH-BSPF application increased plant fresh weight by 50.29%, plant height by 13.27%, root length by 4.93%, chlorophyll content by 4.93%, and total phosphorus content by 30.82%. This study reveals the intrinsic mechanism of HA pre-intercalation in enhancing the saline-alkali structural stability of LDH, providing a theoretical basis for long-term phosphorus management in saline-alkali soils.