Effect of freeze-thaw cycles on nitrogen retention in an alpine topsoil amended with peanut shell hydrochar

Soil nitrogen (N) loss, which can be enhanced by freeze-thaw (FT) cycles in cold regions, has threatened global water security and ecosystem stability. Hydrochar, a cost-effective alternative to conventional biochar, shows promise for soil improvement and remediation, yet its effects on soil water and N retention under FT cycles remain poorly understood. This study investigated the influence of hydrothermal carbonization (HTC) temperatures (130, 200, and 270 °C) on the physicochemical properties of peanut shell-derived hydrochars (PSHs) (denoted as PSH-130, PSH-200, and PSH-270) and evaluated their effectiveness in mitigating nitrate N (NO₃^--N) and ammonium N (NH₄⁺-N) leaching from the alpine soil amended with PSHs at a ratio of 4 % (w/w, dry soil basis) under conditions of sequential leaching and FT cycles. Compared to the unamended soil, PSH-200 amendment reduced the cumulative leachate volume by 5.9 % and initial leachate NO₃^--N concentration by 97.2 % while increased the initial leachate NH₄⁺-N concentration by 216.4 % over the 12-day sequential leaching. Under FT cycles, PSH-200 amendment reduced the initial leachate NO₃^--N concentration by 98.4 % while increased the initial leachate NH₄⁺-N concentration by 463.1 % compared to CK. The evelated cumulative NH₄⁺-N leaching is likely due to the high organic N in the hydrochar. PSH-200 outperformed PSH-130 (limited by labile organic matter release) and PSH-270 (hindered by pore occlusion) in mitigating water and NO₃^--N losses due to its resistance to FT-induced macropore collapse, maintained hydraulic conductivity, and synergistic physical-chemical adsorption. Hydrochar application provides a sustainable solution for conserving N in cold regions.