Effects of biochar addition on soil water migration in red soil slope farmland

To explore effects of biochar addition on soil water migration in red soil slope farmland, unsaturated infiltration experiment was carried out under different biochar addition conditions．Experiments were designed with five biochar addition gradients（0%, 1%, 3%, 5% and 10%）．By employing one-dimensional vertical constant head laboratory test, wetting peak migration rate, infiltration rate, cumulative infiltration rate, and infiltration characteristic parameters during soil infiltration process were analyzed, with infiltration process further fitted and evaluated for applicability．Results have indicated that biochar addition significantly inhibits migration distance of wetting front per unit time．As amount of biochar added increases, soil wetting front migration distance gradually decreases．Under different biochar addition treatments, soil infiltration process in red soil slope farmland exhibited a consistent pattern: infiltration rate was high in initial stage; as infiltration time increased, rate gradually decreased and eventually stabilized; while cumulative infiltration amount gradually increased with advance of infiltration time, but increase range decreased as biochar application rate increased．Soil infiltration rates and cumulative infiltration volumes of red soil slope farmland under different biochar addition treatments follow pattern: 0% > 1% > 3% > 5% > 10%．All soil infiltration parameters decreased with increasing biochar addition．Stable infiltration rates decreased by 8.5%, 31.7%, 34.1%, and 53.7%, respectively, compared to the 0% addition treatment, while average infiltration rates decreased by 16.1%, 29.4%, 37.6%, and 55.6%, respectively．Fitting results have indicated that among three infiltration models of Horton model, Green-Ampt model, and Philip model, Horton model demonstrated superior fitting accuracy and performance compared to the other two models．Biochar addition to red soil slope farmland significant inhibited soil water migration processes, and inhibitory effect was positively correlated with biochar addition．Under biochar-amended conditions, Horton model can be prioritized for fitting unsaturated infiltration processes in red soil slope farmland．