A Study on the Effects of Modified Biochar on Water Movement and Nitrogen Leaching in Reclaimed Soils in Southern Henan

Reclaimed soil is an important reserve land resource, and improving its water and nitrogen retention was essential for sustainable agriculture. In this study, corn straw biochar (BC) and biochars modified with H3PO4 (HBC) and NaOH (NBC) were applied to reclaimed yellow-cinnamon soil from southern Henan at rates of 1%, 2%, and 4%. Soil column experiments and simulations were conducted to investigate their effects on soil water movement and the leaching of NH4+-N and NO3−-N. The results showed that all biochar treatments significantly delayed wetting front migration, reduced cumulative infiltration and saturated hydraulic conductivity (Ksat), and increased soil water content and retention. The overall effectiveness followed HBC > NBC > BC. Compared with the control (CK), HBC prolonged the time for the wetting front to reach 50 cm by 19.25–43.92% and reduced the 180-min cumulative infiltration by 10.97–27.72%. At the 4% addition rate, water retention was increased by 15.84%, 11.85%, and 7.30%, and Ksat was decreased by 30.74%, 28.14%, and 18.18% for HBC, NBC, and BC, respectively. NH4+-N leaching was significantly reduced by all biochar treatments, with the maximum reduction (57.13%) achieved under 4% NBC. However, NO3−-N leaching from the biochar-amended soils was generally higher than that from CK, except for the 4% NBC treatment during the 10–20 h period. In conclusion, the biochars effectively improved soil water retention and reduced NH4+-N leaching but had limited effect on NO3−-N control. Further investigation into the mechanisms is needed.