Biochar as a Capillary Barrier Outperforms Straw and Peat in Ameliorating Coastal Saline Soil and Promoting Plant Growth

Abstract

Coastal saline soils can be ameliorated by establishing a capillary barrier at certain depths, yet the outcomes are contingent on barrier type. This study evaluated the performance of straw, biochar, and peat as capillary barriers in ameliorating coastal saline soil and improving plant growth. A 5-cm-thick barrier layer was buried in soil boxes at the 20-cm depth, and phreatic evaporation was simulated over the growth cycle of Bermuda grass (Cynodon dactylon (L.) Pers.). Distinct patterns of water, salt and nutrient distributions were observed in the 0–20 cm soil layers across treatments. Soil water content notably increased in biochar treatment compared with the control. Soil salt accumulation was strongly inhibited above different capillary barriers, as indicated by decreased electrical conductivity. Both soil Na+ and SO42− concentrations decreased in response to biochar treatment, despite the elevation of Ca2+ and Mg2+ concentrations. Biochar treatment exhibited the greatest positive effects on soil organic carbon, Olsen phosphorus and ammonium nitrogen concentrations, leading to the most significant improvements in root morphology and plant biomass. Path analysis revealed that the presence of capillary barriers modified root morphology through contrasting effects on salinity, ammonium nitrogen, and Olsen phosphorus in the upper soil layers, which conferred benefits to C. dactylon growth. This study demonstrates that biochar is a highly effective capillary barrier material, offering a promising strategy for sustainable soil amelioration and plant growth promotion in coastal saline areas.