Rhizosphere management by biochar and leaching improved plant performance in fresh bauxite residue sand

High sodicity and Low nutrient retention in bauxite-processing residue sand (BRS) disposal areas restrict sustainable vegetation management in this highly alkaline environment. Although previous investigations have reported the beneficial effect of organic amendments on BRS rehabilitation, little is known about the underlying mechanisms of this complicated process, particularly after supplementary leaching of biochar amended BRS. We have investigated the coupled effect of supplementary leaching process, biochar [aged acidic (AC) vs alkaline pine (PC)] amendment and di-ammonium phosphate (DAP) fertilisation on rhizosphere nutrient dynamic and ryegrass performance in a 116-day glasshouse study. Biochar amendment in DAP + PC and DAP + AC treatments significantly increased and decreased pH (0.3–0.5 units) in all BRS rhizosphere and root-free zones, respectively. The application of alkaline and acidic biochars to BRS have reduced ammonia volatilisation (25–80%) and increased nitrogen retention (9–72%) in comparison with DAP treatment. Supplementary leaching had no significant effect on BRS rhizosphere pH, but reduced the EC values by ca. 62% in biochar-amended treatments. The leachates’ pH remained unchanged (ca. 8) throughout the experiment, while their EC reduced in AC (6 folds) and PC (9 folds) amended treatments, with lower reduction in rhizosphere than root-free zones. The interaction of applied biochars and plant roots generally decreased nutrient leaching from rhizosphere in comparison with root-free zones. Sodium was the dominant cation in the leachate of all treatments and cumulative abundance of exchangeable cations were in the order of Na+ > Ca2+ > K+ > Mg2+ > Al3+. The reduction of BRS salinity has increased plant biomass (ca. 47%) in biochar amended treatments by improving the capacity of fresh BRS rhizosphere for plant establishment