Aged biochar alters nitrogen pathways in bauxite-processing residue sand: Environmental impact and biogeochemical...

Low nitrogen (N) content and retention in bauxite-processing residue sand (BRS) disposal areas pose a great challenge to the establishment of sustainable vegetation cover in this highly alkaline environment. The budget and fate of applied N in BRS and its potential environmental impacts are largely unknown. We investigated the effect of combined application of biochars [aged acidic (AC) vs alkaline pine (PC)] and di-ammonium phosphate (DAP) fertiliser on ammonia (NH₃) volatilisation, nitrous oxide (N₂O) emission and N retention in a 116-day glasshouse study. The application of AC to BRS decreased pH (≈0.5 units) in BRS, while PC biochar increased pH (≈0.3 units). The application of AC reduced NH₃ volatilisation by ca. 80%, while PC by ca. 25%. On the other hand, the AC treatment increased N₂O emission by 5 folds. However, the N loss via N₂O emission in the AC treatment only accounted for ca. 0.4% of applied N. The reduction in BRS pH and increased retention of mineral N due to the presence of oxygen-containing (phenolic and carboxylic) functional groups in AC may be responsible for reduced NH₃ volatilisation and increased N₂O emission. This study has highlighted the potential of biochar (particularly aged biochar) in improving N retention and minimising environmental impacts in highly alkaline environments.