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

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.

Assessment of N2O emissions from a fertilised vegetable cropping soil under different plant residue management strategies using 15N tracing techniques.

Combined application of plant residues and N fertilisers strongly affect soil mineral N dynamics and N2O emissions depending on the quality of the plant residues, their application methods and other management strategies. We investigated the effect of combined application of two vegetable plant residues (cauliflower and sweet corn) and 15N fertiliser on N dynamics and N2O emission in a glasshouse pot study. The experiment was conducted under two residue management practices (soil incorporation vs surface mulching) over 98days with growing basil (Ocimum basilicum) plants. We also assessed the efficacy of applying the nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP) to the plant residues, for reducing N loss and mitigating N2O emissions. Application of plant residues, both on the soil surface or into soil, resulted in net N mineralisation and increased cumulative N2O emission compared with the application of N fertiliser alone. Soil surface mulching of sweet corn decreased total and residue-induced cumulative N2O emission compared with the incorporation method, while it showed opposite effect on N2O emissions from cauliflower residue. The application of DMPP with sweet corn residue reduced total, residue- and fertiliser-induced N2O emissions; however its application with cauliflower residue did not show any mitigating effect on the N2O emissions. The residue application methods and the use of DMPP did not significantly affect 15N recovery by the basil plants. In contrast, soil incorporation of these residues doubled the microbial immobilisation of applied 15N into soil organic matter. Linear regression analysis of N2O emission during the experimental period indicated that in the treatments without DMPP application, soil NO3--N concentration was the most important factor in controlling the magnitude of N2O emissions, while the application of DMPP changed the dominant regulating factor from NO3--N to NH4+-N concentration.