Background and Aims
The objective of this study was to test the suitability of greenwaste biochar to aid nitrogen (N) retention in rehabilitated bauxite-processing residue sand (BRS).
Bauxite residue sand was collected from the Alcoa of Australia Pinjarra refinery. The pH of BRS was adjusted to values of 5, 7, 8 and 9 and subsequently amended with different rates (1, 5, 10 and 20 %, w/w) of greenwaste biochar. The loss of N via NH3 volatilization following addition of di-ammonium phosphate (DAP) was determined using an acid trapping method.
At low pH (5), increasing pH rather than adsorption capacity, resulting from biochar addition, caused greater losses of N through volatilization from BRS. In BRS with medium pH (7, 8), increasing adsorption capacity, induced by biochar addition, played the more dominant role in enhancing adsorption of NH4+-N /NH3-N and lowering NH3 volatilization. In the BRS with high pH (9), the majority of NH4+-N /NH3-N pools was lost via NH3 volatilization due to the strong acid-base reaction at this pH.
It is concluded that the interaction of changes in pH and adsorption capacity induced by greenwaste biochar addition affects the availability and dynamics of NH4+-N/ NH3-N in BRS amended with DAP.