Aged acidic biochar increases nitrogen retention and decreases ammonia volatilization in alkaline bauxite residue sand

•Nitrogen (N) use efficiency and retention are low in bauxite residue sand (BRS).•Acidic and alkaline biochars were tested for their capacity of retaining N.•Aged acidic biochars can adsorb much more NH4+-N than other biochars in BRS.•Acidic biochar had a larger amount of oxygen-containing functionalities.•Acidic biochar can be used as an effective amendment for rehabilitation of BRS.

Bauxite residue sand (BRS) is the primary growth medium for rehabilitating Alcoa's residue storage areas in south-west Western Australia. Successful revegetation of highly alkaline BRS can be hindered by its low nitrogen (N) use efficiency. Biochar, a carbon (C)-rich material, has been suggested to have the potential to improve water and nutrient retention in soil. However, little is still known about the effect of biochar amendment on N use efficiency in the alkaline BRS environment. This incubation study aimed to evaluate the impact of biochars with different characteristics on N retention and dynamics in BRS. The BRS (pH 9.5 after being pre-treated with 1% gypsum and leached with water) was amended with the acidic biochar (pH 3.86; AC, from wild fire) and alkaline biochars (pH 9.58-10.8; greenwaste, GW; Jarrah, JL; mallee, ML) at a rate of 10% (w/w). The N loss via NH3 volatilization was much lower from the AC treatment (24% of di-ammonia phosphate (DAP)-N added) than the alkaline biochars treatments (76-80% of the DAP-N added). The AC treatment can retain about 73% of N added to BRS, compared with <25% in alkaline biochar treatments. This can be attributed to the acidic nature and the greater NH4+-N sorption capacity arising from the presence of a high density of the oxygen-containing functional groups on the surface of acidic biochar as revealed by the FTIR spectroscopy. These results imply acidic biochar can be used as an effective amendment for increasing N use efficiency by plants growing in alkaline BRS.