Pathways of different forms of nitrogen and role of ammonia-oxidizing bacteria in alkaline residue sand from bauxite processing

Abstract

Nitrogen (N) dynamics and associated processes are often overlooked in the rehabilitation protocols of disturbed landscapes. This study reports on the transformations of N fertilizers and the microbial community, and plant growth responses in rehabilitated strongly alkaline residue sand from bauxite processing (BRS). Ryegrass was grown in specifically designed growth chambers in a two-factorial completely randomized design. Different forms of N fertilizer, such as ammonium sulphate (AS), potassium nitrate (KN) and glycine (GL), were applied at two rates. Nitrogen uptake by plants, residual inorganic N and N losses through leaching and volatilization were determined and quantified throughout the growing period. The abundance of both ammonia-oxidizing archaea (AOA) and bacteria (AOB) was determined by quantitative polymerase chain reaction. The results showed that N uptake was greater with KN fertilizer (31.3–56.4%) than with AS (23.4–47.8%) and GL (16.4–38.1%), in spite of the substantial leaching loss of NO3−. Combined N losses by volatilization and leaching with GL (39–53%) and AS (40–60%) fertilizers indicated both physico-chemical and biological transformations of N by mineralization and nitrification. Ammonia-oxidizing bacteria, rather than AOA, were the dominant nitrifiers that colonized the freshly rehabilitated BRS growth media. The gene copy number of AOB correlated with both soil extractable NO3−-N (r = 0.92, P < 0.001) and ryegrass leaf biomass N (r = 0.89, P < 0.001). We concluded that AOB play a pivotal role in the cycling of N in BRS, whereas NO3−-N is critical for plant N nutrition and rehabilitation in the alkaline BRS disposal areas.