Selecting a nitrogen availability index for understanding plant nutrient dynamics in rehabilitated bauxite-processing residue sand

Understanding nutrient dynamics in non-typical soil materials such as bauxite-processing residue sand (pH > 10; EC > 30 dS m−1) is critical for developing fertilizer strategies and evaluating ecological restoration performance. Indices relating nitrogen (N) concentration in soil to plant N uptake are well-established for natural soils but their application to non-typical soils has received little attention. This study investigated a range of soil-based methods [i.e. 2 M KCl extractable inorganic N (NH4+, NO3−), potentially mineralizable nitrogen (PMN), and 0.01 M CaCl2 extractable N] to identify their suitability for describing soil-plant N relations in highly alkaline bauxite-processing residue sand. Nitrogen availability indices were measured under laboratory (pot trial) and field conditions. Pot trial was established using residue sand that had been amended (10%, v/v, basis) with various organic (greenwaste compost, biochar and biosolids) and inorganic (zeolite) materials. Both the field study and pot trial showed that 2 M KCl extractable NO3−-N was most highly correlated with plant biomass N compared with the other N availability indices. Findings from this study suggest that 2 M KCl extractable NO3−-N can be used as a soil quality indicator in developing fertilizer management strategies and assessing ecological status of the residue storage areas.