Mineralisation of nitrogen in a chronosequence of rehabilitated bauxite mines

Abstract

Mine site rehabilitation should aim to establish quickly and maintain the processes of nutrient cycling at rates comparable with, or approaching, those of native forests. Current management strategies for rehabilitating bauxite mines and other mine sites in Australia usually include planting fast-growing understorey species at high densities and applying fertiliser. We provide the first detailed study of nitrogen (N) availability and N transformations (mineralisation/immobilisation) in such rehabilitated mine sites. Mean concentrations of NO₃^– (0–5 cm) in a chronosequence (7, 13, 22, and 27 years old) of rehabilitated mine sites ranged from 0.5 to 1.3 kg/ha, and NH₄⁺ from 4.0 to 9.5 kg/ha. In burnt and unburnt native jarrah (E. marginata Donn ex. Smith) forests adjacent to the mine site, mean NO₃^– concentrations in surface soil (0–5 cm) were 0.8 kg/ha (burnt) and 1.1 kg/ha (unburnt), and mean NH₄⁺ concentrations were 6.8 kg/ha (burnt) and 7.8 kg/ha (unburnt). Concentration of NH₄⁺ at 0–5 cm was strongly related to soil water content (R² = 0.69, P < 0.05) in rehabilitation sites, but not at 5–10 cm depth. Rates of N mineralisation (0–5 cm) in rehabilitation sites ranged from 34 to 52 kg/ha.year, of the same order as rates in native forest soil. In all rehabilitation and native forest sites, rates of N mineralisation were significantly related to rates of N-uptake at both 0–5 and 5–10 cm depth (R² > 0.63, P < 0.05). Soil C/N ratios (0–5 cm) in rehabilitation sites ranged from 22.4 to 38.8, and in native forests from 35.6 (burnt) to 40.3 (unburnt). Soil C/N ratios increased with depth in both rehabilitation and native forest sites (ranged from 31.2 to 51.6). Availability of water was the major determinant of nitrogen availability in this strongly Mediterranean climate.