Aluminium-organic matter-fluoride interactions during soil development in oxidised mine waste

A three year field trial assessing the suitability of mine waste rock as a surrogate subsoil with different depths of topsoil showed that pasture yield was significantly lower in the unmodified waste rock (UWR) treatments compared to waste rock modified with lime and potassic superphosphate (MWR) during the first two years but there was no difference in yield in the third year between the waste rock treatments. The reduced yield in UWR during the first two years was considered to be due to phytotoxic levels of aluminium (Al) in the UWR soil solution. Soil samples from the waste rock interface (A) and waste rock at depth ≥ 300 mm (B) collected at the end of the three year trial were analysed for different forms of soil Al, total soluble fluoride (F), dissolved organic carbon (DOC) and soil pH to determine whether organic matter and F leachate from the topsoil reduced Al toxicity at the waste rock interface (A) during soil development over 3 years. Results showed that after 3 years, 0.02 M CaCl₂ extractable Al (Al_Ca) and 1 M KCl extractable exchangeable + soluble Al (Al_K) were effectively reduced in the MWR interface (A) by lime and fertiliser applications but they remained at very high phytotoxic levels (Al_Ca = 17–21 μg g⁻¹, Al_K = 261–339 (μg g⁻¹) in the UWR interface (A) irrespective of topsoil depth treatment. The corresponding organic bound Al (Al_OM) ranged from 200 to 214 μg g⁻¹ and DOC was in the range 169–203 μg g⁻¹ in both UWR and MWR interfaces (A). These values were a significant two-fold higher than the values at depth ≥ 300 mm (B) which were considered to be the original levels of Al_OM and DOC in UWR. The increase in Al_OM and DOC by the third year is probably due to progressive leaching of organic ligands produced from the decomposition of organic matter in the topsoil subject to an annual rainfall of 2500 mm. This evidence for the reduction in Al toxicity by organic matter is further supported by significant dry root density (DRD) in the UWR interface (A). This indicated that organic matter may have complexed toxic Al in UWR and alleviated Al toxicity in the third year of the field trial. Total soluble F levels were similar between interface (A) and waste rock depth (B) of the UWR, suggesting that F levels have not changed during the three years of the trial and therefore there was no evidence of F complexing with phytotoxic Al in the soil solution in the presence of organic ligands.