Chemical characterisation of deep profile Ferrosols under sugarcane in wet tropical northern Queensland

Abstract

The chemical properties of deep profile samples (up to 12 m) of Ferrosols from northern Queensland were investigated to provide an understanding of the accumulation of nitrate (NO₃) within these soil profiles. The influence of other cations and anions present in the soil solution or on the exchange and the charge chemistry of the profiles were examined with respect to the NO₃ accumulations. The major ions in the soil solution were Na, NO₃, and chloride (Cl). Distinct regions of anion accumulation were observed; SO₄ accumulated in the upper profile of all cores, whereas NO₃ and Cl accumulations were restricted to the lower profile of cores with appreciable AEC (>1 cmol_c/kg). Gaines-Thomas selectivity coefficients were used to indicate exchange preference for cations and anions, and are as follows: Al > Ca ~ Mg > K > Na and sulfate (SO₄) > Cl ~ NO₃. The selectivity of SO₄ increased and the extractable SO₄ decreased in the lower profile of all cores. This has important implications for the adsorption of NO₃ and Cl. The NO₃ and Cl accumulations were shown to correspond to a region of low SO₄ occupancy of the exchange sites in the lower profile. Along with the high SO₄ selectivity, this suggests that SO₄ may control the positioning of the NO₃ accumulations. It was concluded that the NO₃ accumulations were relatively stable under current management practices, although the reduction in NO₃ inputs would likely see the gradual replacement of NO₃ with Cl as a result of their comparable selectivity for exchange sites.