The Role of Iron Oxides in Organic Matter Preservation and Phosphate Sorption in New South Wales Soils

Abstract

Evaluating the role of iron (Fe) and aluminium (Al) oxides in the preservation of soil organic carbon (SOC) and phosphate sorption is important for better management of soils. Thirty-seven surface (0 – 20cm) and 37 subsurface (20 – 40cm) soil samples from 37 sites across different agricultural regions of New South Wales, Australia. Soil samples were subjected to three separate extractions. X-ray absorption spectroscopy (XAS) was used to identify and quantify Fe in different minerals using 18 surface and subsurface bulk soil samples. Surface and subsurface soils with substantial Fe oxides contents from two sites i.e., Wagga Wagga and Tumbarumba were used to evaluate the adsorption of phosphate and DOC behaviour. Results showed that SOC was extracted in the sequence: CPP >CDCB >COX, with mean of 62%, 41% and 28% C, respectively, of the TC in soils. The extraction sequence for Fe was: FeDCB>FeOX>FePP, with mean of 49%, 9% and 3%, respectively, of the total Fe. The sequence for Al was: AlDCB>AlOX>AlPP, with mean of 4%, 3.9% and 2%, respectively, of the total Al. All extractable forms of Fe and Al showed significant positive correlations with extractable C. Linear combination fitting analysis of the X-ray absorption near edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) at the Fe-K edge revealed that crystalline Fe oxides (i.e. hematite and goethite) accounted for 60% and 40% respectively, of the proportion of total Fe present in the bulk soil samples. EXAFS predicted Fe contents in different phases better than XANES in bulk soils. The isotherm showed that both surface and subsurface soils from Tumbarumba had a greater phosphate adsorption capacity than the soils from Wagga Wagga. The greatest capacity for DOC adsorption in both studied soils occurred in the subsurface soils. In the mixed solution of P and DOC, phosphate adsorption promoted DOC desorption in the surface and subsurface soils from Wagga Wagga and Tumbarumba.