Characteristics of Free and Mineral Associated Organic Matter in Soils with Contrasting Mineralogy.

We evaluated the role of minerals on the quantitative and qualitative distribution of OC pools in soils with contrasting mineralogy. OM in the density fractions of these soils was characterized using XRD, DRIFTS, XPS and IRMS in original states, and after 6% NaOCl and 10% HF treatments. NaOCl resistant fraction was considered as relatively stable pool of OC and HF soluble fraction was assumed as the mineral bound OC. Vertisol, Luvisol and Solonetz with phyllosilicates dominated clay fraction contained greater proportion of free OM (<1.8 g cm^-3) than the sesquioxides dominated Ferralsol. Free OM fractions had higher C content, greater C:N ratio, lower δ¹³C and were enriched in aliphatic C as compared to their corresponding mineral bound fractions. Three discrete mineralogies were identified in heavy density fractions (>1.8 g cm^-3): Fe/Al oxides dominated (1.8->2.6 g cm^-3) only in the Ferralsol, phyllosilicates (1.8-2.6 g cm^-3) and, quartz and feldspar dominated (>2.6 g cm^-3) fractions in the other three soils. Sesquioxides fraction had the highest aromatic and oxidised C, and amide N species. The increased C:N ratio of oxidation resistant OM and the highest amount of HF soluble C in the sesquioxide fractions indicate preferred preservation of C over N with this fraction. OM associated with phyllosilicates was enriched with aromatic C, O-alkyl C and protonated amide N. The amount of oxidation resistant and HF soluble C(N) varied between phyllosilicates, with kaolinitic Luvisol had more oxidation resistant C than the smectite-rich Vertisol. Quartz and feldspar associated OM contributed the lowest proportion of C(N) to the total C. The amount of oxidation resistant and HF soluble C and N associated with primary minerals were also lowest. Our results show that sesquioxides have greater potential to preserve OC in soils than the other minerals, and phylloslicates preferentially preserve N containing organic compounds in soils.