An Assessment of Mineral–Organic Matter Associations in Soils

Abstract

This study aimed to obtain a better understanding on the mechanisms and controls of mineral–organic matter (OM) interactions across a range of soil types and land uses. Batch adsorption experiments were conducted to evaluate sorption capacity of different organic carbon (OC) compounds on pure minerals and their interaction mechanisms were determined by infrared spectroscopy. Role of mineralogy on the dynamics of soil OM pools was assessed in four cropped soils with contrasting mineralogy by sequential density fractionation and chemical treatments. Effect of land use change on soil OM pools also investigated using the same soils under native and cropped land uses applying different spectroscopic techniques. Results revealed that Fe and Al oxides can stabilise more OC in soils than the phyllosilicates and primary minerals through organo–mineral association. However, extend of OC sorption and stabilisation highly dependent on composition of OC, soil environments, soil depth and land uses. Preferential association of aromatic and carboxyl C with Fe and Al oxides and protonated amide N with phyllosilicates and primary minerals were noticed. In acidic environment, oxides surface showed affinity to anionic C compound e.g., carboxylate via strong ligand exchange, while the cationic amino acid was preferentially adsorbed by the phyllosilicate surfaces via electrostatic interaction. Mineral associated OC can be vulnerable to biodegradation as an effect of land use change, particularly under OC saturated condition due to the formation of weaker bonds among excessive OC loading and mineral surface. This indicates the importance of sink capacity of distinct mineral surfaces in OC stabilisation and hence should be taken into account in modelling of potential capacity of soil for C sequestration. Land use change impact was less in the sub-surface mineral associated OC, thus; mineral rich sub-surface soils can act as a potential C sink together with improved land use system.