Effect of mono- and divalent cations on sorption of water-extractable organic carbon and microbial activity

Sorption is an important process for retention of organic carbon (C) in soils. The effect of Na+ and Ca2+ on sorption of organic C has been studied in salt-affected soils, but little is known about the effect of K+ and Mg2+ ions on sorption of water-extractable organic C (WEOC). The effect of Na+, K+, Ca2+ and Mg2+ ions on sorption of WEOC and its decomposition were investigated in a loamy sand (7.5 % clay) and a sandy clay loam (34.4 % clay). Salinity was developed with NaCl, KCl, MgCl2 or CaCl2 to obtain different concentrations of exchangeable Na+, K+, Ca2+ and Mg2+ at an electrical conductivity in a 1:5 soil/water extract (EC1:5) of 1 dS m(-1). Water-extractable organic C was derived from wheat straw, and microbial activity after sorption was quantified by measuring CO2 emission from the soils for 27 days. The concentration of sorbed C was higher in the sandy clay loam than in the loamy sand and decreased in the treatment order Ca2+ > Mg2+ > K+ > Na+, but cumulative CO2-C emission after sorption was highest from the Na+ and lowest from the Ca2+ treatments. The strong binding in the Ca2+ and Mg2+ treatments can be explained by the low zeta potential and the high covalency index of cation binding with C, whereas zeta potential was high and the covalency index was low in the Na+ treatments. Although K+ is also monovalent, WEOC was more strongly bound in the K+ than in the Na+ treatment. The weak binding with Na increased the accessibility of the sorbed C to soil microbes and, thus, microbial activity. Our results suggest that monovalent cations may enhance decomposition and leaching of WEOC in saline soils with Na+ having a greater effect than K+. Divalent cations, particularly Ca2+, enhance the binding of organic matter and thus organic C stabilization, whereas Mg2+ ions have a smaller effect.