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

Abstract

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 (EC_1:5) 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.