Effect of exchangeable cation concentration on sorption and desorption of dissolved organic carbon in saline soils

Abstract

Sorption is a very important factor in stabilization of dissolved organic carbon (DOC) in soils and thus C sequestration. Saline soils have significant potential for C sequestration but little is known about the effect of type and concentration of cations on sorption and release of DOC in salt-affected soils. To close this knowledge gap, three batch sorption and desorption experiments were conducted using soils treated with solutions either low or high in salinity. In Experiment 1, salinity was developed with either NaCl or CaCl2 to obtain an electrical conductivity (EC) in a 1:5 soil: water extract (EC1:5) of 2 and 4dSm−1. In Experiments 2 and 3, NaCl and CaCl2 were added in various proportions (between 25 and 100%) to obtain an EC1:5 of 0.5 and 4dSm−1, respectively. At EC1:5 of 4dSm−1, the sorption of DOC (derived from wheat straw) was high even at a low proportion of added Ca2+ and did not change with proportion of Ca added, but at EC1:5 of 0.5dSm−1 increasing proportion of Ca2+ added increased DOC sorption. This can be explained by the differences in exchangeable Ca2+ at the two salinity levels. At EC1:5 of 4dSm−1, the exchangeable Ca2+ concentration did not increase beyond a proportion of 25% Ca2+, whereas it increased with increasing Ca2+ proportion in the treatments at EC1:5 of 0.5dSm−1. The DOC sorption was lowest with a proportion of 100% as Na+. When Ca2+ was added, DOC sorption was highest, but least was desorbed (with deionised water), thus sorption and desorption of added DOC were inversely related. The results of this study suggest that DOC sorption in salt-affected soils is mainly controlled by the levels of exchangeable Ca2+ irrespective of the Ca2+ concentration in the soil solution which has implications on carbon stabilization in salt-affected soils.