The nature and kinetics of organic matter release from soil by salt solutions

Summary Laboratory experiments in soil columns were performed to study the influence of dissolved salts on the amount and composition of organic matter (OM) released from soil. Samples of two surface soils from former wastewater infiltration sites were leached with solutions containing dissolved salts (NaH 2 PO 4 , NaNO 3 , CaCl 2 ) and by deionized water. The NaH 2 PO 4 solution induced strongest release with 0.6% of soil organic carbon (C org ) with 700 ml for 100 g of soil, while CaCl 2 released the least, summing to 0.1–0.2% of C org . The OM released was characterized by UV absorbance (aromaticity), ultrafiltration (molecular size distribution) and solid‐phase extraction (polarity). The results suggest that CaCl 2 preferentially released readily soluble OM. For the other solutions we assume solubilization by enhanced electrostatic repulsion (water), sodium exchange (NaNO 3 ), and sodium exchange and calcium decomplexation and displacement of sorbed organic anions (NaH 2 PO 4 ) to be the major mechanisms of release. In all experiments a phase of spontaneous desorption was observed, followed by a phase of steady‐state desorption. Activation energies for steady‐state release were estimated from kinetic investigations and suggest that the release is controlled by diffusion towards the phase boundary. These investigations emphasize the influence of dissolved salts on the nature and quantity of organic matter released from soil. The method presented seems able to characterize soil organic matter with respect to its availability and its mode of association with the soil matrix.