Dissolved organic matter release and retention in an alkaline soil from the Nile River Delta in relation to surface charge and electrolyte type

The sorption of dissolved organic matter (DOM) on surfaces of soil particles strongly depends on surface charge (SC). As anions and cations in soil solution affect SC, the sorption properties can also be modified by electrolytes. This issue is particularly important in alkaline soils. In order to describe the effect of electrolyte type on SC properties and DOM sorption we comparatively investigated in column and batch experiments the release and retention behaviors of DOM of a Vertisol from the Nile River Delta. As electrolytes we have chosen NaCl, Na2SO4, and CaCl2, which are commonly found in soil solution. CaSO4 was included to investigate the effect of gypsum application to the soil on the tested factors. The SC of the clay fraction was quantified at the initial soil pH of 8.3 at electrolyte concentrations of 0–10 mmolc l− 1 and a DOM application between 0 and 25 mg DOC l− 1. The kind of anion had strong effects on SC of the clay fraction. At 10 mmolc L− 1 the SC was − 2.6 and − 4.1 mmolc kg− 1 for NaCl and Na2SO4, respectively. Strong negative SC at presence of Na2SO4 led to repulsive forces, which hindered the sorption of DOM on clay particles and induced high DOM release in column experiments. At 10 mmolc l− 1 concentration of CaCl2 and CaSO4, the values of SC were −0.9 and −0.7 mmolc kg−1, respectively, indicating that Ca2+ is much stronger adsorbed than SO42− under the alkaline conditions. In the DOM-clay system, most negative SC values were obtained in deionized water as compared to the electrolyte solutions, indicating that in the absence of electrolytes the strongest repulsive forces hindered the sorption of DOM on clay particles. The sorbed amounts of DOM on fine soil were large not only for both CaCl2 and CaSO4 electrolytes, but also for NaCl electrolyte with almost the same values. The fact that NaCl increased DOM sorption can be explained by soluble Ca2+ and ion exchange as distinct amounts of Ca2+ were released during percolation from the sample which stabilize DOM by Ca2+ bridging. Despite some negative impact of applied SO42− on stable aggregation and DOM leaching, for agricultural practice the application of CaSO4 is superior to that of CaCl2, since the former removes Na+ more efficiently.