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

Abstract

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, Na 2SO 4, and CaCl 2, which are commonly found in soil solution. CaSO 4 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 mmol c 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 mmol c L − 1 the SC was − 2.6 and − 4.1 mmol c kg − 1 for NaCl and Na 2SO 4, respectively. Strong negative SC at presence of Na 2SO 4 led to repulsive forces, which hindered the sorption of DOM on clay particles and induced high DOM release in column experiments. At 10 mmol c l − 1 concentration of CaCl 2 and CaSO 4, the values of SC were −0.9 and −0.7 mmol c kg − 1, respectively, indicating that Ca 2+ is much stronger adsorbed than SO 4 2 − 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 CaCl 2 and CaSO 4 electrolytes, but also for NaCl electrolyte with almost the same values. The fact that NaCl increased DOM sorption can be explained by soluble Ca 2+ and ion exchange as distinct amounts of Ca 2+ were released during percolation from the sample which stabilize DOM by Ca 2+ bridging. Despite some negative impact of applied SO 4 2 − on stable aggregation and DOM leaching, for agricultural practice the application of CaSO 4 is superior to that of CaCl 2, since the former removes Na + more efficiently.