Respiration and sorption of water-extractable organic carbon as affected by addition of Ca²⁺, isolated clay or clay-rich subsoil to sand

Abstract

Clay addition to light-textured soils is used to ameliorate water repellency and to increase nutrient retention. However, clay addition may also increase the potential to bind organic matter and thus C sequestration. Divalent calcium ions (Ca2+) play an important role in binding of organic matter to clay because they provide the bridge between the clay particles and organic matter which are both negatively charged. In the first experiment, quartz sand was mixed with clay isolated from a Vertosol at rates of 0, 50 and 300 g kg−1, finely ground mature wheat residues (20 g kg−1) and powdered CaSO4 at 0, 5 and 10 g kg−1. Soil respiration was measured over 28 d. Compared to the sand alone, addition of isolated clay at 300 g kg−1 increased cumulative respiration with a stronger increase than that at 50 g kg−1. Addition of CaSO4 increased electrical conductivity, decreased sodium adsorption ratio and reduced cumulative respiration. The latter can be explained by enhanced sorption of organic matter to clay via Ca2+ bridges. In a second experiment, isolated clay or subsoil of the Vertosol without or with powdered CaSO4 at 10 g kg−1 were used for a batch sorption with water-extractable organic C (WEOC) from wheat straw followed by desorption with water. Addition of 10 g kg−1 CaSO4 increased sorption and decreased desorption of WEOC in both subsoil and isolated clay. In the third experiment, subsoil of the Vertosol was used for a batch sorption in which WEOC was added repeatedly. Repeated addition of WEOC increased the concentration of sorbed C but decreased the sorbed proportion of the added WEOC. This indicates that sorption of WEOC may be underestimated if it is added only once in batch sorption experiments.