Effects of Different Rates of Ca2+ Addition on Respiration and Sorption of Water-Extractable Organic C to a Vertisol Subsoil

It is well known that calcium (Ca2+) plays an important role in binding organic matter to clay. However, most previous studies were conducted with either topsoil or pure aluminosilicates. Less is known about the effect of Ca2+ on binding of organic matter to clay-rich subsoils, which have lower organic-matter contents than topsoils, and their clays are more strongly weathered than pure aluminosilicates. Two experiments were conducted with a Vertisol subsoil (69% clay): a laboratory incubation and a batch sorption. The mineral substrate in the incubation experiment was pure sand alone or sand amended with 300 g clay kg(-1). Powdered calcium sulfate (CaSO4) at rates of 0, 5, 10, and 15 g Ca kg(-1) and mature wheat residue at a rate of 20 g kg(-1) were added to this mineral substrate and the water content was adjusted to 70% of water-holding capacity. Carbon dioxide release was measured for 28 days. Cumulative respiration per g soil organic carbon (C) (SOC from clay and residues) was increased by clay addition. Increasing Ca2+ addition rate decreased cumulative respiration in the sand with clay but had no effect on respiration in the pure sand. Clay and Ca2+ addition had no significant effect on microbial biomass carbon (MBC) per g SOC but clay addition reduced the concentration of potassium sulfate (K2SO4)-extractable C per g SOC. For the batch sorption experiment, the subsoil was mixed with 0 to 15 g Ca kg(-1) and water-extractable organic C (WEOC) derived from mature wheat straw was added at 0, 1485, 3267, and 5099 mg WEOC kg(-1). Increasing Ca2+ addition rate increased sorption of WEOC, particularly at the greatest concentration of WEOC added, and decreased desorption. This study confirmed the importance of Ca2+ in binding organic matter to clay and suggests that Ca2+ addition to clay-rich subsoils could be used to increase their organic C sequestration.