Roychand, P.Marschner, P.2016-11-062016-11-062015Communications in Soil Science and Plant Analysis, 2015; 46(2):185-1940010-36241532-2416http://hdl.handle.net/2440/102213It 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.en© Taylor & Francis Group, LLCCalcium; decomposition; sorption; subsoil clayJournal article003002463310.1080/00103624.2014.9678580003475279000052-s2.0-849271737782-s2.0-84920426027166687Marschner, P. [0000-0001-6808-0244]