Sandy Soil Amended with Clay Soil: Effect of Clay Soil Properties on Soil Respiration, Microbial Biomass, and Water Extractable Organic C

The low organic C (OC) sequestration and low water and nutrient holding capacities of sandy soils can be improved by amendment with clay soils. However, the effect of clay soils may depend on clay soil properties, including cation exchange capacity (CEC) and clay and OC contents. Two experiments were carried out. In the first experiment, sandy soil was mixed with two high clay content soils (Sodosol and Vertosol) at 10% and 40% (w/w). The soil was left unamended or amended with wheat straw, to assess if the effect of clay soil addition was influenced by OC addition. In the second experiment, sandy soil was mixed with two low-clay content soils which differed in CEC without wheat straw amendment. In experiment 1, compared with sandy soil alone, amendment with Sodosol (low OC) reduced cumulative respiration more strongly than addition of Vertosol. Without wheat straw addition, both clay soils increased microbial biomass C (MBC), whereas with wheat straw, they reduced microbial biomass C (MBC) compared with sandy soil alone. Sodosol amendment reduced water-extractable OC (WEOC) without wheat straw but increased it with wheat straw. In experiment 2, only the high CEC clay soil reduced cumulative respiration, MBC and WEOC compared with sandy soil alone. Clay soil addition to sandy soil will result in the greatest OC sequestration if the added soil has high clay but low OC content and its CEC is high.