Clay Addition and Redistribution to Enhance Carbon Sequestration in Soils

The association of organic carbon (SOC) with clay in soils means that additions of clay to soils can increase the capacity of the soils for storage, and, eventually, sequestration of C. Addition of a fine-textured waste from bauxite processing to sandy soils for up to 29 years has led to increases of about 12 Mg C ha(-1), with a strong (r(2) = 0.93, P < 0.001) correlation between clay content and SOC. An increase of 2.2 Mg C ha(-1) has also occurred after 8 years in a sandy topsoil amended with subsoil clay-rich material. Bentonite addition increased plant yield in degraded and light-textured soils in tropical Australia. In Thailand, addition of clay-rich materials, particularly bentonite, but also clayey termite mound material, greatly increased the productivity of a degraded light-textured soil. Examination of soil modified by redistribution of subsoil clay into sandy topsoil by mechanical inversion showed the growth of roots in incorporated lumps of clay. Electron micrographs of clay-rich soils showed that fine mineral material (clay) can become closely associated with roots and other organic matter, which can protect them from decomposition. Roots within added or redistributed clay, along with microbes and their products, may become coated, enabling carbon sequestration in the long-term.