Ion exchange properties of casts of the anecic earthworm (Martiodrilus carimaguensis Jimenez and Moreno) in a Colombian savanna Oxisol

Variable-charge minerals dominate tropical soils. The zero point of charge (ZPC) is a function of the various mineral and organic constituents of a so il and the degree to which they interact determines the expression of varia ble charge. The ZPC can be affected by the presence of organic matter, modi fying the overall charge properties of a soil and thus its ability to retai n ions against leaching. Earthworm casts generally have higher organic matter contents, compared wit h the surrounding bulk soil. The higher organic matter can induce a shift o f the ZPC to lower soil pH, improving the range of charge capacity, cation and anion adsorption, and reducing the risk of leaching. This study analyse s the influence of the earthworm Martiodrilus carimaguensis Jimenez and Mor eno on the shift of ZPC in a native savanna and two improved pastures from the Eastern Plains of Colombia. ZPC was determined for casts and bulk soil by a proton titration method to evaluate the amount of charge on colloidal surfaces with different pH and electrolyte concentrations. In all the syste ms, the ZPC in casts was displaced to more acid values, increasing the gene ration and retention of charge at natural pH levels (pH in H2O). Compared w ith the non-ingested soil, the level of exchangeable Al+3 in casts was redu ced and the levels of Ca+2 and Mg+2 were increased. Improved pastures incre ased the difference between ZPC and pH (+42 % to +56 %), compared with nati ve savanna. These results were mainly attributed to the ability of earthwor ms to concentrate organic matter in their casts when ingesting organic-rich substrates. Modification of the ZPC in casts implies an improvement in the cation exchange capacity that could result in a greater availability and r etention of nutrients.