Subsoil bulk density and organic carbon stock in relation to land use for a Western Australian Sodosol.
Résumé
It has been demonstrated that the high bulk densities of clayey subsoils of Sodosols can result from a process involving shrink/swell cycles and the development of ped coatings containing topsoil material. Our objective was to study this process according to land use and to compute the amount of organic carbon introduced into the subsoil as a consequence of ped coating in the subsoil. In an area located east of Katanning, we carried out an intensive soil survey and selected 2 closely adjacent sites with similar soils but differing in their land use: a cultivated soil and a never cultivated soil in an undisturbed strip of land. Pit faces and horizontal planes of the top of the subsoil were studied. The polygonal network corresponding to the cross-section of the prismatic peds was described. The clay content and bulk density of the prisms was determined. The carbon contents of the coatings and of the sandy material rich in organic carbon that corresponded to remnants of native vegetation was measured. Results showed that the coating process has occurred under native vegetation but is more active when the land has been used for agricultural and pastoral activities, as indicated by thicker sandy-clay coatings on the vertical faces of prisms and the higher bulk densities in the subsoil. The mean bulk density was 1.71 g/cm3 at 20-25 cm depth at the undisturbed site and was 1.86 and 1.82 g/cm3 at 20-25 and 25-30 cm depth, respectively, at the cultivated site. The total soil organic carbon stock was estimated to be close to 68.9 and 61.0 Mg/ha at the undisturbed and cultivated sites, respectively. The organic carbon stock in the subsoil was 38.5 and 23.3% of the respective total stocks. One fourth of the carbon stock in the subsoil corresponded to materials rich in organic carbon that originated from roots of native vegetation. The organic carbon stock in the ped coatings was <1% of the total stock at the 2 sites. Finally, our results support the adoption of zero-till agricultural system for the soil studied to restrict subsoil densification.