Soil chemical and physical constraints to pasture productivity on rehabilitated land after bauxite mining

The reconstruction and rehabilitation of soil profiles to facilitate sustainable land use as a pasture, requires careful planning and management. Pastures are among the most common form of land rehabilitation after mining, but the productivity of these pastures is less than that of comparable unmined land. Here we assessed the soil properties and pasture production on recently rehabilitated farmland after bauxite mining, which are considered to have diminished productivity in comparison to adjacent unmined grassland. This was to understand the limitation to achieving levels of soil fertility, pasture productivity and sustainability similar to those of the pre-mined landscapes. After comparing soil pit profile descriptions, we assessed several soil and plant parameters to identify the constraints to pasture growth. Several edaphic parameters differed between unmined and rehabilitated soils, including pH, salinity, particle size distribution, soil strength, soil organic carbon and cation exchange capacity. Both rehabilitated and unmined areas had some deficiencies in plant available nutrients in both topsoils and subsoils. Many of the edaphic parameters that showed significant differences were likely due to the mixing of materials during excavation, handling and re-spreading as part of the mining and rehabilitation process. The pasture plants also exhibited different responses between rehabilitated and unmined soils. Root mass penetration through the rehabilitated profiles was generally less than the unmined profiles. Differences in pasture dry matter production were identified between the mined and unmined areas. No significant difference was found in pasture species composition on the rehabilitated sites. In summary, no single constraint to pasture production on the rehabilitated land was identified. However, the handling and mixing of the soil materials leading to the creation of impenetrable zones may be an important aspect. In addition, organic matter concentrations remain low in rehabilitated topsoils and fertile topsoils were likely diluted through the disturbance and the profile reconstruction process. These three factors may be attributed to the underlying cause of reduced pasture production