Effect of land use change and overgrazing on runoff, sediment and soil phosphorus losses in the Soolegan Watershed



The discharge of total suspended sediments (TSS) and dissolved nutrients from watersheds into receiving waters is a serious problem when natural vegetation is degraded by agriculture or overgrazing. This study was conducted in Soolegan Watershed to investigate land use change and overgrazing effects on phosphorus loss (dissolved & particulate), particle size associated loss of P and effect of time of rainfall on total dissolved P concentration in runoff. Much higher runoff, sediment and nutrient losses observed under cultivated land and HDP (high degraded pasture) than those under MDP (moderately degraded pasture). Contribution of particulate phosphorus exported from micro plot via surface runoff was increased by decreasing plant cover. Largest amount of dissolved phosphorus loss measured in HDP (16.4 mg/m2) and particulate P loss in cultivated land (301.3 mg/m2). Concentration of total dissolved phosphorus (TDP) in runoff was significantly (p<0.05) decreased with time because of increase in runoff volume. Loss of clay associated total phosphorus was 10.9 times greater in cultivated land than that under MDP. Significantly higher amount of labile inorganic P loss, associated with clay and silt particles, founded in cultivated land than that under MDP, which showed the same trend for total P loss by clay and silt particles. The results showed that the degradation of natural plant cover causes an increase in transfer of dissolved P and nutrient rich particles through catchment onto receiving waters. This may lead to phosphorus release into overlaying waters, which make suitable condition for eutrophication.