Phosphorus mobility in a karst landscape under pasture grazing system

The spatial distribution and partitioning of water dissolved phosphorus fractions in the soil profile of a grazed karst sinkhole landscape were investigated. We also measured P fractions in surface runoff entering a sinkhole drain and in karst spring flow draining the study area. Grazing increased total N, C, and all forms of P of soil. Dissolved inorganic orthophosphate (DPi) was the highest in the surface soil layer and diminished significantly with depth. The proportion of dissolved unreactive phosphorus (DPu) increased with soil depth. Changes in DPu with landscape position and depth were closely correlated with changes in dissolved organic carbon (DOC) suggesting that the mobility and transport of DPu was mediated by DOC. Landscape position sampling showed molybdate reactive phosphorus (MRP) and DPu increased toward the bottom and center of sinkholes. The distribution of DPi and DPu in surface runoff and that occurring in underground watercourses confirms the significance of DPu transported into karst groundwater.