Pronounced surface stratification of soil phosphorus, potassium and sulfur under pastures upstream of a eutrophic wetland and estuarine system

High concentrations of nutrients in surface soil present a risk of nutrient movement into waterways through surface water pathways and leaching. Phosphorus (P) is of particular concern because of its role in aquatic system eutrophication. We measured nutrients under annual pastures on a beef farm and a dairy farm in the Peel-Harvey catchment, Western Australia. Soils were sampled in 10-mm increments to 100mm depth in March, June and September. Plant litter contained approximately 300-550mg kg-1 Colwell-extractable P. Extractable soil P was strongly stratified, being approximately 100-225mg kg-1 (dairy) and 50-110mg kg-1 (beef) in the top 10mm and <40mg kg-1 at 40-50mm depth. Total P and extractable potassium were also highly stratified, whereas sulfur was less strongly stratified. Shoot nutrient concentrations indicated that nitrogen was often limiting and sulfur was sometimes limiting for pasture growth: concentrations of P were often much greater than required for adequate growth (>4mg g-1). We conclude that high P concentrations at the soil surface and in litter and shoots are a source of risk for movement of P from farms into waterways in the Peel-Harvey catchment.