Phosphorus sorption in soils and sediments: implications for phosphate supply to a subtropical river in southeast Queensland, Australia

Phosphorus (P) is often a key limiting nutrient in freshwater systems, and excessive P can result in algal blooms, with flow-on effects to aquatic food webs. P sorption is an important process in aquatic and terrestrial ecosystems whereby phosphate (PO43−) is exchanged between liquid and solid phases. This study shows that differences in the concentration of PO43− in a subtropical river system during high and low flow can be attributed to differences in P sorption characterises of its catchment soils and sediments. The sediments have lower Equilibrium Phosphate Concentrations (EPC0) and higher binding energy (Kd); the surface soils have higher EPC0 and higher easily desorbed P (NH4Cl–P). A comparison of filterable reactive phosphorus (frP) in water samples collected at high and low flows, with soil and sediment EPC0, suggested that during event flows, the high EPC0 and NH4Cl–P of surface soils is producing a net movement of PO43− from the soil/sediment system into runoff and stream flow. At baseflow, there is more likely a net movement of PO43− into the riverbed sediments. This has important implications for management actions aimed at reducing P loads to river systems and downstream water storages, namely the need to increase the infiltration of rainfall to decrease the amount of PO43− being flushed from the surface soil.