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

Abstract

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 (PO4 3-) is exchanged between liquid and solid phases. This study shows that differences in the concentration of PO4 3- 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 PO4 3- from the soil/sediment system into runoff and stream flow. At baseflow, there is more likely a net movement of PO4 3- 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 PO4 3- being flushed from the surface soil.