Phosphorus dynamics in agricultural drainage ditches: An influence of landscape properties

Excessive loading of phosphorus (P) into aquatic systems from adjacent terrestrial ecosystems often results in eutrophication and subsequent degradation of aquatic resources. This study investigated the dynamics of P within replicated experimental conventional and controlled (with weirs) agricultural drainage systems during simulated storm runoff events (summer and winter). The study objective was to examine the capacity of experimental agricultural drainage ditches to sorb and act as a P sink. In the summer runoff event, ditches with weirs had significantly lower (0.01) bioavailable P (Pw) (0.018 mg g−1) compared with conventional ditches with no weirs (0.021 mg g−1), suggesting inundation enhanced by weirs could play a vital role in increasing P sorption capacity of agricultural drainage ditches during the growing season via a decrease in sediment pH, which facilitates P adsorption to sediment and also reduces P solubility. Mean P sorption maxima (Smax) (139 to 671.8 mg kg−1 in summer versus 525 to 1,288 mg kg−1 in winter) and P binding energy (K) (0.63 to 1.34 L mg−1 in summer; 0.09 to 0.30 L mg−1 in winter) were high for all drainage ditch sediments, indicating experimental ditches were capable of sorbing P and could therefore be described as a P sink. These results will assist farm managers in making informed decisions on effective P management in relation to agricultural runoff directed through drainage ditches.