Evaluation of two management options to improve the water quality of Lake Brunner, New Zealand

Lake Brunner, located on the west coast of the South Island of New Zealand, has been identified as threatened by phosphorus (P) inputs from neighbouring dairy farms. A two-year study was undertaken to determine if the application of reactive phosphate rock (RPR) with or without the addition of aluminium (Al) sulphate (alum) at 20 kg Al ha-1 could decrease P losses in surface runoff from dairy pastures compared to the same rate of P applied as superphosphate (30 kg P ha-1). The study was conducted on nine humped and hollowed plots (1100-3500 m2). Frequent and reliable rainfall year round (totalling 4000-5000 mm yr-1) translated into more than 50 surface runoff events per year and large losses of P from superphosphate-treated plots (flow-weighted mean filterable reactive P (FRP) = 0.072 mg l-1; mean load 8.2 kg P ha-1 yr-1). In contrast, the flow-weighted mean FRP concentration in the RPR-treated plots was significantly less (0.034 mg l-1; mean load 4.0 kg P ha-1 yr-1). No effect was found on other P forms. Alum did not decrease P losses, most likely due to Al being washed off in surface runoff before it could bind to the soil. Enhanced P losses from superphosphate-treated plots were attributed to the greater availability of P within 21 days of application (water solubility 80-95%) compared with RPR (approximate to 1% water soluble), and the occurrence of surface runoff during this period (especially in year 1). The use of RPR is recommended as one management practice to decrease FRP inputs to Lake Brunner and may be useful in other regions where rainfall and soil pH allow.