Calculating nitrogen leaching losses and critical nitrogen application rates in dairy pasture systems using a semi-empirical model

A simple, semi-empirical model for estimating nitrogen (N) leaching losses and critical N application rates in dairy pasture systems is described. The model uses the annual rates of major N flux processes in the soil-plant system to determine the potentially leachable N pool(mineral N and mineralisable N), and estimates the N leaching loss based on measured relationships between the N leaching loss and the potentially leachable N in the soil. The N flux processes considered in the model include fertiliser or effluent N applications, biological N fixation, soil N mineralisation and immobilisation, plant N uptake, animal N return at the urine patches, ammonium volatilisation, and denitrification. The impact of drainage on N leaching is taken into account by normalising the N leaching loss to a per 100 mm drainage basis. A quadratic equation is used to describe the relationship between the N leaching loss and potentially leachable N. Tests of the model predictions against other experimental data showed reasonable agreements between the estimated N leaching losses with those measured. The modelled critical N application rates which would cause the annual average N concentration in the drainage water to reach the drinking water standard (11.3 mg N l(-1)) are: 390-392 kg N ha(-1) for cut and carry and 162-192 kg N ha(-1) for grazed pastures if urea is used; and 588-600 kg N ha(-1) for cut and carry, and 248-301 kg N ha(-1) for grazed pastures if dairy shed effluent is used.