Modelling optimal strategies for decreasing nitrate loss with variation in weather - a farm-level approach

A farm-level framework for assessing the economic impact of measures to reduce nitrate loss by leaching is described. The framework links a database of crop treatments and nitrogen loss generated with the IACR SUNDIAL model for 10 years of weather and an economic model, Farm-adapt, for a root-cropping farm on sandy loam in the East Midlands of England. Weather induced variation in nitrate loss over time was greater than that resulting from differences in management practice. Limits on nitrate loss per hectare resulted in a relatively small annual mean cost to the farm when allowed to choose the optimal management practice (including doing nothing) in each year (e.g. £8 ha−1 for a 30 kg ha−1 limit, resulting in a 6.2 kg ha−1 and 3.2 mg l−1 reduction in mean nitrate-N loss and mean nitrate-N concentration, respectively). In no years was it feasible with the treatments tested to reduce concentration of nitrate-N to the EU limit of 11.3 mg l−1 in every week of the year. A mean annual loss of 11.3 mg l−1 was feasible in four out of 10 years at a mean cost of £10 ha−1. The most cost-effective reductions of loss (in terms of £ kg−1 nitrate-N ha−1) were achieved by targeted reductions in N application followed by a combination of reduced N and growing winter cover before spring crops. Untargeted limits (quotas) on nitrogen, nitrogen taxes and application of single management practices were less cost effective than combinations of practices. Three management strategies, based on these combinations, were imposed for all years. Mean costs were greater than where the farm could choose the optimal management practice in each year; a 4.67 mg l−1 reduction in nitrate-N concentration cost £19 ha−1 and a 5.88 mg l−1 reduction £33 ha−1.