Using field and farm nitrogen budgets to assess the effectiveness of actions mitigating N loss to water

Many water bodies are at risk from diffuse nitrogen (N) pollution from farm sources. Catchment buffering and long transition times mean responses to mitigation practices implemented on farms may not be detected by short-term measurement. We investigated the feasibility of using field- and farm-scale N budgets to evaluate mitigation effectiveness in the short-term, and the implications of application scale on such evaluations. Farm data were collected from 34 farms for four cropping seasons (2004–2008) in two catchments in SW England. All farms adopted one or more mitigation practices to try to decrease N leaching in the final year of data collection. Nitrogen budgets were compared before and after their implementation to explore the sensitivity of calculations to mitigation at the two scales. Post mitigation, field-scale surplus (on a crop average basis) decreased significantly (p < 0.05), and exposed opportunities to reduce N surpluses through improved manure management. At the farm scale, N surplus improved on 27 farms (79%) with surplus reductions averaging 23 kg N ha−1 (29%). However, sensitivity to a range of other factors including cropping, stocking density, weather (yield) and fertiliser price, combined with a likely small effect of the mitigation practices, restricted the extent and certainty with which reductions in N surplus could be attributed to mitigation. A longer run of data post-mitigation is required to add certainty. However, the analysis suggests that N budgets are of value for identifying mitigation strategies.