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Challenges and solutions in identifying agricultural pollution hotspots using gross nitrogen balances

Serra, João, Cordovil, Cláudia M.d.S., Cruz, Soraia, Cameira, Maria Rosário, Hutchings, Nicholas J.
Agriculture, ecosystems & environment 2019 v.283 pp. 106568
cost effectiveness, crop production, issues and policy, livestock, mineral fertilizers, models, nitrogen, pollution, roughage, soil, Denmark, Portugal
Gross nitrogen balances (GNB) at the national level are a broad indicator of reactive nitrogen (N) pollution but the identification of pollution hotspots is necessary for designing cost-effective abatement policies. This requires a spatial disaggregation of GNBs to finer resolutions, but key inputs are often only available at high spatial scales. Modelling offers a method to provide these inputs but introduces additional error. Here we develop methods to estimate the main inputs (manure, synthetic fertiliser) and outputs (roughage feed and crop products) for mainland Portugal for the NUTS2, NUTS3 and municipality levels for the years 1989, 1999 and 2009. Our results show that despite the declining of the mainland GNBs over this period (47 to 38 kg N ha-1),the range of GNBs at progressively finer resolutions increased from 26 to 95 at the NUTS3 to −50–963 kg N ha−1 at the municipality levels. The increased concentration of livestock in some areas appears to be leading to an inefficient use of manure for crop production whereas there appears to be a depletion of soil N stocks in other areas. A comparison of our results with those from Denmark leads us to conclude that themunicipality level is the most suitable to identify hotspots, even thougherrors can arise when there is a poor correspondence between agrienvironmental conditions and the socioeconomic administrative boundaries at which statistical data are often available.