Regional freshwater nitrogen budgets for Scotland

Following Carnell et al. (2019) which provided a nitrogen (N) budget for Scotland, this short project aims to provide freshwater N budgets for 10 Scottish hydrometric regions. The regional N budgets quantify key N flows across the hydrosphere and terrestrial systems (agricultural and semi-natural) and include N from human waste and atmospheric deposition. The freshwater model used here ‘LTLS-FM’ provides estimates of total N inputs to freshwater in 2010. The results indicate that total N inputs vary between regions, from 1.7 kT N for Orkney and Shetland, to 33.2 kT N for NE Scotland. However, when N inputs are divided by the area of the region, it becomes apparent that two of the smallest regions, Tweed and Forth, have the highest N inputs/km2. Across Scotland, LTLS-FM estimates of losses of freshwater N to coastal waters (151 kT N) and to the atmosphere (17.3 kT N) in 2010 were substantial, with more than half of the N losses originating from just 3 regions (Tay, Solway and NE Scotland). As much as 68% of Scotland’s N in freshwaters comes from agricultural land management, for which improved nutrient use efficiency could increase environmental as well as economic benefits. These data will contribute to the development of the Scottish Nitrogen Balance Sheet (SNBS), which is mandated under the Scottish Climate Change Act 2019.