Uncovering nitrogen accumulation in a large mixed land-use catchment: Implications for national-scale budget studies and environmental management

• Developed a comprehensive total nitrogen budget for an 8231 km 2 catchment area. • The catchment was a total N sink, with a total 35 ktonnes N/yr, or 42.5 kg N/ha/yr. • 69% of the catchment acts as a net nitrogen sink, while 31% contributes as a nitrogen source. • Used the total N budget to test for nitrogen accumulation in the subsoils. • Analysis revealed N accumulation in grassland subsoils rather than arable land. Accurately quantifying the location and extent of nitrogen accumulation is crucial for mitigating its severe impacts on climate and the environment. Here we estimated a spatial total N budget and its input/output fluxes from different land uses on a 1 km 2 grid scale across the whole of a large, mixed land use catchment (Trent, UK). With a long history of water quality monitoring, the Trent catchment provides a unique and ideal test bed for developing a detailed nitrogen budget and determining where N accumulation occurs. In 2015, a significant 35 (±5) ktonnes N accumulation was found, with 31 % of the area acting as a net source and 69 % as a net sink. The spatial budget ranged from −16 (±5) to 45 (±7) tonnes N/km 2 /year. Using this budget, we identified N accumulation and loss areas under diverse land uses and conducted strategic soil sampling and C/N analysis. Notably, grassland subsoil exhibited nitrogen buildup compared to arable land, spotlighting intricate land use, nitrogen, and soil dynamics. The study emphasizes the need for targeted nutrient management to prevent potential environmental repercussions linked to subsoil nitrogen accumulation, especially in grassland contexts.