The UK’s total nitrogen budget from 1990 to 2020: a transition from source to sink?

This study estimates the annual total nitrogen balance of the UK from 1990 to 2020. The following inputs of nitrogen are considered: inorganic fertilizer, atmospheric deposition; food and feed imports; and biological nitrogen fixation. The outputs considered compose: atmospheric emissions; direct losses of sewage and industrial effluent to the sea; fluvial losses at source; food and feed exports; and terrestrial denitrification. It is shown that: (1) Inputs of inorganic fertilizer declined significantly over the study period with both atmospheric deposition and food and feed imports significantly increasing. (2) Outputs of total N also significantly declined with all output pathways decreasing except for atmospheric emissions and terrestrial denitrification to N₂. (3) The UK was a net source of total nitrogen in 1990 of approximately −1941 ± 224 kilotonnes N/year (−8 tonnes N/km²/year; inter-quartile range of ±0.9 tonnes/km²/year). However, by 2012, this net nitrogen source had decreased to about −1446 ± 195 kilotonnes N/year (−5.9 tonnes N/km²/year). The future total N balance of the UK is being driven by declines in outputs rather than changes in inputs. The largest declines are in the atmospheric emissions of reactive nitrogen (N_r) and the fluvial flux of N such that by 2020 to the total N budget is predicted to be −1042 (±246) kilotonnes N/year (−4.2 tonnes N/km²/year) and by 2031 the UK would be a net sink of total N.