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

Abstract

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 (Nr) 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.