Njk Howden

Evidence for nitrogen accumulation: the total nitrogen budget of the terrestrial biosphere of a lowland agricultural catchment

Several national-scale studies have shown that reactive N is accumulating in developed countries even when only the terrestrial biosphere is considered. However, none of these studies was able to consider the total N budget and so any discrepancy in budgets could be dismissed as being accounted for by N2 exchange. This study considered a large (9,948xa0km2), mixed agricultural catchment where records of N flux, land use, climate and population go back at least to 1883. The N inputs were: biological nitrogen fixation, food and feed transfers, atmospheric deposition and inorganic fertilizers. The N outputs were atmospheric emissions (NH3, N2O, NO, N2), direct waste losses and fluvial losses at the soil source. The results showed that, prior to the large-scale use of inorganic fertilizers, the total N budget of the catchment was at steady state with only a small net loss of total N. After the widespread introduction of inorganic fertilizers, the balance of the catchment shifts in favour of the net accumulation. Even accounting for losses to groundwater, the catchment was found to have accumulated 315xa0ktonnes N (315xa0tonnes/km2) at a rate of 5.5xa0tonnes N/km2/yr (55xa0kgxa0N/ha/yr) over 35xa0years since 1973. We propose that the accumulation of N could be occurring in subsoils of the catchment.

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 −1941xa0±xa0224xa0kilotonnes N/year (−8xa0tonnes N/km2/year; inter-quartile range of ±0.9xa0tonnes/km2/year). However, by 2012, this net nitrogen source had decreased to about −1446xa0±xa0195xa0kilotonnes N/year (−5.9xa0tonnes 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 (−4.2xa0tonnes N/km2/year) and by 2031 the UK would be a net sink of total N.