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

Abstract

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 (9,948 km2), 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 315 ktonnes N (315 tonnes/km2) at a rate of 5.5 tonnes N/km2/yr (55 kg N/ha/yr) over 35 years since 1973. We propose that the accumulation of N could be occurring in subsoils of the catchment.