The Fluvial Flux of Nitrogen from Great Britain 1974 - 2005 in the context of the terrestrial nitrogen budget of Great Britain.

This study compiles long term records of the concentration of nitrogen species from British rivers to assess temporal change in the total fluvial nitrogen flux compared to the other inputs to, and outputs from, the UK terrestrial biosphere. The following nitrogen species are considered: ammonia, nitrate, nitrite, dissolved organic nitrogen and particulate organic nitrogen. Concentration and flow records were reconstructed from 1974 to 2005 for ammonia, nitrate, nitrate and dissolved organic nitrogen (DON) and from 1992 for particulate organic nitrogen (PON). The reconstructed fluvial nitrogen time series was compared to records for inorganic fertiliser, atmospheric emissions, industrial and sewage effluent, and imports. .The results of the study show that the total dissolved nitrogen flux over the study period, after flow correction, varied from 470 to 980 ktonnes N, which, on average, comprised: 69% nitrate-N; 26% dissolved organic-N; 4% ammonia-N; and 1% nitrite-N. Including PON varied from 504--1004 ktonnes N/yr which made the UK the largest exporter of nitrogen to the oceans yet reported. The flux of ammonia-N shows a significant decline over the study period, but significant increases in both nitrate-N and dissolved organic-N mean that the total dissolved nitrogen flux still shows a significant increase at a rate of 6.3 ktonnes N/yr2. The dissolved nitrogen flux record shows both a steady increase over the period 1974 to 2005 and sharp discrete rises in response to severe droughts. The rise in response to severe droughts is not consistent with a storage effect of reduced flows but appears to represent new production in the year of the drought. The long-term rise of fluvial nitrogen flux from British rivers is in contrast to declines in inputs and other N outputs, so that, although Great Britain is a net sink of total nitrogen, the size of this sink is diminishing.