The fluvial flux of nitrate from the UK terrestrial biosphere - An estimate of national-scale in-stream nitrate loss using an export coefficient model

This paper compares the fluvial flux of nitrate at the UK's tidal limit with an estimate of losses from the terrestrial biosphere at the point of entry to the river system to estimate in-stream losses across the UK's entire river network between 1925 and 2007. These estimates were used to support interpretation of the current trend in the fluvial flux of nitrogen from the UK, and to give an assessment of the fluvial component of the total nitrogen budget of UK. The release of nitrate to the point at which it enters the stream network is estimated, using a Monte-Carlo framework, from a description of annual soil N mineralisation and immobilisation combined with an export coefficient model. Long-term records of land use (including - agricultural, forestry and urban uses); livestock; human population and atmospheric deposition are used. The study shows that: (i) The flux of nitrate from UK soils in the period since 1925 varied between 420 and 1463 ktonnes N yr(-1), with two peaks, one in 1944 and one in 1967. The first of these peaks was caused by mineralisation of soil organic matter following radical land use change in the 1940s, and the second was a multifactorial response to land use change and agricultural intensification in the 1960s. (ii) The current trend in N release from soils appears to be downward but the current fluvial flux at the tidal limit is slowly increasing. It is possible that in the near future nitrate-N losses at the tidal limit will, in some years, exceed N losses from soils implying a net N gain into the fluvial network. This apparent gain can be explained by the breakthrough of high nitrate groundwater into surface waters. (iii) The predicted export from UK soils at source as water enters the stream network implies that the UK will soon become a net source of total reactive N species, as the capacity for in-stream removal becomes exhausted by the ingress of N-rich groundwater to the river system. (C) 2011 Elsevier B.V. All rights reserved.