『Abstract
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 limits 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.
Keywords: Nitrogen; Export coefficients; Rivers; In-stream losses;
Flux』
1. Introduction
2. Approach and methodology
2.1. Modelling approach
2.2. Model parametarisations
3. Results
3.1. Land-use change in the UK
3.2. Export model
4. Discussion
5. Conclusions
References