European Journal of Agronomy 13 2000 207 – 223
Nitrogen fluxes in three arable soils in the UK
J. Webb
a,
, R. Harrison
b
, S. Ellis
b
a
ADAS Consulting Ltd, Woodthorne, Wergs Road, Wol6erhampton WV
6 8
TQ, UK
b
ADAS Consulting Ltd, ADAS Boxworth, Battlegate Lane, Boxworth, Cambridge CB
3 8
NN, UK Received 19 February 1999; received in revised form 6 August 1999; accepted 13 April 2000
Abstract
Measurements were made of nitrate leaching, ammonia NH
3
, nitrous oxide N
2
O and dinitrogen N
2
emissions, and crop offtake of N, together with wet N deposition in order to estimate annual fluxes of N inputs and N outputs
at three sites, Gleadthorpe GL, Terrington TE and Rosemaund RO, in the UK over the three seasons 199596, 199697 and 199798. The soils were loamy sand, alluvial silt and silty clay loam, respectively. The objective of the
project is to quantify all the major N fluxes over two arable rotations on contrasting soil types. Soil N at GL at 0.07 – 0.08 was about half that measured at TE and RO 0.11 – 0.17. These differences were consistent with those
usually found between soils of different clay content in an arable rotation. Over the first two winters excess winter rainfall EWR at all sites, especially TE, was less than average. In consequence amounts of nitrate-N leached were
better related to EWR, than to the previous crop. Estimates of mineralization overwinter 199596 at 50 – 60 kgha, did not differ consistently between sites or previous crops. Over the following winter data suggest net immobilization of
soil mineral N SMN. Using only measured fluxes, annual N
2
O losses of 0.5 – 2.7 of fertilizer-N applied were estimated. Wet deposition of 20 kg Nha at RO was greater than wet deposition at GL and TE of 7 kg Nha. These
differences were greater than expected from current national estimates of deposition. Wet deposition of N has contributed 5 of total N inputs and NH
3
fluxes have usually been negligible. Outputs of N were dominated by crop offtake and nitrate leaching. Balances for winter wheat ranged from − 85 to + 57 kgha, largely in consequence of
variation in N offtake, due to differences in yield and fate of straw, and also in nitrate leaching. Gaseous losses were usually small, and in total appear to be no greater than N inputs from wet deposition. Thus in arable systems where
no organic manures are applied, priority in reducing losses of N to the environment needs to be given to nitrate leaching. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Nitrogen cycle; Nitrogen balance; Emissions; Pollution www.elsevier.comlocateeja
1. Introduction
Environmental pollution may be caused when nitrogen is lost from the soilcrop system. Nitrate
NO
3 −
pollutes ground and surface waters Foster et al. 1982, ammonia NH
3
when deposited to land increases soil acidification and N eutrophica
tion Roelofs and Houdijk, 1991 and nitrous oxide N
2
O contributes to global warming Bouwman, 1990 and breakdown of stratospheric
ozone Crutzen, 1981. These losses of N are
Corresponding author: Tel.: + 44-1902-693235; fax: + 44- 1902-693166.
E-mail address
:
jim.webbadas.co.uk J. Webb. 1161-030100 - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S 1 1 6 1 - 0 3 0 1 0 0 0 0 0 7 5 - 7
linked by the nitrogen cycle Johnston and Jenkinson, 1989, such that measures to reduce
one N pollutant may lead to increased emissions of another. Thus there is a need for integrated
studies of pollutants at the systems level. Such work will assist the development of strategies to
minimize the environmental impact of N losses from agriculture.
The EC Nitrate Directive EEC, 1991 requires member states to implement measures that will
reduce NO
3 −
losses from soils. The United Na- tions Economic Commission for Europe is revis-
ing the
Convention on
Long-Range Transboundary Air Pollution to include recom-
mendations to reduce NH
3
emissions. Under the UN Framework on Climate Change, signatories
are committed to returning emissions of green- house gases to 1990 levels by 2000. Manipulation
of farm practices to minimize N pollution de- pends on developing an understanding of the
system. Despite a considerable number of detailed studies on the fate of fertilizer-N Bhogal et al.,
1997 and references therein, many have concen- trated on a single loss pathway, and our under-
standing of the full cycle remains imperfect. The arable system is perhaps the simplest within agri-
culture, since inputs to and exports from fields are relatively well defined, and distribution of N in-
puts is spatially more uniform than on grazed grassland. Nevertheless, attempts to account for
all N within systems have usually resulted in considerable imbalances Jarvis, 1993. The risk is
that such imbalances will be attributed to the pathway that was not measured. Recent advances
in the measurement of losses, and in more precise estimation of soil N concentration, open the pos-
sibility of measuring all main loss pathways, and hence improving our understanding of the rela-
tionship between them, and their interaction with agricultural practice. Although not covered in this
paper, this would allow an evaluation of the models which currently calculate C and N bal-
ances on a field by field basis e.g. Bradbury et al., 1993. The objective of the work reported here is
to measure net N outputs via leaching, NH
3
volatilization, N
2
O and dinitrogen N
2
emissions, and crop offtake, together with wet deposition of
N over two arable rotations on contrasting soil types. These fluxes will be measured for 5 years,
and the N balance compared with detailed mea- surements of total soil N taken at the beginning
and end of the study. This will enable us to determine which are the greatest N outputs in
arable cropping systems, and comparison of the N balance with measured changes in soil N will
indicate how well current measurements account for N fluxes within arable systems. This paper
presents the results of the first 3 year’s measure- ments.
2. Materials and methods
