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
2
.
1
. Crop rotations and management The study began in 1995 at three ADAS exper-
imental farms. These farms provided contrasts in soil type; free-draining sandy soil at ADAS
Gleadthorpe GL; deep moisture- and NO
3 −
- re- tentive alluvial soil at ADAS Terrington TE;
silty clay loam at ADAS Rosemaund RO. Soil analyses are given in Table 1. The objective of the
project is to quantify all the major N fluxes over a complete 5-year rotation. Our study is of net
inputs and outputs, and no measurements were made of N mineralization, although estimates
were made during the field capacity FC period and between harvest and the beginning of FC. No
measurements were made of N fixation. Estimates for legume crops were taken from Sylvester-Bra-
dley 1993. Two rotations were studied; cereals winter wheat, Triticum aesti6um or winter rye,
Secale cereale, sugarbeet Beta 6ulgaris L. and potatoes Solanum tuberosum at GL and TE
with linseed, Linum usitatissimum in place of a second cereal at TE; cereals winter wheat and
winter oats, A6ena sati6a, oilseed rape Brassica napus, OSR and winter beans Vicia faba at RO.
At each site two fields were selected for monitor- ing, and soil and crop measurements were made
from within an area of c. 2 ha within each field. Measurements at RO ceased in 1997 after 2 years.
No catch crops were grown during the phases of the rotations reported here. Fertilizer-N appli-
cations were typical of those applied in the UK for commercial crops Anon., 1994 and are given
in Table 2. Pests, diseases and weeds were con- trolled according to prevailing commercial prac-
tice, and no problems were observed.
2
.
2
. Soil N In order to compare topsoil N concentrations
at the beginning and end of the study, we took 200 soil samples from the area to be sampled in
each experimental field, in autumn 1995. The sample size needed was based on estimates of
variability in soil N obtained from previous stud- ies of the total N concentration in the cultivated
horizon of UK arable soils, and assumed equal variability between the mean values at the begin-
ning and end of the project. Soil samples were taken to the depth of cultivation 23 cm from
each field. Each sample was analyzed separately for total N Anon., 1986. Samples were also
taken at RO in autumn 1997, when the study finished at that site.
2
.
3
. Soil mineral nitrogen Soil samples to determine soil mineral N
SMN were taken in 30 cm increments to 90 cm at the beginning and end of the FC periods, and
at harvest. The FC period was identified using the IRRIGUIDE model of soil moisture status Bai-
ley Spackman, 1996. Semi-cylindrical augers were used, with decreasing diameters for the lower
depths to minimize contamination, taking at least nine cores for each sample. Three replicate sam-
ples were taken from each field. NH
4 +
- and NO
3 −
- N were extracted by shaking 40 g moist soil with
200 ml 2 M KCl for 2 h before filtering and analysis using standard methods Anon., 1986.
Soil organic carbon SOC was determined from the 0 – 15 cm horizon by loss on ignition, SOC was
multiplied by 1.7 to give soil organic matter.
Table 1 Soil analyses of fields where measurements were made of N fluxes in arable rotations Autumn, 1995
Gleadthorpe Terrington
Rosemaund Farm
Propagation Welbeck
Top Kingston Field
Flat Field Prestons
Shepherds Gate 7.1
7.4 pH
8.3 7.8
6.8 7.1
43 43
P mgl 63
43 19
44 K mgl
81 74
283 215
182 326
Mg mgl 103
115 284
254 97
148 ND
ND
a
2.14 3.19
SOM 2.11
3.25 0.076
0.173 0.111
Total N 0.134
0.071 0.171
0.0013 0.0013
0.0024 SE
0.0018 0.0023
0.0023 Soil depth cm
0–23 0–23
0–30 0–30
26 26
77 85
Sand Silt
11 17
45 45
6 29
Clay 29
4 30–60
30–60 Soil depth
83 71
Sand 12
23 Silt
6 5
Clay 60–90
60–90 Soil depth
Sand 85
84 Silt
10 11
4 6
Clay
a
ND not determined.
J .
Webb et
al .
Europ .
J .
Agronomy
13 2000
207 –
223
Table 2 N inputs to fields, crop yields and N offtakes where N fluxes were measured in arable rotations
a
Rosemaund Terrington
Farm Gleadthorpe
Prestons Shepherds Gate
Flat Field Propagation
Fields Welbeck
Top Kingston Potatoes
W. beans W. wheat
W. beans W. wheat
Crop 19951996 W. wheat
3 3
8.5 3
3 3
Seed N kgha 180
190 180
285
c
205 190
Fertilizer N kgha 0.12
38.2 4.0
3.9 7.2
9.0 4.2
0.28 Yield tha
4.7 0.22
146 93
2.7 144
Rb
14.5 139
Rb
139 2.7
87 4.1
N offtake in product removed from field kgha Total N offtake kgha
5.0 134
Rb
ND 171
189
Rb
3.6 126
Rb
9.6 184
Rb
W. wheat W. wheat
W.oats Sugar beet
Crop 19961997 W.rye
W. rye 3
3 3
Seed N kgha 3
3 0.3
160 196
102 115
130 Fertilizer N kgha
130 58.9
5.9 2.50
8.9 0.32
9.4 0.10
9.0 0.32
0.56 5.1
0.06 Yield tha
7.2 151
5.5 174
4.0 131
7.1 N offtake in product removed from field kgha
77
Rb
8.0 70
Rb
0.7 102
216
Rb
6.9 227
Rb
5.2 174
Rb
ND 9.4
5.6 137
Total N offtake kgha 135
7.6 Linseed
Sugarbeet Sugarbeet
Potatoes Crop 19971998
Seed N kgha 0.3
0.3 8.5
3 40
255 Fertilizer N kgha
130 1.8
63.6 0.01
53.1 1.83
1.19 55.5
0.21 Yield tha
0.2 N offtake in product removed from field kgha
70
Rb
87
Rb
2.0 4.7
165
Rb
7.4 45
Rb
2.4 ND
60 Total N offtake kgha
249 22.4
ND
a
SE in brackets; ND, not measured.
b
R, removed from field
c
Estimated N fixation according to Sylvester-Bradley 1993.
2
.
4
. Nitrate leaching Before the first test crops were sown 20 porous
ceramic pots were installed ten at RO to 90 cm in each field following the method described by
Lord and Shepherd 1993. After the onset of the FC period, water samples were taken from the
porous pots every 2 weeks or following 50 mm rainfall, whichever came first. The onset of the FC
period was estimated using IRRIGUIDE. Due to variations in crop growth, and hence water ex-
traction, there will be some error in this estimate. Porous cups may be used as a check since water
cannot be extracted from them until the soil has reached at least 70 – 80 kPa, close to the water
content at FC Lord and Shepherd, 1993. The actual date of the onset of FC was taken as the
time when 50 of the porous pots were yielding leachate. On-site meteorological data were used in
the calculations of overwinter drainage. Leachate may bypass porous pots in structured soils with
macropone flow. However, Webster et al. 1993 showed that the method is accurate for structure-
less sandy soils such as GL. While macropone flow can occur at TE and RO, the fields used here
were not underdrained and so water draining via macropores will have rejoined the soil matrix
above the depth to which the pots were installed.
Samples were frozen for transport and thawed at room temperature overnight before analysis for
NH
4 +
- and NO
3 −
-N Anon., 1986. The total quantity of N leached below 90 cm was calculated
from NH
4 +
- and NO
3 −
-N concentrations in the drainage water and estimates of drainage volumes
from IRRIGUIDE. Nitrate-N losses between sampling dates were estimated using the trape-
zoidal rule, i.e. linear interpolation between sam- pling points Lord and Shepherd, 1993. Summing
these for all sampling occasions gave total min- eral-N leached each winter. Total mineral-N
leached overwinter N
leach
, together with mea- surements of SMN at the beginning SMN
a
and end SMN
s
of the FC period, enabled net over- winter mineralization OvMin to be calculated
as: OvMin = SMN
s
+ N
leach
− SMN
a
2
.
5
. Nitrous oxide and dinitrogen emissions Nitrous oxide emissions follow a seasonal pat-
tern in the UK, being greatest in the spring and autumn when conditions are wettest. They also
exhibit a marked response to fertilizer-N applica- tion Goulding et al., 1993; Kaiser and Heine-
meyer, 1996, which tend to be applied in the spring. Measurements were made in each experi-
mental year 199596, 199697, 199798 between October to December and March to June, when
N
2
O fluxes were expected to be greatest. Fluxes were measured weekly during these periods at the
two fields at each experimental site. The sampling frequency was increased to one measurement per
day for 5 days following all fertilizer-N applica- tions. Nitrous oxide fluxes were measured in real
time using 12 closed chambers radius 7.5 cm, height 15 cm and photo-acoustic infra-red spec-
troscopy PAIRS Velthof et al., 1996. The flux from each field was calculated as the mean of the
12 individual chambers. Nitrous oxide emissions under field conditions respond very rapidly to a
stimulating event, e.g. rainfall, this response can be positive, leading to a large peak in emissions,
but unless the stimulus is maintained, then the peak should rapidly return to an appropriate
background level. The daily N
2
O measurements were extrapolated to a yearly emission using a
system that avoids carrying over large peak emis- sions and therefore overestimating the flux. All
data were plotted against time and the extrapola- tion carried forward at the level of the previously
measured emission if the subsequent emission had increased, but extrapolating backwards at the
level of the newly measured emission if this had decreased relative to the previous emission. Where
no measurements had been made following an event, such as fertilizer application or cultivation,
then previous data collected in the project was used. The cumulative area of the resultant joined
peaks was then calculated and determined the annual emission in kg Nha.
To estimate N
2
emissions from denitrification activity at times of large emission, the acetylene
inhibition technique AIT was used Ryden et al., 1978. An experimental area of 48 m
2
was estab- lished at the GL and TE sites. Ammonium nitrate
fertilizer was applied at 40 and 120 kg Nha to simulate the split-fertilizer applications typical in
the UK. The resultant N
2
emissions were moni- tored over the subsequent 3 days. The N
2
concen- trations of gas samples taken from closed
chambers over a 1 h closure period were deter- mined using continuous flow isotope-ratio mass
spectrometry CF-IRMS, Stevens et al., 1993.
2
.
6
. Ammonia fluxes Measurements of NH
3
exchange were carried out using the aerodynamic gradient method Den-
mead, 1983 as modified by Schjørring 1995 with passive flux samplers Leuning et al. 1985. Wind-
speed and NH
3
concentration profiles were made linear with respect to the logarithm of height
using the relationship between zero plane dis- placement and crop height Stanhill, 1969.
For the NH
3
flux measurements, only one field at each site was monitored. These were Top
Kingston GL, Shepherds Gate TE and Flat field RO. Anemometers and duplicate passive
flux samplers were at three TE or four GL, RO heights on a mast located in the centre of the field.
These heights were varied during the growing season to ensure that the bottom anemometer and
samplers were above the height of the growing crop. The top anemometer and samplers were
always \ 2.4 m above the ground surface. Passive flux samplers were exposed on the mast for peri-
ods of 1 month during the main growing periods April – July of 1996 and 1997.
Unfortunately, dataloggers used to capture windspeed data in 1996 proved to be unreliable.
Thus, windspeed data was only available for short periods during NH
3
sampling. Consequently, av- erage windspeeds were estimated by extrapolating
from the relationship between the windspeed at a nearby meteorological station at 2 m height and
the windspeed profile obtained at the site for a period when the datalogger was working. Essen-
tially this consisted of the following steps:
– For each 24 h period for which site data was available, the windspeed profile was fitted to the
equation lnh − d = m · u + c
1 where h is the above-ground height, d is the zero
plane displacement, u is the windspeed, and m and c are constants von Karman’s constant divided
by the eddy velocity, and the logarithm of the roughness length, respectively.
– For each 24 h period for which site data were available, the windspeed was calculated at 2
m height. – Calculated
site windspeed
was plotted
against the local meteorological station wind- speed, for 2 m height.
– Using the relationship between site and local windspeed at 2 m height, the site windspeed was
calculated at 2 m for the whole NH
3
sampling period using local windspeed data.
– The windspeed profile was reconstructed for the site from the 2 m calculated windspeed Eq.
1, above using the median value of c obtained earlier.
In 1998 Willems badge samplers Willems, 1993 were used in place of passive flux samplers
to estimate NH
3
concentrations. In this season measurements were made during crop growth and
senescence from late summer onward.
2
.
7
. Wet N deposition Wet N deposition was measured at one field per
site, by the method developed for this purpose in the UK Environmental Change Network Hall,
1986. Rainfall was collected through a filter fun- nel into 3-l polythene bottles. These were held
inside double-skinned metal cylinders designed to insulate the bottles and minimize evaporation.
Samples were analyzed for NH
4 +
- and NO
3 −
-N using standard methods Hall, 1986.
2
.
8
. Crop measurements Cereals, linseed and winter beans were har-
vested by a combine cutting three 2.3 m swathes through the crop for 10 m. The grain was weighed
on the combine. Grain samples were taken for dry matter DM and N concentrations, the latter
determined by near infra-red reflectance Anon., 1986. Above-ground crop samples, comprising
five ‘grab’ samples of c. 100 fertile shoots, cut at ground level, were taken from each area just prior
to harvest to determine DM and N harvest in- dices. Plant uptake of N was calculated by divid-
ing the combine-derived values for grain N by the N harvest index. The straw of cereal crops was
removed from the fields at all sites, except when winter rye was grown at GL. Straw, leaf and
haulm residues of beans, sugarbeet and potatoes were incorporated in the soil. Yields of sugarbeet
and potatoes were determined from three or four subsamples of 10 m length. No haulm samples
were taken from potatoes to estimate crop residue N, N in leaves was estimated for sugarbeet at GL,
but not at TE.
3. Results
