Nitrogen fluxes in three arable soils in the UK
J. Webb
a,*, R. Harrison
b, S. Ellis
baADAS Consulting Ltd,Woodthorne,Wergs Road,Wol6erhampton WV6 8TQ, UK bADAS Consulting Ltd,ADAS Boxworth,Battlegate Lane,Boxworth,Cambridge CB3 8NN, UK
Received 19 February 1999; received in revised form 6 August 1999; accepted 13 April 2000
Abstract
Measurements were made of nitrate leaching, ammonia (NH3), nitrous oxide (N2O) and dinitrogen (N2) 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 1995/96, 1996/97 and 1997/98. 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 1995/96 at 50 – 60 kg/ha, 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 N2O losses of 0.5 – 2.7% of fertilizer-N applied were
estimated. Wet deposition of 20 kg N/ha at RO was greater than wet deposition at GL and TE of 7 kg N/ha. These differences were greater than expected from current national estimates of deposition. Wet deposition of N has contributed 5% of total N inputs and NH3fluxes have usually been negligible. Outputs of N were dominated by crop
offtake and nitrate leaching. Balances for winter wheat ranged from −85 to +57 kg/ha, 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.com/locate/eja
1. Introduction
Environmental pollution may be caused when nitrogen is lost from the soil/crop system. Nitrate
(NO3
−
) pollutes ground and surface waters (Foster et al. 1982), ammonia (NH3) when deposited to land increases soil acidification and N eutrophica tion (Roelofs and Houdijk, 1991) and nitrous oxide (N2O) 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:[email protected] (J. Webb).
J.Webb et al./Europ.J.Agronomy13 (2000) 207 – 223 208
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 NO3− 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 NH3 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, NH3 volatilization, N2O and dinitrogen (N2) 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 NO3
−-
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.
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. NH4
+- 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 Shepherds Gate Flat Field Prestons
7.1 7.4
pH 8.3 7.8 6.8 7.1
43 43
P (mg/l) 43 63 19 44
K (mg/l) 81 74 283 215 182 326
Mg (mg/l) 103 115 284 254 97 148
ND NDa
2.14 3.19
SOM (%) 3.25 2.11
0.076 0.173 0.111
Total N 0.071 0.134 0.171
(0.0013) (0.0013) (0.0024)
(SE) (0.0018) (0.0023) (0.0023)
Soil depth (cm) 0–30 0–30 0–23 0–23
26 26
77 85
% Sand
% Silt 11 17 45 45
6 29
% Clay 4 29
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 11 10
4 6
% Clay
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Table 2
N inputs to fields, crop yields and N offtakes where N fluxes were measured in arable rotationsa
Rosemaund Terrington
Farm Gleadthorpe
Prestons Shepherds Gate Flat Field
Propagation
Fields Top Kingston Welbeck
Potatoes W. beans W. wheat
W. beans W. wheat
Crop 1995/1996 W. wheat
3 3 3 8.5 3 3
Seed N (kg/ha)
180
190 190 180 285c 205
Fertilizer N (kg/ha)
(0.12) 38.2 (4.0) 3.9 7.2
9.0 4.2 (0.28)
Yield (t/ha) 4.7 (0.22)
146
93 (2.7) 87 (4.1) (2.7) 144Rb (14.5) 139Rb 139
N offtake in product removed from field (kg/ha)
Total N offtake (kg/ha) 134Rb (3.6) 126Rb (9.6) 184Rb (5.0) ND 171 189Rb
W. wheat W. wheat W.oats
Sugar beet
Crop 1996/1997 W. rye W.rye
3 3 3
Seed N (kg/ha) 3 3 0.3
160 196 102
115 130
Fertilizer N (kg/ha) 130
58.9
5.9 (0.56) 5.1 (0.06) (2.50) 8.9 (0.32) 9.4 (0.10) 9.0 (0.32)
Yield (t/ha)
(7.2) 151 (5.5) 174 (4.0) 131 (7.1)
N offtake in product removed from field (kg/ha) 77Rb (8.0) 70Rb (0.7) 102
216Rb (6.9) 227Rb (5.2) 174Rb
ND (9.4)
(5.6) 137
Total N offtake (kg/ha) 135 (7.6)
Linseed
Sugarbeet Potatoes Sugarbeet
Crop 1997/1998
Seed N (kg/ha) 0.3 8.5 3 0.3
40 0
255
Fertilizer N (kg/ha) 130
1.8
63.6 (1.19) 55.5 (0.21) (0.01) 53.1 (1.83)
Yield (t/ha)
(0.2)
N offtake in product removed from field (kg/ha) 87Rb (4.7) 165Rb (7.4) 45Rb 70Rb (2.0) (2.4) ND
60
Total N offtake (kg/ha) 249 (22.4) ND
aSE in brackets; ND, not measured. bR, removed from field
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 NH4
+- and NO
3
−-N (Anon., 1986). The total
quantity of N leached below 90 cm was calculated from NH4
+- 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 minmin-eral-N leached overwinter (Nleach), together with mea-surements of SMN at the beginning (SMNa) and end (SMNs) of the FC period, enabled net over-winter mineralization (OvMin) to be calculated as:
OvMin=SMNs+Nleach−SMNa
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 (1995/96, 1996/97, 1997/98) between October to December and March to June, when N2O 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 N2O 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 N/ha.
J.Webb et al./Europ.J.Agronomy13 (2000) 207 – 223 212
fertilizer was applied at 40 and 120 kg N/ha to simulate the split-fertilizer applications typical in the UK. The resultant N2 emissions were moni-tored over the subsequent 3 days. The N2 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 NH3 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 NH3 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 NH3flux 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 periperi-ods (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 NH3 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
ln(h−d)=m · u+c (1)
where h is the above-ground height, d is the zero plane displacement,uis the windspeed, andmand
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 NH3 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 cobtained earlier.
In 1998 Willems badge samplers (Willems, 1993) were used in place of passive flux samplers to estimate NH3 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 NH4
+- and NO
3
−-N
using standard methods (Hall, 1986).
2.8. Crop measurements
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
3.1. Crop yields and nitrogen offtake
Crop yields were usually satisfactory (Table 2). The exceptions were after the hot and dry summer of 1996 when winter wheat yields at GL (4.5 t/ha) and potato yields at TE (38 t/ha) were less than expected for those sites at Nopt (6 – 7 and 60 – 70 t/ha, respectively).
Nitrogen offtake was usually least (B100 kg N/ha) for those crops (e.g. winter rye and sugar-beet) where residues remained in the field. Great-est removals of N were by large winter wheat crops when the straw was removed (\200 kg N/ha, TE and RO).
The greatest removals of N from the systems were crop N offtakes. These ranged from 45 kg/ha for linseed at TE in 1997/98 to 227 kg/ha for winter wheat (following winter beans) at RO (Table 2). Such differences partly reflect differ-ences in straw management. In all cases, except winter rye at GL, cereal straw was removed. There was also great variation in grain N offtake, from 70 kg/ha at GL to 146 kg/ha at TE, reflect-ing much greater grain yields on this more mois-ture-retentive soil. Offtakes from sugarbeet were also relatively small, and less than those by pota-toes, a consequence of the smaller N concentra-tions in sugarbeet roots.
3.2. Soil nitrogen
Soil N at GL in autumn 1995 at 0.07 and 0.08%, was about half that measured at TE and RO (Table 1). Such differences are partly in con-sequence of soil type, soil C and N tending to be greater on soils of greater clay content (Stevenson, 1982). The soil N concentrations measured in 1995 were equivalent to between 2400945 (GL) and 5000970 (RO) kg N/ha, based on a sample depth of 23 cm and soil dry bulk densities of 1.5 g/cm3
for GL and 1.3 g/cm3
for TE and RO.
3.3. Soil mineral nitrogen
Measurements of SMNa (Table 3) were typical for UK conditions following potatoes, OSR and winter beans at 110 – 130 kg/ha (Harrison, 1995), and sugarbeet at 30 – 40 kg/ha (Sylvester-Bradley and Shepherd, 1997), except following sugarbeet at TE overwinter 1997/98. However, SMNa fol-lowing winter wheat (110 – 130 kg/ha) at GL in 1996 was much more than expected for this soil type (Webb et al., 1997). This is likely to be a consequence of large amounts of fertilizer-N re-maining unrecovered due to small wheat yields in the hot, dry summer of 1996. By the following spring SMN was reduced to 40 – 50 kg/ha at GL (Table 3) despite smaller than average leaching losses. For other crops SMNs were similar to those reported by Harrison (1995), albeit SMNsat TE and RO following winter wheat were no greater than average despite below-average excess winter rainfall (EWR) and nitrate leaching. The large SMNsmeasured following winter rye at GL in 1997 was due to the samples not being taken until after fertilizer-N had been applied for the potatoes.
3.4. Nitrate leaching
Nitrogen leaching losses were dominated by NO3
−-N (results not shown), as expected from
previous studies (Webster et al. 1993). Henceforth the term Nleach will be used to describe losses of both NH4
+- and NO
3
−-N.
J.Webb et al./Europ.J.Agronomy13 (2000) 207 – 223 214
Table 3
Soil mineral nitrogen (SMN kg/ha) to 90 cm and total nitrogen (%) and dates of sampling in fields where N fluxes were measured in arable rotationsa
Terrington
Farm Gleadthorpe Rosemaund
Welbeck
Top Kingston Propagation Shepherds Gate Flat Field Preston
SMN
28
Autumn 1995 41 41 28 92 110
(24/11/95) (17/10/95) (17/10/95)
(14/11/95) (11/12/95) (14/11/95)
W. wheat W. wheat Potatoes
W. wheat W. beans
Crop W. wheat
Spring 1996 70 65 88 81 89 129
(23/4/96) (8/3/96) (3/4/96)
(23/4/96) (5/2/96) (5/2/96)
82 23 93
Post harvest 1996 66 NDa ND
(6/9/96) (11/9/96) (11/9/96) (6/9/96)
Autumn 1996 126 107 50 128 116 115
(30/10/96) (17/12/96) (17/12/96)
(30/10/96) (12/12/96) (4/11/96)
W. rye
Crop W. rye Sugarbeet W. wheat W. wheat W. oats
41 96 94 50
Spring 1997 54 61
(12/3/97) (14/3/97) (14/3/97)
(12/3/97) (18/3/97) (19/3/97)
79 75 29
Post harvest 1997 51 116 108
(19/8/97) (18/12/97) (23/8/97)
(19/8/97) (1/9/97) (17/8/97)
ND 75
Autumn 1997 ND 56
(18/12/97) (18/12/98)
353b 92
Spring 1998 88 93
(8/4/98) (20/4/98) (20/4/98) (8/4/98)
Potatoes
Crop Sugarbeet Linseed Sugarbeet
85 73
Post harvest 1998 43 19
(5/10/98) (25/8/98) (11/11/98) (3/12/98)
aND, not determined.
bTaken after fertilizer-N applied to the potatoes.
the three winters reported here, this was less than 220 mm at all sites, and at TE was less than 100 mm in the first two winters (Table 4). Overwinter
Nleach at TE in 1995/96, when EWR was only 22 – 43 mm after winter wheat, was particularly small (2 – 7 kg N/ha). Under UK conditionsNleach of 35 kg N/ha would be expected following winter wheat receiving Nopt (Lord, 1992). Overwinter
Nleachin 1996/97 following potatoes at TE, at only 27 kg N/ha, was also less than expected (80 kg/ha, Lord, 1992). In contrast, losses following winter wheat at RO overwinter 1995/96 were about twice those expected, despite EWR being c. 130 mm less than the long-term average for that site. This crop followed winter beans, and apparently insufficient
allowance had been made for the residual N from that crop, when applying fertilizer-N to the winter wheat (Table 2). At GL overwinter 1996/97,Nleach after winter wheat (37 – 52 kg N/ha) were also quite large. In both fields the winter wheat crops followed sugarbeet, and the fertilizer-N applica-tions were larger than is now considered appropri-ate for winter wheat on a light sandy soil (Webb et al., 1998). Moreover, large Nleach in the second winter after the sugarbeet harvest have been ob-served elsewhere (Shepherd et al. 1997). At GL
Nleach were greater overwinter 1997/98, partly in consequence of the much greater EWR. At TE
Table 4
Excess winter rainfall and nitrate leaching (kg NO3-N/ha) from fields where N fluxes were measured in arable rotationsa
Terrington
Farm Gleadthorpe Rosemaund
Field Top Kingston Welbeck Propagation Shepherds Gate Flat Field Preston Previous crop 1995/96 Sugarbeet Sugarbeet W. wheat W. wheat W. wheat Oilseed rape
91 22
Excess winter rainfall (mm) 91 43 192 167
17 (1.3) 2 (0.2) 7 (0.8)
NO3-N leached (kg/ha) 24 (1.5) 67 (4.6) 33 (2.8)
W. wheat W. wheat
Previous crop W. wheat Potatoes W. beans W. wheat
115 64 100 166 102
Excess winter rainfall (mm) 115
37 (8.7) 6 (1.6) 27 (1.7)
52 (5.8) 78 (6.4)
NO3-N leached (kg/ha) 22 (2.6)
W. rye Sugarbeet
Previous crop 1997/98 W. rye W. wheat
213 137 213
213 Excess winter rainfall (mm)
75 (2.3) 6 (0.6) 30 (3.2) NO3-N leached (kg/ha) 57 (2.5)
aSE in brackets.
3.5. O6erwinter N mineralization
Mineralization of 25 – 60 kg N/ha was estimated between harvest and the onset of the FC period at GL and TE in the autumns of 1996 and 1997. Mineralization overwinter 1995/96 at 20 – 60 kg N/ha following cereals and sugarbeet (Table 5), was similar to the modelled estimates reported by Webb et al. (1997). However, over the following winter estimates of OvMin were much less, usu-ally suggesting net immobilization. Net mineral-ization of 20 – 60 kg N/ha were also estimated overwinter 1997/98.
3.6. Nitrous oxide and dinitrogen
Measurements of N2O from autumn to late spring of 1995/96 showed maximum daily fluxes of 29, 66 and 17 g N/ha.d at GL, TE and RO, respectively, although the largest emission was thought to result from a ‘degassing’ event. Cumu-lative losses over the season were 0.2 – 0.3, B0.1 – 0.1, and 0.1 – 0.3 kg N/ha per 6 months for GL, TE and RO, respectively. In 1996/97 cumulative emissions were 0.7 – 1.0, 0.3 and 0.3 – 1.0 kg N/ha per 6 months for the three sites, respectively. In 1997/98 losses of N2O at GL were slightly
Table 5
Estimates of overwinter-N mineralization (kg N/ha) in fields where N fluxes were measured in arable rotations
Farm Gleadthorpe Terrington Rosemaund
Top Kingston Welbeck Propagation
Field Shepherds Gate Flat Field Prestons
1995/96
W. wheat
Previous crop Sugarbeet Sugarbeet W. wheat W. wheat OSR
49 53
Overwinter mineralizationa 17 60 64 52
1996/97
W. wheat W. wheat
Previous crop W. wheat W. wheat Potatoes Beans
52 −7
Overwinter mineralization −16 −8 22 −43
1997/98
Previous crop W. rye W. rye Sugarbeet W. wheat
Overwinter mineralization NA NA 23 67
aCalculated as NO
J.Webb et al./Europ.J.Agronomy13 (2000) 207 – 223 216
Table 6
Summary of ammonia flux measurements and calculations from fields where N fluxes were measured in arable rotations Crop heighta(cm) Dheight (cm)
Site Period Flux (gN/ha per day) Fit of profilesb,c
End
Start R2 (NH
3)c R2(u)
18/6/96
GL 3/9/96 45 29 19 70
22/7/97 120 84
25/6/97 6¡ 1 99.1
4/4/96
22/3/96 5
RO 7
6/6/96 20 13 25
9/5/96 18
4/7/96 70 45
6/6/96 34¡ 19
4/7/96 15/8/96 110 70 46¡ 74 53
4/6/97 3/7/97 90 63 15 2 96.1
7/8/97 95 67
3/7/97 29¡ 9 99.6
22/3/96
TE 30/4/96 3 0
18/6/96
30/4/96 131 54
30/8/96 60 39 85
4/7/96 56
8/5/97 30 22
2/4/97 22¡ 28 81.2
8/5/97 4/6/97 50 36 65 82 99.7
2/7/97 87 61
4/6/97 19¡ 11 95.2
2/7/97 29/7/97 90 63 5 2 97.2
9/7/98
GL 24/7/98 50 7¡ 3 92.0
17/8/98 50
24/7/98 34¡ 24 97.6
17/898 4/9/98 50 4¡ 9 74.4
23/9/98 50
4/9/98 16¡ 30 93.7
13/10/98 50 5
23/9/98 9 57.7
23/7/98 20
8/7/98 8
TE 5 99.4
23/7/98 12/8/98 30 32¡ 63 99.6
3/9/98 30
12/8/98 2¡ 0 99.7
25/9/98 30 14 3 99.7
3/9/98
15/10/98 30 47¡ 68 99.8
25/9/98
aMean height of crop during measurement period. bPlane of zero displacement.
cResults of regression of [NH
3] or wind-speed (u) vs. ln(h−d) (x)-where no value is shown wind-speed data based on nearby site
(see text); , ammonia emission;¡, ammonia deposition.
greater than in previous years. This may be partly a consequence of the 1997/98 cropping season being wetter than the 2 previous years.
Dinitrogen emissions were between 5 and 20 g N/ha.d, equivalent to N2O-N emissions measured at a similar time in the season 1996/97 (data not shown). Using these limited data it was suggested for the purpose of an annual estimate of total N loss as N2O and N2, that the cumulative N2O flux should be doubled.
3.7. Ammonia fluxes
The results are summarised in Table 6, and in
general indicate rather small fluxes. In contrast to 1996, when emissions dominated, there was no consistent pattern for the 1997 data. However,R2 values for the NH3 profiles were generally very small indicating only poor fits to the linear regres-sion. The best fit in 1997 was for 8 May to 4 June at Terrington, when an emission of 65 g NH3-N/ ha.day was measured.
10% of the average. Conversely, in order to in-clude 90% of measurements, duplicates which are 935% of the average are required. The effect of the poor linear regression fits for the NH3 concen-tration on the calculated fluxes can be calculated. If this is done, although fluxes appear to be generally small, the 95% confidence interval spans the range of emission to deposition.
Measurement of NH3 concentrations with flux samplers gave windspeed-weighted rather than time-averaged values. Furthermore, concentra-tions for 1996 were calculated (in general) from extrapolated windspeed data. Hence, the concen-tration data should be treated with some caution. Average NH3 concentrations at each site were: GL 1.6 and 0.5mg NH3-N/m
3
(SE 0.15 and 0.11,
n=8 and 8), TE 3.4 and 1.8 mg NH3-N/m3 (SE 0.15 and 0.71,n=26 and 24), and RO 2.5 and 2.6 mg NH3-N/m
3
(SE 0.10 and 0.12, n=39 and 16) for 1996 and 1997, respectively.
Previous work had shown that the errors asso-ciated with passive flux samplers were large at the small NH3 concentrations measured. Further-more, the errors in estimating NH3concentrations using passive flux samplers were compounded by errors in estimating average windspeeds, since the horizontal flux vertical profile was measured, a vertical profile was required to calculate the verti-cal flux. Willems badge samplers measure NH3 concentrations directly, and so their use both simplified flux calculation and allowed better reso-lution of the overall error in vertical flux measure-ment. This error was estimated to be +30 g NH3-N/ha.d. These results were therefore within, or very close to the experimental error associated with the methodology. The net NH3 flux for July to November 1998 at both GL and TE was 10 – 12930 g NH3-N/ha.d deposition. This gives a range of between 3 kg NH3-N/ha emission to 6 kg NH3-N/ha deposition.
3.8. Wet N deposition
Nitrogen inputs from wet deposition (Table 2) ranged from 5 kg/ha (GL 1996/97) to 22 kg/ha (RO 1996/97), and were two to four times greater at RO than GL or TE.
4. Discussion
4.1. Soil nitrogen
The amounts of soil N measured at GL at 0.07 and 0.08% were rather small compared with the 0.11 – 0.13% N reported by Webb et al. (1997) on sites of similar soil type and in the same arable rotation. Organic manures had been applied regu-larly to the root crops at those sites, and poultry manure had been applied previously to sugarbeet at GL, so there is no clear explanation for the smaller soil N at GL.
4.2. Inputs of nitrogen
The greatest N inputs were of fertilizer-N, which were between 80 and 95% of the total, except at TE in 1997/98 and RO in 1995/96 where linseed and winter beans respectively were grown, and no fertilizer-N was applied. At the sites re-ported here most fertilizer-N applications approx-imated to current recommendations of Nopt (Anon., 1994), and so there is little scope for reducing fertilizer-N application without incurring an economic penalty, albeit in two instances more fertilizer-N was applied than currently recom-mended. However, recently published data (Webb et al., 1998) indicate that current recommenda-tions for wheat grown on sandy soils, such as those at GL, are 10 – 25 kg N/ha in excess. Cur-rent recommendations for cereals grown after sug-arbeet on sandy soils may be 0 – 45 kg N/ha greater than necessary (Webb et al., 2000). This may account for the large leaching losses mea-sured at GL in the second winter after sugarbeet. The adoption of these new proposals would have reduced N inputs at GL to winter rye after sugar-beet, and may have led to lessNleachin the follow-ing winter and potentially less N2O emission. A target-oriented approach is not applicable to UK conditions as Nopt is only very poorly correlated with grain yield (e.g. Shepherd, 1993; Webb et al. 1995).
J.Webb et al./Europ.J.Agronomy13 (2000) 207 – 223 218
deposition at all three sites. Goulding et al. (1998) measured c. 9 kg N/ha wet deposition in E. England, similar to the amounts reported here for GL and TE. The greater amounts measured at RO may be a consequence of the greater amount of livestock farming in that part of the UK lead-ing to local redeposition of NH3.
4.3. Nitrogen outputs
Outputs of N were mainly as crop offtake, and ranged from 60% of the total outputs for winter rye to 85% of the total for winter wheat. Straw from winter rye was incorporated, thereby reduc-ing crop offtake in comparison with winter wheat, for which the straw was removed at all sites. Haulm from the winter bean crop was also incor-porated after harvest, and crop N offtake was only 66% of total N outputs. An average of 77% of the N outputs were accounted for by crop N offtake. However, these large differences in the proportions of N outputs accounted for by differ-ences in the fate of non-harvested residues were confounded with seasonal, and between-site dif-ferences in EWR, and hence Nleach. Overwinter rainfall at GL of 115 mm following winter rye was less than average, but greater than at TE in either of the first two winters. WhileNleach follow-ing sugarbeet has usually been found to be only moderate (Shepherd & Lord, 1996), losses of only 6 kg N/ha at TE in 1996/97 are considered to be due to Nleach being restricted by only 64 mm EWR. For the remainder of the project we pro-pose to reduceNleachby growing cover crops when the cereals are to be followed by a spring-sown crop.
The amounts of Nleach that may take place, without exceeding the 50 mg/l limit stipulated by the EU Nitrate Directive (EEC, 1991), depend upon EWR (Wild & Cameron, 1980). At GL and TE, where average EWR is c. 200 mm, on average no more than 22 kg N/ha NO3−-N may be leached if the 50 mg/l limit is to be met. At RO, where average EWR is c. 300 mm, then up to 34 kg/ha NO3−-N may be leached. Thus at GL, following winter wheat grown after sugarbeet, even in a year of average rainfall, NO3−-N concentrations in drainage water would have greatly exceeded 50 mg/l.
The peak N2O emission of 66 g N/ha.d oc-curred on the silty clay loam RO soil two days after 131 kg N/ha was applied coincident with 16 mm of rainfall. It was hypothesised that intense denitrification activity and N2O production was stimulated by the combination of fertilizer appli-cation and heavy rainfall, that rapidly filled the soil pores with water, confining the N2O pro-duced. Good drying conditions the following day, i.e. a long sunny, warm day, allowed the soil pores to dry rapidly, such that the previously trapped N2O was released and emitted at the soil surface-a ‘degassing’ event. This phenomenon has been demonstrated by Young and Ashby (unpub-lished data) for a silty clay soil. They measured a peak ‘de-gassing’ rate of 16.8 kg N/ha.d, which lasted for only a few minutes.
The major factors to increase N2O emissions were fertilizer application and rainfall events as Sexstone et al. (1985) and Kaiser and Heinemeyer (1996) concluded earlier, although losses were generally less than 1 kg N/ha over 6 months. The annual N emissions (as N2O and N2) averaged over the first two seasons were 2.0 – 4.0, 0.4 – 4.4, and 1.6 – 4.0 kg N/ha for GL, TE and RO, respec-tively. These losses represent between 1.3 and 2.5, 0.2 and 2.7, and 1.0 and 2.5% of the total mineral fertilizer-N applied to the three sites, respectively, and are in general agreement with emissions esti-mated by Mosier (1994) of approximately 0.3 – 2.3% of applied fertilizer-N lost as N2O, corresponding to 0.6 and 4.5 kg N/ha.y for a routine application of 200 kg N/ha to winter cereals. Greater emissions of N2O and N2 were measured in 1997/98, but while these data have yet to be fully evaluated, greater losses could have been due to generally wetter weather in that season.
4.4. O6erwinter mineralization
The greater N concentrations at RO and TE might have been expected to produce greater overwinter mineralization at those sites than at GL. However several recent studies have sug-gested that mineralization is related not to total soil N concentrations, but to the size of organic residues returned to the soil over the previous 3 – 5 years (e.g. Matus and Rodrı´guez, 1994).
4.5. Nitrogen balance
Considering only inputs to and outputs from the fields, then over the three seasons reported here, inputs of N have exceeded outputs in only three of the six fields monitored (Table 7). At GL, net inputs of 33 and 117 kg N/ha have been measured at Top Kingston and Welbeck, respec-tively. These surpluses have been mainly a conse-quence of large (190 kg/ha) applications of fertilizer-N to winter wheat crops in 1995/96, and recent work now suggests such applications are unnecessarily large on these sandy soils (Webb et al., 1998). In addition straw from the subsequent winter rye crops was incorporated, thus reducing outputs of N from those fields, and other workers have noted the importance of the fate of unhar-vested residues in determining whether N balances in arable rotations lead to net loss or net accum-mulation of N (e.g. Van Faassen & Lebbink, 1990). At TE, where cereal straw was removed in both years, outputs have exceeded inputs (by c. 24 kg N/ha). This is partly in consequence of grow-ing sugarbeet and linseed crops given only 115, 40 and 0 fertilizer-N. Moreover, because of the two dry winters, Nleach has been small. In seasons of average rainfall greater Nleach would be expected at TE, and in consequence a net loss of N from the soil. At RO, N was approximately in balance for Flat field (+16 kg N/ha), however at Prestons there was a net loss of N of 68 kg/ha over the two seasons. This was due to both straw being re-moved from that field in both years, and EWR being sufficient to leach much of the residual N over winter. While these results suggest that N inputs and outputs were broadly in balance in these arable rotations, there are two caveats to
this observation. The first is that the balance depends greatly on straw disposal; incorporation is likely to lead to net accumulation of N, while removal may lead to net loss. The second is that in two of the seasons reported here, Nleach was usually less than expected. In more ‘normal’ sea-sons Nleach is likely to be greater, and in conse-quence net losses of N from the soil system may be expected, unless measures such as cover crop-ping are taken to conserve N over winter. Thus influence of winter rainfall on N balance has been noted by Ju¨rgens-Gschwind and Jung (1979).
If, in order to give a better account of N fluxes in these soils, measurements of SMN and N in unharvested crop residues are included in the balance (Table 7) then at harvest 1996 between 40 and 88 kg N/ha could not be accounted for. At RO much of this N could have been as SMN, since this was not measured in that year. At TE, after potatoes, some of the N would have been as haulm. However, following winter wheat at GL and TE 40 – 60 kg N/ha was not accounted for. This may have been due to our measurements underestimating outputs, but the calculated errors do not suggest this. Immobilization of mineral-N by soil organic matter is more likely. Over the following season there appears to have been net N mineralization under the cereal crops. Only under sugarbeet was any N unaccounted for, and this may have been present as unharvested crop residues. These were not measured at TE in 1997, and the unaccounted N (32 kg N/ha) was much less than that measured in sugarbeet residues at GL in the following year. In 1998 net N mineral-ization at TE under linseed was estimated as 26 kg N/ha.
4.6. Reducing nitrogen pollution
J.Webb et al./Europ.J.Agronomy13 (2000) 207 – 223 220
Table 7
Balance sheet of N inputs and outputs in fields where N fluxes were measured in arable rotationsa
Farm Gleadthorpe Terrington Rosemaund
Field Top Kingston Welbeck Propagation Shepherds Gate Flat Field Prestons W. wheat W. wheat Potatoes W. beans
Crop W. wheat W. wheat
1996 Inputs(kg/ha)
Total 202 189 267 307 227
Outputs(kg/ha)
134 (3.6)
Crop offtake 126 (9.6) 184 (6.0) 144 (14.5) 139 189
17 (1.7) 2 (0.2) 7 (0.8) 67 (4.6) 33 (2.8)
Total 145 188 167 206 224
57 1 100
SMN post harvest 66 ND
– – ND
– 32 –
Unharvested residues
40 66 88 NA
N not accounted for 45 NA
Crop
W. rye W. rye Sugarbeet W. wheat W.wheat W. oats 1997
Inputs(kg/ha)
130
Fertilizer –N 130 115 160 196 102
0 0 0
N deposition 5 22
138 121 172
Crop offtake 174 (9.4)
56 (5.8)
SMN in spring 50
51
SMN post harvest 79 75 29 116 108
67 ND
Unharvested residues 58 − – –
−76 32 −9
−52 −140
N not accounted for −129
Linseed Sugarbeet Crop Sugarbeet Potatoes
1998 Inputs(kg/ha)
130
Fertilizer –N 255 0 40
0 0 0
N deposition 8 10
Table 7 (Continued)
Terrington Rosemaund
Gleadthorpe Farm
Welbeck Propagation Shepherds Gate Flat Field
Field Top Kingston Prestons
NA Unharvested residues 162 ND 15
N not accounted for −128 34 −34 25
−24
117 −23 16 −68
Total net outputs 33
aSE in brackets; ND, not determined; NA, not available. bCalculated as NO
3-N leached+soil mineral N in spring-soil mineral N in autumn.
argued that the damage done by N2O emissions (ozone destruction, climate change), means that its impact, per kg of N emitted, is greater than that of NO3−-N. However, a substantial propor-tion of N2O losses from agriculture is estimated to derive from NO3− leached (IPPC/OECD, 1997), thus reducing Nleach may also reduce N2O emis-sions. Whether this proves to be so is likely to depend upon the means to reduce Nleach. Reduc-ing fertilizer-N applications, takReduc-ing into account the results of recent research, should have a dual benefit. Conserving N by means of cover crops may lead to increased emissions of N2O when those crops are incorporated.
Acknowledgements
We thank staff at ADAS Gleadthorpe, Rose-maund and Terrington for carrying out the field-work. Funding by the UK Ministry of Agriculture, Fisheries and Food is also gratefully acknowledged.
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