Assessment of tillage strategies to decrease nitrate leaching in the
Brimstone Farm Experiment, Oxfordshire, UK
J.A. Catt
a,*, K.R. Howse
a, D.G. Christian
b, P.W. Lane
c, G.L. Harris
d, M.J. Goss
eaSoil Science Department, IACR, Rothamsted, Harpenden, Herts AL5 2JQ, UK bCrop and Disease Management Department, IACR, Rothamsted, Harpenden, Herts AL5 2JQ, UK
cStatistics Department, IACR, Rothamsted, Harpenden, Herts AL5 2JQ, UK dADAS Gleadthorpe, Meden Vale, Mans®eld, Notts NG20 9P, UK
eCentre for Land and Water Stewardship, Richards Building, University of Guelph, Guelph, Ont., Canada N1G 2W1
Accepted 13 August 1999
Abstract
Tillage may in¯uence nitrate losses from agricultural soils. Losses of nitrate were measured in drain¯ow at 60 cm depth and in combined surface runoff and inter¯ow in the A horizon (surface layer ¯ow) on hydrologically sealed plots with a two-year comparison (1988±1990) of shallow-tine cultivation vs. mouldboard ploughing. Ploughing increased concentrations and loadings of nitrate in drain¯ow and surface layer ¯ow, especially in the ®rst year. After these two years the shallow-tined plots were ploughed to plant winter beans (Vicia fabaL.), and nitrate in drain¯ow then increased over the next three winters, slightly exceeding that from the plots which had been ploughed throughout for winter cereals. The composition of the surface layer ¯ow did not show this effect, however. Calculations of net winter mineralisation of soil organic nitrogen showed that shallow-tine cultivation may have decreased mineralisation slightly compared with ploughing in the ®rst two years. These calculations did not indicate any increase in mineralisation for two winters after the minimally cultivated plots were ploughed in autumn 1990, probably because the soil was then very dry. This increase was apparently delayed until the ®fth winter (1992/1993), which was much wetter than any since autumn 1990. In the previous eight years (1980±1988) half of the plots had been ploughed and half had been direct drilled. Averaged over the ®ve winters 1988/1989±1992/1993, the ®ve measures of nitrate loss in drain¯ow from plots previously direct drilled were 6±57% more than from plots previously ploughed, and winter mineralisation was 20% more, with no evidence of any decline in either with time. The nitrate produced by mineralisation of organic matter conserved by the eight years of direct drilling was mainly lost by leaching or denitri®cation; it was of little or no bene®t to the crops. The results suggest that in the long term more nitrate is leached from land subject to periods of minimal or zero tillage and ploughing than from land ploughed every year.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Drain¯ow; Surface layer ¯ow; Nitrate loss; Mineralisation; Leaching; Shallow tillage; Direct drilling
1. Introduction
The Brimstone Farm Experiment, situated near Faringdon, Oxfordshire, UK (National Grid Reference
*Corresponding author. Tel.:44-1582-763133; fax:
44-1582-760981.
E-mail address: john.catt@bbsrc.ac.uk (J.A. Catt).
SU 248946), has twenty 0.2 ha hydrologically isolated plots (Cannell et al., 1984; Harris et al., 1984), which have been used since 1978 to study the effects of various soil and crop management techniques on nitrate leach-ing. Brimstone is one of the few experimental sites where nitrate loss can be measured directly in plots large enough for use of standard farm machinery.
Results from Phase I of the experiment (1978± 1988) indicated the importance of mineralisation of crop residues and other soil organic matter in produ-cing nitrate that is leached during the autumn and winter. For example, on land sown to winter cereals after mouldboard ploughing with no seedbed (autumn) fertiliser N, a mean of 21.8 kg N haÿ1
(SE 6.0) was leached as nitrate over the following winter periods (Goss et al., 1993). Although some of this leached N could have come from atmospheric deposition or ®xation by free-living soil micro-organisms, such as blue-green algae, a tentative N balance showed that most of it must have been derived from mineralisation of soil organic matter. Mineralisation also contributes a large proportion of the nitrate leached from arable land at Rothamsted (Addiscott, 1988; Jenkinson and Parry, 1989). Consequently, in Phase II of the Brimstone Experiment (1988±1993) several possible strategies for limiting mineralisation of soil organic matter were tested. This paper gives results for the technique of minimal (shallow-tine) cultivation, which was com-pared with mouldboard ploughing, and also examines the effect on nitrate leaching of ploughing plots that had previously been direct drilled for eight years.
2. Site characteristics and experimental procedures
2.1. The Brimstone Farm Experiment
Brimstone Farm is on a cracking clay soil (pelo-stagnogley soil of Avery, 1980, Verti-eutric Gleysol of FAO-Unesco, 1988) of the Denchworth series (Jarvis, 1973) developed on Upper Jurassic Oxford Clay. The Ap horizon contains 540 g kgÿ1
clay (<2mm) and
390 g kgÿ1
silt (2±60mm), and in dry summer periods
cracks up to ca. 10 cm wide at the surface extend to ca. 1.0 m depth. The plots are on a 2% slope at 100±106 m OD and measure 59 m41 m in total, but include discard areas to allow ploughing and other ®eld
opera-tions at the correct depth over a central agronomic area of 40 m28 m (0.112 ha). The plots are isolated on either side by polythene membranes to 1.1 m depth parallel to the slope. Water moving down the slope towards each plot is intercepted by gravel-®lled trenches 1 m deep and 10 cm wide located in the permanent grass discard area upslope of each plot, and surface runoff is prevented from moving between plots across the slope by an inverted PVC gutter anchored to the soil surface over the polythene mem-branes. Movement of water below these barriers is negligible in the very slowly permeable Oxford Clay (Youngs and Goss, 1988).
Water moving through the soil is collected in par-allel mole drains 2 m apart and at 60 cm depth. On each plot these feed into a trench containing a pipe drain at 90 cm depth, which is covered with permeable back®ll to within 45 cm of the surface. Surface runoff and inter¯ow to a depth of approximately 25 cm (together termed surface layer ¯ow) are collected in a deep plough furrow across the lower end of each plot a short distance upslope of the pipe drain trench. Both the drain¯ow and surface layer ¯ow are measured by V-notch weirs, the head level being recorded by a ¯oat system attached to a chart recorder and also to a rotary potentiometer and datalogger. In Phase II of the experiment fresh water samples were collected every 3 h during ¯ow events from U-bends situated upstream of the ¯oat chambers, and were analysed for nitrate-N colorimetrically after reduction to nitrite using a cadmium column in a ¯ow injection system. Loadings of nitrate-N for winter (harvest to spring top-dressing) and summer (top-dressing to harvest) were calculated by integrating ¯ow rates and nitrate con-centrations over time using Simpson's rule. As the loadings were strongly in¯uenced by differences in the ¯ow volumes, they were also normalised to a unit volume of ¯ow (100 mm). Surface layer ¯ow was not measured or analysed in 1992/1993.
2.2. Analysis of nitrogen in crops and soils
were measured at harvest on 9.2 m28 m areas of each plot. Soil mineral-N was measured at each spring (before fertiliser application) and autumn (immedi-ately after harvest) by extraction with 2 M aqueous potassium chloride solution; in the ®rst three years two cores were taken per plot, but because the amounts in subsoil horizons were small and changed little with season, the samples analysed in the last two years were bulked from 12 topsoil (0±20 cm) samples per plot.
2.3. Tillage and crop comparisons
Two plots (4 and 18) were shallow-tine cultivated to 7 cm depth at 25 cm lateral spacing in autumn 1988 and 1989 for the cultivation of winter oats (Avena sativa L.) and winter wheat (Triticum aestivum L.), respectively, but were then ploughed to 20 cm depth in autumn 1990 to incorporate winter ®eld bean (Vicia faba L.) seeds that had previously been broadcast. They were also ploughed (a) in autumn 1991 to incorporate the bean residues and sow winter barley (Hordeum vulgareL.), and (b) in autumn 1992 prior to a bare winter fallow, which was followed in 1993 by a crop of spring oats (Avena sativaL.). Two other plots (10 and 20) were ploughed to 20 cm depth and power-harrowed to 7 cm depth in all ®ve years and, apart from growing fertilised winter wheat instead of unfer-tilised beans in 1990/1991, received the same treat-ments of residue disposal (i.e. burning in all ®ve autumns), cropping and fertiliser applications as plots 4 and 18. The rates of spring-applied N fertiliser were 100 kg haÿ1
to winter oats in 1989, 150 kg to winter wheat in 1990, 179 kg to winter wheat (plots 10 and 20 only) in 1991, 122 kg to winter barley in 1992 and nil to spring oats in 1993. The results presented therefore (a) compare the effects of ploughing and shallow-tine cultivation in the ®rst two years, and (b) show the effect in the remaining three years of ploughing land that was previously shallow-tined, though this effect was combined with that of different crops (winter beans and winter wheat) in the third year.
Both pairs of plots included one which had been ploughed in the previous eight autumns of Phase I (plots 10 and 18) and one on which the crops had been established by direct drilling (plots 4 and 20). The crops in Phase I were mainly winter cereals, the residues of which were burned after harvest. Compar-ison of these plots in the 1988±1993 period therefore
provides further information on the effect of plough-ing land that had previously been direct drilled.
2.4. Statistical methods
Preliminary statistical analysis of results for the various measures of nitrate loss was by Genstat (Gen-stat 5 Committee, 1993). As there were only two replicate plots for each treatment and the annual or half-yearly differences in nitrate loadings between these were always too large for treatment differences to be statistically signi®cant, we compared loadings and mean concentrations for shorter periods to show whether differences were associated with treatments rather than arising by chance. For ease of interpreta-tion we chose periods of equal durainterpreta-tion, starting on the same date for all plots. To avoid as far as possible the zero values resulting from lack of ¯ow, the analysis was based on eight 4-week periods for each winter starting on the ®rst day of drain¯ow on any of the plots in each year. The mean values for each treatment in each winter could then be displayed as pro®les show-ing the change with time. However, there were wide ranges of values and we cannot assume constant variance for each 4-week value, as small values are bound to be less variable than large ones. Conse-quently, we transformed the values to a log scale after replacing the few zero values for loading by the smallest non-zero value (0.01 kg N haÿ1
); this effec-tively stabilised the variance. However, no substitute was suitable for the zero values for concentration, so the lack of ¯ow in 1991/1992 prevented any result based on concentrations being obtained for that year. We used a split-plot analysis with Greenhouse± Geisser correction to the degrees of freedom (Green-house and Geisser, 1959) to assess whether the pro®les for the two treatments were parallel, i.e. whether there was an interaction between time and treatment. For both loadings and concentrations there was, as expected from Fig. 1, a large effect of time, but the interaction was signi®cant neither for loadings nor for concentrations in any year.
of Phase II. This means that once the relationship between values in two successive periods has been described, it adequately accounts for those between other periods.
3. Results
3.1. Rainfall, drain¯ows and surface layer ¯ows
In the ®rst four years (1 September±31 August) of Phase II, annual rainfall was much less than the long
term (1941±1970) average for the Faringdon area (686 mm). As a result the drain¯ows in these years (Table 1) were less than in Phase I: mean annual drain¯ows for the four plots ranged from 4.4 mm in 1991/1992 to 188.5 mm in 1989/1990 (mean over all four years90.8 mm), whereas those for 10 drained plots between 1978/1979 and 1987/1988 were 73± 276 mm (mean over all 10 years199 mm). However, in the last year of Phase II (1992/1993) the rainfall was 784 mm, which increased mean drain¯ow for the four plots to 477.5 mm. As percentages of the annual rainfall, the mean drain¯ows for treatments ranged
from 0.4% on plots 4 and 18 in 1991/1992 to 63.4% on plots 10 and 20 in 1992/1993; the mean for all four plots over the ®ve years was 29.9%.
In 1988/1989 and 1989/1990 the mean amounts of drain¯ow were slightly greater on tined plots (means of 137.7 mm in 1988/1989 and 196.5 mm in 1989/ 1990) than on ploughed plots (132.5 mm in 1988/1989 and 180.6 mm in 1989/1990). This difference was not signi®cant, but agrees with the difference in drain¯ow between ploughed and direct drilled plots observed in Phase I, which Harris and Colbourn (1986) attributed to the better continuity between topsoil and subsoil of the undisturbed macropores in direct drilled soil.
The amounts of surface layer ¯ow were much less than drain¯ow, ranging from 0 to 1.0% of the rainfall (mean for all four plots over the four years monitored was 0.3%). In contrast to drain¯ow, they were slightly greater on the ploughed plots (4.7 mm in 1988/1989 and 3.6 mm in 1989/1990) than on the shallow-tined plots (1.9 mm in 1988/1989 and 2.6 mm in 1989/ 1990). Although it was not signi®cant, this difference can probably be attributed to the greater total macro-porosity of ploughed topsoil, the disruption of vertical channels into the subsoil resulting from ploughing and the greater tendency for lateral ¯ow above a smeared or compacted base to the ploughed layer.
3.2. Nitrate in drain¯ow
As in Phase I (Goss et al., 1993), nitrate concentra-tions in drain¯ow were highest (usually in the range 50±100 mg NO3-N l
ÿ1
) in the ®rst ¯ows of winter,
then progressively declined often to values <11.3 mg NO3-N lÿ1, the EU drinking water limit (Anon., 1980) by the late winter, and then increased again after spring top-dressing (Fig. 1A±E).
Several aspects of the drain¯ow results for 1988/ 1989 and 1989/1990 suggest that mouldboard plough-ing increased nitrate losses compared with shallow-tine cultivation:
1. The mean annual concentrations for the two ploughed plots were 18.1 mg NO3-N l
ÿ1
in 1988/ 1989 and 18.8 mg NO3-N l
ÿ1
in 1989/1990, a mean of 54% greater than on the shallow-tined plots (9.8 mg NO3-N lÿ1 in 1988/1989 and 15.2 mg NO3-N lÿ1 in 1989/1990) (Table 2). Based on the numerous individual determinations, this difference was highly signi®cant (P<0.001), and conforms to the pattern noted in Phase I, when greater nitrate concentrations were often measured on ploughed than direct drilled plots, especially in autumn and winter drain¯ows before spring top-dressing. The difference in Phase I was attributed by Goss et al. (1993) to conservation (less mineralisation) of organic matter and more by-pass ¯ow in direct drilled land.
2. The mean percentage of annual water samples exceeding the EU drinking water limit was greater on the ploughed plots (45% in 1988/1989 and 66% in 1989/1990) than on the shallow-tined +plots (29% in 1988/1989 and 51% in 1989/1990) (Table 2). The mean difference over the two years was 16% of samples (not signi®cant).
Table 1
Annual rainfall, drain¯ows and surface layer ¯ows for selected plots in the Brimstone Experiment 1988/1989±1992/1993 (1 September±31 August), all values in mm
Plots 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993
Ra Da Ca R D C R D C R D C R D C
4b(ST/P) 138.0 0.0 181.0 3.9 48.4 0.1 3.7 0.0 547 ±e
18 (ST/P)c 138.0 3.8 212.0 1.2 41.9 0.0 0.1 0.1 368 ±e
461 574 483 512 784
10 (P)d 167.0 1.6 202.0 5.1 24.6 0.4 3.4 0.1 533 ±e
20b(P) 97.9 7.8 159.0 2.0 24.9 0.0 10.3 0.0 462 ±e
aR: annual rainfall; D: drain¯ow; C: cultivated layer ¯ow.
bDirect drilled in 1980±1987, other plots ploughed 1980±1987.
cShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
dPloughed in all ®ve autumns.
3. The mean winter loading of nitrate for the ploughed plots was 15.94 kg N haÿ1
, 16% more than for the shallow-tined plots (13.74 kg N haÿ1
) (Table 3). Most of this increase was in the ®rst year when the difference between treatments was 69% (not signi®cant).
4. The mean summer loading of nitrate for the ploughed plots was 1.64 kg N haÿ1
, 20% more than for the shallow-tined plots (1.37 kg N haÿ1
) (Table 4). As with the winter loadings, most of this (non-signi®cant) difference was in the ®rst year.
5. The mean annual nitrate loss per 100 mm drain-¯ow was 11.64 kg N for the ploughed plots, 34% more than for the shallow-tined plots (8.69 kg N) (Table 5). Again most of the increase was in the ®rst year when the difference between treatments was 75% (not signi®cant).
In the third year (1990/1991) there were interesting changes in all the measures of nitrate loss in drain¯ow:
1. Mean annual nitrate concentrations were 8% greater (P0.013 based on individual
determina-Table 2
Mean annual nitrate concentrations (mg NO3-N l
ÿ1) in drain¯ow from selected plots and (in parenthesis) the percentages of drainwater
samples per year exceeding the EC drinking water limit (11.3 mg NO3-N l
ÿ1), Brimstone Farm 1988/1989±1992/1993
Plot 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993 Five year means
4a(ST/P) 9.8 (29.8) 17.3 (54.0) 47.1 (100) 45.0 (100) 18.9 (73.2) 27.6 (71.4)
18 (ST/P)b 9.7 (27.6) 13.0 (48.2) 30.5 (100) 41.0 (100) 16.5 (53.0) 22.1 (65.8)
10 (P)c 13.9 (35.7) 18.6 (62.0) 27.0 (47.6) 37.8 (99.4) 9.3 (37.8) 21.3 (56.5)
20a(P) 22.2 (54.9) 18.9 (70.3) 44.8 (98.0) 55.2 (100) 16.4 (52.1) 31.5 (75.1)
aDirect drilled in 1980±1987, other plots ploughed 1980±1987.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
cPloughed in all ®ve autumns.
Table 3
Nitrate loadings in drain¯ow (kg N haÿ1) between harvest and the ®rst spring application of fertiliser to any plot, Brimstone Farm 1988/1989±
1992/1993
Plot 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993 Total
4a(ST/P) 4.81 22.93 13.52 0.53 79.57 121.36
18 (ST/P)b 7.09 20.12 8.13 0.04 37.83 73.21
10 (P)c 8.18 21.76 2.67 0.93 34.99 68.53
20a(P) 11.92 21.89 5.83 5.56 55.78 100.98
aDirect drilled in 1980±1987, other plots ploughed 1980±1987.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
cPloughed in all ®ve autumns.
Table 4
Nitrate loadings in drain¯ow (kg N haÿ1) between the ®rst spring application of fertiliser to any plot and harvest, Brimstone Farm 1989±1993
Plot 1989 1990 1991 1992 1993 Total
4a(ST/P) 2.71 0.02 2.65 1.28 6.62 13.28
18 (ST/P)b 2.73 0.02 1.54 0.00 8.51 12.80
10 (P)c 4.05 0.01 0.69 0.24 6.67 11.66
20a(P) 2.42 0.08 0.99 0.29 8.73 12.51
aDirect drilled in 1980±1987, other plots ploughed 1980±1987.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
tions) on the plots previously shallow-tined (4 and 18) than on the plots previously ploughed (10 and 20) (Table 2).
2. Whereas all the drainwater samples on plots 4 and 18 exceeded the EU drinking water limit, only 73% of those on plots 10 and 20 exceeded it (Table 2), though this difference was not sig-ni®cant.
3. The mean winter loading was 155% more (not signi®cant) on plots 4 and 18 than on 10 and 20 (Table 3).
4. The mean summer loading was 149% more (not signi®cant) on plots 4 and 18 (Table 4).
5. The mean annual loss per unit volume of drain-¯ow was 38% greater (not signi®cant) on plots 4 and 18 (Table 5).
The increases after ploughing the plots which had been shallow-tined can be attributed to three possible factors: ®rst, the bean plants on plots 4 and 18 were more widely spaced than the wheat on plots 10 and 20 and thus may have taken up less N than the wheat, leaving more available for leaching; second, there may have been N inputs to the soil of plots 4 and 18 through ®xation of atmospheric N by the beans; third, the additional organic matter conserved on plots 4 and 18 during the two years of shallow-tine cultivation was perhaps mineralised by the ploughing and harrowing in autumn 1990. The increases cannot be attributed to losses of fertiliser, because the beans on plots 4 and 18 received no N fertiliser.
Averaged over the two following years, drain¯ow from plots 4 and 18 continued to provide greater mean annual nitrate concentrations, more samples exceed-ing the EU drinkexceed-ing water limit, greater winter and
summer loadings of nitrate and larger annual losses per unit volume of drain¯ow than plots 10 and 20, though the differences became progressively smaller (Tables 2±5). As a result, the nitrate losses averaged over the ®ve years of Phase II were approximately the same from the two pairs of plots: on plots 4 and 18 the number of drainwater samples exceeding the EU limit was 4% greater, the winter loading was 15% greater, the summer loading was 8% greater and the annual loss per unit volume of drain¯ow was 4% greater, but mean annual nitrate concentrations were 6% greater on plots 10 and 20 (all differences non-signi®cant).
The mean values of the same measures of nitrate loss for plots 4 and 20 (direct drilled between 1980 and 1987) and plots 10 and 18 (ploughed in the same eight years) suggest longer-term effects of different tillage practices on nitrate losses in drain¯ow. In all ®ve years of Phase II, the mean annual concentrations, numbers of samples exceeding the EU drinking water limit, winter loadings and annual losses per unit volume of drain¯ow were greater on plots 4 and 20 than on 10 and 18 (Tables 2, 3 and 5). Based on the individual determinations, the ®rst of these was highly signi®cant (P<0.001), though other differences were not signi®-cant. The mean summer loadings were also greater on plots 4 and 20, but only in four of the ®ve years (Table 4). Nevertheless, averaged over all ®ve years, the plots previously direct drilled showed larger mean values of all ®ve drain¯ow measures: mean annual concentration was 36% more (P<0.001 based on all individual determinations), the number of samples exceeding the EU limit was 20% more, winter loading was 57% more, summer loading was 6% more, com-bined winter and summer loadings were 49% more and annual loss per unit volume of drain¯ow was 46%
Table 5
Annual nitrate loadings in drain¯ow (kg N haÿ1) normalised to unit volume of drain¯ow (100 mm depth of water), Brimstone Farm 1988/
1989±1992/1993
Plot 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993 Five year mean
4a(ST/P) 5.47 12.68 33.41 48.92 15.75 23.25
18 (ST/P)b 7.12 9.50 23.08 40.00 12.59 18.46
10 (P)c 7.32 10.78 13.66 34.41 7.82 14.80
20a(P) 14.65 13.82 27.39 56.80 13.96 25.32
aDirect drilled in 1980±1987, other plots ploughed 1980±1987.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
more (the last ®ve all non-signi®cant). Also, none of the measures shows any decline over the ®ve years in the differences between the two pairs of plots.
3.3. Nitrate in surface layer ¯ow
In the ®rst two years some of the measures of nitrate loss in the surface layer ¯ow showed a similar pattern to those in drain¯ow, but often in a less consistent way:
1. Averaged over the two years the concentrations of nitrate were 28% greater (P<0.009 based on individual determinations) on the ploughed plots than the shallow-tined plots (Table 6), but the effect was con®ned to the ®rst year, because in 1989/1990 the mean concentration was slightly greater on the shallow-tined plots.
2. The number of surface layer ¯ow samples exceeding the EU drinking water limit was 252% greater on the ploughed plots in 1988/1989 but 195% greater on the shallow-tined plots in 1989/ 1990 (Table 6). Neither difference was signi®cant.
3. The mean annual loadings of nitrate in surface layer ¯ow over the two years were 240% greater on ploughed than shallow-tined plots (Table 7), but the effect was much greater in 1988/1989 when the difference between treatments was 490% (not signi®cant).
4. The mean annual nitrate loss per 100 mm surface layer ¯ow was 229% greater for the ploughed plots (9.54 kg N haÿ1
) than the shallow-tined plots (2.90 kg N haÿ1
), but the effect was almost entirely con®ned to the ®rst year when the dif-ference between treatments was 535% (Table 8), though this was not signi®cant.
The ploughing of plots 4 and 18 in autumn 1990 also had inconsistent effects on the various measures of nitrate loss in surface layer ¯ow. Only the mean annual loss per unit volume of ¯ow showed an increase on plots 4 and 18 in both the subsequent years in which surface layer ¯ow was monitored (Table 8); averaged over both years it was 132% greater than on plots 10 and 20 (not signi®cant).
Table 6
Mean annual nitrate concentrations (mg NO3-N l
ÿ1) on selected plots and (in parenthesis) the percentages of surface layer ¯ow samples per
year exceeding the EC drinking water limit (11.3 mg NO3-N l
ÿ1), Brimstone Farm 1988/1989±1991/1992
Plot 1988/1989 1989/1990 1990/1991 1991/1992 Means
4a(ST/P) 6.7 (0.0) 10.2 (25.7) 6.4 (0.0) ±d 7.8 (8.6)
18 (ST/P)b 15.5 (29.2) 3.8 (0.0) 20.6 (100) 39.0 (100) 19.7 (57.3)
10 (P)c 24.1 (85.3) 8.9 (8.7) 25.0 (95.5) 33.3 (100) 22.8 (72.4)
20a(P) 11.5 (17.4) 1.8 (0.0) ±d ±d 6.7 (8.7)
aDirect drilled in 1980±1987, other plots ploughed.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990 and 1991.
cPloughed in all four autumns.
dIndicates no cultivated layer ¯ow.
Table 7
Annual nitrate loadings (kg N haÿ1) in surface layer ¯ow, Brimstone Farm 1988/1989±1991/1992
Plot 1988/1989 1989/1990 1990/1991 1991/1992 Four year totals
4a(ST/P) 0.00 0.16 0.01 0.00 0.17
18 (ST/P)b 0.19 0.03 0.01 0.05 0.28
10 (P)c 0.35 0.20 0.11 0.02 0.68
20a(P) 0.77 0.05 0.00 0.00 0.82
aDirect drilled in 1980±1987, other plots ploughed 1980±1987.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990 and 1991.
The effects of the cultivation contrast in Phase I on the measures of nitrate loss in surface layer ¯ow in Phase II were more consistent, but mainly opposite to those in the drain¯ow. Averaged over all four years of Phase II in which surface layer ¯ow was monitored, the mean annual concentrations of nitrate were 193% greater (P<0.001 based on all individual determina-tions), the percentage of samples exceeding the EU limit was 650% greater and the mean annual loss per unit volume of ¯ow was 583% greater on plots 10 and 18 (ploughed in Phase I) than on plots 4 and 20 (direct drilled in Phase I), but the annual loading was 3% greater on plots 4 and 20. None of the last three differences was signi®cant.
3.4. Differences in four weekly nitrate loadings and concentrations in winter drain¯ow
In the ®rst year of Phase II (1988/1989) the 4-week loadings and concentrations for plots 10 and 20 (ploughed) were both signi®cantly greater than for plots 4 and 18 (P0.041 for loadings and 0.014 for concentrations). The loadings were 85% greater and the concentrations 144% greater. In the second year the loadings were 51% greater (P0.002) on plots 4 and 18 (shallow-tined) than on plots 10 and 20, but the concentrations were not signi®cantly different (P>0.05). In the third year, after plots 4 and 18 had been ploughed, both loadings and concentrations were greater on these plots than on 10 and 20; loadings were 138% greater (P0.045) and concentrations 18% greater (P0.012). In 1991/1992 the loadings were not signi®cantly different (P0.132) and concentra-tions could not be compared because of the very small ¯ows.
The 4-week concentrations in the ®rst four years of Phase II (1988/1989±1991/1992) were consistently greater for the plots (4 and 20) which had been direct drilled in Phase I (1980±1987) than for those which had been ploughed in Phase I (plots 10 and 18). The differences ranged from 22% in 1989/1990 to 63% in 1990/1991, and were signi®cant in these two years (P0.014 and 0.007, respectively), but not in the other years. The 4-week loadings were greater for the plots previously direct drilled in only two of the four years and none of the differences in loadings was signi®cant.
As the effect of the Phase I treatments (ploughing vs. direct drilling) is tested in this way against a background of the Phase II treatment effect already established, rather than against a random effect, it cannot be regarded as very reliable. There were insuf-®cient plots to quantify satisfactorily both the immedi-ate effect of Phase II cultivation treatments and the longer term effect inherited from Phase I treatments. However, the agreement between several Phase II measures that more nitrate was lost in drain¯ow from plots previously direct drilled strongly suggests that the eight years of tillage contrast in Phase I did in¯uence subsequent losses.
3.5. Crop yields and nitrogen contents at harvest
In 1991 the mean yield of wheat on plots 10 and 20 was 50% more than that of beans on plots 4 and 18, and in 1992 the mean yield of barley after beans was 17% greater than after wheat. Despite these expected (though non-signi®cant) differences, the total grain yields over the ®ve years were similar on all four plots (Table 9).
Table 8
Annual nitrate loadings in surface layer ¯ow (kg N haÿ1) normalised to unit volume of ¯ow (100 mm depth of water), Brimstone Farm 1988/
1989±1991/1992
Plot 1988/1989 1989/1990 1990/1991 1991/1992 Four year mean
4a(ST/P) 0.00 4.10 10.00 0.00 3.53
18 (ST/P)c 5.00 2.50 50.00b 50.00 26.88
10 (P)d 21.88 3.92 27.50 20.00 18.33
20a(P) 9.87 2.50 0.00 0.00 3.09
aDirect drilled in 1980±1987, other plots ploughed 1980±1987.
bFlow was <0.05 mm, so too small to show in Table 2.
cShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990 and 1991.
In the ®rst two years the mean amounts of N taken up in the grain were 14% greater on the ploughed plots than the shallow-tined, and the amounts taken up in straw were 17% greater. In the third year (1990/1991) the winter beans on plots 4 and 18 contained 87% more N in grain plus straw than the winter wheat on plots 10 and 20 (Table 9), even though they received no N fertiliser and the wheat received 179 kg N haÿ1
. This was presumably because of N ®xation by the beans. In 1991/1992 the total N uptake by winter barley after beans was 17% more than after winter wheat, similar to the difference in grain yields, but in 1992/1993 N uptakes were approximately the same on all four plots. Because of the differences in 1990/1991 and 1991/1992, the total uptakes (grain plus straw) over the ®ve years were 24% greater on plots 4 and 18 than on 10 and 20. However, none of these differences was signi®cant.
Differences in N uptakes of crops grown on land that had been ploughed or direct drilled in Phase I (1980±1987) were small in all years of Phase II except
1992/1993 (20% more on plots 4 and 20 than on 10 and 18) so that over the ®ve years only 1.5% (13 kg N haÿ1
) more was taken up in grain and straw from land previously direct drilled. Any extra N released by mineralisation of the additional organic matter conserved by eight years of direct drilling was therefore of little bene®t in terms of either crop N uptake or yield, except possibly in the ®fth year after the direct drilled plots were ploughed up when the spring oats crop received no fertiliser. By comparison, the difference in total N lost in drain¯owsurface layer ¯ow (Tables 3, 4 and 7) was 49% (41.0 kg haÿ1
) more from the land direct drilled in 1980±1987. This is more than three times the difference in total crop N uptake.
3.6. Soil mineral-N and winter N balances
In autumn 1988 the two plots (4 and 20) which had been direct drilled for the previous eight years con-tained a mean of 10 kg haÿ1
(58%) more mineral-N
Table 9
Grain yields (at 85% dry matter), total nitrogen uptakes and winter uptakes (sowing to spring fertiliser application) for selected plots in the Brimstone Experiment 1988/1989±1992/1993
Plots 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993 Totals
4a(ST/P) Grain yield (t haÿ1) and crop 5.88 (WO)d 7.64 (WW)e 3.86 (WBe)f 6.03 (WBa)g 3.95 (SO)h 27.36
N uptake grain (kg haÿ1) 70 155 298 152 43 718
N uptake straw (kg haÿ1) 23 15 107 84 11 240
N uptake winter (kg haÿ1) 24 18 7 51 ± 100
18 (ST/P)b Grain yield (t haÿ1) and crop 5.21 (WO)d 7.19 (WW)e 3.92 (WBe)f 6.89 (WBa)g 4.04 (SO)h 27.25
N uptake grain (kg haÿ1) 67 153 323 182 37 762
N uptake straw (kg haÿ1) 26 13 89 58 10 196
N uptake winter (kg haÿ1) 30 15 10 33 ± 88
10 (P)c Grain yield (t haÿ1) and crop 5.70 (WO)d 7.64 (WW)e 6.55 (WW)e 5.45 (WBa)g 2.02 (SO)h 27.36
N uptake grain (kg haÿ1) 93 158 183 148 34 616
N uptake straw (kg haÿ1) 29 14 42 46 11 142
N uptake winter (kg haÿ1) 31 13 42 54 ± 140
20a(P) Grain yield (t haÿ1) and crop 5.96 (WO)d 7.44 (WW)e 5.12 (WW)e 5.59 (WBa)g 2.80 (SO)h 26.91
N uptake grain (kg haÿ1) 90 166 191 133 45 625
N uptake straw (kg haÿ1) 31 16 22 79 11 159
N uptake winter (kg haÿ1) 29 17 46 58 ± 150
aDirect drilled in 1980±1987, other plots ploughed.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
cPloughed in all ®ve autumns.
dWinter oats.
eWinter wheat.
fWinter beans.
gWinter barley.
than plots 10 and 18, which had been ploughed for at least the same period (Table 10). Thereafter the amounts of nitrate-N generated by net mineralisation of soil organic matter (mineralisation minus immobi-lisation) during each winter period (harvest to spring fertiliser application) can be estimated from the bal-ance between (a) winter inputs, i.e. the amount of mineral-N in the soil at harvest (Table 10) plus the N in atmospheric deposition, non-symbiotic N ®xation and seed, and for 1991/1992 the N in residues of beans incorporated after harvest (Table 9), and (b) outputs, i.e. nitrate-N lost in drain¯ow (Table 3) and surface layer ¯ow (Table 7), the N in the crop just before fertiliser was applied in spring (Table 9), the amount of soil mineral-N just before spring fertiliser applica-tion (Table 10) plus N lost by denitri®caapplica-tion. The amount of N deposited from the atmosphere (17 kg N haÿ1
) was taken as half the annual value measured in 1985/1986 at Harwell, 15 km east of Brimstone Farm (Goulding, 1990). A value of 2 kg N haÿ1
was assumed for non-symbiotic N ®xa-tion during the winter by blue-green algae (Witty et al., 1979; Barry et al., 1993) and heterotrophs (Wild, 1988, p. 633), though with some allowance for
increased activity of heterotrophs because of the large clay and organic matter contents of the Brimstone soil. The amounts in seed (Table 11) were based on ana-lyses of the grain produced. The N lost by winter denitri®cation (7 kg N haÿ1
per year) was based on measurements of nitrous oxide emission in the pre-sence of acetylene from a drained plot at Brimstone Farm through the winter of 1980/1981 by Colbourn and Harper (1987).
Soil mineral-N is spatially very variable and, as each determination of mineral-N was based only on two cores or twelve bulked topsoil samples, the values used may not have been representative of the whole plot. Consequently, the values for net mineralisation given in Table 12, which are based on two mineral-N determinations, one in autumn and the other in spring, must be treated with caution, especially as the differ-ences between replicate plots are often larger than between treatments. Negative values for net miner-alisation in Table 12 indicate that immobilisation of N in the microbial biomass exceeded mineralisation.
Over the ®rst two years of Phase II (1988/1989 and 1989/1990), when there was a direct comparison between ploughing and shallow-tine cultivation, the
Table 10
Amounts of mineral-N (kg N haÿ1) in topsoil (0±20 cm) samples taken in spring (S) just before fertiliser application and after harvest (H) for
selected plots in the Brimstone Experiment 1988/1989±1992/1993
Plots 1988/1989 1989/1990 1990/1991 1991/1992 1992/1993
H S H S H S H S H S
4a(ST/P) 25 18 15 24 22 17 46 28 37 23
18 (ST/P)b 12 22 16 22 22 14 32 18 42 18
10 (P)c 21 18 13 25 24 15 19 14 30 21
20a(P) 27 27 15 20 31 16 46 19 56 15
aDirect-drilled in 1980±1987, other plots ploughed.
bShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
cPloughed in all ®ve autumns.
Table 11
Nitrogen content of seed sown at Brimstone Farm in Phase II
Year Crop Variety Sowing rate (kg haÿ1) N in seed (g kg±1) N in seed (kg haÿ1)
1988±1989 Winter oats Pennal 160 15.4 2.46
1989±1990 Winter wheat Pastiche 195 26.9 5.25
1990±1991 Winter wheat C2 Pastiche 209 26.9 5.62
1991±1992 Winter barley Marinka 184 20.9 3.85
mean net winter mineralisation was 10% greater on the ploughed plots than the shallow-tined (Table 12). The difference was 26% (not signi®cant) in the ®rst year, but zero in the second. However, after plots 4 and 18 were ploughed in autumn 1990, immobilisation unexpectedly exceeded mineralisation on these plots, giving small negative values for net mineralisation on both plots. On plots 10 and 20 (ploughed in all three years) net mineralisation was less than in the two previous winters, so the decrease in mineralisation of plots 4 and 18 may be partly attributable to seasonal weather or some other factor limiting mineralisation over all plots. In the winter of 1991/1992 the negative values for net mineralisation on plots 4 and 18 were much larger, even though the incorporated residues of winter beans must have added much mineralisable organic matter. However, in the ®nal year (1992/1993) the mean value for the same plots was positive
(27 kg N haÿ1
), indicating a considerable increase in net mineralisation, mainly on plot 4, yet on plots 10 and 20 there was a decrease in net mineralisation compared with 1991/1992. Over the whole ®ve year period the total net winter mineralisation was a mean of 124 kg N haÿ1
greater (not signi®cant) on plots 10 and 20 than on 4 and 18. These differences suggest that (a) shallow-tine cultivation decreased mineralisa-tion slightly compared with ploughing in the ®rst year; (b) subsequent ploughing and incorporation of bean residues did not immediately increase net mineralisa-tion on 4 and 18 Ð this was probably because of the low rainfall and dry soil conditions in 1990/1991 and 1991/1992 (Table 1); (c) the much wetter winter of 1992/1993 led on plot 4 to the largest calculated net winter mineralisation value of any plot or year (53 kg N haÿ1
), though this was not repeated on plot 18; and (d) despite the wet conditions in 1992/1993
Table 12
Calculation of net amounts of nitrate-N (kg haÿ1) generated by winter mineralisation of soil organic matter, Brimstone Farm, 1988/1989±
1992/1993a
Year and calculation Plots
4b(ST/P) 18 (ST/P)c 10 (P)d 20b(P)
1988/1989
Inputs 47 34 43 49
Outputs 54 66 65 76
Net N mineralisation (outputs±inputs) 7 32 22 27
1989/1990
Inputs 39 40 37 39
Outputs 72 64 67 66
Net N mineralisation (outputs±inputs) 33 24 30 27
1990/1991
Inputs 47 47 49 57
Outputs 45 39 67 75
Net N mineralisation (outputs±inputs) ÿ2 ÿ8 18 18
1991/1992
Inputs 176 144 42 69
Outputs 87 58 76 90
Net N mineralisation (outputs±inputs) ÿ89 ÿ86 34 21
1992/1993
Inputs 57 62 50 76
Outputs 110 63 63 78
Net N mineralisation (outputs±inputs) 53 1 13 2
Five year totals of net N winter mineralisation 2 ÿ37 117 95
aNegative values indicate immobilisation>mineralisation.
bDirect drilled in 1980±1987, other plots ploughed.
cShallow-tined in autumn 1988 and 1989, ploughed in autumn 1990±1992.
there was very little net mineralisation on plots 10 and 20. Taken together (c) and (d) suggest that the amount of easily mineralisable organic-N remaining in the soil at harvest 1992 was greater on plots 4 and 18 than on plots 10 and 20; this additional organic-N must have been derived either from the bean crop grown in 1990/ 1991 or from the organic matter conserved during the two years of shallow-tine cultivation (1988/1989 and 1989/1990).
Over the ®rst four years of Phase II the mean total net winter mineralisation on the two plots (10 and 18) which had been ploughed in the previous eight years was 33 kg N haÿ1
, 12 kg more than on the plots which were direct drilled in Phase I (Table 12). In the same four winters this contributed neither to increased losses of nitrate in drain¯owsurface layer ¯ow nor to increased N uptake by the crops in winter; in this period the ®rst was 9 kg N haÿ1
more on plots 4 and 20 (Tables 3 and 7) and the second was 11 kg N haÿ1 more on 4 and 20 (Table 9). However, summed over all ®ve years of Phase II (i.e. including the much wetter winter of 1992/1993) the net mineralisation was 8.5 kg N haÿ1
(21%) more on plots 4 and 20 than on 10 and 18, suggesting that much of the N in the additional organic matter conserved by direct drilling in Phase I was not mineralised until the ®fth year of Phase II. There was no winter crop in 1992/1993, so the N mineralised in that winter contributed mainly to the large loss of nitrate in drain¯ow, though the loss by denitri®cation could have been greater than the 7 kg N haÿ1
per year measured by Colbourn and Harper (1987).
4. Discussion
It is clear from the signi®cantly greater mean annual nitrate concentrations in drain¯ow and surface layer ¯ow and in 4-week loadings and mean concentrations of nitrate in drain¯ow from plots 10 and 20 in 1988/ 1989 and the non-signi®cant but often quite large differences in (a) percentages of drainwater samples exceeding the EU drinking water nitrate limit, (b) the winter and summer loadings of nitrate in drain¯ow, (c) the annual nitrate losses per unit volume of drain¯ow, (d) the annual loading of nitrate in surface layer ¯ow and (e) the annual nitrate losses per unit volume of surface layer ¯ow, all averaged over 1988/1989 and
1989/1990, that shallow-tine cultivation decreased nitrate losses compared with mouldboard ploughing. Shallow-tine cultivation probably decreases nitrate leaching by limiting the aeration of the soil and therefore the mineralisation of soil organic matter compared with ploughing. This was supported by the estimated amounts of net winter mineralisation, which were 20% greater on plots 10 and 20 than on 4 and 18 in 1988/1989. However, in the following year (the second of the direct ploughing/shallow-tine cul-tivation comparison), there was no difference in esti-mated net mineralisation, the 4-week winter loadings of nitrate in drain¯ow were signi®cantly greater on the shallow-tined plots, there was no signi®cant difference in 4-week mean concentrations, and the differences in winter loadings in drain¯ow, summer loadings in drain¯ow, annual loss per unit volume of drain¯ow, annual loadings in surface layer ¯ow and losses per unit volume of surface layer ¯ow were all much less than in 1988/1989. These results suggest that in the dry conditions of the early years of Phase II the bene®t of shallow-tine cultivation in decreasing mineralisa-tion and nitrate leaching was limited to the ®rst year. Further evidence for greater loss of nitrate after ploughing is provided by the increases noted after plots 4 and 18 were ploughed in 1990. In that year the annual mean concentrations and the 4-week loadings and mean concentrations in drain¯ow were signi®-cantly greater on plots 4 and 18, and there were non-signi®cant but quite large increases in winter loading in drain¯ow, summer loading in drain¯ow, annual losses per unit volume of drain¯ow and surface layer ¯ow and percentage of drainwater samples exceeding the EU limit on plots 4 and 18 compared with 10 and 20. However, N ®xation by the winter beans and the wide spacing of bean plants compared with wheat could have contributed to the greater losses on plots 4 and 18. In the fourth and ®fth years of Phase II, plots 4 and 18 continued to show small, non-signi®cant increases over plots 10 and 20 in many of these measures of nitrate leaching, but these could have resulted partly from the mineralisation of winter bean residues incorporated at harvest 1991.
annual concentrations over the ®ve years and in 4-week concentration values in two of the ®ve years, suggesting that ploughing up land which was pre-viously direct drilled increased nitrate leaching. Summed over all ®ve winters of Phase II, net miner-alisation of N was 8.5 kg haÿ1
greater on plots 4 and 20 than on 10 and 18, indicating that approximately 20% of the increased nitrate loss in drain¯owsurface layer ¯ow over the whole of Phase II (40.98 kg N haÿ1
) resulted from the additional mineralisation caused by ploughing land that was previously direct drilled. However, despite a large input of organic N in the form of the bean residues, none of this overall difference in mineralisation resulted from the ®rst four years of Phase II, which were unusually dry. It arose mainly in the much wetter winter of 1992/1993, ®ve years after the direct drilling regime ceased, and then almost entirely on only one of the two plots. This suggests that mineralisation of soil organic matter is a rather irregular process, largely but not entirely depen-dent on soil moisture, and which can be delayed for several years by dry conditions.
Compared with the clear pattern of nitrate loss in drain¯ow, that of nitrate loss in surface layer ¯ow showed less consistent effects of the Phase I and II cultivation treatments. In particular, three of the four measures of nitrate loss in surface layer ¯ow over the four years in which this was monitored showed much greater mean values on the plots that were ploughed in Phase I; two of these were measures of the actual nitrate concentration and the third (loss per unit volume of ¯ow) is a measure of overall nitrate con-centration. The fourth measure (mean annual loading) was approximately the same for both pairs of plots. The mineral-N contents of the surface soil horizon do not explain the greater concentrations in surface layer ¯ow from the plots previously ploughed, because over the ®rst four years of Phase II they were slightly greater at both harvest and spring samplings on the plots previously direct drilled (Table 10). Instead we suggest that the lateral movement of water through the topsoil of the plots previously ploughed was slower than through those previously direct drilled, thus allowing more time for the water to dissolve nitrate. This implies that some structural differences between soil which had been repeatedly ploughed or direct drilled in Phase I survived the changed cultivation regime through at least the earlier dry years of Phase II.
The very small differences in yield and total N uptakes of crops grown over the ®ve years of Phase II between land previously ploughed or direct drilled suggest that most of the nitrate released by miner-alisation of the additional organic matter conserved in the direct drilled soil in Phase I was lost by leaching or denitri®cation. For leaching this was con®rmed by the increase of 49% in nitrate loading in drain¯ow sur-face layer ¯ow from plots direct drilled in Phase I. It is likely that most of the mineralisation occurred at times in the autumn and winter when the crops could not take advantage of the nitrate released. The only year in which there was any bene®t to the crop was 1992/1993 when the spring oats were unfertilised and grown after a bare winter fallow, both of which would have maximised the effect of any differences in soil mineral-N resulting from mineralisation of organic matter conserved in Phase I.
In Phase I of the Brimstone Experiment, the mean annual nitrate loading in the drain¯ow from direct drilled plots was 26.7 kg N haÿ1
and that from ploughed plots was 34.9 kg N haÿ1
(Goss et al., 1993, Table 4), an increase of 31%. As this percentage is less than the increase of 49% in the mean annual nitrate loadings in drain¯ow resulting in Phase II from ploughing some of the plots that had been direct drilled in Phase I, it is likely that more nitrate is leached in drain¯ow in the long term from land that is direct drilled for a period and then ploughed for several years than from land which is ploughed every year. This may be because the regularly ploughed land is closer to a steady state with less production of entropy and less degradation of substances with large molecular weights (i.e. humus) to small molecules (i.e. nitrate) (Addiscott, 1995).
5. Conclusions
1. Ploughing increased nitrate losses in drain¯ow compared with shallow-tine cultivation in the ®rst two years of the experiment (1988/1989 and 1989/ 1990), but the effect was much stronger in the ®rst year than the second.
which had been ploughed in all three years. However, this could have been caused by a change of crop from winter cereals to winter beans as well as by mineralisation of additional soil organic matter conserved during the two years of shallow-tine cultivation.
3. The losses on the plots that had been shallow-tined in the ®rst two years continued to exceed those from the plots that had been ploughed so that after ®ve years the losses from the two pairs of plots were approximately the same.
4. In all ®ve years the mean nitrate losses for plots which had been direct drilled in the preceding eight years (1980±1987) exceeded those from plots that had been ploughed in the same eight years, suggesting that the additional organic matter con-served by the direct drilling continued to miner-alise for at least ®ve years. However, some of the mineralisation was probably delayed by dry conditions in the ®rst four years (1988/1989± 1991/1992). Nitrate formed by the additional mineralisation on the plots previously direct drilled did not bene®t the crops, probably because it was released at times when they could not take it up. 5. Comparison of the nitrate losses in drain¯ow
between different treatments in Phase I with those in Phase II suggests that in the long term more nitrate is lost from land that is direct drilled for several years and then ploughed annually for a further period than from land which is ploughed every year.
6. In the ®rst year of the comparison between ploughing and shallow-tine cultivation, the nitrate losses in surface layer ¯ow (surface runoff plus inter¯ow in the A horizon) were also greater on ploughed plots and, as with drain¯ow, the effect was less in the second year. However, unlike drain¯ow, the nitrate content of the surface layer ¯ow was scarcely increased in the third winter by ploughing plots that had been shallow-tined in the two previous years, and the concentration of nitrate in the surface layer ¯ow of plots that had been ploughed between 1980 and 1987 was greater than that of plots direct drilled in the same period. This suggests that the different topsoil structures resulting from the tillage con-trast of the 1980±1987 period were not completely destroyed by four years of ploughing in dry
conditions, and continued to in¯uence the rate of lateral water movement in the 1988±1993 period.
Acknowledgements
We thank the Ministry of Agriculture, Fisheries and Food for continuing ®nancial support for the Brim-stone Farm Experiment throughout the period con-sidered in this paper. IACR receives grant-aided support from the Biotechnology and Biological Sciences Research Council. D. Brockie (IACR) and T.J. Pepper (ADAS) provided technical support for ®eld and laboratory work.
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