Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol54.Issue1-2.Mar2000:
Soil & Tillage Research 54 (2000) 91±100
Tillage caused dispersion of phosphorus and
soil in four 16-year old ®eld experiments
E. Sibbesena,1, F. Skjùthb,2, G.H. Rubñka,*
a
Department of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences,
Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
b
Department of Agricultural Systems and Land Use, Danish Institute of Agricultural Sciences,
Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
Received 19 July 1999; received in revised form 16 November 1999; accepted 6 December 1999
Abstract
Long-term ®eld experiments are important for studies of the long-term effects of agricultural management practices.
Unfortunately tillage caused dispersion of soil from plot to plot is a serious problem in such experiments if the plots are
uncon®ned and tillage takes place across plot borders. The extent of this problem is only documented in few relatively old
®eld experiments and these do not re¯ect present day tillage operations. In this study, four 16-year old ®eld experiments with
different phosphorus fertilisation treatments were used to quantify the contemporary extent of this problem. A twodimensional dispersion model ®tted well to measurements of total soil P content in transects across and along plots of the four
experiments. We found that tillage caused soil dispersion across and along the plots on average were 0.34 and 0.72 m2 per
tillage year. This is signi®cantly higher than found in previous studies, re¯ecting that contemporary tillage operations move
soil more around than previous tillage practices. Already after 5 years of tillage, 3±11% of the net added P had left the plots.
After 15 or 16 years of experimentation, at the time of soil sampling this had increased to 14±18%. We therefore conclude,
that contemporary tillage operations move soil around to an extent, which is not compatible with experimental designs having
no permanent borders between plots and we recommend that designs of new long-term ®eld experiments take this into
account. Regarding existing old long-term ®eld experiment with uncon®ned plots, it is important to acknowledge the fact that
soil movement between plots has taken place and that accumulated treatment effects therefore are seriously blurred. Relating
treatments to responses in soil and crops may therefore be seriously wrong. However, it is still possible to utilise the actual
differences of measured parameters between plots in such experiments and relate these to each other. Therefore, in spite of the
problems with soil dispersion between plots, such old long-term ®eld experiments still play an important role as living soil
archives providing important material and information for a wide range of process-oriented studies. # 2000 Elsevier Science
B.V. All rights reserved.
Keywords: Long-term ®eld experiments; Tillage; Soil movement; Carry-over; Border effects; Tillage erosion; Modelling
*
Corresponding author. Tel.: 45-89-99-18-59; fax: 45-89-99-17-19.
E-mail address: [email protected] (G.H. Rubñk).
1
Died on 23 October 1998.
2
Ê rhus, Denmark.
Present address: Veterinary and Milk Quality Department, Danish Dairy Board, Frederiks Alle 22, DK-8000 A
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 0 8 8 - X
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E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
1. Introduction
Long-term ®eld experiments are needed to study
long-term effects of agricultural management treatments on soils and crops. Unfortunately, a number of
experimental problems are associated with such
experiments. The most severe one is probably the
exchange of soil between plots. Exchange of soil
between plots can not only be mediated by tillage,
wind and water erosion, but also by faunal activity.
Such soil movement can blur the treatment derived
effects in the plough layer soil, e.g. by increasing or
decreasing concentrations of nutrients, organic matter
or disease organisms. Clearly, this has implications for
our understanding of long-term processes in the agroecosystems under study. In the oldest long-term ®eld
experiments with non-con®ned plots, this exchange of
soil between plots has probably reached a stage where
little of the original plough layer soil has remained
inside the same plots (Christensen, 1989). However,
even 10-year old ®eld experiments may have this
problem too.
The problem of soil movement in long-term ®eld
experiments has been known for many years (Kofoed,
1960; Warren and Johnston, 1967; Smith, 1971;
Dorph-Petersen, 1972; MacDonald and Peck, 1976;
Hiroche et al., 1979). Nevertheless, the problem is
overlooked in general by research people and maybe
even neglected by some. Clearly it takes great courage
to realise this problem after having spent most of the
lifetime on a long-term ®eld experiment. Often clear
crop colour differences and sharp boundaries between
differently treated plots are referred to as a proof of
limited soil exchange. However, such differences and
boundaries may just as well be caused by different
nitrogen fertilisation rates.
It is dif®cult to quantify the soil movement over
time. Tracers that follow the moving soil are needed.
Long-term ®eld experiments with additions of phosphorus, heavy metals or other substances that bind
strongly to the soil can be used to study the process if
information is available on yearly addition rates and
crop removals of the substances.
Kofoed (1960) measured the movement of 32P
labelled superphosphate across 3 m wide plots
during 2 days of tillage equivalent to normal
tillage for 7 years. Inspired by Kofoeds results,
Sibbesen et al. (1985) and Sibbesen and Andersen
(1985) developed a simple two-dimensional model for
tillage induced dispersion of soil and accumulating
substances. The model describes the development
with time of a concentration gradient of substance,
by the means of the solution to a diffusion equation.
The model is in agreement with the central limit
theorem, when it is used for the situation where
the same cultivations are repeated many times
in alternating directions. The model includes a
diffusion coef®cient, D, which has the dimension
m2 per tillage operation or tillage year. Fitting the
model to Kofoed's (1960) data yielded D values of
0.33 m2 yrÿ1 for a sandy loam and 0.42 m2 yrÿ1 for a
coarse sandy soil.
Sibbesen (1986) subsequently used the model to
simulate tillage derived soil and substance dispersion
in 21 more than 50-year old ®eld experiments and
estimated the mean content of original plot soil
remaining in the plots. Assuming D values of
0.4 m2 yrÿ1 along and across plots, most of the experiments had less than 30% of the original plot soil left in
the central quarter of the plots.
McGrath and Lane (1989) were the ®rst to ®t the
model to real ®eld conditions. They examined the
dispersion of heavy metals across plot borders in ``The
Market Garden Experiment'', Woburn, England,
where metal contaminated sewage sludge had been
applied from 1942 to 1961. The model ®tted well
to the observed dispersion of metals and produced
D values of 0.24 and 0.13 m2 per standard tillage
operation parallel and perpendicular to the ploughing
direction, respectively. By considering the tillage
caused soil dispersion, McGrath and Lane (1989)
were able to account for about 80% of the heavy
metals applied. Without considering the soil dispersion only about 40% the applied heavy metals could
be found in the soil within the original treated area
of a plot.
The D values estimated by McGrath and Lane
(1989) mainly re¯ect the tillage practice current
before 1974. To estimate the movement of soil under
contemporary tillage practices, we examined four,
current long-term ®eld experiments on phosphorus
(P). The soil dispersion in these experiments is
expected to re¯ect the effects of more modern tillage
operations.
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
2. Material and methods
2.1. Four 16-year old ®eld experiments on
phosphorus
The experiments, having plots with different P addition rates, were located at Askov, LundgaÊrd, Rùnhave
and édum Experimental Stations in Denmark. The
®rst three experiments started in 1975 and the last in
1976, so they had run for 16 and 15 years, respectively,
when the soil movement study was initiated.
Treatments and layout are shown in Fig. 1. Phosphorus was given as evenly broadcasted superphosphate. To gain information on the soil dispersion
across plot boundaries soil samples were taken from
the plough layer along two crossing transects in spring
1991 before seedbed preparation (Fig. 1). Two replicate samples were taken for each 0.5 m with a single
gouge auger (3 cm diameter). In order to gain information on the ®eld heterogeneity additional plough
layer samples were taken immediately after harvest in
1991 from a 0.70.7 m2 square in the centre of each
plot, 13 samples were taken per plot and pooled. At the
same time, dry bulk densities of plough layer soil were
determined by sampling intact soil cores from the
93
plough layer with of a steel cylinder of 8.5 cm diameter, 12 cores per location. The cores were dried and
sieved (2 mm) and the dry weight of sieved soil
expressed relative to the sample volume were measured. All samples were taken to the ploughing depth
at each site (Table 1). Total P content was determined
in all the augered samples with a perchloric acid
method (Rubaek and Sibbesen, 1993). Texture and
volume weights of the plough layer soils are shown in
Table 1, where also the soil types according to soil
taxonomy are indicated.
Information was obtained from the four locations
on the tillage performed, including type of implement,
working depth, travelling speed, travelling direction
and number of operations for each implement (Table 2).
In Rùnhave and édum, mouldboard ploughing had
been done only for 12 years due to intervening grass
leys. Information was obtained also on fertilisation of
each plot and the guarding areas surrounding the plots
and on harvested crop yield and crop P concentration
of each plot. Net P additions to the plots are given in
Table 3.
Based on the information on P added to and harvested from both the plots and the guarding areas and
the number of tillage years during the experimental
Fig. 1. Layout and treatments of four long-term ®eld experiments on phosphorus. The lines in each experiment indicate the position of soil
sampling transects. Plot dimensions (m) are indicated in the lower right plots of each experiment. Numbers in plots indicate P treatment. `1'
refers to no P fertilisation, `2' refers to 15 kg haÿ1 yrÿ1 P, `3' refers to 30 kg haÿ1 yrÿ1 P, `4' refers to 75 kg haÿ1 yrÿ1 P every ®fth year, `5'
refers to 15 kg haÿ1 yrÿ1 P as liquid animal manure and `6' refers to 15 kg haÿ1 yrÿ1 P as solid animal manure. `A' indicates that the column
below was fertilised with the normal N rate and `B' indicates fertilisation with 50% more than normal N rate.
94
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
Table 1
Properties of plough layer soil in the four experiments
a
Soil classification
Plough layer depth (cm)
Clay (%)
Silt (%)
Fine sand (%)
Coarse sand (%)
Organic matter (%)
Dry bulk density (g cmÿ3)
a
Askov
LundgaÊrd
Rùnhave
édum
Typic Agrudalf
21
12.2
13.3
33.7
39.5
2.2
1.46
Orthic Haplohumod
21
5.0
4.3
22.6
65.5
2.6
1.36
Typic Agrudalf
25
13.6
18.5
43.4
21.8
2.7
1.42
Oxyagric Agriudoll
24
10.4
15.5
48.0
23.6
2.6
1.48
According to soil taxonomy.
Table 2
Tillage of four long-term ®eld experiments from spring 1975 to autumn 1991
Implement
Working
depth (cm)
Mouldboard ploughc
Travel speed
(km hÿ1)
Rotary cultivator
S-tine cultivatore
21±24d
25
9±10
6±7
5±6
6
3±4
6±8
Seed drill
Beet drill
Inter-row cultivator
C-tine cultivatorf
3±5
3
1.5±3
7±10
5±6
3
2.5±4
7±8
Beet lifter
3±6
2±4
Travel
directiona
Along
Across
Along
Along
Aslant
Along
Along
Along
Along
Aslant
Along
Number of passages
Askov
LundgaÊrd
16
16
Rùnhave
édumb
12
12
16
32
8
8
27
2
2
8
32
12
4
9
13
12
24
12
2
18
12
12
12
24
11
11
4
a
Travel direction relative to the longest side of the plots.
Started a year later in 1976.
c
The plough base width was 37 cm in Askov, LundgaÊrd and Rùnhave most years and 42 cm in édum. A reversible plough type was used
in Askov, Rùnhave and édum. A non-reversible plough type was used in LundgaÊrd most years.
d
Ploughing depth was 21 cm at Askov and LundgaÊrd, 24 cm at édum and 25 cm at Rùnhave.
e
S-tine cultivator for seedbed preparation.
f
C-tine cultivator for stubble cultivation.
b
Table 3
Net-P addition to plots with given treatment factor combinationa
1
Askov
LundgaÊrd
Rùnhave
édum
2
3
4
5
6
A
B
A
B
A
B
A
B
A
B
A
B
ÿ352
ÿ147
ÿ374
ÿ310
ÿ355
ÿ156
ÿ378
ÿ325
ÿ118
71
ÿ139
ÿ104
ÿ134
42
ÿ151
ÿ127
117
314
91
104
99
283
70
94
ÿ47
168
ÿ71
ÿ117
ÿ62
136
ÿ94
ÿ132
ÿ123
56
ÿ144
44
ÿ121
62
ÿ130
49
P level: `1' refers to no P fertilisation, `2' refers to 15 kg haÿ1 yrÿ1 P, `3' refers to 30 kg haÿ1 yrÿ1 P, `4' refers to 75 kg haÿ1 yrÿ1 P every
®fth year, `5' refers to 15 kg haÿ1 yrÿ1 P as liquid animal manure and `6' refers to 15 kg haÿ1 yrÿ1 P as solid animal manure. N level: `A' refers
to normal N fertilisation and `B' refers to fertilisation with 50% more than normal N rate.
a
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
95
period, the soil dispersion model was ®tted to the
measured total soil P content along the transects and in
the plot centres as outlined below. Furthermore, differences in total soil P content between individual
replicate plots and treatment means were assumed to
represent the variation of total soil P content between
plots, when the experiments started.
2.2. Fundamental dispersion model
Tillage caused, one-dimensional dispersion of soil
particles originally positioned uniformly in a band,
can be approximated with the equation suggested by
Sibbesen et al. (1985):
C0
W=2 x S
p
erf
c x; r
2
2 Dr
W=2 ÿ x ÿ S
p
(1)
erf
2 Dr
where c(x, r) is the concentration of soil particles in a
distance x from the centre of the band after r tillage
operations or tillage years. The parameter W represents the width of the band, C0 is the initial concentration in the band and D is the dispersion coef®cient.
S accounts for possible unidirectional shifts, which
e.g. occur on sloping land. D has the dimension
m2 yrÿ1 if x, W and S are in metres and r in years.
It is noted that Eq. (1) is a slightly modi®ed solution of
the one-dimensional diffusion equation with a constant diffusion coef®cient.
R x The error function is
de®ned as erf x 2=p 0 exp ÿy2 dy.
To give an impression of the relative signi®cance of
D and r the average movement of soil in one dimension was calculated for different D and r (Fig. 1). The
average movement doubles, when either D or r
increases four times, i.e. the average movement is
proportional to the square root of D or r.
2.3. Fitting the dispersion model
We consider a generalisation of Eq. (1) with dispersion in two dimensions (Sibbesen and Andersen,
1985) with repeated fertilisation and tillage operations. The model gives an estimate of the concentration of total soil P at any point in the ®eld at the time
when the soil cores were sampled from the transects.
Let M and N represent the number of columns and
rows of the experimental ®eld, let the width of a plot
Fig. 2. Average movement, x(4Dtpÿ1)1/2 in metres, across a
vertical plane of soil particles, originally positioned in the plane, as
a function of time t in years for different tillage intensities (D
values (m2 yrÿ1) indicated on the curves).
be W and the length be L (Fig. 2). Let (xm, yn) be the coordinates of the plot centre in column m and row n.
Assume that an amount of Ct(xm, yn) total soil P
(kg haÿ1) is uniformly distributed to the plot with
centre in (xm, yn) at time t. After this disposition a
number of rt repeated tillage operations is performed.
Then according to the model the current concentration
in any point (xi, yi) is given as
c xi ; yi C0
M X
N X
T
X
1
Ct xm ; yn
4
Ax
Bx
erf p erf p
2 Dx rt
2 Dx rt
(
!
!)
Ay
By
(2)
erf p erf p
2 D y rt
2 Dy rt
m1 n1 t1
where AxW/2(xmÿxi)Sx, BxW/2ÿ(xmÿxi)ÿSx,
AyL/2(ynÿyi)Sy and ByL/2(ynÿyi)ÿSy. The
parameters Dx and Sx represent the dispersion coef®cient and the shift across columns, respectively, Dy and
Sy are analogously de®ned along columns. The parameter C0 is the initial background concentration of
total soil P in the experimental ®eld. These are the
parameters of the model to be estimated.
2.4. Parameter estimation
The model was ®tted by minimising the residual sum
of squares between the observed, c, and estimated, ^c,
96
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
P
concentrations: RSS i c xi ; yi ÿ ^c xi ; yi 2 using
the gradient method of Levenberg±Marquardt (Press
et al., 1988), which when converged gives the
estimated hessian and thereby an estimate of the
parameter covariance matrix. The method requires
the calculation of partial derivatives of RSS with
respect to the parameters of the dispersion model.
These gradients are expressed in terms of the
partial derivatives
P of the dispersion model:
@RSS=@aj ÿ2 i ci ÿ ^ci @^ci =@aj , which are
fairly easy to derive from Eq. (2) and given here
for completion.
T
N X
M X
X
@^c xi ; yi
1
1
Ct xm ; yn p
ÿ
@Dx
8
pD3x rt
m1 n1 t1
A2x
B2x
Ax exp ÿ
Bx exp ÿ
4Dx rt
4Dx rt
(
!
!)
Ay
By
erf p erf p
2 Dy rt
2 D y rt
M X
N X
T
@^c xi ; yi X
1
1
Ct xm ; yn p
@Sx
4
pDx rt
m1 n1 t1
2
Ax
B2x
ÿ exp ÿ
exp ÿ
4Dx rt
4Dx rt
(
!
!)
Ay
By
erf p erf p
2 Dy rt
2 Dy rt
As the dispersion parameters are non-negative, the
model was ®tted with respect to the logarithmic
transformed dispersion parameters to avoid boundary
problems.
2.5. Initial ®eld heterogeneity
No plough layer soil samples were available from
the start of these experiments and the initial concentration of total soil P in the plots at the start is therefore
unknown. However, it is unlikely that the concentration was constant from plot to plot as total P normally
varies over ®elds, even within short distances.
Besides, the Askov ®eld had been used for other
experiments previously.
To estimate the initial ®eld heterogeneity, let DPy,ij
denote the net total P (P added minus P removed with
the harvested crop) added to plot i with treatment j
during year y. Then the accumulated amount of total
P
P added to a given plot is DP;ij t DPt;ij . The
deviation from the average of plots with the same
treatment is DP;ij ÿ DP;j . Assuming a constant initial
concentration and no transport between plots, then
these deviations should represent the random deviation between plots given the same treatment. However,
the deviations were mostly systematic, i.e. they identi®ed regions with concentrations below or above the
treatment averages. Probably these regions had different concentrations of total soil P from the start of
the experiment. Assuming plot ij is in column m and
row n, we use DP;ij ÿ DP;j as an estimate of C0(xm,
yn) the plot speci®c deviation from the initial ®eld
concentration C0 and let the inner summation in Eq.
(2) run from t 0.
3. Results and discussion
3.1. Sensitivity analysis
The model was ®tted to the data from each experimental site under varying assumptions and initial
conditions. The estimation routine requires initial
values for each parameter, but sensitivity analyses
showed that the outcome was independent of the
choice of initial values. We also investigated how
the outcome was affected by exclusion of the total
soil P data from the plot centres. Inclusion of these
observations is attractive as it gives information from
the entire ®eld, but since part of the plot centre
observations are positioned far from the transects
(Fig. 1), the plot centre observations could potentially be very in¯uential. However, we found that
the in¯uence was generally small compared with
the variance on the estimates. The guarding areas
were grown with the same crops as the plots, but
fertilised according to some reference rate. Most
likely, the tillage has moved soil and P from this area
to the experimental plots and vice versa. However,
analyses showed that inclusion or exclusion of soil
transport from the guarding areas had very little effect
on the results. We also investigated the effect of
including initial soil heterogeneity as suggested
above. As expected it had some in¯uence on the
obtained parameter estimates, hence it makes a difference whether or not the experimental ®eld is
assumed homogenous initially.
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
97
Fig. 3. Observed and ®tted total P of plough layer soil along transects at Askov, LundgaÊrd, Rùnhave and édum across (left) and along (right)
plots. Dotted lines indicate plot borders. Numbers 1±6 are treatments (see Fig. 1). The horizontal `staircase' lines indicate the expected P level
in the treatments assuming that no tillage caused soil dispersion had occurred. The small steps in the across ®gures are due to small variations
in yields in the two N treatments.
98
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
Table 4
Estimated soil dispersion coef®cients (D values), horizontal shifts (S values) and initial amounts of total P in plough layer soil (C0) of four
long-term ®eld experiments on Pa 95% con®dence intervals are shown in brackets.
LundgaÊrd
Askov
Dx, across (m2/tillage year)
Dy, along (m2/tillage year)
Sx, across (m)
Sy, along (m)
C0 (total P kg haÿ1)
a
Rùnhave
édum
0.28 (0.17 to 0.44)
0.39 (0.23 to 0.67)
0.29 (0.14 to 0.63)
0.41 (0.29 to 0.59)
1.48 (0.78 to 2.81)
0.49 (0.29 to 0.83)
0.31 (0.12 to 0.77)
0.84 (0.55 to 1.28)
ÿ1.01 (ÿ1.52 to ÿ0.49)
2.22 (1.74 to 2.70)
ÿ1.01 (ÿ1.49 to ÿ0.52) ÿ1.55 (ÿ1.91 to ÿ1.19)
ÿ0.28 (ÿ1.47 to ÿ0.91) ÿ0.97 (ÿ1.65 to ÿ0.28)
1.10 (0.64 to 1.55)
0.40 (ÿ0.12 to 0.93)
1888 (1868 to 1908)
1411 (1399 to 1422)
1906 (1896 to 1915)
2276 (2268 to 2284)
One tillage year includes one passage of mouldboard plough and other associated tillage operations (Table 2).
3.2. Fitting the model
The following results were obtained by ®tting the
two-dimensional model to the complete data sets
including total P in plough layer soil along the transects and in the plot centres and also including guarding
areas and assuming initial soil heterogeneity. Fig. 3
shows that the model ®tted well to the transect data. It
also shows how the tillage caused soil dispersion has
smoothed the treatment-induced soil-P differences
between plots as P-enriched soil from treatment no.
3 (30 kg haÿ1 yrÿ1 P) and P-depleted soil from treatment no. 1 (P unmanured) has been gradually mixed
with each other and the other treatments. Without this
dispersion the total-P content should have followed
the staircase pattern indicated in Fig. 3 along and
across the plots which has levels of steps below that
®tted in unmanured plots (treatment no. 1) and above
in P-enriched plots (treatment no. 3).
The estimated D values are given in Table 4 together
with S and C0 values. The Dx values, representing the
soil dispersion for the dimension across the plots,
differed relatively little between locations, whereas
the Dy values, representing the dimension along plots,
differed much more. The Dy values were much higher
than the Dx values in Askov and édum and a little
higher in LundgaÊrd. However, in Rùnhave they were
almost identical. The reason for this may be that the
direction of mouldboard ploughing was along the
plots in Askov, LundgaÊrd and édum but across in
Rùnhave. However, apart from this, it is dif®cult to
explain the order or variation of Dy and Dx relative to
the soil types and tillage of the different locations
(Tables 1 and 2).
The soil dispersion across the plots was calculated
based on the square root of the D values (Table 5),
which is proportional to the soil dispersion. Soil
dispersion across columns was about two-thirds of
that along and the Dx and Dy values were 0.34 and
0.72 m2 per tillage year on average of the four locations. Apart from tillage practice, D may depend on
the chemical substance, which is dispersed, the soil
taxon and upon the prevailing meteorological conditions within the time span of the experiment. The
present D values were clearly higher than the 0.13 and
0.24 m2 per tillage round (mouldboard ploughing and
seedbed preparation) across and along plots reported
by McGrath and Lane (1989). On average the soil
dispersion in our study was 60±70% higher than that
found in their study. Furthermore, our average D
values were higher than the ones estimated by Sibbesen et al. (1985) from Kofoed's (1960) model tillage
experiments, 0.33 m2 yrÿ1 for a sandy loam and
0.42 m2 yrÿ1 for a sandy soil. We believe that contemporary tillage practices (including type of machinery and travel speed across the ®eld) are the main
Table 5
Derived soil dispersion parameters from estimated D values (Table 4)
p
ÿ1/2
x, across columns (m yrÿ1/2 )
pD
)
D
,
along
columns
(m
yr
y
p
Dx Dy m2 yrÿ1
Askov
LundgaÊrd
Rùnhave
édum
Mean
0.53
1.22
0.64
0.62
0.70
0.44
0.54
0.56
0.30
0.64
0.92
0.59
0.58
0.85
0.49
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
Fig. 4. Proportion of total P net-addition remaining in the net-plots
at Askov, LundgaÊrd, Rùnhave and édum as a function of tillage
years calculated from estimated D values (Table 4).
reason for the increased soil movement found in this
study.
Coef®cients of variation, based on square root D,
were 9% across and 29% along. This variation is
caused by differences in tillage and soil type, but it
is dif®cult to point out the speci®c cause of the
variation observed.
Based on the square root of the products of Dy and
Dx, the soil dispersion was greatest at Askov followed
by édum, LundgaÊrd and Rùnhave. The proportion of
accumulating or depleting P, remaining in the net-plot
at a given time, was calculated from plot dimensions
and estimated D values (Fig. 4). Already after 5 years
of tillage, 3±11% of the net-added P had left the netplots. In 1991 when soils were sampled, the percentages left had increased to 18 in Askov, 16 in LundgaÊrd, 17 in Rùnhave and 14 in édum.
3.3. What to do about the soil movement problem?
Contemporary tillage operations unfortunately
move soil to such a degree that it simply is not
compatible with long-term ®eld experiments with
no barriers between plots. In current long-term ®eld
experiments which have run for a long time, little can
be done about it. It is of course possible to calculate
the extent of soil movement and then estimate the
``true'' level of various soil and crop parameters,
assuming that no soil movement had taken place,
but the interpretation of such data is obviously uncertain. Therefore, in general, it is not possible to relate
99
directly the measured parameters to the treatments as
such. However, it is still possible to utilise actual
differences of measured parameters between plots
and relate them to each other, e.g. measured soil P
content to crop response. The function as a living soil
archive is still embedded in such long-term ®eld
experiments. They are important sources for a wide
range of process oriented experiments.
For new long-term ®eld experiments, which has to
run for more than 4±5 years and where normal tillage
is necessary, ploughing and harrowing simply should
not be allowed to cross plot borders. A system of
ridges, which should never be ploughed or harrowed,
could separate the plots. Alternatively, very large
gross-plots are needed where only a small inner net
plot is used for actual experimentation (4±5 years). In
addition, no tillage or minimum tillage systems could
be employed, but this changes the top soil conditions
relative to normally tilled soil, and may therefore not
be suitable for many types of experiments.
4. Conclusions
We conclude, that tillage operations move soil
around to an extent, which is not compatible with
experimental designs having no permanent borders
between plots, and we recommend that designs of new
long-term ®eld experiments take this into account.
References
Christensen, B.T., 1989. Askov 1894±1989: research on animal
manure and mineral fertilizers. Proceedings of the Sanborn
Field Centennial. University of Missouri-Columbia, SR-415,
pp. 28±48.
Dorph-Petersen, K., 1972. Markforsùg. Statens Planteavlsudvalg.
Lyngby, Denmark, 429 pp. (in Danish.)
Hiroche, R., de Miranda, L.T., Soares, E., 1979. Corn plot
experiment contamination from fertilization of adjacent plots.
Revista Brasileira de CieÂncia do Solo 3, 190±193.
Kofoed, A.D., 1960. The effect of soil tillage on the movement of
superphosphate in soil using labelled phosphorus. Tidsskrift for
Planteavl 63, 285±306 (in Danish with English summary.)
MacDonald, G.E., Peck, N.H., 1976. Border effects in a long-term
fertility experiment. Agro. J. 68, 530±532.
McGrath, S.P., Lane, P.W., 1989. An explanation for the apparent
losses of metals in a long-term ®eld experiment with sewage
sludge. Environ. Pollution 60, 235±256.
Press, W.H., Flannery, B.P., Teukolsky, S.A., Vetterling, W.T.,
100
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
1988. Numerical Recipes in C, The Art of Scienti®c
Computing. Cambridge University Press, Cambridge, MA.
Rubaek, G.H., Sibbesen, E., 1993. Resin extraction of labile, soil
organic phosphorus. J. Soil Sci. 44, 467±478.
Sibbesen, E., 1986. Soil movement in long-term ®eld experiments.
Plant and Soil 91, 73±85.
Sibbesen, E., Andersen, C.E., 1985. Soil movement in long-term
®eld experiments as a result of cultivations II. How to estimate
the two-dimensional movement of substances accumulating in
the soil. Exp. Agric. 21, 109±117.
Sibbesen, E., Andersen, C.E., Andersen, S., Flensted-Jensen, M.,
1985. Soil movement in long-term ®eld experiments as a result
of cultivations I. A model for approximating soil movement in
one horizontal dimension by repeated tillage. Exp. Agric. 21,
101±107.
Smith, A.N., 1971. Lateral movement of phosphate in soil during
cultivation. J. Aust. Inst. Agric. Sci. 37, 352±354.
Warren R.G., Johnston, A.E., 1967. Hoos®eld continuous Barley.
Rothamsted Experimental Station Report for 1966, pp. 320±
338.
Tillage caused dispersion of phosphorus and
soil in four 16-year old ®eld experiments
E. Sibbesena,1, F. Skjùthb,2, G.H. Rubñka,*
a
Department of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences,
Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
b
Department of Agricultural Systems and Land Use, Danish Institute of Agricultural Sciences,
Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
Received 19 July 1999; received in revised form 16 November 1999; accepted 6 December 1999
Abstract
Long-term ®eld experiments are important for studies of the long-term effects of agricultural management practices.
Unfortunately tillage caused dispersion of soil from plot to plot is a serious problem in such experiments if the plots are
uncon®ned and tillage takes place across plot borders. The extent of this problem is only documented in few relatively old
®eld experiments and these do not re¯ect present day tillage operations. In this study, four 16-year old ®eld experiments with
different phosphorus fertilisation treatments were used to quantify the contemporary extent of this problem. A twodimensional dispersion model ®tted well to measurements of total soil P content in transects across and along plots of the four
experiments. We found that tillage caused soil dispersion across and along the plots on average were 0.34 and 0.72 m2 per
tillage year. This is signi®cantly higher than found in previous studies, re¯ecting that contemporary tillage operations move
soil more around than previous tillage practices. Already after 5 years of tillage, 3±11% of the net added P had left the plots.
After 15 or 16 years of experimentation, at the time of soil sampling this had increased to 14±18%. We therefore conclude,
that contemporary tillage operations move soil around to an extent, which is not compatible with experimental designs having
no permanent borders between plots and we recommend that designs of new long-term ®eld experiments take this into
account. Regarding existing old long-term ®eld experiment with uncon®ned plots, it is important to acknowledge the fact that
soil movement between plots has taken place and that accumulated treatment effects therefore are seriously blurred. Relating
treatments to responses in soil and crops may therefore be seriously wrong. However, it is still possible to utilise the actual
differences of measured parameters between plots in such experiments and relate these to each other. Therefore, in spite of the
problems with soil dispersion between plots, such old long-term ®eld experiments still play an important role as living soil
archives providing important material and information for a wide range of process-oriented studies. # 2000 Elsevier Science
B.V. All rights reserved.
Keywords: Long-term ®eld experiments; Tillage; Soil movement; Carry-over; Border effects; Tillage erosion; Modelling
*
Corresponding author. Tel.: 45-89-99-18-59; fax: 45-89-99-17-19.
E-mail address: [email protected] (G.H. Rubñk).
1
Died on 23 October 1998.
2
Ê rhus, Denmark.
Present address: Veterinary and Milk Quality Department, Danish Dairy Board, Frederiks Alle 22, DK-8000 A
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 0 0 ) 0 0 0 8 8 - X
92
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
1. Introduction
Long-term ®eld experiments are needed to study
long-term effects of agricultural management treatments on soils and crops. Unfortunately, a number of
experimental problems are associated with such
experiments. The most severe one is probably the
exchange of soil between plots. Exchange of soil
between plots can not only be mediated by tillage,
wind and water erosion, but also by faunal activity.
Such soil movement can blur the treatment derived
effects in the plough layer soil, e.g. by increasing or
decreasing concentrations of nutrients, organic matter
or disease organisms. Clearly, this has implications for
our understanding of long-term processes in the agroecosystems under study. In the oldest long-term ®eld
experiments with non-con®ned plots, this exchange of
soil between plots has probably reached a stage where
little of the original plough layer soil has remained
inside the same plots (Christensen, 1989). However,
even 10-year old ®eld experiments may have this
problem too.
The problem of soil movement in long-term ®eld
experiments has been known for many years (Kofoed,
1960; Warren and Johnston, 1967; Smith, 1971;
Dorph-Petersen, 1972; MacDonald and Peck, 1976;
Hiroche et al., 1979). Nevertheless, the problem is
overlooked in general by research people and maybe
even neglected by some. Clearly it takes great courage
to realise this problem after having spent most of the
lifetime on a long-term ®eld experiment. Often clear
crop colour differences and sharp boundaries between
differently treated plots are referred to as a proof of
limited soil exchange. However, such differences and
boundaries may just as well be caused by different
nitrogen fertilisation rates.
It is dif®cult to quantify the soil movement over
time. Tracers that follow the moving soil are needed.
Long-term ®eld experiments with additions of phosphorus, heavy metals or other substances that bind
strongly to the soil can be used to study the process if
information is available on yearly addition rates and
crop removals of the substances.
Kofoed (1960) measured the movement of 32P
labelled superphosphate across 3 m wide plots
during 2 days of tillage equivalent to normal
tillage for 7 years. Inspired by Kofoeds results,
Sibbesen et al. (1985) and Sibbesen and Andersen
(1985) developed a simple two-dimensional model for
tillage induced dispersion of soil and accumulating
substances. The model describes the development
with time of a concentration gradient of substance,
by the means of the solution to a diffusion equation.
The model is in agreement with the central limit
theorem, when it is used for the situation where
the same cultivations are repeated many times
in alternating directions. The model includes a
diffusion coef®cient, D, which has the dimension
m2 per tillage operation or tillage year. Fitting the
model to Kofoed's (1960) data yielded D values of
0.33 m2 yrÿ1 for a sandy loam and 0.42 m2 yrÿ1 for a
coarse sandy soil.
Sibbesen (1986) subsequently used the model to
simulate tillage derived soil and substance dispersion
in 21 more than 50-year old ®eld experiments and
estimated the mean content of original plot soil
remaining in the plots. Assuming D values of
0.4 m2 yrÿ1 along and across plots, most of the experiments had less than 30% of the original plot soil left in
the central quarter of the plots.
McGrath and Lane (1989) were the ®rst to ®t the
model to real ®eld conditions. They examined the
dispersion of heavy metals across plot borders in ``The
Market Garden Experiment'', Woburn, England,
where metal contaminated sewage sludge had been
applied from 1942 to 1961. The model ®tted well
to the observed dispersion of metals and produced
D values of 0.24 and 0.13 m2 per standard tillage
operation parallel and perpendicular to the ploughing
direction, respectively. By considering the tillage
caused soil dispersion, McGrath and Lane (1989)
were able to account for about 80% of the heavy
metals applied. Without considering the soil dispersion only about 40% the applied heavy metals could
be found in the soil within the original treated area
of a plot.
The D values estimated by McGrath and Lane
(1989) mainly re¯ect the tillage practice current
before 1974. To estimate the movement of soil under
contemporary tillage practices, we examined four,
current long-term ®eld experiments on phosphorus
(P). The soil dispersion in these experiments is
expected to re¯ect the effects of more modern tillage
operations.
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
2. Material and methods
2.1. Four 16-year old ®eld experiments on
phosphorus
The experiments, having plots with different P addition rates, were located at Askov, LundgaÊrd, Rùnhave
and édum Experimental Stations in Denmark. The
®rst three experiments started in 1975 and the last in
1976, so they had run for 16 and 15 years, respectively,
when the soil movement study was initiated.
Treatments and layout are shown in Fig. 1. Phosphorus was given as evenly broadcasted superphosphate. To gain information on the soil dispersion
across plot boundaries soil samples were taken from
the plough layer along two crossing transects in spring
1991 before seedbed preparation (Fig. 1). Two replicate samples were taken for each 0.5 m with a single
gouge auger (3 cm diameter). In order to gain information on the ®eld heterogeneity additional plough
layer samples were taken immediately after harvest in
1991 from a 0.70.7 m2 square in the centre of each
plot, 13 samples were taken per plot and pooled. At the
same time, dry bulk densities of plough layer soil were
determined by sampling intact soil cores from the
93
plough layer with of a steel cylinder of 8.5 cm diameter, 12 cores per location. The cores were dried and
sieved (2 mm) and the dry weight of sieved soil
expressed relative to the sample volume were measured. All samples were taken to the ploughing depth
at each site (Table 1). Total P content was determined
in all the augered samples with a perchloric acid
method (Rubaek and Sibbesen, 1993). Texture and
volume weights of the plough layer soils are shown in
Table 1, where also the soil types according to soil
taxonomy are indicated.
Information was obtained from the four locations
on the tillage performed, including type of implement,
working depth, travelling speed, travelling direction
and number of operations for each implement (Table 2).
In Rùnhave and édum, mouldboard ploughing had
been done only for 12 years due to intervening grass
leys. Information was obtained also on fertilisation of
each plot and the guarding areas surrounding the plots
and on harvested crop yield and crop P concentration
of each plot. Net P additions to the plots are given in
Table 3.
Based on the information on P added to and harvested from both the plots and the guarding areas and
the number of tillage years during the experimental
Fig. 1. Layout and treatments of four long-term ®eld experiments on phosphorus. The lines in each experiment indicate the position of soil
sampling transects. Plot dimensions (m) are indicated in the lower right plots of each experiment. Numbers in plots indicate P treatment. `1'
refers to no P fertilisation, `2' refers to 15 kg haÿ1 yrÿ1 P, `3' refers to 30 kg haÿ1 yrÿ1 P, `4' refers to 75 kg haÿ1 yrÿ1 P every ®fth year, `5'
refers to 15 kg haÿ1 yrÿ1 P as liquid animal manure and `6' refers to 15 kg haÿ1 yrÿ1 P as solid animal manure. `A' indicates that the column
below was fertilised with the normal N rate and `B' indicates fertilisation with 50% more than normal N rate.
94
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
Table 1
Properties of plough layer soil in the four experiments
a
Soil classification
Plough layer depth (cm)
Clay (%)
Silt (%)
Fine sand (%)
Coarse sand (%)
Organic matter (%)
Dry bulk density (g cmÿ3)
a
Askov
LundgaÊrd
Rùnhave
édum
Typic Agrudalf
21
12.2
13.3
33.7
39.5
2.2
1.46
Orthic Haplohumod
21
5.0
4.3
22.6
65.5
2.6
1.36
Typic Agrudalf
25
13.6
18.5
43.4
21.8
2.7
1.42
Oxyagric Agriudoll
24
10.4
15.5
48.0
23.6
2.6
1.48
According to soil taxonomy.
Table 2
Tillage of four long-term ®eld experiments from spring 1975 to autumn 1991
Implement
Working
depth (cm)
Mouldboard ploughc
Travel speed
(km hÿ1)
Rotary cultivator
S-tine cultivatore
21±24d
25
9±10
6±7
5±6
6
3±4
6±8
Seed drill
Beet drill
Inter-row cultivator
C-tine cultivatorf
3±5
3
1.5±3
7±10
5±6
3
2.5±4
7±8
Beet lifter
3±6
2±4
Travel
directiona
Along
Across
Along
Along
Aslant
Along
Along
Along
Along
Aslant
Along
Number of passages
Askov
LundgaÊrd
16
16
Rùnhave
édumb
12
12
16
32
8
8
27
2
2
8
32
12
4
9
13
12
24
12
2
18
12
12
12
24
11
11
4
a
Travel direction relative to the longest side of the plots.
Started a year later in 1976.
c
The plough base width was 37 cm in Askov, LundgaÊrd and Rùnhave most years and 42 cm in édum. A reversible plough type was used
in Askov, Rùnhave and édum. A non-reversible plough type was used in LundgaÊrd most years.
d
Ploughing depth was 21 cm at Askov and LundgaÊrd, 24 cm at édum and 25 cm at Rùnhave.
e
S-tine cultivator for seedbed preparation.
f
C-tine cultivator for stubble cultivation.
b
Table 3
Net-P addition to plots with given treatment factor combinationa
1
Askov
LundgaÊrd
Rùnhave
édum
2
3
4
5
6
A
B
A
B
A
B
A
B
A
B
A
B
ÿ352
ÿ147
ÿ374
ÿ310
ÿ355
ÿ156
ÿ378
ÿ325
ÿ118
71
ÿ139
ÿ104
ÿ134
42
ÿ151
ÿ127
117
314
91
104
99
283
70
94
ÿ47
168
ÿ71
ÿ117
ÿ62
136
ÿ94
ÿ132
ÿ123
56
ÿ144
44
ÿ121
62
ÿ130
49
P level: `1' refers to no P fertilisation, `2' refers to 15 kg haÿ1 yrÿ1 P, `3' refers to 30 kg haÿ1 yrÿ1 P, `4' refers to 75 kg haÿ1 yrÿ1 P every
®fth year, `5' refers to 15 kg haÿ1 yrÿ1 P as liquid animal manure and `6' refers to 15 kg haÿ1 yrÿ1 P as solid animal manure. N level: `A' refers
to normal N fertilisation and `B' refers to fertilisation with 50% more than normal N rate.
a
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
95
period, the soil dispersion model was ®tted to the
measured total soil P content along the transects and in
the plot centres as outlined below. Furthermore, differences in total soil P content between individual
replicate plots and treatment means were assumed to
represent the variation of total soil P content between
plots, when the experiments started.
2.2. Fundamental dispersion model
Tillage caused, one-dimensional dispersion of soil
particles originally positioned uniformly in a band,
can be approximated with the equation suggested by
Sibbesen et al. (1985):
C0
W=2 x S
p
erf
c x; r
2
2 Dr
W=2 ÿ x ÿ S
p
(1)
erf
2 Dr
where c(x, r) is the concentration of soil particles in a
distance x from the centre of the band after r tillage
operations or tillage years. The parameter W represents the width of the band, C0 is the initial concentration in the band and D is the dispersion coef®cient.
S accounts for possible unidirectional shifts, which
e.g. occur on sloping land. D has the dimension
m2 yrÿ1 if x, W and S are in metres and r in years.
It is noted that Eq. (1) is a slightly modi®ed solution of
the one-dimensional diffusion equation with a constant diffusion coef®cient.
R x The error function is
de®ned as erf x 2=p 0 exp ÿy2 dy.
To give an impression of the relative signi®cance of
D and r the average movement of soil in one dimension was calculated for different D and r (Fig. 1). The
average movement doubles, when either D or r
increases four times, i.e. the average movement is
proportional to the square root of D or r.
2.3. Fitting the dispersion model
We consider a generalisation of Eq. (1) with dispersion in two dimensions (Sibbesen and Andersen,
1985) with repeated fertilisation and tillage operations. The model gives an estimate of the concentration of total soil P at any point in the ®eld at the time
when the soil cores were sampled from the transects.
Let M and N represent the number of columns and
rows of the experimental ®eld, let the width of a plot
Fig. 2. Average movement, x(4Dtpÿ1)1/2 in metres, across a
vertical plane of soil particles, originally positioned in the plane, as
a function of time t in years for different tillage intensities (D
values (m2 yrÿ1) indicated on the curves).
be W and the length be L (Fig. 2). Let (xm, yn) be the coordinates of the plot centre in column m and row n.
Assume that an amount of Ct(xm, yn) total soil P
(kg haÿ1) is uniformly distributed to the plot with
centre in (xm, yn) at time t. After this disposition a
number of rt repeated tillage operations is performed.
Then according to the model the current concentration
in any point (xi, yi) is given as
c xi ; yi C0
M X
N X
T
X
1
Ct xm ; yn
4
Ax
Bx
erf p erf p
2 Dx rt
2 Dx rt
(
!
!)
Ay
By
(2)
erf p erf p
2 D y rt
2 Dy rt
m1 n1 t1
where AxW/2(xmÿxi)Sx, BxW/2ÿ(xmÿxi)ÿSx,
AyL/2(ynÿyi)Sy and ByL/2(ynÿyi)ÿSy. The
parameters Dx and Sx represent the dispersion coef®cient and the shift across columns, respectively, Dy and
Sy are analogously de®ned along columns. The parameter C0 is the initial background concentration of
total soil P in the experimental ®eld. These are the
parameters of the model to be estimated.
2.4. Parameter estimation
The model was ®tted by minimising the residual sum
of squares between the observed, c, and estimated, ^c,
96
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
P
concentrations: RSS i c xi ; yi ÿ ^c xi ; yi 2 using
the gradient method of Levenberg±Marquardt (Press
et al., 1988), which when converged gives the
estimated hessian and thereby an estimate of the
parameter covariance matrix. The method requires
the calculation of partial derivatives of RSS with
respect to the parameters of the dispersion model.
These gradients are expressed in terms of the
partial derivatives
P of the dispersion model:
@RSS=@aj ÿ2 i ci ÿ ^ci @^ci =@aj , which are
fairly easy to derive from Eq. (2) and given here
for completion.
T
N X
M X
X
@^c xi ; yi
1
1
Ct xm ; yn p
ÿ
@Dx
8
pD3x rt
m1 n1 t1
A2x
B2x
Ax exp ÿ
Bx exp ÿ
4Dx rt
4Dx rt
(
!
!)
Ay
By
erf p erf p
2 Dy rt
2 D y rt
M X
N X
T
@^c xi ; yi X
1
1
Ct xm ; yn p
@Sx
4
pDx rt
m1 n1 t1
2
Ax
B2x
ÿ exp ÿ
exp ÿ
4Dx rt
4Dx rt
(
!
!)
Ay
By
erf p erf p
2 Dy rt
2 Dy rt
As the dispersion parameters are non-negative, the
model was ®tted with respect to the logarithmic
transformed dispersion parameters to avoid boundary
problems.
2.5. Initial ®eld heterogeneity
No plough layer soil samples were available from
the start of these experiments and the initial concentration of total soil P in the plots at the start is therefore
unknown. However, it is unlikely that the concentration was constant from plot to plot as total P normally
varies over ®elds, even within short distances.
Besides, the Askov ®eld had been used for other
experiments previously.
To estimate the initial ®eld heterogeneity, let DPy,ij
denote the net total P (P added minus P removed with
the harvested crop) added to plot i with treatment j
during year y. Then the accumulated amount of total
P
P added to a given plot is DP;ij t DPt;ij . The
deviation from the average of plots with the same
treatment is DP;ij ÿ DP;j . Assuming a constant initial
concentration and no transport between plots, then
these deviations should represent the random deviation between plots given the same treatment. However,
the deviations were mostly systematic, i.e. they identi®ed regions with concentrations below or above the
treatment averages. Probably these regions had different concentrations of total soil P from the start of
the experiment. Assuming plot ij is in column m and
row n, we use DP;ij ÿ DP;j as an estimate of C0(xm,
yn) the plot speci®c deviation from the initial ®eld
concentration C0 and let the inner summation in Eq.
(2) run from t 0.
3. Results and discussion
3.1. Sensitivity analysis
The model was ®tted to the data from each experimental site under varying assumptions and initial
conditions. The estimation routine requires initial
values for each parameter, but sensitivity analyses
showed that the outcome was independent of the
choice of initial values. We also investigated how
the outcome was affected by exclusion of the total
soil P data from the plot centres. Inclusion of these
observations is attractive as it gives information from
the entire ®eld, but since part of the plot centre
observations are positioned far from the transects
(Fig. 1), the plot centre observations could potentially be very in¯uential. However, we found that
the in¯uence was generally small compared with
the variance on the estimates. The guarding areas
were grown with the same crops as the plots, but
fertilised according to some reference rate. Most
likely, the tillage has moved soil and P from this area
to the experimental plots and vice versa. However,
analyses showed that inclusion or exclusion of soil
transport from the guarding areas had very little effect
on the results. We also investigated the effect of
including initial soil heterogeneity as suggested
above. As expected it had some in¯uence on the
obtained parameter estimates, hence it makes a difference whether or not the experimental ®eld is
assumed homogenous initially.
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
97
Fig. 3. Observed and ®tted total P of plough layer soil along transects at Askov, LundgaÊrd, Rùnhave and édum across (left) and along (right)
plots. Dotted lines indicate plot borders. Numbers 1±6 are treatments (see Fig. 1). The horizontal `staircase' lines indicate the expected P level
in the treatments assuming that no tillage caused soil dispersion had occurred. The small steps in the across ®gures are due to small variations
in yields in the two N treatments.
98
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
Table 4
Estimated soil dispersion coef®cients (D values), horizontal shifts (S values) and initial amounts of total P in plough layer soil (C0) of four
long-term ®eld experiments on Pa 95% con®dence intervals are shown in brackets.
LundgaÊrd
Askov
Dx, across (m2/tillage year)
Dy, along (m2/tillage year)
Sx, across (m)
Sy, along (m)
C0 (total P kg haÿ1)
a
Rùnhave
édum
0.28 (0.17 to 0.44)
0.39 (0.23 to 0.67)
0.29 (0.14 to 0.63)
0.41 (0.29 to 0.59)
1.48 (0.78 to 2.81)
0.49 (0.29 to 0.83)
0.31 (0.12 to 0.77)
0.84 (0.55 to 1.28)
ÿ1.01 (ÿ1.52 to ÿ0.49)
2.22 (1.74 to 2.70)
ÿ1.01 (ÿ1.49 to ÿ0.52) ÿ1.55 (ÿ1.91 to ÿ1.19)
ÿ0.28 (ÿ1.47 to ÿ0.91) ÿ0.97 (ÿ1.65 to ÿ0.28)
1.10 (0.64 to 1.55)
0.40 (ÿ0.12 to 0.93)
1888 (1868 to 1908)
1411 (1399 to 1422)
1906 (1896 to 1915)
2276 (2268 to 2284)
One tillage year includes one passage of mouldboard plough and other associated tillage operations (Table 2).
3.2. Fitting the model
The following results were obtained by ®tting the
two-dimensional model to the complete data sets
including total P in plough layer soil along the transects and in the plot centres and also including guarding
areas and assuming initial soil heterogeneity. Fig. 3
shows that the model ®tted well to the transect data. It
also shows how the tillage caused soil dispersion has
smoothed the treatment-induced soil-P differences
between plots as P-enriched soil from treatment no.
3 (30 kg haÿ1 yrÿ1 P) and P-depleted soil from treatment no. 1 (P unmanured) has been gradually mixed
with each other and the other treatments. Without this
dispersion the total-P content should have followed
the staircase pattern indicated in Fig. 3 along and
across the plots which has levels of steps below that
®tted in unmanured plots (treatment no. 1) and above
in P-enriched plots (treatment no. 3).
The estimated D values are given in Table 4 together
with S and C0 values. The Dx values, representing the
soil dispersion for the dimension across the plots,
differed relatively little between locations, whereas
the Dy values, representing the dimension along plots,
differed much more. The Dy values were much higher
than the Dx values in Askov and édum and a little
higher in LundgaÊrd. However, in Rùnhave they were
almost identical. The reason for this may be that the
direction of mouldboard ploughing was along the
plots in Askov, LundgaÊrd and édum but across in
Rùnhave. However, apart from this, it is dif®cult to
explain the order or variation of Dy and Dx relative to
the soil types and tillage of the different locations
(Tables 1 and 2).
The soil dispersion across the plots was calculated
based on the square root of the D values (Table 5),
which is proportional to the soil dispersion. Soil
dispersion across columns was about two-thirds of
that along and the Dx and Dy values were 0.34 and
0.72 m2 per tillage year on average of the four locations. Apart from tillage practice, D may depend on
the chemical substance, which is dispersed, the soil
taxon and upon the prevailing meteorological conditions within the time span of the experiment. The
present D values were clearly higher than the 0.13 and
0.24 m2 per tillage round (mouldboard ploughing and
seedbed preparation) across and along plots reported
by McGrath and Lane (1989). On average the soil
dispersion in our study was 60±70% higher than that
found in their study. Furthermore, our average D
values were higher than the ones estimated by Sibbesen et al. (1985) from Kofoed's (1960) model tillage
experiments, 0.33 m2 yrÿ1 for a sandy loam and
0.42 m2 yrÿ1 for a sandy soil. We believe that contemporary tillage practices (including type of machinery and travel speed across the ®eld) are the main
Table 5
Derived soil dispersion parameters from estimated D values (Table 4)
p
ÿ1/2
x, across columns (m yrÿ1/2 )
pD
)
D
,
along
columns
(m
yr
y
p
Dx Dy m2 yrÿ1
Askov
LundgaÊrd
Rùnhave
édum
Mean
0.53
1.22
0.64
0.62
0.70
0.44
0.54
0.56
0.30
0.64
0.92
0.59
0.58
0.85
0.49
E. Sibbesen et al. / Soil & Tillage Research 54 (2000) 91±100
Fig. 4. Proportion of total P net-addition remaining in the net-plots
at Askov, LundgaÊrd, Rùnhave and édum as a function of tillage
years calculated from estimated D values (Table 4).
reason for the increased soil movement found in this
study.
Coef®cients of variation, based on square root D,
were 9% across and 29% along. This variation is
caused by differences in tillage and soil type, but it
is dif®cult to point out the speci®c cause of the
variation observed.
Based on the square root of the products of Dy and
Dx, the soil dispersion was greatest at Askov followed
by édum, LundgaÊrd and Rùnhave. The proportion of
accumulating or depleting P, remaining in the net-plot
at a given time, was calculated from plot dimensions
and estimated D values (Fig. 4). Already after 5 years
of tillage, 3±11% of the net-added P had left the netplots. In 1991 when soils were sampled, the percentages left had increased to 18 in Askov, 16 in LundgaÊrd, 17 in Rùnhave and 14 in édum.
3.3. What to do about the soil movement problem?
Contemporary tillage operations unfortunately
move soil to such a degree that it simply is not
compatible with long-term ®eld experiments with
no barriers between plots. In current long-term ®eld
experiments which have run for a long time, little can
be done about it. It is of course possible to calculate
the extent of soil movement and then estimate the
``true'' level of various soil and crop parameters,
assuming that no soil movement had taken place,
but the interpretation of such data is obviously uncertain. Therefore, in general, it is not possible to relate
99
directly the measured parameters to the treatments as
such. However, it is still possible to utilise actual
differences of measured parameters between plots
and relate them to each other, e.g. measured soil P
content to crop response. The function as a living soil
archive is still embedded in such long-term ®eld
experiments. They are important sources for a wide
range of process oriented experiments.
For new long-term ®eld experiments, which has to
run for more than 4±5 years and where normal tillage
is necessary, ploughing and harrowing simply should
not be allowed to cross plot borders. A system of
ridges, which should never be ploughed or harrowed,
could separate the plots. Alternatively, very large
gross-plots are needed where only a small inner net
plot is used for actual experimentation (4±5 years). In
addition, no tillage or minimum tillage systems could
be employed, but this changes the top soil conditions
relative to normally tilled soil, and may therefore not
be suitable for many types of experiments.
4. Conclusions
We conclude, that tillage operations move soil
around to an extent, which is not compatible with
experimental designs having no permanent borders
between plots, and we recommend that designs of new
long-term ®eld experiments take this into account.
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