Tillage caused dispersion of phosphorus and
soil in four 16-year old ®eld experiments
E. Sibbesen
a,1, F. Skjùth
b,2, G.H. Rub
ñ
k
a,*aDepartment of Crop Physiology and Soil Science, Danish Institute of Agricultural Sciences, Research Centre Foulum, PO Box 50, DK-8830 Tjele, Denmark
bDepartment 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 two-dimensional 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 m2per
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).
1Died on 23 October 1998.
2Present address: Veterinary and Milk Quality Department, Danish Dairy Board, Frederiks Alle 22, DK-8000 AÊ rhus, Denmark.
1. Introduction
Long-term ®eld experiments are needed to study long-term effects of agricultural management treat-ments 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 agro-ecosystems 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 phos-phorus, 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 Dvalues of 0.33 m2yrÿ1for a sandy loam and 0.42 m2yrÿ1for 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 m2yrÿ1along and across plots, most of the experi-ments 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 disper-sion only about 40% the applied heavy metals could be found in the soil within the original treated area of a plot.
2. Material and methods
2.1. Four 16-year old ®eld experiments on phosphorus
The experiments, having plots with different P addi-tion 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. Phos-phorus was given as evenly broadcasted superpho-sphate. 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 repli-cate samples were taken for each 0.5 m with a single gouge auger (3 cm diameter). In order to gain infor-mation on the ®eld heterogeneity additional plough layer samples were taken immediately after harvest in 1991 from a 0.70.7 m2square 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
plough layer with of a steel cylinder of 8.5 cm dia-meter, 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 mea-sured. 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 har-vested 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 haÿ1yrÿ1P, `3' refers to 30 kg haÿ1yrÿ1P, `4' refers to 75 kg haÿ1yrÿ1P every ®fth year, `5'
refers to 15 kg haÿ1yrÿ1P as liquid animal manure and `6' refers to 15 kg haÿ1yrÿ1P as solid animal manure. `A' indicates that the column
Table 1
Properties of plough layer soil in the four experiments
Askov LundgaÊrd Rùnhave édum
Soil classificationa Typic Agrudalf Orthic Haplohumod Typic Agrudalf Oxyagric Agriudoll
Plough layer depth (cm) 21 21 25 24
Clay (%) 12.2 5.0 13.6 10.4
Silt (%) 13.3 4.3 18.5 15.5
Fine sand (%) 33.7 22.6 43.4 48.0
Coarse sand (%) 39.5 65.5 21.8 23.6
Organic matter (%) 2.2 2.6 2.7 2.6
Dry bulk density (g cmÿ3) 1.46 1.36 1.42 1.48
aAccording to soil taxonomy.
Table 2
Tillage of four long-term ®eld experiments from spring 1975 to autumn 1991
Implement Working
depth (cm)
Travel speed (km hÿ1)
Travel directiona
Number of passages
Askov LundgaÊrd Rùnhave édumb
Mouldboard ploughc 21±24d 5±6 Along 16 16 12
25 6 Across 12
Rotary cultivator 9±10 3±4 Along 2
S-tine cultivatore 6±7 6±8 Along 16 12 18
Aslant 32 32 24 12
Seed drill 3±5 5±6 Along 8 12 12 12
Beet drill 3 3 Along 8 4
Inter-row cultivator 1.5±3 2.5±4 Along 27 9
C-tine cultivatorf 7±10 7±8 Along 2 13 12 11
Aslant 2 24 11
Beet lifter 3±6 2±4 Along 8 4
aTravel direction relative to the longest side of the plots. bStarted a year later in 1976.
cThe 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.
dPloughing depth was 21 cm at Askov and LundgaÊrd, 24 cm at édum and 25 cm at Rùnhave. eS-tine cultivator for seedbed preparation.
fC-tine cultivator for stubble cultivation.
Table 3
Net-P addition to plots with given treatment factor combinationa
1 2 3 4 5 6
A B A B A B A B A B A B
Askov ÿ352 ÿ355 ÿ118 ÿ134 117 99 ÿ47 ÿ62 ÿ123 ÿ144 ÿ121 ÿ130
LundgaÊrd ÿ147 ÿ156 71 42 314 283 168 136 56 44 62 49
Rùnhave ÿ374 ÿ378 ÿ139 ÿ151 91 70 ÿ71 ÿ94
édum ÿ310 ÿ325 ÿ104 ÿ127 104 94 ÿ117 ÿ132
aP level: `1' refers to no P fertilisation, `2' refers to 15 kg haÿ1yrÿ1P, `3' refers to 30 kg haÿ1yrÿ1P, `4' refers to 75 kg haÿ1yrÿ1P every
®fth year, `5' refers to 15 kg haÿ1yrÿ1P as liquid animal manure and `6' refers to 15 kg haÿ1yrÿ1P as solid animal manure. N level: `A' refers
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, dif-ferences 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): distancexfrom the centre of the band afterrtillage operations or tillage years. The parameter W repre-sents the width of the band,C0is the initial concen-tration in the band andDis the dispersion coef®cient.
S accounts for possible unidirectional shifts, which e.g. occur on sloping land. D has the dimension m2yrÿ1if x, Wand 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 con-stant diffusion coef®cient. The error function is de®ned as erf x 2=pRx
0exp ÿy2dy.
To give an impression of the relative signi®cance of
Dandrthe average movement of soil in one dimen-sion was calculated for differentDandr(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 ofDorr.
2.3. Fitting the dispersion model
We consider a generalisation of Eq. (1) with dis-persion in two dimensions (Sibbesen and Andersen, 1985) with repeated fertilisation and tillage opera-tions. The model gives an estimate of the concentra-tion of total soil P at any point in the ®eld at the time when the soil cores were sampled from the transects. LetMandNrepresent the number of columns and rows of the experimental ®eld, let the width of a plot
beWand the length beL(Fig. 2). Let (xm,yn) be the
co-ordinates of the plot centre in columnm and rown. 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 ofrtrepeated tillage operations is performed.
Then according to the model the current concentration in any point (xi,yi) is given as
parametersDxandSxrepresent the dispersion coef®-cient and the shift across columns, respectively,Dyand Syare analogously de®ned along columns. The
para-meter 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, 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 timet in years for different tillage intensities (D
concentrations: RSSP
i c xi;yi ÿ^c xi;yi2using 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 of the dispersion model:
@RSS=@aj ÿ2P
i ciÿ^ci@^ci=@aj, which are
fairly easy to derive from Eq. (2) and given here for completion.
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 concen-tration of total soil P in the plots at the start is therefore unknown. However, it is unlikely that the concentra-tion 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, letDPy,ij
denote the net total P (P added minus P removed with the harvested crop) added to plot i with treatment j
during yeary. Then the accumulated amount of total
P added to a given plot is DP;ijPtDPt;ij. The deviation from the average of plots with the same treatment isDP;ijÿDP;j. Assuming a constant initial concentration and no transport between plots, then these deviations should represent the random devia-tion between plots given the same treatment. However, the deviations were mostly systematic, i.e. they iden-ti®ed regions with concentrations below or above the treatment averages. Probably these regions had dif-ferent concentrations of total soil P from the start of the experiment. Assuming plotijis in columnmand rown, we useDP;ijÿDP;jas an estimate ofC0(xm, yn) the plot speci®c deviation from the initial ®eld
concentrationC0and let the inner summation in Eq. (2) run fromt0.
3. Results and discussion
3.1. Sensitivity analysis
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 trans-ects 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 haÿ1yrÿ1P) and P-depleted soil from treat-ment 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 estimatedDvalues are given in Table 4 together withSandC0values. TheDxvalues, representing the
soil dispersion for the dimension across the plots, differed relatively little between locations, whereas theDyvalues, representing the dimension along plots, differed much more. TheDyvalues 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 ofDyandDxrelative 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 Dyvalues were 0.34 and
0.72 m2per tillage year on average of the four loca-tions. Apart from tillage practice, D may depend on the chemical substance, which is dispersed, the soil taxon and upon the prevailing meteorological condi-tions within the time span of the experiment. The presentDvalues were clearly higher than the 0.13 and 0.24 m2per 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 Sibbe-sen et al. (1985) from Kofoed's (1960) model tillage experiments, 0.33 m2yrÿ1 for a sandy loam and 0.42 m2yrÿ1 for a sandy soil. We believe that con-temporary tillage practices (including type of machin-ery and travel speed across the ®eld) are the main Table 4
Estimated soil dispersion coef®cients (Dvalues), horizontal shifts (Svalues) and initial amounts of total P in plough layer soil (C0) of four long-term ®eld experiments on Pa95% con®dence intervals are shown in brackets.
Askov LundgaÊrd Rùnhave édum
Dx, across (m2/tillage year) 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) Dy, along (m2/tillage year) 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) Sx, across (m) ÿ1.01 (ÿ1.52 toÿ0.49) 2.22 (1.74 to 2.70) ÿ1.01 (ÿ1.49 toÿ0.52) ÿ1.55 (ÿ1.91 toÿ1.19) Sy, along (m) ÿ0.28 (ÿ1.47 toÿ0.91) ÿ0.97 (ÿ1.65 toÿ0.28) 1.10 (0.64 to 1.55) 0.40 (ÿ0.12 to 0.93) C0(total P kg haÿ1) 1888 (1868 to 1908) 1411 (1399 to 1422) 1906 (1896 to 1915) 2276 (2268 to 2284)
aOne tillage year includes one passage of mouldboard plough and other associated tillage operations (Table 2).
Table 5
Derived soil dispersion parameters from estimatedDvalues (Table 4)
Askov LundgaÊrd Rùnhave édum Mean
Dx
p , across columns (m yrÿ1/2) 0.53 0.62 0.54 0.64 0.58
Dy
p
, along columns (m yrÿ1/2) 1.22 0.70 0.56 0.92 0.85
DxDy
p
m2yrÿ1
reason for the increased soil movement found in this study.
Coef®cients of variation, based on square rootD, 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 ofDyand 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 estimatedDvalues (Fig. 4). Already after 5 years of tillage, 3±11% of the added P had left the net-plots. In 1991 when soils were sampled, the percen-tages left had increased to 18 in Askov, 16 in Lund-gaÊrd, 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 uncer-tain. Therefore, in general, it is not possible to relate
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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