214 L.A. Bouwman, W.B.M. Arts Applied Soil Ecology 14 2000 213–222
grasslands, compaction affects many physical and bio- logical soil properties and processes by various direct
and indirect mechanisms. To unravel these, we should know the relationships between the load of the passing
traffic and the resulting soil compaction, bulk density, root distribution over the soil profile, crop yield, and
finally the structure of the community of soil organ- isms and its functioning. This knowledge may help
farmers in their choice of machinery load, the width and the pressure of the tires and the frequency of traf-
fic, depending on soil type, weather conditions, and acceptable degree of compaction Arts et al., 1994.
Nematodes are involved in a wide range of activities and connections to other organisms in the soil food
web, comprising species feeding on living roots her- bivores H, on microorganisms bacterivores B;
fungivores F, and species feeding on larger organ- isms predators P or on mixed diets omnivores O.
Nematode communities in permanent grasslands are relatively stable with respect to numbers and have
a high species diversity. Numbers of specimens in trophic groups often decrease in the order BF
HOP Boström and Sohlenius, 1986. The various nematode trophic groups reflect important microbio-
logical and phytopathogenic processes such as organic matter decomposition, nutrient mineralization and her-
bivory. Although numbers of nematodes belonging to a particular feeding category are not always correlated
to the amount of specific food, numbers are correlated with process rates, e.g. numbers of bacterivorous ne-
matodes correlate with bacterial production Clarholm et al., 1981 and N mineralization rate, in particular,
in sandy soils with pore diameters between 30 and 90 mm Hassink et al., 1993, and herbivores correlate
with root growth, etc. Soil compaction affects nema- todes directly as well as indirectly. Vulnerable species
can be damaged during passage of the traffic Boag, 1985; decreased habitable pore space after passage
of the traffic affects probably all taxa except those that are intimately connected with root development, and
finally root dynamics in particular affects the numbers of herbivores and some root-generated microbivorous
taxa Griffiths et al., 1991; Bouwman et al., 1993. The effect of tillage and compaction regimes on total
numbers and dynamics of nematodes can therefore not be generalized, but feeding categories and taxa should
be examined individually Fortnum and Karlen, 1985; Parmelee and Aston, 1986; Thomas, 1978.
Trophic differences among nematode populations were observed to be indicative of variations in crop
yields Edwards, 1988; Edwards and Kimpinski, 1997 and also of decomposition and, in particular, N
mineralization processes Hassink et al., 1993.
A field experiment was carried out to study the relationships between traffic load and physical, bio-
logical and crop production characteristics of the soil. In 1988 the experiment Arts et al., 1994 was
established on a marine loamy sand soil in The Netherlands, where grass was sown on plots strips
previously tilled to a depth of 50 cm and consequently subjected to different degrees of soil compaction
over the whole surface of the plots. Soil physical characteristics as well as crop yields were measured
throughout the 5-year trial period and finally, in the last year of the experiment 1992, the distribution of
the roots over the soil profile and the nematode fauna were recorded in two plots with different degrees
of compaction. It was hypothesized that compaction may have positive as well as negative effects on
crop yield and thus, a site-specific optimum degree of soil compaction exists. Single season changes in
soil management regimes have little lasting effect on nematode communities Baird and Bernard, 1984;
Sohlenius and Sandor, 1989, but the nematode fauna in the upper 10 cm of the grass-sod of the experimen-
tal plots is assumed to be in a new equilibrium after 4 years of growth under experimental conditions.
2. Materials and methods
2.1. The experimental site The research was carried out at the Oostwaardhoeve
experimental farm longitude 52
◦
18
′
N, latitude 4
◦
85
′
E in the Wieringermeer, a reclaimed polder, used for agricultural production since 1934. The soil is a cal-
careous loamy 30 silt marine sand M50: 50–105, containing 5 organic matter Arts et al., 1994.
2.2. The establishment of the field experiment In April 1988 an area of ca. 2 ha, 180 m×120 m,
was tilled with a rotodigger to a depth of 50 cm to loosen the soil, resulting in a loose layer of 60 cm
depth. The field was divided into three blocks of
L.A. Bouwman, W.B.M. Arts Applied Soil Ecology 14 2000 213–222 215
equal size, on which four plots strips were assigned 180 m×2 m to four loading treatments: 0, 4.5, 8.5
and 14.5 t. For this purpose, a loading frame with a steel roller, 2 m wide and 1.2 m in diameter, carrying
the various loads passed over the plots; the roller was drawn by a tractor with 3 m trail-width. After the
first compaction treatment perennial ryegrass, Lolium perenne mixture BG3 was sown, and harvested up
to four times annually. After each harvest the specific traffic loads passed over the various plots. A load of
1 t exerts a pressure of 100–500 kg per dm
2
, depend- ing on the actual compactability of the upper soil
layer e.g. dry versus wet conditions. The experiment was extended with different rates of
nitrogen fertilization 0, 140, 280 and 420 kg N ha
− 1
per year within various subplots. 2.3. Physical measurements
Over the period 1988–1992, soil porosity, perme- ability and penetrability were reduced. To characterize
the soil condition, measurements were carried out at different depths, after each harvest: thickness of the
initially loosened layer, soil bulk density, penetration resistance, and the soil moisture content measured
weekly.
To measure the thickness of the loosened layer, in each plot five tiles 30 cm×30 cm were buried on top
of the undisturbed subsoil, i.e. at a depth of 60 cm, after rotodigging in 1988. The thickness of the soil layer
above these tiles was measured as well as the position of the tile with regard to a fixed reference point. This
latter measurement was necessary to correct the first one for sinking of the tile.
Soil bulk density was measured in the 15–20 and 30–35 cm layers below the soil surface, in undisturbed
ring samples containing 100 ml soil. The penetration resistance was measured with a
Bush Penetrometer conus type B, ASAE Standards, 1993. This parameter, expressed as average val-
ues of 10 penetrations per event in the soil layers 0–5, 5–10, 10–15 cm, etc. indicates the pressures
roots have to exert to overcome soil compaction, effective root-zone depth Gregory, 1988, as well
as soil porosity and compaction Groenevelt et al., 1984.
Soil moisture contents ww in different soil layers, including the 0–5 cm layer, were measured weekly in
mixed samples, originating from 10 subsamples, by overnight drying 20 g soil at 105
◦
C. 2.4. Crop measurements
Grass yield and distribution of grass roots over the soil profile were measured. Crop responses are likely
to be directly related to properties such as pore size distribution, permeability, aeration, etc. and indirectly
to bulk density and compaction.
Grass dry matter yield was determined after each harvest of 20 m×1.2 m strips. Annual grass yields are
the sums of three or four harvests. The distribution of the roots over the soil profile
was measured once by the pin-board method Böhm, 1979 in June 1992, in the 4.5 and 14.5 t treatments,
fertilized with 280 kg N ha
− 1
per year. 2.5. Nematode densities
Nematodes were isolated from the soil in 1992, the fifth year of the experiment, in March, June,
September, and December, in the 4.5 and 14.5 t treatments 280 kg N. The organisms were isolated
from 3×100 g fresh soil from the upper 10 cm with Oostenbrinks’s elutriation method Jacob and Van
Bezooyen, 1984. Total numbers were counted and expressed per 100 g dry soil; the figures presented are
averages of three samples. Sixty specimens per sam- ple were identified to genus level and assigned to the
following feeding types: bacterivores Rhabditidae; Cephalobidae, including genera such as Cephalobus,
Acrobeloides, Acrobeles, Chiloplacus but also a small number of non-cephalobid Teratocephalidae; Monhys-
teridae including a small number of Prismatolaimidae; Plectidae; Panagrolaimidae; Alaimidae, fungivores
Aphelenchoididae including mainly Aphelenchoides and few Aphelenchus avenae; Tylenchidae; Psilenchi-
dae, herbivores Paratylenchidae; Pratylenchidae and omnivorespredators Dorylaimidae.
2.6. Statistical analysis Effects of compaction were tested by analysis of
variance ANOVA: all pairwise multiple compar- ison procedures were carried out according to the
Student–Newman–Keuls method.
216 L.A. Bouwman, W.B.M. Arts Applied Soil Ecology 14 2000 213–222
3. Results
