216 L.A. Bouwman, W.B.M. Arts Applied Soil Ecology 14 2000 213–222
3. Results
3.1. Physical measurements The thickness of the initially loosened layer of
60 cm depth had already stabilized during the first year 1988 of the experiment, and only fluctuated season-
ally, due to moisture content in the subsequent years. Thickness decreased monotonously with load from
0 → 14.5 t, it changed from 55 → 43 cm Fig. 1a.
Soil bulk density, not measured in the sod but in the 15–20 and 30–35 cm depth layers, increased
with load treatment during the first 2 years, and sub- sequently stabilized; in the 0 t treatment 15–20 cm
layer bulk density only increased slightly in the last 2 years of the experiment 1.10 → 1.15 g cm
− 3
. In the 4.5 t treatment the bulk density in the 15–20 cm
layer increased from 1.10 g cm
− 3
in the loosened soil in 1988 to 1.41 g cm
− 3
in 1992. The resistance to penetration in the various soil lay-
ers varied between seasons along with soil moisture, with the highest values in summer dry and the lowest
in winter wet conditions. The resistance increased in all load treatments from 1988 to 1989 and then
stabilized Fig. 1b.The resistance also increased with soil depth but only in the upper soil layers. Resistance
was highest in the 14.5 t treatment and lowest in the 0 t treatment. In the 0–5 cm layer the lowest values
were measured in the 0 t treatment in winter 1991, 0.55 MPa and the highest in the 14.5 t treatment in
summer 1991 4 MPa; Fig. 1b; the latter resistance was much too high to be broken by monocotyledons
Dexter, 1986.
Soil moisture content in the 0–5 cm layer showed a considerable increase parallel to the weight of the
load-traffic Fig. 1c. Moisture contents in the 14.5 t treatment were up to 30 higher than in the 0 t treat-
ment. In winter moisture contents in the 14.5 t treat- ment in the upper 5 cm soil layer approached to 50
ww, decreasing down to 10 in summer.
3.2. Crop measurements Above ground dry matter yields as affected by
N-application and load-treatment are presented in Fig. 2. The effect of the load-treatment was signif-
icant p0.01 and the N-responseload-treatment interaction effect was also significant p0.01, but
not when values for 368 kg N and 0 t load were left out of the calculations; this means that in practice the
loss of yield due to soil compaction was independent of the level of N application Keen, 1993.
In all years, grass yield was highest in the 4.5 t treat- ment Fig. 2, on average it was 12,000 kg dry matter
per hectare 280 kg N plots and it was 5, 6 and 12 less in the 8.5, 0 and 14.5 t treatments, respectively
Table 1.
The distribution of roots over the soil profile dif- fered significantly between the 4.5 and 14.5 t plots,
although the total mass of root dry matter per hectare was the same in both treatments, 3.6 t Fig. 3. In the
most compacted soil 14.5 t almost no roots had pen- etrated the soil below 20 cm depth, while in the 4.5 t
treatment more than 20 of the total root mass was found between 20 and 50 cm depth. Consequently, root
density in the upper soil layers was considerably lower in the less compacted soil.
3.3. Nematode densities After 4 years of grass growth, total numbers of
nematodes in the slightly 4.5 t, and in the heavily 14.5 t compacted soil did not differ significantly
throughout the year, and amounted to 123 and 124 ind. per gram fresh soil, respectively Table 2. How-
ever, numbers of bacterivores, herbivores and omni- vorespredators were significantly different under both
compaction regimes at all sampling dates. The effects of compaction were tested by analysis of
variance: numbers of bacterivores in heavily 14.5 t compacted soil were significantly p0.035 lower
than in the less 4.5 t compacted soil, fungivores were not significantly lower in the 14.5 t treatment than
in the 4.5 t treatment, herbivores were significantly p0.0001 higher in the 14.5 t treatment than in the
4.5 t treatment and finally omnivorespredators were significantly p0.0001 lower in the 14.5 t treatment
than in the 4.5 t treatment. In the heavily compacted soil the average proportion of the herbivores almost
all Paratylenchus sp. was ca. 50, and in the slightly compacted soil 13. Numbers and proportions of
the other feeding types were much higher in the less compacted soil, in particular the omnivorespredators
and the bacterivores.
With respect to the taxa, herbivorous Paratylenchus sp. were significantly more numerous 3.7× in the
L.A. Bouwman, W.B.M. Arts Applied Soil Ecology 14 2000 213–222 217
Fig. 1. Effects of the various load treatments 0, 4.5, 8.5, 14.5 t on three physical properties of the grassland soil from 1988 to 1991: a thickness cm of the initially loosened layer; b cone index MPa in the 0–5 cm layer; c soil moisture content 00 ww in the 0–5 cm
layer for the 0 and 14.5 t loads.
218 L.A. Bouwman, W.B.M. Arts Applied Soil Ecology 14 2000 213–222
Fig. 2. Effect of various load treatments on above ground dry matter yield of grass for four nitrogen fertilizer rates in 1992. Table 1
Average annual dry matter yield kg ha
− 1
per year over 1988– 1992, under four load treatments 280 kg N ha
− 1
per year Load treatment t
Dry matter yield kg ha
− 1
Index 11270
94 4.5
11960 100
8.5 11310
95 14.5
10580 88
heavily compacted soil, while the fungivorous Aphe- lenchoides sp. was equally abundant in both plots. All
other taxa were more numerous in the less compacted soil, with the exception of Panagrolaimus sp., Aphe-
lenchus avenae, Psilenchus sp., and Pratylenchus sp., taxa that only occurred in very small numbers, less
than two specimens per gram soil Table 3.
4. Discussion
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