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Soil Biology & Biochemistry 32 (2000) 1731±1741
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Responses of trophic groups of soil nematodes to residue
application under conventional tillage and no-till regimes
Shenglei Fu*, David C. Coleman, Paul F. Hendrix, D.A. Crossley Jr.
Institute of Ecology, University of Georgia, Athens, GA 30602-2202, USA
Accepted 13 April 2000

Abstract
A laboratory and a ®eld study were conducted to monitor the increase in numbers and 14C uptake of di€erent trophic groups
of soil nematodes in response to residue addition and to examine the relative importance of bacterivorous and fungivorous
nematodes in conventional (CT) and no-till (NT) agroecosystems. In general, soil nematode numbers increased more rapidly in
response to residue addition and became much more abundant (greater than ®ve-fold) under laboratory conditions than in the
®eld. Our results showed that bacterivorous nematodes responded to residue addition earlier than fungivorous nematodes under
both CT and NT regimes in the laboratory and ®eld studies. A depth e€ect was observed in NT, but not in the CT treatment;
this re¯ected the vertical residue distribution in both tillage regimes. Soil nematodes were more abundant under NT than under
CT in the ®eld. The same pattern was observed at the beginning of the laboratory study but it reversed later. The ratios of
fungivorous-to-bacterivorous nematodes (FN-to-BN) were not signi®cantly di€erent between CT and NT treatments at the
beginning of the experiment. They were very low (less than 0.2) in both tillage regimes, indicating that bacterivorous nematodes
were relatively more important than fungivorous nematodes in both tillage agroecosystems. However, the FN-to-BN ratios

increased with time after residue decomposition started, particularly in the CT treatment. This suggested that the relative
importance of fungivorous nematodes increased with the progress of residue decomposition. It was more pronounced in the CT
treatment during the short period after residue application. In both the laboratory and ®eld studies, the 14C speci®c activity of
soil nematodes and the ratio of 14C bound in nematode biomass to total 14C decayed in the experiment (reported elsewhere)
were signi®cantly higher under CT than under NT, suggesting that soil nematodes use carbon more eciently under CT than
under NT. No signi®cant di€erence of 14C speci®c activity of soil nematodes was found between the two depths under CT in
both the studies; however, 14C speci®c activity was signi®cantly higher in the 0±2.5 cm than in the 2.5±5.0 cm layer under NT in
the laboratory study. 7 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Trophic groups; Soil nematodes;

14

C speci®c activity; Conventional tillage; No-till

1. Introduction
Nematodes are one of the most abundant groups of
soil invertebrates. More than four out of ®ve metazoan
individuals on earth are nematodes, often reaching several millions per square meter (Bongers and Bongers,
1998). Although the contribution of soil nematodes to

* Corresponding author. Department of Environmental Studies,
University of California, 339 Natural Sciences 2, Santa Cruz, CA
95064, USA. Tel.: +1-831-459-3685; fax: +1-831-459-4015.
E-mail address: fushenglei@hotmail.com (S. Fu).

total soil respiration is very low, soil nematodes are
believed to have profound e€ects on soil processes
through their in¯uence on the composition and activity
of soil micro¯ora (Petersen and Luxton, 1982). Several
microcosm studies have shown that the presence of
soil animals (e.g. nematodes) can directly a€ect the
biomass and activity of the microbial community
through feeding on fungi and bacteria (Bardgett et al.,
1993a, 1993b; Ferris et al., 1997). Soil nematodes are
signi®cant regulators of residue decomposition and
nutrient release in natural ecosystems through their
high turnover rates and their interactions with micro-

0038-0717/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 0 7 1 7 ( 0 0 ) 0 0 0 9 1 - 2

1732

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

¯ora (Santos et al., 1981; Coleman et al., 1984; Parker
et al., 1984; Ingham, et al., 1985; Moore et al., 1988).
Based on model calculations, approximately 30% of
the annual N mineralization in Lovinkhoeve agricultural soil was due to the contribution of bacterivorous
nematodes (de Ruiter et al., 1993).
Trophic structure is a functional classi®cation that
contributes to understand the structure of the nematode community, and how each group a€ects the transfer of matter or energy in the ecosystem (Freckman
and Caswell, 1985). Many studies have focused on the
nematode response to organic amendments and pollutants, seasonal cycles, and the spatial distribution of
di€erent trophic groups of soil nematodes in natural
and agricultural ecosystems (Stinner and Crossley,
1982; Parmelee and Alston, 1986; Ettema and Bongers,
1993; Freckman and Ettema, 1993; Ruess, 1995).
`Colonizers', with high reproduction rates and short
life cycles, are thought to respond rapidly to high

nutrient availability; and `persisters', with low reproduction rates and longer life cycles, are believed to be
more sensitive to soil disturbance (Bongers, 1990).
Any soil disturbance can a€ect soil nematode
trophic structure and total abundance. In agroecosystems, tillage is the major disturbance to soil and it
causes the redistribution of plant residue and soil organic matter, subsequently changing microbial structure and nematode trophic structure. Parmelee and
Alston (1986) found that bacterivorous nematodes
were more abundant in conventional tillage (CT) than
in no-till (NT) plots over an annual cycle, whereas fungivorous nematodes were more abundant in NT plots
during the dry summer cropping season, but more numerous in CT during winters. Beare et al. (1992) concluded that fungivorous microarthopods were
relatively more important in determining litter C losses
in NT, while bacterivorous nematodes had a greater
in¯uence in CT. Bardgett (1998) also reported that
fungivores were twice as abundant in organically managed grassland systems as in conventionally managed
soils. However, the detrital food web at Lovinkhoeve
was dominated by bacteria and bacterivores under
both conventional and integrated farm management
(Brussaard et al., 1990; de Ruiter et al., 1993). Stinner
and Crossley (1982) found that total numbers of nematodes and free-living nematodes (bacterivores and fungivores) were not signi®cantly di€erent in CT and NT,
but phytophages were signi®cantly di€erent in the two
tillage treatments. The di€erences between the ®ndings

of various studies indicate that the dynamics of trophic
groups of soil nematodes under di€erent tillage
regimes need more detailed investigation. The importance of a bacterial-based food web versus a fungalbased food web in di€erent tillage regimes needs to be
con®rmed.
Our objectives were to monitor the response of

di€erent trophic groups of soil nematodes to residue
decomposition and to test the relative importance of
bacterivorous and fungivorous nematodes in CT and
NT agroecosystems. In addition, 14C speci®c activity
of soil nematodes was determined to compare the carbon use eciencies under di€erent tillage regimes. A
laboratory and a ®eld study were conducted separately
for these purposes.

2. Materials and methods
2.1. Site description
The study was conducted at the Horseshoe Bend experimental area, near Athens, GA. The soil is characterized as a well-drained moderately acidic sandy clay
loam (Thermic Kanhapludult). Annual mean minimum
and maximum soil temperatures were 8.3 and 19.38C
for CT, and 9.5 and 17.58C for NT plots. The research

plots in this experiment consisted of six alternating CT
and NT plots, collectively occupying approximately
0.1 ha. These plots have been managed as CT and NT
since 1984. In CT, the soil was moldboard plowed,
disked and then rotary tilled before planting. In NT,
the soil remained undisturbed except for a surface slit
cut at the time of planting. Maize (Zea mays ) was
grown as a summer crop, following wheat (Triticum
aestivum ) or clover (Trifolium incarnatum ) cover crops
during winter months (Hendrix et al., 1986; Parmelee
and Alston, 1986; Beare et al., 1992).
2.2. Experimental
2.2.1. Laboratory study
On 16 March 1997, intact soil cores were taken
from the CT and NT ®eld plots. Each soil core from
the CT ®eld plot was fragmented by hand and visible
residues were picked out by hand. Then 1.0 g of 14C
labeled corn leaf litter was mixed with the soil (equivalent to 509 g mÿ2). The corn leaf litter had been cut
into 1 cm2 pieces. Soils were ®tted back to the metal
core (5 cm in diameter, 5 cm in height) of the soil

corer, representing the CT treatment. Soil cores from
the NT ®eld plots were kept intact in the metal core
but old surface residues were carefully removed. Then
1.0 g of 14C labeled corn leaf litter was applied on the
surface of each soil core, representing the NT treatment. We placed each CT or NT soil core in a 1 l
Mason jar with some wet ®lter paper to keep the
moisture relatively constant. We set all Mason jars in
a growth chamber maintained at a range of 22±258C.
The Mason jars were opened periodically to moisten
the ®lter paper and to allow for aeration. We destructively sampled three CT and NT soil cores on each
sampling date, respectively.

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

2.2.2. Field study
On 6 April 1998, the CT plots were cultivated by
hand using a spade and visible old residues were
removed. PVC tubes (10 cm in diameter, 10 cm in
height) were placed into the soil at a depth of 5.0 cm.
Then 4.0 g of 14C labeled corn leaf litter were incorporated into the soil in each core (equivalent to 509 g

mÿ2). Old surface residue was removed from the NT
plots and PVC tubes were set into the soil at a depth
of 5.0 cm, and 4.0 g of 14C labeled corn leaf litter was
applied on the soil surface inside each tube. We
sampled three CT and NT soil cores on each sampling
date.
2.3. Nematode extraction and identi®cation
Soil cores were cut into two layers as 0±2.5 and 2.5±
5.0 cm, and at each layer, approximately 50 g of moist
soil was obtained and used for nematode extraction.
Soil nematodes were extracted using the Baermann
funnel method (McSorley, 1987). After ®xation in 4%
formaldehyde solution, nematodes were counted under
an inverted microscope. To identify the nematodes, we
evenly divided the microscopic ®eld into four sections
making a cross mark under the Petri dish. Then 50
nematodes were randomly selected in each section for
identi®cation such that a total of 200 nematodes per
sample was identi®ed into ®ve trophic groups: bacterivores, fungivores, phytophages, predators and omnivores (Yeates et al., 1993). The whole sample was
identi®ed when total numbers of nematodes were less

than 200.
We measured 1000 randomly selected individual
nematodes for width and length. Nematode biomass
was then calculated according to Andrassy's (1956)
formula and converted to dry weight assuming a drymass content of 25% (Yeates, 1979). Nematode
samples were ®ltered with glass micro®bre ®lters (25
mm in diameter, Cat No. 1820-025, Whatman International). Each ®lter with nematodes was then put
into a 20 ml vial. Nematodes were then digested with 1
ml of Scintigest (Fisher Scienti®c, Fair Lawn, NJ) for
at least 48 h. The samples were then diluted with 1 ml
deionized water and neutralized with 1 ml acetic acid
(0.6 M). Then 20 ml of Scintiverse (Fisher Scienti®c,
Fair Lawn, NJ) was added to each vial, and 14C activity was measured on a liquid scintillation counter.
All values were corrected by quench curve and background counts were subtracted. The 14C speci®c activity was calculated by dividing the 14C activity by the
total biomass of soil nematodes.
2.4. Statistical analysis
Unless otherwise stated, all measurements were
made on samples from 0±5.0 cm depth, and all data

1733

were expressed on a dry mass basis. Statistical analyses
for all data were performed using SAS software (SAS
Institute, 1985). Three-way ANOVA was carried out
for decomposition time, soil depth and tillage. Comparison among means was carried out using Tukey's
test. Signi®cance levels were set at P < 0:05:

3. Results
3.1. Total nematode numbers
In the laboratory study, total nematode numbers
increased rapidly and peaked 1 month after residue application in both the 0±2.5 and 2.5±5.0 cm layers in
the CT treatment, averaging 152 and 191 individuals
gÿ1 soil over the entire experiment, respectively
(Fig. 1A and B). In the NT, total nematodes increased
signi®cantly in the 0±2.5 cm layer and only marginally
in the 2.5±5.0 cm layer, averaging 140 and 74 individuals gÿ1 soil, respectively (Fig. 1A and B). At the
beginning, soil nematode numbers were higher in NT
than in CT, but the pattern was reversed 2 weeks after
residue application.
In the ®eld study, soil nematode numbers did not

show signi®cant changes under either tillage regime,
but increased signi®cantly 40 days after residue application in the 0±2.5 cm layer and 67 days after the experiment began in the 2.5±5.0 cm layer in CT (Fig. 1C
and D). Total nematode numbers were signi®cantly
higher under NT than under CT in the ®eld.
In general, total soil nematode numbers increased
more rapidly and became much more abundant under
the laboratory conditions than those in the ®eld
(Fig. 1A±D). The mean numbers from the laboratory
study were 20±30 and 5±6 times more in the CT and
NT than from the ®eld study, respectively.
3.2. Microbivorous nematodes and FN-to-BN ratio
In both the laboratory and ®eld studies, bacterivores followed exactly the same pattern as that of
total nematodes (Fig. 2A±D). Fungivores showed a
di€erent response to residue decomposition in di€erent treatments. In the laboratory study, fungivores
showed a delayed response to the residue addition
in both CT and NT treatments compared with the
bacterivores. In CT, fungivores increased only
slightly during the ®rst 2 weeks of the experiment
and increased rapidly thereafter, showing no depth
e€ect. In NT, fungivores did not show any signi®cant change at either depth throughout the experiment (Fig. 3A and B). In the ®eld study,
fungivores showed an even more prolonged response
to the residue application. They did not increase in
numbers until 40 days after the experiment started,

1734

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

Fig. 1. Dynamics of total nematodes after residue application: (A) 0±2.5 cm layer in laboratory, (B) 2.5±5.0 cm layer in laboratory, (C) 0±2.5 cm
layer in ®eld, (D) 2.5±5.0 cm layer in ®eld. Means and standard errors are from three replicates. Note that scales of (A) and (B) are di€erent
from those of (C) and (D).

and the tillage e€ect was not as great as in the laboratory during this short period (Fig. 3C and D).
The ratio of fungivorous-to-bacterivorous nematodes (FN-to-BN) was not signi®cantly di€erent in CT
and NT at the beginning of the residue application
(Tables 1 and 2). The ratio was very low under both

tillage regimes for both laboratory and ®eld studies.
However, it increased signi®cantly with time at the
later stages of residue decomposition under CT for
both laboratory and ®eld studies. In NT, however, no
signi®cant change was observed except at the end of
the experiment when there was an increase in the 2.5±

Table 1
Temporal changes of ratio of FN-to-BN after residue application in laboratory
Tillage

CT
NT

a
b

Depth (cm)

0±2.5
2.5±5.0
0±2.5
2.5±5.0

Time after residue application (days)
1

8

17

32

40

0.1420.02b
0.1020.01
0.1620.03
0.1220.03

0.1720.03
0.2520.05
0.03a 20.00
0.0220.00

0.80a 20.19
0.49a 20.04
0.2220.05
0.2120.08

1.44a 20.12
1.13a 20.02
0.2220.04
0.2620.08

2.10a 20.25
1.3520.99
0.1420.05
0.43a 20.09

Signi®cant at P < 0:05.
Means and standard errors from three replicates.

1735

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

Fig. 2. Changes of bacterivorous nematodes after residue application: (A) 0±2.5 cm layer in laboratory, (B) 2.5±5.0 cm layer in laboratory, (C)
0±2.5 cm layer in ®eld, (D) 2.5±5.0 cm layer in ®eld. Means and standard errors are from three replicates. Note that scales of (A) and (B) are
di€erent from those of (C) and (D).

Table 2
Temporal changes of ratio of FN-to-BN after residue application in ®eld
Tillage

CT
NT

a
b

Depth (cm)

0±2.5
2.5±5.0
0±2.5
2.5±5.0

Time after residue application (days)
1

13

32

40

49

67

0.2220.06b
0.1620.05
0.1720.03
0.2520.05

0.1120.03
0.1221.00
0.1020.02
0.2220.13

0.2220.04
0.2420.03
0.0820.01
0.1820.02

0.3720.09
0.2020.01
0.1520.02
0.1020.00

1.18a 20.41
0.77a 20.19
0.64a 20.13
0.2420.13

1.00a 20.47
0.90a 20.22
0.34a 20.06
0.0920.07

Signi®cant at P < 0:05.
Means and standard errors from three replicates.

1736

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

5.0 cm layer in the laboratory study and in the 0±2.5
cm layer in the ®eld study (Tables 1 and 2).
3.3. Other trophic groups
In the laboratory study (Table 3), phytophages
showed a signi®cant increase on days 32 and 40 in the
0±2.5 cm layer, and on days 17 and 32 in the 2.5±5.0
cm layer in CT, and increased signi®cantly on the days
8 and 40 in the 0±2.5 cm layer in NT. Predators
showed a signi®cant increase on day 17 in the 2.5±5.0
cm layer and numbers declined thereafter, however,
they did not increase signi®cantly until the end of the
experiment in the 0±2.5 cm layer in CT. Predators also
increased signi®cantly on days 8 and 32 after residue

application in the 0±2.5 cm layer in NT, but remained
unchanged in the 2.5±5.0 cm layer throughout the experiment. Overall, predators accounted for only a
small portion (less than 2%) of the total soil nematodes. Omnivore numbers showed a trend of increasing
with time after residue application in both layers and
under both tillage regimes; however, the di€erences
were not statistically signi®cant.
In the ®eld study (Table 4), phytophage numbers
remained unchanged in all cases. There were more
phytophages under NT than under CT, but no depth
e€ect was observed in either tillage treatment. Predator
numbers did not show any changes in CT, but showed
a signi®cant increase at the end of the experiment at
both depths in NT. Omnivore numbers remained

Fig. 3. Changes of fungivorous nematodes after residue application: (A) 0±2.5 cm layer in laboratory, (B) 2.5±5.0 cm layer in laboratory, (C) 0±
2.5 cm layer in ®eld, (D) 2.5±5.0 cm layer in ®eld. Means and standard errors are from three replicates. Note that scales of (A) and (B) are di€erent from those of (C) and (D).

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741
Table 3
Dynamics of other trophic groups of soil nematodes after residue addition in laboratory (individual gÿ1 soil)
Tillage Group Depth (cm) Time after residue application (days)

Pa

CT

Pr
Om
NT

P
Pr
Om

0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0

1

8

17

32

40

7.3b
11.6
0.17
0.01
3.4
4.4
6.0
18.7
0.01
0.01
2.3
2.8

13.8
23.1
0.42
0.01
1.1
2.3
66.9c
50.7
2.57c
0.83
3.0
2.7

21.1
87.2c
0.01
11.25c
4.9
9.3
51.8
28.0
1.05
1.73
3.0
1.4

34.1c
98.1c
1.10
4.37
4.3
10.7
30.5
32.6
4.75c
0.12
4.0
2.7

36.1c
46.6
4.02c
1.19
7.3
15.1
80.7c
39.8
0.69
0.63
7.2
3.5

a

P, Om and Pr refer to phytophages, omnivores and predators, respectively.
b
Means of three replicates. Standard errors are not reported here
due to limited space.
c
Signi®cant at P < 0:05.

unchanged at both depths in the CT, but there was an
increase on days 32 and 40 in the 0±2.5 cm layer and
day 40 in the 2.5±5.0 cm layer in NT. Overall, phytophages, predators and omnivores were more abundant
under NT than under CT in the ®eld study.
3.4.

14

C speci®c activity of soil nematodes

In the laboratory study, 14C speci®c activity of soil
nematodes was the highest on day 8 of the experiment
Table 4
Dynamics of other trophic groups of soil nematodes after residue addition in ®eld (individual gÿ1 soil)
Tillage Group Depth (cm) Time after residue application (days)

CT

Pa
Pr
Om

NT

P
Pr
Om

0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0
0±2.5
2.5±5.0

1

13

32

40

49

67

3.7b
4.3
0.14
0.27
1.6
0.9
6.4
8.9
0.46
0.73
3.0
3.0

1.0
0.9
0.10
0.09
0.6
0.7
4.6
4.7
0.14
0.91
2.5
3.0

0.8
0.9
0.12
0.01
0.7
0.2
7.9
8.1
0.17
0.27
6.2c
3.0

3.6
2.4
0.07
0.05
2.6
0.5
5.5
5.8
0.29
0.16
6.7c
4.4c

2.3
3.0
0.16
0.11
1.4
0.8
5.6
6.6
0.16
0.27
2.7
1.4

4.6
4.8
0.07
0.03
2.0
0.6
6.4
5.1
0.91c
1.37c
3.2
1.4

a
P, Om and Pr refer to phytophages, omnivores and predators, respectively.
b
Means of three replicates. Standard errors are not reported here
due to limited space.
c
Signi®cant at P < 0:05.

1737

and declined thereafter in both the 0±2.5 and 2.5±5.0
cm layers under CT; and it increased gradually but did
not peak until 1 month later under NT. The 14C
speci®c activity of soil nematodes was signi®cantly
higher under CT than under NT. No signi®cant di€erence of 14C speci®c activity of soil nematodes was
found between the two layers under CT, but it was signi®cantly higher in the 0±2.5 cm layer than in the 2.5±
5.0 cm layer under NT (Fig. 4A and B).
In the ®eld study, the 14C speci®c activity of soil
nematodes was signi®cantly higher under CT than
under NT. The 14C speci®c activity peaked on day 13
of the experiment and declined thereafter in both the
0±2.5 and 2.5±5.0 cm layers under CT; and it was
hardly detectable until 1 month after residue application under NT. There was no signi®cant di€erence
of 14C speci®c activity of soil nematodes between the
two depths under CT and NT (Fig. 4C and D).

4. Discussion
4.1. Early response of bacterivorous nematodes
In our study, bacterivorous nematodes responded
much earlier and faster to residue application than
fungivorous nematodes, while predators and omnivores did not show any response until towards the end
of the experiment. Our results were generally in agreement with other studies (Freckman, 1988; Ettema and
Bongers, 1993; Griths et al., 1993; Bouwman and
Zwart, 1994). In a plowed system in Sweden, Sohlenius
and Bostrom (1984) found bacterivorous nematodes to
be most abundant in buried plant residue during the
early, rapid phase of decomposition, but fungivorous
nematodes were most abundant later as decomposition
slowed. Griths et al. (1993) found a large increase in
the numbers of populations of microbivorous nematodes during the decomposition of barley roots, with
bacterivores initially dominant and then fungivores
becoming dominant. Bouwman and Zwart (1994)
observed that the Rhabditidae (bacterivores), in particular, bloomed during the early stages of decomposition, whereas Cephalobidae (bacterivores) and
Aphelenchoididae (fungivores), and ®nally, Tylenchidae (phytophages, in part, feed on fungi) became numerous at the later stages of decomposition of organic
matter. Freckman (1988) concluded that the pattern of
succession of bacterivores followed by fungivores is a
common feature of organic matter decomposition, mirroring microbial succession.
In contrast, our ®ndings challenged the explanation
of Griths et al. (1993) for the fast increase of nematode populations. Griths et al. (1993) pointed out
that the initial increase in nematode numbers was
likely to result from preferential migration of nema-

1738

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

todes, and not from reproduction because nematodes
could not reproduce rapidly enough to account for the
increase. Nevertheless, rapid reproduction of soil
nematodes was the only pathway to result in the
increase of nematode populations in our laboratory experiment. Since soil cores were maintained in Mason
jars in a growth chamber, no migration of nematodes
was possible. This was more convincing in the CT
treatment because the nematode numbers increased
rapidly in both surface and deep soil layers. Anderson
et al. (1981) examined the life cycles of two soil nematodes in a laboratory microcosm study and found
Mesodiplogaster lheritieri had a very fast generation
time, 4 days, but the other species Acrobeloides sp. had
a slower generation time of 11 days. It is apparent that

generation times of soil nematodes are species speci®c
(Anderson and Coleman, 1981). Species composition
might be di€erent in our study as compared to the
study of Griths et al. (1993) because corn residue
rather than barley roots was used in our study. The
quality of detritus in¯uences the type and growth of
micro¯ora, and subsequently, their grazers (Freckman,
1988).
4.2. FN-to-BN ratio
Twinn (1974) reviewed the ratio of FN-to-BN for
several sites, noting it is an indication of the importance of the two groups in the decomposition pathway.
Bongers and Bongers (1998) also noted that changes in

Fig. 4. 14C speci®c activity of soil nematodes: (A) 0±2.5 cm layer in laboratory, (B) 2.5±5.0 cm layer in laboratory, (C) 0±2.5 cm layer in ®eld,
(D) 2.5±5.0 cm layer in ®eld. Means and standard errors are from three replicates. Note that scales of (A) and (B) are di€erent from those of (C)
and (D).

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

the relative abundance of bacterivores or fungivores
mirror changes in the decomposition route. A low FNto-BN ratio re¯ects the dominance of bacterivorous
nematodes and may indicate abundant bacterial populations (Freckman, 1988) and vice versa. Yeates et al.
(1993) addressed the same issue by using the reverse
ratio, BN-to-FN. They proposed that additional information on food sources for nematodes within the
decomposer food web could be obtained by using the
ratio of bacterivores-to-fungivores. This ratio is based
solely on the ratio of numerical abundance and takes
no account of body size, nematode activity or the grazing e€ects of the nematodes. A signi®cantly higher
ratio implies a greater availability of food for bacterivores (Yeates et al., 1993).
In our experiments, the FN-to-BN ratio increased
with time after residue application in the CT treatment
and did not increase in the NT treatment until the end
of the experiment. However, FN-to-BN ratios were
not signi®cantly di€erent at the beginning of the experiment between the CT and NT treatments, and were
very low under both tillage regimes. In general, the absolute numbers of both bacterivores and fungivores
were higher in NT than in CT. Our results indicated
that bacterivorous nematodes had a greater in¯uence
than fungivorous nematodes under both CT and NT
treatments at the beginning of the experiment. Nevertheless, the importance of fungivorous nematodes
increased with time in the progress of residue decomposition under both tillage regimes, so the fungal
pathway might be more important in CT than in NT
shortly after residue application. Though fungivores
did not show much increase in NT in our studies, their
importance in NT might increase later as residue decomposition proceeds. To some extent, our results
were in agreement with Brussaard et al. (1990) and
Stinner and Crossley's (1982) studies; however, these
were not consistent with Parmelee and Alston (1986)
and the ®ndings of Beare et al. (1992). The timing of
the sampling and the design of the experiment might
be responsible for these consistencies and discrepancies.
4.3. Tillage and environmental e€ects
In agroecosystems, disturbance can have a strong in¯uence on soil microbial populations and, subsequently, nematode communities. Coleman et al.
(1983) concluded that intensively managed grasslands
appear to correspond to the `fast cycle' dominated by
labile substrates and bacteria, while less productive,
organically fertilized grasslands relate to the `slow
cycle' dominated by more resistant substrates and
fungi. Sohlenius and Bostrom (1984) found that recovery of nematode populations from a moderate disturbance, such as tillage or fertilization, resulted in an

1739

increase in the dominance of opportunistic bacterivores. Dmowska and Kozlowska (1988) also pointed
out that plowing stimulates mineralization and results
in increase of nematode numbers and dominance of
the opportunistic taxa.
In our studies, signi®cant tillage e€ects were
observed in most cases. All trophic groups were signi®cantly higher in NT than in CT in the ®eld study, and
at the beginning of the laboratory study. Microbivorous
nematodes (bacterivores
and fungivores)
responded to residue addition much earlier and their
numbers increased much faster under CT than under
NT. 14C speci®c activity was signi®cantly higher under
CT than under NT. Our results agreed with that disturbance (e.g. plowing) can have a strong in¯uence on
soil nematodes (Dmowska and Kozlowska, 1988;
Ettema and Bongers, 1993; Freckman and Ettema,
1993). However, there were two sources of soil disturbances in our experiment treatments: application of
crop residue (resource enrichment) and plowing (physical disturbance). The disturbances in CT were greater
than in NT treatment since crop residue was well
mixed into soil under CT by plowing, while crop residue was only applied on soil surface without plowing
under NT. How to di€erentiate from each other the
e€ects of the two sources of disturbances on soil nematodes under di€erent tillage regimes? We suggest that
another set of treatment with or without plowing in
the absence of crop residue addition be carried out
simultaneously.
There was a signi®cant depth e€ect on the response
of total nematodes, bacterivores, fungivores and 14C
speci®c activity under NT, but not under the CT treatment in both the laboratory and ®eld studies. Furthermore, in the laboratory study, fungivore numbers
started increasing 2 weeks after residue application in
CT but did not increase throughout the entire period
in NT. In the ®eld study, fungivore numbers started
increasing 1 month after residue application at both
layers in CT and at the surface layer, but not in the
deep layer of the NT treatment. Vertical distribution
of trophic groups of soil nematodes re¯ected the distribution and abundance of their food sources. Faster
residue decomposition rate and more uniformly distributed organic matter by plowing eliminated the
depth e€ect in CT, whereas the strati®cation of organic
matter on the soil surface resulted in the di€erent response of nematodes at di€erent soil depths in NT.
The di€erence of microclimate (e.g. temperature and
moisture) under di€erent tillage regimes and di€erent
environments also has e€ects on the abundance, activity and survival of soil nematodes (Holland and
Coleman, 1987). These e€ects may be more pronounced in our ®eld study. In the ®eld study, the residue on the soil surface in NT acts as a radiant energy
barrier that reduces evaporative water loss and moder-

1740

S. Fu et al. / Soil Biology & Biochemistry 32 (2000) 1731±1741

ate soil temperature ¯uctuations. In contrast, the relatively bare soil in CT resulting from residue incorporation has high potential evaporative water losses and
large daily temperature ¯uctuations (Holland and
Coleman, 1987). Favorable soil temperature and
moisture enable soil microorganisms and soil animals
to reproduce faster in the laboratory than in the ®eld
where soil temperature and moisture ¯uctuates.
Since the 14C speci®c activity of soil nematodes and
the ratio of 14C bound in nematode biomass to total
14
C decayed in the experiment (reported elsewhere)
were signi®cantly higher under CT than under NT in
both the laboratory and ®eld studies, it is logical to
infer that soil nematodes use carbon more eciently
under CT than under NT. Although the 14C speci®c
activity of soil nematodes was higher in the ®eld than
in the laboratory study, the ratio of 14C bound in
nematode biomass to total 14C decayed in the experiment (reported elsewhere) was lower in the ®eld than
in the laboratory. Where do soil nematodes use carbon
more eciently, in the ®eld or in the laboratory? The
answer remains uncertain.
The 14C speci®c activities of soil nematodes in our
study were lower compared with Yeates et al.'s (1998)
study. Is it because of di€erent substrates used in two
studies (crop residue in our study and root exudates in
theirs) or because of di€erent groups of nematodes
selected for measurements (the entire nematode community considered in our study and only one sedentary
species selected in their study)? The question cannot be
answered without substantial evidence of C-to-N ratio,
lignin contents of the substrates and other characteristics of the nematode community.

Acknowledgements
We thank Keith W. Kisselle, Carol J. Garrett, Betty
Weise, Paula Marcinek, Patricia Huback, Kathy Sasser, Brent Andrews and Sherry Farly for their ®eld
and laboratory assistance. Special thanks from the
senior author to Dr Christien H. Ettema for her technical instruction and insightful discussion. Dr Christien
H. Ettema, Stephanie Madson, two anonymous
reviewers and the Editor-in-Chief (Dr John Waid) of
the journal greatly improved the manuscript. This
study was supported by a grant from the National
Science Foundation to the Institute of Ecology, University of Georgia.

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