Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue11-12.oct2000:

Soil Biology & Biochemistry 32 (2000) 1763±1772
www.elsevier.com/locate/soilbio

Plant N capture and microfaunal dynamics from decomposing
grass and earthworm residues in soil
A. Hodge a,*, J. Stewart b, D. Robinson b,c, B.S. Griths b, A.H. Fitter a
a

Department of Biology, University of York, P.O. Box 373, York YO10 5YW, UK
b
Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK
c
Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UU, UK
Accepted 25 April 2000

Abstract
Plant roots may be e€ective competitors with micro-organisms for the nutrients released from decomposing organic patches
buried in soil. We aimed to establish whether this was because they were more e€ective at acquiring nutrients or simply because
they represent a slower turnover pool. Over 30 days we followed decomposition of, and plant N capture from, dual labelled
(15N/13C) earthworms (Lumbricus terrestris L.) and grass (Lolium perenne L. shoots) added as discrete patches to soil microcosm
units containing L. perenne plants. Both patches decomposed rapidly as shown by the amounts of 13C, as 13CO2, released into

the soil atmosphere, which peaked after 8 h for the earthworm patches and 48 h for the grass patches. In the decomposing grass
patches the amounts of 13C and 15N remained co-varied and declined with time. No 13C added in the earthworm patches was
detected in the soils, even after 3 days, con®rming that decomposition of these patches was rapid. Grass patches supported
greater microfaunal (nematode and protozoan) biomass than the earthworm patches, and microfaunal biomass peaked at day 7
on both. Plant N capture from both patches increased with dry weight increment although N capture from the earthworm patch
was greater than that from the grass patch. By day 30 plants had captured 29% (from earthworms) and 22% (from grass) of the
N originally available in the patches. No 13C enrichments from the patches were detected in the plant tissues indicating that
organic compounds were not being taken up by the plant roots. As plants only took up inorganic N from the patch, our results
indicate that microbes initially out-compete plants for the added N, but with time, plants capture more of the N originally
added as they represent a slower turnover pool. 7 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Decomposition; Organic patches; Earthworms (Lumbricus terrestris L.); Lolium perenne L; Protozoa; Nematodes

1. Introduction
Decomposition of organic material in soil is a major
source of plant nutrients, especially in low input ecosystems, such as pastures. A clearer understanding of
the factors governing nitrogen recycling in particular is
important as this nutrient is most likely to be the limiting factor in plant growth, in such ecosystems (Vitousek and Howarth, 1991). The distribution of organic

* Corresponding author. Tel.: +44-1904-432878; fax: +44-1904432860.
E-mail address: ah29@york.ac.uk (A. Hodge).

inputs, and hence nutrient availability, however, is
both spatially and temporally heterogeneous at scales
relevant to plant roots (Gupta and Rorison, 1975;
Jackson and Caldwell, 1993; Robertson et al., 1993;
Stark, 1994; Farley and Fitter, 1999). Thus, plants
may bene®t from recognising and exploiting such
nutrient-rich zones or patches in competition both
with micro-organisms and other root systems. Mechanisms by which roots may exploit nutrient-rich
patches in competition with other root systems include
localised root proliferation within the patch (Hodge et
al., 1999a; Robinson et al., 1999), or increased rates of
nutrient uptake per unit of root (Jackson et al., 1990).
Root proliferation within a nutrient-rich patch can be

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 5 - X

1764

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

triggered by nitrate ions (Zhang and Forde, 1998;
Zhang et al., 1999), although root systems of di€erent
plant species show varying degrees of proliferation response (Campbell et al., 1991; Hodge et al., 1998;
Einsmann et al., 1999). The nutrient-rich patches
themselves will also vary widely in a range of characteristics, such as frequency, distribution and concentration (see Fitter, 1994) which may also in¯uence the
ability of roots to exploit them. We have shown that
N capture from a range of patches di€ering in their
chemical and physical complexity was related to the Cto-N ratio of the added patches (Hodge et al., 2000a).
Furthermore, 15N but not 13C, enrichments were
detected in the plant tissues originating from the dual
labelled (15N/13C) patches. This implies that microbes
were decomposing the patches prior to plant N capture
(i.e., the plants were not taking up intact organic compounds) and that the speed of microbial decomposition
could be important in determining the resulting N capture by the plants.
Nutrient-rich organic patches are zones of high microbial activity and population densities (Griths et
al., 1994) even in the absence of plant roots (Christensen et al., 1992; Griths et al., 1995). Thus, microbes
may initially sequester available nutrients before roots
can gain access to them. However, ultimately some of

the sequestered nutrients will become available through
microbial turnover and be released back into the rhizosphere. Roots of wheat seedlings grown in the pots
containing a complex organic patch (Lolium perenne
shoot material) acquired most of their N 10±22 days
after patch addition, at a time when the populations of
both ¯agellates and nematodes (and hence, presumably
those of other microbes) were declining (Griths et

al., 1994; van Vuuren et al., 1996). After addition of a
simple organic patch (L-lysine) L. perenne seedlings
captured 57±61% of the N originally available after 35
days (Hodge et al., 1999b) suggesting that, in the
longer term, roots are e€ective competitors for released
N. In this experiment, we examined the timing of
patch decomposition and subsequent changes in microfaunal biomass and plant N capture from the patch.
We compared the decomposition dynamics of two
patch types: grass shoots and dead earthworms, which
resemble patches likely to be encountered in the natural environment and which contrast in their chemical
characteristics, particularly in their C-to-N ratio.
Patches were added to supply the same amount of

total N and were dual-labelled with 15N and 13C, so
that the dynamics of plant N capture and patch decomposition could be followed.
We tested the following hypotheses:
1. Decomposition of the earthworm patch would be
more rapid than that of the grass patch because of
its lower C-to-N ratio. Hodge et al. (2000a) have
shown that mineralisation and plant N uptake
increased as C-to-N ratio decreased.
2. Microfaunal biomass, an indicator of microbial activity and biomass (AndreÂn et al., 1988; Christensen
et al., 1996) would peak in the soil receiving the
earthworm patch before that in the grass patch as
decomposition, and hence release of N, would be
more rapid in the former patch type. In the grass
patch the increase in protozoan biomass would not
be as steep, but sustained for longer, because of the
slower rate of decomposition.
3. Plants would capture most of their N from the

Fig. 1. Amount of 13C, as 13CO2, recovered from the soil atmosphere as a percentage of 13C originally added in the grass (*) and earthworm
(q) patches with time. Data are means across both experimental runs …n ˆ 6† with SE bars.

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

patches following the peak in microfaunal biomass,
and N capture from the earthworm patch would be
greater than from the grass patch which would
release N more slowly.

1765

at plant level during the ®rst experimental run and 50±
197 mmol mÿ2 sÿ1 at plant level during the second experimental run.

2. Materials and methods
2.1. Experimental design
All experimental plants were grown in the microcosm units. The microcosm units consisted of a section
of PVC pipe (length 20 cm, I.D. 10 cm) with a small
hole cut near the top to allow insertion of a gas
sampling tube (15 cm long  0.3 cm I.D.) at an angle
of 458 to the horizontal. Each microcosm unit was

®lled with a mixture of sand:soil as described in Hodge
et al. (1999c) to a depth of 10 cm before a wooden
pole (15.5 cm long  2.7 cm E.D.) was placed in the
centre of the microcosm tube. The purpose of this pole
was to allow precise placement of the patches once the
seedlings had developed suitably while ensuring minimal disturbance to the system. At the top of each
microcosm unit the top 2 cm section (7.5 cm wide at
base) of a PVC funnel was placed to direct the roots
into the middle section of the tube where the organic
patch was to be placed. The remaining space in the
microcosm unit was ®lled with the sand:soil mixture
ready for planting. The microcosm units were then
placed within four large …60  40  30 cm) freely draining insulated boxes (six microcosm units per box) containing a mixed turf of Trifolium repens L. (white
clover) and Lolium perenne L. cv. Fennema (perennial
ryegrass; all seeds were supplied by Johnson Seeds,
Lincolnshire, UK) plants to bu€er the microcosm
tubes against ¯uctuations in external temperature and
to produce a realistic microclimate around the microcosm units. Twelve L. perenne seeds were planted in
each microcosm unit, with seeds germinating within 5
days and seedlings being left for a further 27 days

before patches were added. The boxes were maintained
in a heated glasshouse throughout.
The experiment was repeated: 12 replicate units of
each patch type (grass and earthworms) were established on 11 November 1998 and a second run of 12
replicate units of each patch type set up on 6 December 1998. The mean temperature, mean daily maximum and mean daily minimum did not vary between
the two experimental runs and was 198C (SE 2 0.1),
208C (SE20.3), and 158C (SE20.3), respectively. The
plants were grown under 16 h days with natural light
supplemented by 400 W halogen bulbs. Photosynthetically active radiation (PAR) ¯ux was recorded weekly
at noon and ranged between 50 and 320 mmol mÿ2 sÿ1

Fig. 2. The relationship between (a) mg 15N in the decomposing
grass (*) and earthworm (q) patches and (b) mg 13C in the decomposing grass (*) patch with time d. (c) The relationship between mg
15
N and mg 13C in the grass (*) patch. Regression equations are
given in Table 1. Points shown in (a) and (b) are the means …n ˆ 6†
with standard error bars, while points in (c) are the raw data from
the six replicate grass patches harvested at 3, 7 and 14 days. Note
the log scale in graphs (a) and (b).

1766

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

Table 1
Parameters of linear regressions of (A) mg 15N in the organic patches against time (d ), (B) mg
15
N against 13C remaining in the grass patcha

(A)
log mg15N in the grass patch
log mg15N in the earthworm patch
(B)
log mg13C in the grass patch
(C)
log mg15N in the grass patch
a

C in the grass patch against time and (C) mg

Regression equation y ˆ c ‡ mx

Independent variable (x )

P

F1, 28

R2

0:438 ÿ 0:071x
ÿ1:58 ÿ 0:070x

d
d

< 0.001
< 0.001

70.2

73.6

1:04 ÿ 0:111x

d

< 0.001

ÿ0:087 ‡ 0:795x

log mg13C

< 0.001

69.17
81.99
F1, 22
35.68
F1, 16
90.64

60.1
84.1

The probability (P ), F statistic (F ) and R 2 values are for the signi®cance and goodness-of-®t of the regression.

Three replicates were used for each of four destructive
harvests in each of the two experimental runs.

2.2. Organic patch material
Organic material added as patches was either 320
mg of grass (Lolium perenne L. cv. Miranda shoots) or
105 mg of earthworm (Lumbricus terrestris L.) material. Both types of organic material were dual
labelled with 15N and 13C. The grass material was produced as described in Hodge et al. (1998). The labelled
earthworm material was produced by keeping earthworms in containers with soil and feeding them on a
diet of 15N/13C labelled L. perenne shoot material for
10 months. The soil was changed regularly during this
time. Earthworms were then removed from containers,
washed and starved overnight to void their guts before
being killed by freezing. Both the earthworm and grass
were added as a ®nely milled powder to the microcosm
units. The material was placed in the space created by
removal of the wooden pole at 10 cm depth. The
remainder of the space was ®lled with sand:soil mix
only. The earthworm material added to the tubes contained 10.1% N (2.55 at.% 15N) and 40.8% C (1.34
at.% 13C), with a C-to-N ratio of 4.1:1. The grass material added contained 3.3% N (13.3 at.% 15N) and
40.4% C (3.1 at.% 13C), with a C-to-N ratio of 12.2:1.
Thus, 10.6 mg N was added to each microcosm unit.

Table 2
Percentage of original patch
vested with timea
Time (day)

15

N and

13

C recovered in the soil har-

Grass patches
(%)

3
7
14
30

13

13

C

3124.5
3024.1
1621.8
NDb

Earthworm patches
(%)

15

N

75210.9
7127.0
5228.3
1524.5

(%)

15

N

4524.6
3624.6
3523.3
1022.6

a
There was no 13C enrichments detected in soil receiving the earthworm patches at any harvest date. Mean data …n ˆ 6† with standard
errors are shown.
b
ND, Not detected.

2.3. Plant and soil analysis
To follow the decomposition of the added organic
patches 13C respired (as 13CO2) from the organic
patches was monitored by sampling gas from within
the soil. Gas samples were taken by inserting a syringe
needle (11.5 cm long) into the gas sampling tube and
removing 10 ml of the soil air from the patch zone.
The sample was then injected into an evacuated gas
sample container (PDZ-Europa Ltd, Crewe, UK) for
13
C analysis (see below). Soil gas was sampled 1, 2, 3,
4, 8, 12, 24, 48, 72, 120, 168, 240, 336, 432, 528, 624
and 720 h after patches were added.
Plant uptake of N (as 15N) and C (as 13C) was
determined by destructively harvesting three microcosm units containing the worm or grass patch 3, 7, 14
and 30 days after patch addition. The microcosm units
were removed from their containers and once the gas
sampling tube had been removed, each soil core was
removed intact from its tube. The core was then cut
into three sections: top, middle (containing the patch
zone) and bottom, each of 6 cm thickness. Roots were
extracted by hand from the di€erent soil sections (top,
middle and bottom) and washed thoroughly. Shoots
were cut at the upper surface of the top section. Roots
and shoots were oven-dried at 608C, weighed, milled
and analysed for total N, C, 15N and 13C (see below).
For analysis, the roots from all sections of the
microcosm unit were combined. A subsample of the
milled root and shoot material was analysed for total
N, C, 15N and 13C by continuous-¯ow isotope ratio
mass spectrometry (CF-IRMS). The at.% 15N and 13C
excess was calculated by subtracting 0.366 and 1.088
(atmospheric background), respectively, from the
measured values. The percentage of the patch N originally added which was captured by the L. perenne
sward was calculated as:

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

1767

ÿ1
ÿ
Fig. 3. (a) NH+
4 ±N and (b) NO3 ±N concentrations (mg g ) recorded at harvest from soil which had received the grass (*) or earthworm (q)
+
patches. NH4 ±N concentrations declined with time (regression equations for log NH+
4 ±N in the grass patches (ÿ) 0.077±0.048 days, P ˆ 0:002,
F1, 22 ˆ 11:94, R 2 ˆ 32:2% and earthworm patches 0.505±0.064 days, P ˆ 0:002, F1, 22 ˆ 13:06, R 2 ˆ 34:4%). There was no signi®cant di€erence
between patch types in the soil NH+
4 ±N concentrations recorded. Di€erent letters on graph (b) indicate signi®cant …P < 0:05† di€erences between
ÿ
harvest dates in the concentration of NOÿ
3 ±N recorded in the soil. NO3 ±N concentrations were higher …P ˆ 0:020† in the earthworm than grass
patches. Data are means across both experimental runs …n ˆ 6† with SE bars.

"

mg 15 N in plant tissue
mg 15 N in original patch material

!

 100

#

Subsamples of the soil from each tube were used for
moisture content determinations (1058C) but only the
middle soil section (containing the patch) was used for
total C, N, 13C and 15N analysis. The soil from the
middle section was also used to determine the inorganic N content, while top and middle sections were
used to determine nematode and protozoan biomass
(as described in Hodge et al., 1998).
2.4. Statistical analysis
Data on the gas samples were analysed using the

General Linear Model (repeated measurements) command in SPSS v 7.0 for the period 1±72 h when all
tubes were still intact. Some gas samples taken after
240 h in the second experimental run were contaminated after sample collection; those data were discarded. Data from destructive harvests were analysed
using the General Linear Model (factorial design) command in SPSS v 7.0. In all cases, a randomised block
design was used. Di€erences referred to in the text
were statistically signi®cant with P < 0:05 as determined by a Bonferroni post-hoc test, unless otherwise
stated. Data on protozoa numbers in the grass patches
at 7 days in the second run were incorrectly recorded
and have not been used.
Plant growth in the ®rst experimental run was

1768

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

greater than in the second when the solar radiation
receipt was less. To allow for di€erences in plant size
between experimental runs the total plant dry weights
at time zero (i.e. time of patch addition) were calculated as the intercept of a regression analysis of natural
logarithm (ln) of tissue dry weight versus time. The
back-transformed value of the intercept was then subtracted from the tissue dry weight at all harvests to
obtain the increment in growth with time. Tissue dry
weight increment values and percentage of N capture
from the patches were analysed by linear regressions
and signi®cant di€erences between the slopes of the
®tted lines for the grass and earthworm patches compared using the F-ratio method (Sokal and Rohlf,
1981; Potvin et al., 1990)

3. Results
3.1. Patch decomposition
Similar amounts of 13C (as 13CO2) were recovered in
the soil gas samples in the two experimental runs. The
earthworm patch decomposed more rapidly than the
grass patch, with 13CO2 recovery peaking 8 h after
patch addition (Fig. 1). CO2 release from the grass
patch peaked 48 h after addition to the microcosm
tubes, but amounts recovered were never signi®cantly
greater than from the earthworm patch. Thereafter,
amounts of 13CO2 recovered declined steadily in both
patches (Fig. 1).
At all harvests the soil which had received the earthworm and grass patches contained more 15N than
background. In contrast, 13C enrichments were
detected only in the soil which had received the grass
patches and only until 14 days (Fig. 2). The amounts
of 15N, and for the grass patches, 13C, recovered in the
soil declined progressively with time (Fig. 2a and b;
Table 1). The linear relationship between mg 15N and
13
C remaining in the grass patches (Fig. 2c; Table 1)
suggested that release of N and C from this patch material was coupled. Since the two patch materials initially had contained di€erent 15N and 13C
enrichments, the percentage of original patch N and C
recovered in the soil at harvest are presented in
Table 2, which shows there was a more rapid initial
loss of N from the earthworm patch.
3.2. Inorganic N and microfauna
ÿ
Soil NH+
4 ±N and NO3 ±N concentrations di€ered
between the two experimental runs only on day 3 in
the grass patch and day 14 in the worm patch, respectively, when they were higher in the second experiment
than the ®rst. The data from the two runs were therefore combined. Soil NH+
4 ±N concentrations declined

Fig. 4. Populations of nematodes and biomass of protozoa in the
grass (*) and earthworm (q) patches, and in the top soil section
(control, (w)) with time. Data are means across both experimental
runs …n ˆ 6† and were transformed, loge, for analysis. Means are
detransformed data plotted on a log scale and bar represents the
SED of the transformed data.

signi®cantly with time (Fig. 3a). There was no di€erence …P ˆ 0:112† in soil NH+
4 ±N concentrations due to
the patch type present. Concentrations of NOÿ
3 ±N
were higher than those of NH+
4 ±N, and peaked at 7
and 14 day (Fig. 3b), i.e. after the peak in NH+
4 ±N
concentration, suggesting active nitri®cation processes
in the soil. NOÿ
3 ±N concentrations were greater in the
soil which had received the earthworm patches than
that which had received the grass patches …P ˆ 0:020†
but there was no signi®cant interaction between patch
 day …P ˆ 0:086).
The dynamics of nematodes and protozoa exhibited
some similarities (Fig. 4). Populations reached a peak
after 7 days and subsequently declined, although the
decline was less marked for protozoa which probably
re¯ects that the MPN method counts both active and
encysted forms. While nematode numbers on both the
grass and earthworm patches were signi®cantly greater
than the control samples, protozoa only signi®cantly
increased on the grass patch and not on the earthworm

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

1769

Fig. 5. Relationship between plant dry weight (D.W.) increment and percentage N capture from the grass (*) and earthworm (q) patches. The
lines shown are ®tted regression lines (solid for grass data; broken for worm data) for the original data of percentage N capture from the organic
patches regressed against plant D.W. increment. Regression lines were signi®cantly …P < 0:05† di€erent as determined by the F-ratio method for
statistically comparing ®tted lines (Potvin et al., 1990) and show N capture (as a percentage of the patch N originally added) from the earthworm
patch was faster than that from the grass patch. Values of raw data are shown.

patch (Fig. 4). On day 7 (peak of nematode numbers)
in both experimental runs the population on both
patch types was mainly (>95%) bacterial-feeding
nematodes. The mean biomass of the bacterial-feeders
from both patch types in experimental run 1 (1.08 mg
gÿ1) was smaller …P < 0:05† than in run 2 (4.71 mg
gÿ1), but there were no signi®cant di€erences between
the grass and earthworm patches.
3.3. Plant N capture
Plants were larger in the ®rst experimental run than
the second i.e. by day 30, mean plant weight was 960
mg in the ®rst run and only 470 mg in the second.
However, there was no signi®cant di€erence in relative
growth rate (RGR) between the experimental runs or
between the two patch treatments and the combined
RGR was 50 mg gÿ1 dÿ1. Total plant N concentrations started the same in plants grown in both
patch types …48:720:83 mg N gÿ1) and declined with
time. This decline was steeper in the ®rst experiment
where the plants were larger. By day 30 in both experimental runs however, plant N concentrations were
higher from the earthworm than the grass patches (i.e.,
mean across runs = 39:221:0 mg N gÿ1 for grass
patches and 42:922:0 mg N gÿ1 for earthworm
patches).
Plant N capture from the patch increased linearly
with plant dry weight increment over both experimental runs (Fig. 5). N capture from the earthworm patch

was greater than from the grass patch. Shoots contained up to 40% of patch N, but roots never contained more than 2.5% of original patch N. Plant
tissues were never enriched with 13C from the patch.

4. Discussion
Decomposition of the earthworm patches, with a
lower C-to-N ratio, was more rapid than that of the
grass patches as shown by the amounts of 13CO2
recovered and analysis of the patch soil, thus con®rming our ®rst hypothesis. The C-to-N ratio of the substrate has a large in¯uence on decomposition (Berg
and Staaf, 1981) and generally net immobilisation
occurs when the C-to-N ratio is >20±30:1 (Bartholomew, 1965), higher than the C-to-N ratio of both
types of patch used in this experiment. The patches
used in our experiment decomposed and released N
throughout. These results also con®rm our other work
on patches: at low C-to-N ratios (i.e. 50% of the patch N originally available within
49 days (Hodge et al., 2000a), while at higher C-to-N
ratios of 21:1 and 31:1 plant N capture was substantially reduced to only 11% after 49 days and >6%
after 35 days, respectively (Hodge et al., 1998, 2000a).
After only 3 days, 50% of earthworm N initially
added was lost from the soil-plant system. By day 7
the amount of N lost had risen to 64%, more than the
50% N mineralisation of earthworm tissue after 7 days

1770

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

reported by Christensen (1988). In contrast, 71% of
the original patch N was still detectable in the soil
from the grass patches. Although plant biomass production was lower in the second experimental run, decomposition of the patches did not di€er between
runs.
The observation that protozoan populations did not
increase on the earthworm patch, in contrast to the
bacterial-feeding nematodes, was unexpected. It is
possible that the earthworm patches, with their high N
content, may have locally altered the pH of the soil
adversely a€ecting the protozoan, but not the nematode, populations. In addition, protozoan growth is
a€ected by the species of bacteria present (Weekers et
al., 1993), which may have been very di€erent between
the two patch types. Earthworms also have antibiotic
properties which may be due to feeding on actinomycetes found in their gut (Brown, 1995). However, as
the earthworms used in this experiment had their guts
voided prior to freezing and milling, antibiotic release
was probably not a signi®cant factor in our study. The
soil receiving the grass material as patches supported
more microfaunal biomass than that receiving the
earthworm material (equal nematode biomass but
greater protozoan biomass). This is not unexpected,
because both substrates had relatively low C-to-N
ratios, and therefore, the greater amount of C added
with the grass patch (to maintain constant N) could
support a larger microfaunal biomass. Nematodes in
both patches peaked at day 7, which does not support
our second hypothesis that earthworm patches would
produce an earlier peak in microfauna due to rapid decomposition. Plant N capture from both patches at
day 7 was less than 2% of the N originally available
but increased over the next two harvest dates (i.e., 8±
10% at day 14 and 22±29% at day 30) when microfaunal biomass was declining, thus partially supporting
our third hypothesis. Griths et al. (1994) also
observed an increase in plant N capture from a similar
grass patch as ¯agellates and nematode numbers in the
patch declined.
Plant N capture from the patches re¯ected their dry
weight increment, although N capture was higher from
the earthworm patch, which decomposed more rapidly.
This result gives further support to our third hypothesis that N capture would be greater from the patch of
lower C-to-N ratio. In addition, by the end of the experiment, total plant N concentrations were also
higher in plants grown in the presence of an earthworm patch. After 30 days, the plants had captured
29% (earthworms) and 22% (grass) of the N originally
available, a much lower ®gure than the c. 70% capture
reported by Whalen et al. (1999) using similar patch
material (15N labelled Lumbricus terrestris ) and the
same plant species (L. perenne ). However, in the study
by Whalen et al. (1999) the ambient temperature ran-

ged from 20 to 278C, whereas in ours, it ranged from
15 to 208C. Temperature can have a large e€ect on N
transformations (see Swift et al., 1979) which probably
explains the discrepancy in plant N capture values. In
contrast, the % N capture from the grass patches
agrees well with our other study using the same material added to a L. perenne sward, but altering the
physical complexity and spatial placement: in that case
N capture from the patch was c. 26% over 70 days
(Hodge et al., 2000b).
These observations emphasise the temporal aspects
of competition between microbes and plants for nitrogen. The bulk of microbial activity was over before
roots were able to grow into and exploit the patch.
Previous gnotobiotic studies with microfauna have
shown that microbial-feeding nematodes and protozoa
enhance the cycling of N, prevent the immobilisation
of N in the microbial biomass and enhance plant N
uptake (Ingham et al., 1985; Griths, 1994). The
assumption has been that roots would need to be present while this transformation activity was proceeding
to take advantage of the N. However, we now know
from numerous studies (van Vuuren et al., 1996;
Hodge et al., 1998, 2000a) that roots typically proliferate in a patch after the early ¯ush of microbial activity
has passed and when faunal populations are declining.
Thus, rather than bene®ting from the gross mineralisation (i.e., intercepting actively transformed N) the
plants bene®t from the net mineralisation (i.e., the
increased pool of mineral N) together with the slower
rates of mineralisation that occur in the secondary
phases of decomposition. Even in situations where the
patch might occur adjacent to an active root system
(e.g., the injection of slurry into an established crop,
death of an earthworm in the rooting zone) the longevity of roots compared to microbes would enable the
plant to absorb N after the initial ¯ush of microbial
activity. Thus, plants may be e€ective competitors for
N because they are a slower turnover pool than the
microbes.

5. Conclusions
Two of our initial hypotheses, i.e. that (1) decomposition of the earthworm patch would be more rapid
than the grass and that (2) plant N capture would be
greater from the earthworm patch due to its lower Cto-N ratio, were supported by our data. Indeed, patch
decomposition was astonishingly rapid, particularly in
the case of earthworm material, and this was re¯ected
in the greater concentrations of NOÿ
3 ±N produced in
the patch. Although 13C from the earthworm patch
was not detectable in the patch soil even after 3 days,
the presence of 13CO2 in the soil atmosphere was still
detectable after this time. Presumably the 13C was pre-

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

sent in gaseous form and lost as the soil was harvested
and dried. Plant N capture from the patches used in
this study was in agreement with our previous work
(Hodge et al., 2000a): more capture occurred from the
patches with the lower C-to-N ratio. Moreover, plant
N was captured only after N had been mineralised by
soil microbes. Although some studies suggest that
plants can take up simple organic compounds intact
(NaÈsholm et al., 1998; Lipson and Monson, 1998), we
have never observed this phenomenon even though
such simple compounds would inevitably have been
released during patch decomposition and made available for uptake. However, even in the absence of direct
uptake of organic N compounds plants, with time, can
capture considerable amounts of N present from Nrich patches.

Acknowledgements
This work was funded by the Biotechnology and
Biological Sciences Research Council (BBSRC). The
Scottish Crop Research Institute receives grant-in-aid
from the Scottish Executive Rural A€airs Department.
We thank Michael Bonkowski, Charles Scrimgeour
and Winnie Stein for their technical assistance.

References
AndreÂn, O., Paustian, K., Rosswall, T., 1988. Soil biotic interactions
in the functioning of agroecosystems. Agriculture, Ecosystems
and Environment 24, 57±67.
Bartholomew, W.V., 1965. Mineralization and immobilization of
nitrogen in the decomposition of plant and animal residues. In:
Bartholomew, W.V., Clark, F.E. (Eds.), Soil Nitrogen. American
Society of Agronomy, Madison, pp. 285±306.
Berg, B., Staaf, H., 1981. Leaching, accumulation and release of
nitrogen in decomposing forest litter. Ecological Bulletins
(Stockholm) 33, 163±178.
Brown, G.G., 1995. How do earthworms a€ect micro¯oral and faunal community diversity. Plant and Soil 170, 209±231.
Campbell, B.D., Grime, J.P., Mackey, J.M.L., 1991. A trade-o€
between scale and precision in resource foraging. Oecologia 87,
532±538.
Christensen, O., 1988. The direct e€ects of earthworms on nitrogen
turnover in cultivated soils. Ecological Bulletins 39, 41±44.
Christensen, S., Griths, B.S., Ekelund, F., Rùnn, R., 1992. Huge
increase in bacterivores on freshly killed barley roots. FEMS
Microbiology Ecology 86, 303±310.
Christensen, S., Rùnn, R., Ekelund, F., Andersen, B., Damgaard, J.,
Friberg-Jensen, U., Jensen, L., Kiil, H., Larsen, B., Larsen, J.,
Riis, C., Thingsgaard, K., Thirup, C., Tom-Petersen, A.,
Vesterdal, L., 1996. Soil respiration pro®les and protozoan enumeration agree as microbial growth indicators. Soil Biology &
Biochemistry 28, 865±868.
Einsmann, J.C., Jones, R.H., Pu, M., Mitchell, R.J., 1999. Nutrient
foraging traits in 10 co-occurring plant species of contrasting life
forms. Journal of Ecology 87, 609±619.
Farley, R.A., Fitter, A.H., 1999. Temporal and spatial variation in

1771

soil resources in a deciduous woodland. Journal of Ecology 87,
688±696.
Fitter, A.H., 1994. Architecture and biomass allocation as components of the plastic response of root systems to soil heterogeneity. In: Caldwell, M.M., Pearcy, R.W (Eds.), Exploitation of
Environmental Heterogeneity by Plants. Academic Press, New
York, pp. 305±323.
Griths, B.S., 1994. Microbial-feeding nematodes and protozoa in
soil: their e€ects on microbial activity and nitrogen mineralization
in decomposition hotspots and the rhizosphere. Plant and Soil
164, 25±33.
Griths, B.S., van Vuuren, M.M.I., Robinson, D., 1994. Microbial
grazer populations in a 15N labelled organic residue and the
uptake of residue N by wheat. European Journal of Agronomy 3,
321±325.
Griths, B.S., Young, I.M., Caul, S., 1995. Nematode and protozoan population dynamics on decomposing barley leaves incubated at di€erent soil matric potentials. Pedobiologia 39, 454±
461.
Gupta, P.L., Rorison, I.H., 1975. Seasonal di€erences in the availability of nutrients down a podzolic pro®le. Journal of Ecology
63, 521±534.
Hodge, A., Stewart, J., Robinson, D., Griths, B.S., Fitter, A.H.,
1998. Root proliferation, soil fauna and plant nitrogen capture
from nutrient-rich patches in soil. New Phytologist 139, 479±494.
Hodge, A., Robinson, D., Griths, B.S., Fitter, A.H., 1999a. Why
plants bother: root proliferation results in increased nitrogen capture from an organic patch when two grasses compete. Plant, Cell
and Environment 22, 811±820.
Hodge, A., Stewart, J., Robinson, D., Griths, B.S., Fitter, A.H.,
1999b. Plant, soil fauna and microbial responses to N-rich organic patches of contrasting temporal availability. Soil Biology &
Biochemistry 31, 1517±1530.
Hodge, A., Robinson, D., Griths, B.S., Fitter, A.H., 1999c.
Nitrogen capture by plants grown in N-rich organic patches of
contrasting size and strength. Journal of Experimental Botany 50,
1243±1252.
Hodge, A., Stewart, J., Robinson, D., Griths, B.S., Fitter, A.H.,
2000a. Competition between roots and soil micro-organisms for
nutrients from nitrogen-rich patches of varying complexity.
Journal of Ecology 88, 150±164.
Hodge, A. Stewart, J. Robinson, D. Griths, B.S. Fitter, A.H.,
2000. Spatial and physical heterogeneity of N supply from soil
does not in¯uence N capture by two grass species. Functional
Ecology, in press.
Ingham, R.E., Trofymow, J.A., Ingham, E.R., Coleman, D.C., 1985.
Interactions of bacteria, fungi, and their nematode grazers: e€ects
on nutrient cycling and plant growth. Ecological Monographs 55,
119±140.
Jackson, R.B., Caldwell, M.M., 1993. Geostatistical patterns of soil
heterogeneity around individual perennial plants. Journal of
Ecology 81, 683±692.
Jackson, R.B., Manwaring, J.H., Caldwell, M.M., 1990. Rapid physiological adjustment of roots to localized soil enrichment. Nature
344, 58±60.
Lipson, D.A., Monson, R.K., 1998. Plant-microbe competition for
soil amino acids in the alpine tundra: e€ects of freeze-thaw and
dry-rewet events. Oecologia 113, 406±414.
NaÈsholm, T., Ekblad, A., Nordin, A., Giesler, R., HoÈgberg, M.,
HoÈgberg, P., 1998. Boreal forest plants take up organic nitrogen.
Nature 392, 914±916.
Potvin, C., Lechowicz, M.J., Tardif, S., 1990. The statistical analysis
of ecophysiological response curves obtained from experiments
involving repeated measures. Ecology 71, 1389±1400.
Robertson, G.P., Crum, J.R., Ellis, B.G., 1993. The spatial variability of soil resources following long-term disturbance. Oecologia
96, 451±456.

1772

A. Hodge et al. / Soil Biology & Biochemistry 32 (2000) 1763±1772

Robinson, D., Hodge, A., Griths, B.S., Fitter, A.H., 1999. Plant
root proliferation in nitrogen-rich patches confers competitive advantage. In: Proceedings of the Royal Society (London) Series B
266, 431±435.
Sokal, R.R., Rohlf, F.J., 1981. Biometry, 3rd ed. Freeman, San
Francisco.
Stark, J.M., 1994. Causes of soil nutrient heterogeneity at di€erent
scales. In: Caldwell, M.M., Pearcy, R.W. (Eds.), Exploitation of
Environmental Heterogeneity by Plants. Academic Press, New
York, pp. 255±284.
Swift, M.J., Heal, O.W., Anderson, J.M., 1979. Decomposition in
Terrestrial Ecosystems Studies in Ecology, 5. Blackwell Scienti®c
Publications, Oxford, pp. 247±254.
van Vuuren, M.M.I., Robinson, D., Griths, B.S., 1996. Nutrient
in¯ow and root proliferation during the exploitation of a temporally and spatially discrete source of nitrogen in soil. Plant and
Soil 178, 185±192.

Vitousek, P.M., Howarth, R.W., 1991. Nitrogen limitation on land
and in the sea: how can it occur. Biogeochemistry 13, 87±116.
Weekers, P.H.H., Bodelier, P.L.E., Wijen, J.P.H., Vogels, G.D.,
1993. E€ects of grazing by the free-living soil amoebae
Acanthamoeba castellanii, Acanthamoeba polyphaga, and
Hartmanella vermiformis on various bacteria. Applied and
Environmental Microbiology 59, 2317±2319.
Whalen, J.K., Parmelee, R.W., McCartney, D.A., Vanarsdale, J.L.,
1999. Movement of N from decomposing earthworm tissue to
soil, microbial and plant N pools. Soil Biology & Biochemistry
31, 487±492.
Zhang, H., Forde, B.G., 1998. An Arabidopsis MADS box gene that
controls nutrient-induced changes in root architecture. Science
279, 407±409.
Zhang, H., Jennings, A., Barlow, P.W., Forde, B.G., 1999. Dual
pathways for regulation of root branching by nitrate. Proceedings
of the Natural Academy of Sciences USA 96, 6529±6534.