changes to water repellence of soil aggregates caused by substrate-induced microbial activity
TRANSCRIPT
Changes to water repellence of soil aggregates caused bysubstrate-induced microbial activity
P . D . H A L L E T T & I . M . Y O U N G
Soil±Plant Dynamics Unit, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK
Summary
Soil microbes produce exudates which upon drying become water-repellent, thus altering hydraulic
properties. The in¯uence of microbial activity caused by adding plant nutrients on the hydraulic
characteristics of soil aggregates is reported. Soil aggregates were collected from a ®eld that had been
fertilized with different amounts of nitrogen. Aggregates were also incubated with different nutrient
treatments in the laboratory. Their sorptivity, hydraulic conductivity and water repellency were measured
with a new device. Adding nitrogen was found to decrease sorptivity and hydraulic conductivity because
of increased water repellency in the ®eld. In the laboratory studies, the addition of nutrients caused severe
water repellency in the soil aggregates. Respiration studies identi®ed a large increase in biological
activity following nutrient amendment which produces water-repellent materials.
Introduction
Enhanced microbial activity caused by adding plant nutrients
to the soil affects physical processes such as water transport
(Rawitz et al., 1994) and retention (Chenu, 1993) and the
stability of aggregates (Skinner, 1979). Probably the greatest
in¯uence is the production of water-repellent microbial
biomass and exudates which alter the hydraulic characteristics
of the soil (Bond & Harris, 1964; Tillman et al., 1989; Chan,
1992) and strengthen the bonds between soil particles (Chenu,
1993). The in¯uence is most severe for dry soil (Wallis &
Horne, 1992). The reduced wetting rate caused by water
repellence has serious implications for soil management since
it affects runoff, disrupts aggregates on rapid wetting, and
accentuates the ¯ow of water between the aggregates.
Fertilizers are needed to increase crop production, but these
other associated effects may need to be considered in a more
holistic management of soil. There has been much research on
water repellency in soil caused by microbial activity (reviewed
by Wallis & Horne, 1992), but we know little of the effects of
added fertilizers. If adding nutrients enhances biological
activity then we should expect changes in the hydraulic
properties of soil through the production of water-repellent
exudates.
When studying water repellency, it is important to examine
soil aggregates as individual structural entities since they are
prevalent following tillage when microbial activity is usually
greatest (Franzluebbers et al., 1994). Most microbial activity
takes place on the surfaces of aggregates where the microbial
substrates are most available (Hattori, 1988; Nietfeld et al.,
1992). Measurements of hydraulic transport properties using
intact cores, for example, might not isolate the in¯uence of
processes occurring at the aggregate surface. Zhang & Hartge
(1992) examined changes in wetting rates in individual soil
aggregates caused by water repellency, but they used
procedures from which speci®c measurements of hydraulic
properties could not be derived.
We have examined the in¯uence of adding nutrients on the
hydraulic transport properties of soil aggregates. We did so by
adding the nutrients to individual aggregates both in the ®eld
and under controlled laboratory conditions. A new device was
used to measure the hydraulic properties of individual soil
aggregates (Leeds-Harrison et al., 1994). Biological activity
caused by nutrient amendments was measured using standard
respiration procedures.
Materials and methods
Soil aggregates with diameters between 2 and 3 cm were
collected from the surface (0±5 cm) at two different sites,
Beechgrove and Lab®eld in the southeast of Scotland.
Characteristics of these soils important to this study are listed
in Table 1. At the Beechgrove site we sampled two direct-
drilled plots, of which one had received no fertilizer and the
other 120 kg ha±1 of nitrogen fertilizer.
Nutrients and water were added to ®eld-moist aggregates to
reach a water content equivalent to the ±50 cm water potential
(wetting) determined previously from a replicate set of
aggregates using a tension table. Four different nutrient
amendments were used: (i) control, no added nutrients; (ii)
R
Correspondence: P. D. Hallett. E-mail: [email protected]
Received 21 April 1998; revised version accepted 5 October 1998
European Journal of Soil Science, March 1999, 50, 35±40
# 1999 Blackwell Science Ltd 35
soil plus glucose (10 mg C g±1 oven-dry soil); (iii) soil plus
glucose and ammonium nitrate (10 mg C g±1 and 2 mg N g±1
oven-dry soil, respectively); and (iv) soil plus glucose,
ammonium nitrate and Hewitt's nutrient solution (10 mg C
g±1, 2 mg N g±1 and 0.38 ml g±1 oven-dry soil, respectively).
The nutrient treatments and the quantity of water required to
reach the desired water content were added to the surfaces of
the soil aggregates using a pipette. After amending the
aggregates, we placed them in sealed plastic containers and
incubated them ®rst for 2 days at 4°C to allow for nutrient and
water redistribution with minimal microbial degradation of the
substrate and then at a constant temperature of 20°C for
2 weeks. Accumulation of carbon dioxide was reduced by
aerating the containers frequently. A subset of samples was
sealed in Kilner jars from which gas samples were taken to
measure respiration for the duration of incubation. The gas was
analysed for CO2 concentration using gas chromatography.
After 2 weeks' incubation, the aggregates were dried at
40°C to simulate an extreme drying event. Concentrations of
carbon and nitrogen within the aggregates were determined
using a continuous-¯ow mass spectrophotometer. To deter-
mine the hydraulic transport properties of the dry soil
aggregates, porosity was determined from their bulk density,
measured using the paraf®n wax method (Black et al., 1965)
and an assumed particle density of 2.65 g cm±3.
Measurement and theory of in®ltration
Hydraulic conductivity and sorptivity of individual dry soil
aggregates were measured using the methodology of
Leeds-Harrison et al. (1994). In this approach, water in®ltrates
into each aggregate from a small area (4 mm in diameter)
(Figure 1) which produces an expanding wetting bulb that does
not reach the boundary of the aggregate during measurement.
A sponge tip allows for the establishment of a negative
pressure head that is required for determining the hydraulic
conductivity. We modi®ed the original approach slightly by
measuring in®ltration from the mass loss of water in a
reservoir, rather than volume displacement in a horizontal
capillary tube, which is more dif®cult to set up and maintain
(Leeds-Harrison et al., 1994).
The balance used was accurate to 1 mg, which is less
than 2% of the smallest total mass of water in®ltrated
during the test. Error due to evaporative loss during the
short testing time was reduced by applying a thin layer of
silicone oil to the surface of the water reservoir (Figure 1)
and by having a hole in the top of the reservoir only
slightly larger than the tube used to convey the liquid to
the aggregate. Tests in which no aggregate was in contact
with the device showed the error in mass change caused by
evaporation was less than 5% of the mass of water imbibed
during a similar time for the smallest measured amount of
water in®ltration.
The steady rate of water ¯ow, Q, into the aggregate was
used to evaluate sorptivity at zero head, S0, and hydraulic
conductivity, K, using the equation
Q � 4brS20
f� 4rKh; �1�
where b is a parameter that depends on the soil-water
diffusivity function, r is the radius of the in®ltrometer tip, f
is the ®llable air-porosity, and h is the pressure head
(Leeds-Harrison & Youngs, 1997). The value of b can be
in the range 0.5 < b < p/4 with 0.55 being an `average'
value (White & Sully, 1987) used here. Using Equation (1),
K is evaluated from the slope of a plot of Q against h
(Leeds-Harrison & Youngs, 1997). For this study, h was 0
L
Table 1 Characteristics of the soils used in this study
Lab®eld Beechgrove
Series Macmerry Carpow
Particle size
Sand /% 59 47
Silt /% 34 39
Clay /% 7 14
Field organic matter /% 6.3 6 0.2a 5.1 6 0.1
Aggregate density /g cm±3 1.64 6 0.01 1.37 6 0.02
Fillable porosity, f 0.38 6 0.00 0.48 6 0.01
Water content at ±50 cm /g 100 g±1 28.5 6 0.91 32.5 6 1.53
a Mean 6 standard error.
Figure 1 The in®ltration device used to measure the sorptivity and
hydraulic conductivity of individual soil aggregates.
36 P. D. Hallett & I. M. Young
# 1999 Blackwell Science Ltd, European Journal of Soil Science, 50, 35±40
and ±2 cm in the measurements, so that S0 and K could be
determined.
A water-repellency index (R) was determined from the
sorptivity measurements of two wetting liquids with different
soil±liquid contact angles (Tillman et al., 1989). It was
evaluated from sorptivity measurements conducted at ±2 cm
pressure head for both water and a 95% ethanol to water
solution. Sorptivity at ±2 cm pressure is given by
Q�ÿ2� �4bS2
�ÿ2�r
f; �2�
where the subscript ±2 signi®es the pressure head at which the
measurements were made. For non-repellent soils, the
sorptivity of a 95% ethanol to water solution, SE, is related
to the sorptivity of pure water, SW, by
Sw � ��E=
E�1=2
��W=
W�1=2
" #S
E; �3�
where mE is the viscosity of 95% ethanol at 20°C
(0.0012 N s m±2), gE is the surface tension of 95% ethanol
at 20°C (0.023 N m±1), mW is the viscosity of water at 20°C
(0.0010 N s m±2), and gW the surface tension of water at
20°C (0.073 N m±1). Using these values, Equation (3)
reduces to
SW� 1:95S
E: �4�
The index R therefore becomes
R � 1:95S
E
SW
� �; �5�
with R = 1.0 signifying a totally non-repellent soil. Tillman
et al. (1989) suggested that a soil with SE < SW (R < 1.95) is
non-repellent. The terms SE and SW are denoted as SE(±2) and
SW(±2) hereafter to avoid confusion with standard sorptivity
measurements made at zero pressure head.
Results and discussion
In the ®eld, application of nitrogen fertilizer caused a
signi®cant reduction in in®ltration to air-dry soil which is
re¯ected in the measured sorptivity and hydraulic conductivity
of the Beechgrove soil (Table 2). It is evident from the
sorptivity measurements with the 95% ethanol solution and the
repellency index values that these differences are caused by
increased water repellence of the aggregates collected from the
fertilized soil. The coarser-textured Lab®eld soil had a smaller
sorptivity, greater hydraulic conductivity, and was less water-
repellent than the Beechgrove soil. The R values for all the soil
aggregates in the ®eld condition are less than for intact cores
tested by Tillman et al. (1989). This probably arose because
R
Table 2 Hydraulic characteristics of the soils as in¯uenced by ®eld and laboratory nutrient amendment
Hydraulic Water sorptivity /mm s±1/2 Ethanol sorptivity
conductivity, K /mm s±1/2 Repellency
Treatment /mm s±1 3 103 0 cm head, SW(0) ±2 cm head, SW(±2) SE(±2) index, R
Lab®eld
Field 11.6 6 2.6a 0.54 6 0.07 0.31 6 0.04 0.35 6 0.02 2.2
C added ² ² 0.42 6 0.01* > 50
C + N added ² ² 0.33 6 0.03 > 50
C + N + Hewitt's added ²² ²² 0.41 6 0.02* > 100
Beechgrove, 0 N
Field 5.9 6 0.7 0.42 6 0.04 0.25 6 0.02 0.58 6 0.07 4.5
C added ² ²² 0.64 6 0.03 > 100
C + N added ² ²² 0.55 6 0.03 > 100
C + N + Hewitt's added ²² ²² 0.58 6 0.03 > 100
Beechgrove, 120 kg ha±1 N
Field 2.1 6 0.2 0.27 6 0.03 0.16 6 0.02 0.54 6 0.04 6.5
C added ²² ²² 0.53 6 0.04 > 100
C + N added ²² ²² 0.49 6 0.03 > 100
C + N + Hewitt's added ²² ²² 0.57 6 0.02 > 100
a Mean 6 standard error.
² No in®ltration within 10 min.
²² No in®ltration within 20 min.
* P < 0.05 between treatment and ®eld control.
Water repellence of soil aggregates 37
# 1999 Blackwell Science Ltd, European Journal of Soil Science, 50, 35±40
the natural drying was less severe than in Australia where
Tillman et al. (1989) made their observations.
The greater availability of nitrogen on the fertilized plot
at Beechgrove might have increased microbial activity, and,
in turn, the increased production of water-repellent
materials. There was certainly a surge in microbial
respiration when the nutrients were added to soil aggregates
in the laboratory (Figure 2). Most signi®cantly, respiration
following nutrient amendment increased much more rapidly
in the early stages of incubation for the fertilized
Beechgrove soil than for the control. Previous research on
substrate-induced respiration suggests that this more rapid
initial respiration is caused by a greater initial active
microbial biomass (Beare et al., 1990). Roberson et al.
(1995) also found that more nitrogen increased microbial
biomass in the ®eld. After 4 days respiration increased
rapidly in the unfertilized Beechgrove soil following
nutrient amendment as the microbial biomass increased.
Respiration increased with each additional nutrient and was
greatest for the soil treated with carbon, nitrogen and Hewitt's
solution (Figure 2). It was also much greater for the
Beechgrove soil than for the Lab®eld soil. Amounts of carbon
and nitrogen, as well their ratio (C:N), for the various soils and
nutrient treatments following incubation are listed in Table 3.
Prior to incubation and nutrient amendment, all of the soils
tested had similar C:N. The addition of 10 mg C g±1 soil
increases the percentage carbon by 1%, and 2 mg N g±1 soil
increased the percentage nitrogen by 0.2% prior to miner-
alization by microbial activity. Much of the added carbon was
lost by respiration. Added nitrogen, however, remained in the
soil, thus diminishing the C:N ratio. Increased nitrogen
availability can enhance the production of polysaccharides
(Williams & Wimpenny, 1977) leading to increased water
repellence.
Fungi were evident on the surfaces of the soil aggregates
to which nutrients were added. The aggregates were broken
to expose and thus visualize internal macropores in which
fungi were proli®c near to the aggregate surfaces. This
accords with observations by Hattori (1988) who found
greater biological activity near to the aggregate and with
theoretical estimates of oxygen availability by Nietfeld et al.
(1992). This is of great importance to hydraulic processes
because the aggregate surface is the transport boundary.
Moreover, the organisms are present mainly in the larger
pores that are responsible for much of the convective
transport in soil.
The observed increase in microbial activity following the
addition of nutrients in the laboratory caused such a signi®cant
reduction in water in®ltration that it was not possible to obtain
meaningful measurements of SW. To obtain a repellency index,
SW(±2) was approximated for these samples from the time-
range required for 5 mm3 of water to in®ltrate the aggregate
(Table 2). For most of the treatments, the addition of nutrients
in the laboratory did not affect SE(±2) signi®cantly. The R
values suggest that the soil is water-repellent following the
addition of nutrients, which is the primary reason for reduced
SW(±2).
Despite the respiration data showing biological activity to
be enhanced with each additional nutrient added, the water
repellence caused by even the smallest nutrient treatment
prevented measuring differences in SW. Water in®ltrated
slightly faster into the Lab®eld soil amended with the two
smallest nutrient amendments, although the sorptivity could
not be determined because steady-state conditions did not
occur in a sensible time (taken as < 20 min). In the other tests
no water was observed to in®ltrate the soil during the testing
time, indicating critical repellency as de®ned by Tillman et al.
(1989). Tests using a wider range of nutrient amendments
would allow for the in¯uence of nutrient concentrations to be
better determined. Clearly, the differences caused by adding
L
Figure 2 The in¯uence of nutrient amendments on microbial
respiration for the soils tested. The symbols indicate the nutrient
amendment: d, control, no added nutrients; s, glucose; ., glucose
plus ammonium nitrate; ,, glucose plus ammonium nitrate and
Hewitt's solution.
38 P. D. Hallett & I. M. Young
# 1999 Blackwell Science Ltd, European Journal of Soil Science, 50, 35±40
fertilizer in the ®eld are detectable with the approach we
presented.
Hydraulic transport in soil can also be reduced by micro-
organisms' clogging pores (Vandevivere & Baveye, 1992;
Seki et al., 1998). However, the similarity in SE(±2) values
suggests that pore clogging has no measurable in¯uence in the
tests we have described.
Conclusions
Adding fertilizer can cause signi®cant changes to the hydraulic
properties of soil aggregates in the ®eld. We found that the
addition of 120 kg ha±1 N reduced water sorptivity and
hydraulic conductivity of soil aggregates signi®cantly because
of increased hydrophobicity. A similar laboratory study in
which microbial activity in soil aggregates was enhanced by
adding selected nutrient treatments showed a similar effect. In
agricultural practice the effect of nutrient amendments on
water transport needs to be considered so that aggregate
stability, contaminant ¯ow through interaggregate pore space,
and over-land runoff do not become problematic.
Acknowledgements
We thank Dr R. Wheatley for assistance in the measurement of
microbial respiration. The Scottish Agricultural College
generously allowed access to the Beechgrove experimental
site, and we thank E. Robertson and M. O'Sullivan for
providing data on the soil and assistance in sampling.
Professor E. G. Youngs provided valuable comments during
the preparation of the script. This work was funded by the
Scottish Of®ce Agriculture, Environment and Fisheries
Department.
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R
Table 3 Carbon and nitrogen in the soils amended with nutrients
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Treatment Carbon /% Nitrogen /% C:N
Lab®eld
Field (pre-incubation) 2.99 0.22 13.6
Field (post-incubation) 2.74 0.20 13.7
C added 3.28 0.20 16.4
C + N added 3.23 0.39 8.3
C + N + Hewitt's added 3.03 0.38 8.0
Beechgrove, 0 N
Field (pre-incubation) 3.66 0.29 12.6
Field (post-incubation) 3.55 0.30 11.8
C added 3.66 0.28 13.1
C + N added 4.28 0.52 8.2
C + N + Hewitt's added 3.86 0.50 7.7
Beechgrove, 120 kg ha±1 N
Field (pre-incubation) 3.89 0.30 13.0
Field (post-incubation) 3.31 0.26 12.7
C added 3.89 0.30 13.0
C + N added 4.44 0.55 8.1
C + N + Hewitt's added 3.86 0.50 7.7
Water repellence of soil aggregates 39
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40 P. D. Hallett & I. M. Young
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