changes to water repellence of soil aggregates caused by substrate-induced microbial activity

6
Changes to water repellence of soil aggregates caused by substrate-induced microbial activity P. D. HALLETT & I. M. YOUNG 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 influence of microbial activity caused by adding plant nutrients on the hydraulic characteristics of soil aggregates is reported. Soil aggregates were collected from a field 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 field. In the laboratory studies, the addition of nutrients caused severe water repellency in the soil aggregates. Respiration studies identified 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 influence 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 influence 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 flow 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 influence 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 specific measurements of hydraulic properties could not be derived. We have examined the influence of adding nutrients on the hydraulic transport properties of soil aggregates. We did so by adding the nutrients to individual aggregates both in the field 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 Labfield 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 field-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

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Page 1: Changes to water repellence of soil aggregates caused by substrate-induced microbial activity

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

Page 2: Changes to water repellence of soil aggregates caused by substrate-induced microbial activity

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

Page 3: Changes to water repellence of soil aggregates caused by substrate-induced microbial activity

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

Page 4: Changes to water repellence of soil aggregates caused by substrate-induced microbial activity

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

Page 5: Changes to water repellence of soil aggregates caused by substrate-induced microbial activity

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.

References

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R

Table 3 Carbon and nitrogen in the soils amended with nutrients

following incubation for two weeks measured using a continuous

¯ow mass spectrophotometer

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

# 1999 Blackwell Science Ltd, European Journal of Soil Science, 50, 35±40

Page 6: Changes to water repellence of soil aggregates caused by substrate-induced microbial activity

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L

40 P. D. Hallett & I. M. Young

# 1999 Blackwell Science Ltd, European Journal of Soil Science, 50, 35±40