Biol Fertil Soils (2000) 31 : 78–84
Q Springer-Verlag 2000
ORIGINAL PAPER
G.D. Bending 7 C. Putland 7 F. Rayns
Changes in microbial community metabolism
and labile organic matter fractions as early indicators
of the impact of management on soil biological quality
Received: 7 April 1999
Abstract Changes to the metabolic profiles of soil microbial communities could have potential for use as
early indicators of the impact of management or other
perturbations on soil functioning and soil quality. We
compared the relative susceptibility to management of
microbial community metabolism with a number of soil
organic matter (OM) and microbial parameters currently used as indicators of changes in soil biological
quality. Following long-term cereal cropping, plots
were subjected to a 16-month treatment period consisting of either a mixed cropping sequence of vetch, spring
barley and clover or a continuous grass-clover ley
which was periodically mown and mulched. The treatments had no effect on soil biomass N or respiration of
microbial populations inoculated into Biolog Gram negative (GN) plates. After 16 months there were no
management-induced changes to total OM, light-fraction OM C and N, labile organic N or water-soluble
carbohydrates. However, patterns of substrate utilization by the soil microbial population following inoculation into Biolog GN plates were found to be highly sensitive to management practice. In the mixed cropping
sequence, substrate utilization changed markedly following plough-in of the vetch crop, with a smaller
change occurring after harvesting of the barley. In the
ley treatment, substrate utilization was not affected until the onset of mowing, when the pattern changed to
become similar to that in the mixed cropping sequence.
Metabolic diversity of the Biolog-culturable microbial
G.D. Bending (Y) 7 C. Putland
Department of Soil and Environmental Sciences,
Horticulture Research International, Wellesbourne,
Warwick CV35 9EF, UK
e-mail: gary.bending6hri.ac.uk
Tel.: c44-1789-470382
Fax: c44-1789-470552
F. Rayns
Henry Doubleday Research Association, Ryton-on-Dunsmore,
Coventry CV8 3LG, UK
population was increased by the ley treatment, but was
not affected by the cropping sequence. We conclude
that patterns of microbial substrate utilization and metabolic diversity are more sensitive to the effects of
management than are OM and biomass pools, and
therefore have value as early indicators of the impacts
of management on soil biological properties, and hence
soil quality.
Key words Biolog substrate-utilization patterns 7
Microbial metabolic diversity 7 Light-fraction organic
matter 7 Microbial biomass 7 Agricultural
management practices
Introduction
The quality of a soil is central to determining the sustainability and productivity of above-ground plant communities (Doran et al. 1994). Recently, greater environmental awareness has led to recognition of the need to
maintain and enhance the quality of soil (Doran et al.
1994). This has highlighted the need for a greater understanding of the factors which contribute to soil quality, so that more reliable methods of assessment can be
made, and in order to identify factors which are particularly sensitive to change. These could be used to provide early indications of the impacts of perturbations
on soil quality (Trasar-Cepeda et al. 1998).
While the chemical characteristics of a soil make a
significant contribution to its quality, and may determine the maximum quality of a particular soil (Hassink
1997), it is the biological and biochemical components
of soil quality which are most susceptible to change,
and therefore to degradation by human activities.
The most widely used biochemical indicator of soil
quality is organic matter (OM) content. Soil OM
(SOM) is crucial for sustaining crop production in agricultural soils. In addition to providing a background
turnover of nutrients to drive plant growth (Jenkinson
1981), SOM also contributes to the maintenance of soil
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structural properties which are necessary for plant
growth and which prevent erosion (Oades 1984). However, changes in the amounts and characteristics of
gross SOM pools occur very slowly, and long-term experiments over decades are needed to determine the
impact of management practices or other perturbations
on SOM dynamics (Beare et al. 1994; Ladd et al.
1994).
SOM is highly heterogeneous and consists of a variety of different fractions which have various origins and
functional roles in the soil (Stevenson 1994). A number
of SOM fractions appear to be more labile than the
gross SOM pool, with the result that changes in these
pools have potential to provide an early indication of
the impacts of management or environmental stress on
soil quality. These pools include labile organic N, lightfraction OM (LFOM) and water-soluble carbohydrates.
Labile organic N and LFOM are considered to govern
patterns of N mineralization in many soils (Bonde and
Roswall 1987; Janzen et al. 1992; Sierra 1996), while the
carbohydrate pool, especially that linked to the heavy
fraction OM, plays a role in the aggregation of soil particles, which determines soil structural properties
(Oades 1984).
The effect of management practices on SOM is
largely mediated by the soil microbial community, and
the C and N content of the biomass is also viewed as an
indicator of soil quality (Rice et al. 1996). A number of
techniques, including phospholipid fatty acid analysis
(Zelles et al. 1992) and patterns of substrate utilization
(Garland and Mills 1991), have been used to describe
the diversity and structure of soil microbial populations. Since these techniques measure the profiles and
activities of communities directly involved in SOM dynamics, they could have great value for the characterization of soil quality.
Soil labile N and carbohydrate pools, and the C and
N content of the soil microbial biomass can respond
rapidly to changes in management practices and climate
(Duxbury and Nkambule 1994; Kaiser and Heinmeyer
1993; Roberson et al. 1995; Rice et al. 1996). However,
the dynamics of LFOM have been investigated only in
soils subjected to long-term differences in management
practice, and the potential of this pool to act as an early
indicator of the impact of management on soil quality is
not known. Further, the impacts of management on the
dynamics of the labile SOM pools have largely been determined in separate studies over widely different timescales. Similarly, soil microbial diversity measurements have been most extensively used to monitor
changes induced by long-term management practices or
to investigate differences between soil communities
from widely different habitats (Zak et al. 1994; Bossiq
and Scow 1995).
The aim of this study was to investigate the susceptibility of labile OM pools, and the size, metabolic profiles and diversity of the soil microbial community, to
short-term changes in management practice, in order to
compare the relative value of changes in these parame-
ters as early indicators of the impacts of management
on soil quality.
Materials and methods
Site details
Measurements were made on treatments included in an experimental conversion of a 12-ha area of farmland from conventional
to organic production, at HRI Wellesbourne, Warwickshire. The
site is on a sandy loam of the Wick series, with 14% clay and a pH
of 5.9. A detailed description of the soil is given in Whitfield
(1974). The site had been under conventionally managed cereals
for at least 20 years prior to the initiation of the study.
Experimental design
The treatments were established in August 1996. Following harvest of a winter wheat (Triticum aestivum L) crop the soil was
ploughed to 20 cm depth and harrowed. Each treatment consisted
of a 1.5-ha area divided into 6 sub-plots. Treatment 1 had a continuous cover of a rye grass (Lolium perenne L)-red clover (Trifolium pratense L) ley, which was mown to 2 cm height and
mulched at monthly intervals from April–August 1997. A total
dry weight biomass of 2.4 t ha –1 was mulched over this period.
Treatment 2 was under winter vetch (Vicia sativa L) from August
1996 until February 1997. The vetch was then ploughed-in to a
depth of 20 cm and the plot harrowed and sown with spring barley (Hordeum vulgare L) and undersown with white clover (Trifolium repens L). The barley crop was harvested in August 1997,
and the white clover left to regenerate. The total dry weight biomass of shoot and root materials ploughed-in over the period
studied was 3.6 t ha –1. The plots received no addition of fertilizer
or pesticide for the duration of the experiment.
Soil sampling
Soils were sampled in August 1996, April 1997, and December
1997. From each sub-plot, 20 cores were taken with a 2.5-cmdiameter steel auger from the 0 to 30-cm layer. The cores were
pooled and passed through a 3-mm sieve.
Soil conditioning
Soils were pre-conditioned prior to the measurement of qualitative characteristics, in order to avoid any effects of environmental
variables such as temperature and moisture content. In this way,
stable changes developing as a consequence of the management
practices could be monitored. Soil samples were each moistened
to a matric potential of P250 kPa. Samples (200 g fresh weight)
were placed into 150-ml polystyrene beakers, and incubated at
15 7C for 14 days inside 15-l containers. Moist air was continuously passed through the containers to ensure that an aerobic atmosphere persisted.
Microbial community metabolic profiles
Substrate-utilization patterns by the soil microbial population
were determined using Biolog Gram negative (GN) microplates
(Biolog, California) by a procedure adapted from Garland and
Mills (1991). Each plate contains 95 separate sole C sources and a
blank well. The C sources include amino acids, carbohydrates,
carboxylic acids and polymers, together with a number of miscellaneous compounds. The rate of utilization of the C sources is
indicated by the reduction of tetrazolium, a redox indicator dye,
which changes from colourless to purple.
80
Soil (5 g fresh weight) was mixed with 50 ml autoclave-sterilized 0.85% w/v NaCl solution and shaken for 30 min, and then
centrifuged at 150 g for 2 min to remove larger suspended soil
particles. A 2-ml volume of the supernatant was diluted with
18 ml NaCl solution, and 0.15 ml inoculated into each microplate
well. The absorbance of wells at 590 nm was measured using a
microplate reader.
Plates were placed into plastic bags, and incubated at 20 7C in
the dark. Absorbance in the microplates was re-measured after a
72-h incubation, at which point average well-colour development
(AWCD) had reached approximately 0.4 absorbance units. For
each well, the increase in absorbance over 72 h was divided by the
AWCD of the microplate (Garland 1996a). Numbers of culturable bacteria were determined by diluting soil from 10 –2 to 10 –5
using 0.85% w/v NaCl solution, and inoculating 1 ml into 19 ml
nutrient agar. The plates were incubated at 20 7C, and bacterial
colonies counted after 6 days.
Table 1 Effect of management on substrate richness, substrate
evenness and Shannon’s diversity index for microbial community
utilization of Biolog Gram negative microplate substrates. Means
followed by different letters are significantly different (P~0.05)
Treatment
Substrate
richness
Substrate
evenness
Shannon’s
diversity index
Treatment 1
(grass-clover ley)
August 1996
April 1997
December 1997
80.3 a
81.3 a
82.3 a
0.925 a
0.922 a
0.943 b
4.05 a
4.05 a
4.16 b
Treatment 2
(crop sequence)
August 1996
April 1997
December 1997
82.5 a
83.6 a
80.8 a
0.925 a
0.936 a
0.931 a
4.08 a
4.14 a
4.09 a
Microbial biomass and SOM characteristics
Microbial biomass N was determined by the fumigation-extraction method (Joergensen and Brookes 1990). Ninhydrin-N released on fumigation was converted to microbial N using a conversion factor of 3.1 (Amato and Ladd 1988). Total OM C and N
were determined at the beginning and end of the experiment using an automated C/N analyser (CN-2000; Leco, Mich.). The
LFOM content was determined by the method of Strickland and
Sollins (1987). The total C and N content of LFOM was determined using the automated C/N analyser. Hot water-soluble carbohydrates were extracted by the method of Brink et al. (1960).
Carbohydrates were determined by a phenol-H2SO4 assay using
glucose standards (Dubois et al. 1956).
Labile organic N was determined by measuring net N mineralization potential during the 2-week conditioning step (Anderson
–
and Ingram 1993). Soil mineral N (NH c
4 and NO 3) was determined before and following incubation using an EnviroFlow 5012
flow injection system (Tecator, Sweden). The mineralization potential was determined by subtracting mineral N present before
incubation from that present following incubation.
Statistical analysis
T-tests were conducted to determine if there were significant
changes to OM and biomass between treatments and within treatments over time. Biolog substrate-utilization patterns were analysed according to Zak et al. (1994) to give substrate richness (the
number of substrates utilized), substrate evenness (the distribution of colour development between the substrates), and Shannon’s diversity index. Patterns of utilization of all Biolog substrates, and those classed as amino acids, carbohydrates, carboxylic acids and polymers, were analysed by principal component
analysis (PCA), using the statistical program Genstat.
Results
Microbial community metabolic profiles
There were no differences in substrate richness between the treatments, and this parameter did not
change over time in either treatment (Table 1). In the
crop sequence, there were no changes to substrate
evenness or diversity over time. However, in the ley
treatment, both evenness and diversity increased significantly between April 1997 and December 1997.
PCA of substrate utilization for all substrates demonstrated that the communities in the two treatments
were very similar at the beginning of the experiment
(Fig. 1a) but that patterns of metabolism diverged with
the imposition of the different management practices.
There was a shift in the pattern of substrate utilization
along PC 2, which was similar in the two treatments,
but which occurred in April 1997 in treatment 2, and
which was delayed until December 1997 in treatment 1.
The pattern of substrate utilization in treatment 2 altered slightly between April 1997 and December 1997,
so that the two treatments had distinctly different patterns of substrate utilization at December 1997. When
the amino acids (Fig. 1b) carbohydrates (Fig. 1c) and
polymer (Fig. 1d) components of the Biolog plates
were analysed by PCA separately, the relative distribution of the treatments were very similar to when all
substrates were analysed together (Fig. 1a). In contrast,
patterns of utilization of carboxylic acids (Fig. 1e)
showed clear changes in substrate use occurring in the
ley between August 1996 and April 1997, while there
was no difference in carboxylic acid utilization between
the two treatments at December 1997.
Microbial biomass and SOM characteristics
There were no significant differences in biomass N or
AWCD of Biolog plates between or within the treatments over the course of the study, with the parameters
remaining at 14.5 mg N g –1 (dry weight) soil (SEM,
B0.5) and 0.4 absorbance units (SEM, B0.01) respectively.
There were no changes to the whole SOM or LFOM
C and N content within or between the treatments over
the course of the experiment, with C remaining at 8600
(SEMB100) and 460.4 (SEMB17.1) mg g –1 (dry
weight) soil in the whole SOM and LFOM respectively,
and N remaining at 960 (SEMB10) and 21.8
(SEMB0.7) mg g –1 (dry weight) soil in the whole SOM
and LFOM respectively.
81
Fig. 1a–e. Principal component (PC) analysis ordination plot
for Biolog substrate utilization in the treatments over time.
a All substrates, b amino acids, c carbohydrates, d carboxylic
acids, e polymers. Solid symbols represent treatment 1, open
symbols represent treatment 2, bars represent BSEM along
each PC axis.
August 1996,
April 1997, D December
1997
Water-soluble carbohydrates
Soil mineral N and N mineralization potential
There was no change in the water-soluble carbohydrate
content in either treatment between August 1996 and
April 1997 (Table 2). However, in both treatments this
pool declined by approximately 40% between April
and December 1997.
At the beginning of the experiment, the soil mineral N
content was the same in both treatments (Table 3).
There was little change to soil mineral N in the ley over
the course of the experiment. However, following
plough-in of the vetch there was a fourfold increase in
soil mineral N in the crop sequence. This was followed
by a decline to the initial level by December 1997.
There was no significant difference in mineralization
82
Table 2 Effect of management on the soil water-soluble carbohydrate pool. Figures in parentheses give BSEM. Means followed by
different letters are significantly different (P~0.05). dw Dry weight
Treatment 1 (grass-clover ley)
Carbohydrate
[mg g –1 (dw) soil]
Treatment 2 (crop sequence)
August 1996
April 1997
December 1997
August 1996
April 1997
December 1997
250 a
(4)
261 a
(19)
162 b
(3)
222 ac
(11)
212 c
(6)
126 d
(6)
Table 3 Effect of management on soil mineral N and N mineralization potential. Figures in parentheses give BSEM. Means followed
by different letters are significantly different (P~0.05)
Treatment 1 (grass-clover ley)
Soil mineral N
[mg N g –1 (dw) soil]
N mineralization potential
[mg N g –1 (dw) soil]
Treatment 2 (crop sequence)
August 1996
April 1997
December 1997
August 1996
April 1997
December 1997
5.8 a
(0.5)
3.3 a
(0.8)
7.2 a
(2.0)
–4.0 b
(2.2)
3.4 a
(0.7)
2.0 a
(0.6)
5.3 a
(0.7)
4.8 a
(0.5)
20.5 b
(3.5)
–2.2 ab
(3.2)
6.1 a
(0.6)
3.2 a
(0.4)
potential between the two treatments at any of the sampling dates. However, in both treatments there was a
significant decrease in mineralization potential between
August 1996 and April 1997, resulting in net immobilization of soil mineral N in April 1997.
Discussion
The results demonstrated that changes in patterns of
substrate utilization and metabolic diversity by the Biolog-culturable soil microbial community are more sensitive indicators of management-induced effects to soil
biological properties, and hence changes in soil quality,
than are gross measures of the microbial biomass and
labile OM components.
Changes to labile OM pools reflect the balance between synthesis and degradation of that pool by the microbial biomass. While the OM pools are all ultimately
derived from plant materials, the nature of the plantderived substrates which give rise to each of the measured pools are different. Further, different groups of
soil microbes are likely to govern the transformations
of the different labile OM pools. The rate at which each
OM pool responds to changes in management or other
perturbation is likely to vary considerably between soil
types and according to the nature of organic inputs.
Since each of the pools has a different role in terms of
soil functioning, a change in any of the measured pools
could be viewed as a change in the functional quality of
SOM. In order to examine soil quality dynamics effectively, a range of OM pools therefore needs to be measured to take account of the varied and overlapping
functional roles of different OM pools.
Despite the incorporation of fresh plant materials
into both treatments, the level of LFOM did not change
in our study, demonstrating that the formation of this
pool was in equilibrium with its degradation. Similarly,
Liang et al. (1998) found that when amounts of corn
crop residues returned to the soil at harvest were altered by manipulating the fertilizer regime, there was
no effect on LFOM C or N over a 3 to 11-year period.
However, in the same study, LFOM C and N increased
markedly within 3 years of the start of a regime in
which manure was added to soil, with the increase related to the amount of OM added. This suggests that
the stabilization of OM added to soil as LFOM could
be related to the biochemical composition of the additions, and the capacity of the soil microbial population
to adapt to new substrates. This was confirmed in laboratory incubation studies for additions of plant materials (Bending et al. 1998).
The dynamics of the labile organic N and water-soluble carbohydrate pools were similar in the two treatments, with changes induced by time of year of sampling, but not management. While the plough-in of
vetch in treatment 2 resulted in a flush of net N mineralization from the incorporated tissues, any residual unmineralized organic N must therefore have been resistant to decomposition, or stabilized by conversion into
non-labile forms.
The apparent immobilization of N in spring, which
occurred in both treatments, may have resulted from
the activities of the saprophytic community during
growth on low C/N ratio root material arising from the
senescence of old roots over winter.
Management practice had no effect on soil biomass
N, probably because there was no change in the total
OM content, or the size of most of the labile OM pools,
which could be considered to represent energy sources
for the biomass.
In contrast to the microbial biomass and SOM characteristics, the substrate-utilization patterns suggested
that the management practices employed caused
83
changes in microbial community composition. There
was no change in the Biolog-culturable microbial community of the ley soil between August 1996 and April
1997. During this period there was effectively no management-induced perturbation imposed. In vitro studies have shown that Biolog-culturable rhizosphere bacterial communities can change with the age of plants,
reflecting changes in amounts and composition of exudates (Garland 1996b). However, in our study there
were no changes to the Biolog-culturable microbial
population as the crop aged between August 1996 and
April 1997, suggesting that crop growth and age had no
effect on the bulk soil microbial population. Following
the addition of OM to the ley soil by mowing and
mulching, the composition of the microbial community
did change. Similarly, in the crop sequence, the community was altered following ploughing-in of the vetch
crop. In comparison, there was only a relatively small
change associated with the harvest of spring barley,
presumably reflecting the lower disturbance of the soil
at that time.
Despite the different types of material added to soil
in the treatments, and the modes by which the organic
materials were incorporated, the changes in the microbial community metabolic profiles following input of
organic material were remarkably similar. This could
have reflected similarities in the biochemical composition of the substrates incorporated, which consisted
largely of N-rich leguminous materials in both treatments. There was a greater amount of OM, and diversity of substrates, incorporated into the crop sequence
than the ley. However, the fact that metabolic diversity
increased in the ley, but not in the crop sequence, could
have reflected the impact of soil disturbance imposed
by tillage. Similarly, Lupwayi et al. (1998) found that
tillage reduced the diversity of Biolog-culturable bacteria, although they found that diversity increased in a sequence of different crop types relative to continuous
wheat.
All the different substrate groups within the Biolog
plates showed changes in patterns of oxidation by the
microbial population. Carboxylic acids, in contrast to
the other substrates, showed clear differences with respect to community metabolism in treatment 1 between
August 1996 and April 1997, but failed to show differences between the treatments in December 1997. This
suggests that carboxylic acid oxidation was modified by
a portion of the Biolog-culturable community which
did not contribute greatly to the oxidation of the other
substrates.
Smalla et al. (1998) showed that the populations
which develop in Biolog plates following inoculation
with soil microbial communities are largely fast-growing species adapted to high substrate concentrations, including Pseudomonas, Enterobacter, Pantoea and Salmonella. Although the contribution of this population
to soil processes is unclear, changes in the metabolic
profile of this community could be useful as a rapid indicator of the effect of management on soil quality.
Acknowledgements We thank the Guild of St Georges at
HDRA and the Ministry of Agriculture, Fisheries and Food for
financial support, and Julie Jones and Kath Phelps for statistical
advice.
References
Amato M, Ladd JN (1988) Assay for microbial biomass based on
ninhydrin-reactive nitrogen in extracts of fumigated soils. Soil
Biol Biochem 20 : 107–114
Anderson JM, Ingram JSI (1993) Tropical soil biology and fertility. A handbook of methods, 2nd edn. CAB International,
Wallingford, UK
Beare MH, Hendrix PF, Coleman DC (1994) Water-stable aggregates and organic-matter fractions in conventional-tillage and
no-tillage soils. Soil Sci Soc Am J 58 : 777–786
Bending GD, Turner MK, Burns IG (1998) Fate of nitrogen from
crop residues as affected by biochemical quality and the microbial biomass. Soil Biol Biochem 30 : 2055–2065
Bonde TA, Rosswall T (1987) Seasonal variation of potentially
mineralizable nitrogen in four cropping systems. Soil Sci Soc
Am J 51 : 1508–1514
Bossiq DA, Scow KM (1995) Impact of carbon and flooding on
the metabolic diversity of microbial communities in soil. Appl
Environ Microbiol 61 : 4043–4050
Brink RH, Dubach P, Lynch DL (1960) Measurement of carbohydrates in soil hydrolyzates with anthrone. Soil Sci
89 : 157–166
Doran JW, Coleman DC, Bezdicek DF, Stewart BA (1994) Defining soil quality for a sustainable environment. SSSA special
publication no. 35. Soil Science Society of America, Madison,
Wis., p 244
Duxbury JM, Nkambule SV (1994) Assessment and significance
of biologically active soil organic nitrogen. In: Doran JW,
Coleman DC, Bezdicek DF, Stewart BA (eds) Defining soil
quality for a sustainable environment. SSSA special publication no. 35, Soil Science Society of America, Madison, Wis.,
pp 125–146
Dubois M, Gilles KA, Hamilton JK, Hofman G, Smith F (1956)
Colorimetric method for the determination of sugars. Anal
Chem 28 : 350–355
Garland JL (1996a) Analytical approaches to the characterisation
of samples of microbial communities using patterns of potential C source utilization. Soil Biol Biochem 28 : 213–221
Garland JL (1996b) Patterns of potential C source utilization by
rhizosphere communities. Soil Biol Biochem 28 : 223–230
Garland JL, Mills AL (1991) Classification and characterization
of heterotrophic microbial communities on the basis of patterns of community level sole carbon source utilization. Appl
Environ Microbiol 57 : 2351–2359
Hassink J (1997) The capacity of soils to preserve C and N by
their association with clay and silt particles. Plant Soil
191 : 77–87
Janzen HH, Campbell CA, Brandt SA, Lafond GP, TownleySmith L (1992) Light fraction OM in soils from long-term crop
sequences. Soil Sci Soc Am J 56 : 1799–1806
Jenkinson DS (1981) The fate of plant and animal residues in soil.
In: Greenland DJ, Hayes MHB (eds) The chemistry of soil
constituents. Chichester. Wiley, UK, pp 505–561
Joergensen RG, Brookes PC (1990) Ninhydrin-reactive measurements of microbial biomass in 0.5 M K2SO4 soil extracts. Soil
Biol Biochem 22 : 1023–1027
Kaiser EA, Heinmeyer O (1993) Seasonal variations of soil microbial biomass carbon within the plough layer. Soil Biol Biochem 25 : 1649–1655
Ladd JN, Amato M, Li-Kai Z, Schultz JE (1994) Differential effects of crop sequence, plant residue and nitrogen fertilizer on
microbial biomass and organic matter in an Australian alfisol.
Soil Biol Biochem 26 : 849–859
84
Liang BC, MacKenzie AF, Schnitzer M, Monreal CM, Voroney
PR, Beyaert RP (1998) Management-induced change in labile
soil organic matter under continuous corn in eastern Canada.
Biol Fertil Soils 26 : 88–94
Lupwayi NZ, Rice WA, Clayton GW (1998) Soil microbial diversity and community structure under wheat as influenced by
tillage and crop sequence. Soil Biol Biochem 30 : 1733–1741
Oades JM (1984) Soil organic matter and structural stability:
mechanisms and implications for management. Plant Soil
76 : 319–337
Rice CW, Moorman TB, Beare M (1996) Role of microbial biomass carbon and nitrogen in soil quality. In: Doran JW, Jones
AJ (eds) Methods for assessing soil quality. SSSA special publication no. 49, Soil Science Society of America, Madison,
Wis., pp 203–215
Roberson EB, Sarig S, Shennan C, Firestone MK (1995) Nutritional management of microbial polysaccharide production
and aggregation in an agricultural soil. Soil Sci Soc Am J
59 : 1587–1594
Sierra J (1996) Nitrogen mineralization and its error of estimation
under field conditions related to the light fraction soil organic
matter. Aust J Soil Res 34 : 755–767
Smalla K, Wachtendorf U, Heuer H, Liu W, Forney L (1998)
Analysis of BIOLOG GN substrate utilization patterns by microbial communities. Appl Environ Microbiol 64 : 1220–1225
Stevenson FJ (1994) Humus chemistry, 2nd edn. Wiley, New
York
Strickland TC, Sollins P (1987) Improved method for separating
light- and heavy-fraction organic material from soil. Soil Sci
Soc Am J 51 : 1390–1393
Trasar-Cepeda C, Leiros C, Gil-Sotres F, Seoane S (1998) Towards a biochemical quality index for soils: an expression relating several biological and biochemical properties. Biol Fertil Soils 26 : 100–106
Whitfield WADI (1974) The soils of the National Vegetable Research Station, Wellesbourne. Report of the National Vegetable Research Station for 1973. National Vegetable Research
Station, Wellesbourne, UK, pp 21–30
Zak JC, Willig MR, Moorhead DL, Wildman HG (1994) Functional diversity of microbial communities: a quantitative approach. Soil Biol Biochem 26 : 1101–1108
Zelles L, Bai QY, Beck T, Beese F (1992) Signature fatty acids in
phospholipids and lipopolysaccharides as indicators of microbial biomass and community structure in agricultural soils.
Soil Biol Biochem 24 : 217–323