Researched by Felix Bast (Formerly Vadakke Madam Sreejith), BBC Researcher No. U248285, [email protected]

Accepted to the journal on 9th September, 2003

What is soil biodiversity?

Soil biodiversity reflects the mix of living organisms in the
soil. These organisms interact with one another and with
plants and small animals forming a web of biological
activity.

Soil is by far the most biologically diverse part of Earth.
The soil food web includes beetles, springtails, mites,
worms, spiders, ants, nematodes, fungi, bacteria, and
other organisms. These organisms improve the entry and
storage of water, resistance to erosion, plant nutrition,
and break down of organic matter. A wide variety of
organisms provides checks and balances to the soil food
web through population control, mobility, and survival
from season to season.

How the soil microorganisms are beneficial to us?

Nutrient cycling

Soil organisms play a key role in nutrient cycling. Fungi,
often the most extensive living organisms in the soil,
produce fungal hyphae. Hyphae frequently appear like
fine white entangled thread in the soil. Some fungal
hyphae (mycorrhizal fungi) help plants extract nutrients
from the soil. They supply nutrients to the plant while
obtaining carbon in exchange and thus extend the root
system. Root exudates also provide food for fungi,
bacteria, and nematodes.

When fungi and bacteria are eaten by various mites,
nematodes, amoebas, flagellates, or ciliates, nitrogen is
released to the soil as ammonium. Decomposition by soil
organisms converts nitrogen from organic forms in
decaying plant residues and organisms to inorganic forms
which plants can use.

Residue decomposition

Soil organisms decompose plant residue. Each organism
in the soil plays an important role. The larger organisms in
the soil shred dead leaves and stems. This stimulates
cycling of nutrients. The larger soil fauna include earthworms,
termites, pseudoscorpions, microspiders, centipedes,
ants, beetles, mites, and springtails.
When mixing the soil, the large organisms bring material
to smaller organisms. The large organisms also carry
smaller organisms within their systems or as “hitchhikers”
on their bodies.

Small organisms feed on the by-products of the larger
organisms. Still smaller organisms feed on the products
of these organisms. The cycle repeats itself several times
with some of the larger organisms feeding on smaller
organisms.
Some larger organisms have a life span of two or more
years. Smaller organisms generally die more quickly, but
they also multiply rapidly when conditions are favorable.
The food web is therefore quick to respond when food
sources are available and moisture and temperature
conditions are good.
Infiltration and storage of water
Channels and aggregates formed by soil organisms
improve the entry and storage of water. Organisms mix
the porous and fluffy organic material with mineral matter
as they move through the soil. This mixing action provides
organic matter to non-burrowing fauna and creates
pockets and pores for the movement and storage of water.
Fungal hyphae bind soil particles together and slime from
bacteria help hold clay particles together. The waterstable
aggregates formed by these processes are more
resistant to erosion than individual soil particles. The
aggregates increase the amount of large pore space which
increases the rate of water infiltration. This reduces
runoff and water erosion and increases soil moisture for
plant growth.

Diversity of the soil biota

The soil biota includes representatives of all
groups of microorganisms and fungi, green and
blue-green algae and of nearly all animal phyla,
Current estimates of the number of species
some groups include bacteria ( 30 000), fungi (
500 000), algae (60 000), protozoa ( 100 000),
nematodes (500 000, and earthworms (3000). In-
dividual organisms range in size from less than
1/an in diameter with a weight of less than I
for the smallest bacteria, to more than 1 m in
length and more than 20 mm in diameter, with a
weight of more than 500 g for the largest earth-
worms. Biomass in a fertile soil may exceed 20 t
ha-1.

Sampling methods differ from group to group
of soil organisms; the different methods vary in
their efficiency and for most, probably all, groups
they are known to underestimate the numbers
and the variety of species. For example, the soil
microflora is known very largely from those or-
ganisms that can be isolated by the conventional
culture methods used by soil microbiologists.
These are known to represent only a small pro-
portion, probably less than 20% of the total mi-
crobial taxa present in soil. The taxa that are selected by the culture
methods may or may not be important or even
significant contributors to the whole range of
biodiversity of active soil microorganisms. Many
bacteria seen in situ by electron microscopy of
soil are unknown species, represented by single
cells less than lpm in diameter; some of these might be dormant stages of
bacteria that change their form and become
readily recognisable when they are provided with
a suitable substrate.

The complexity of communities in the soil
biota and the paucity of taxonomic knowledge of
many common groups make it difficult to define
precisely the diversity of the soil biota. These
problems necessitate the recognition of trophic
or other groups rather than individual species as
units of biodiversity. The grouping tech-
niques have enhanced understanding of the
nature of biodiversity in soils and its impacts on
pedogenesis and soil fertility,
Keystone species, whose activities are critical
for example in organic matter breakdown, de-
serve particular study.

Why are there so many species?

A striking feature of the soil biota is the very
high number of individuals and the variety of
species that make up communities. Wallwork
(1976) likened the diversity of species of soil
animals, even in small areas, to animal diversity
on coral reefs. The numbers and diversity of soil
microorganisms, added to those of soil animals,
must distinguish soils as the most prolific in bio-
diversity of all natural systems. It seems reason-
able to ask why are there so many species?

Bacteria are the most successful and biochem-
ically diverse of soil biota. Many have the ability
to survive as spores or in similar resting stages
for long periods, but respond quickly to-the
availability of a suitable substrate and have a high
intrinsic rate of population increase. Jenkinson
and Ladd ( 1981 ) estimated that in an English
soil bacterial cells divide, on average, once every
2.5 years, spending the remainder of the time in
resting stages. Bacterial activity in soils must be
seen as so disjunct in time and space that com-
mon concepts that are applied to most organisms
(i.e. individuals making up populations of a par-
ticular species, or aggregations of species form-
ing coherent communities), cannot easily be


adopted for soil bacteria. Some bacterial cells
have limited mobility and may move in water
films on soil surfaces over minute distances,
however soil bacteria must generally be seen as
sedentary, waiting for food to come their way,
and not as foragers. Their dispersal results largely
from passive transport by infiltrating water, or
with wind-blown dust, or transport by soil ani-
mals, either on the body surfaces or by excretion
of viable propagules that have passed through the
gut, or by mechanical disturbance of soil aggre-
gates during cultivation.

Very small scale spatial separation of sites of
substrate decomposition provides scope for the
successful coexistence of a variety of species that
exploit a common substrate, but whose activities
do not overlap. Similarly, there is scope for spe-
cies that exploit the same substrate but whose ac-
tivities are episodic and do not coincide because
they have different temperature, moisture, or
other physical environmental optima that are
satisfied at different times. So commonly ac-
cepted concepts of niche specificity and compet-
itive exclusion among species that potentially
share the same niche cannot readily be applied
to the less mobile taxa within the soil biota.
Spores and other propagules of fungi are
moved passively within the soil, in infiltrating
water and by the soil fauna, as are bacteria, but
their ability to modify their growth patterns and
extend their hyphae to follow gradients of nu-
trient concentration gives them a form of mobil-
ity that clearly distinguishes them from bacteria.
Lussenhop ( 1992 ) described how soil fungi for-
age by varying growth patterns from diffuse per-
ennial networks to short-lived colonies, and by
rhizomorphs. The mode of foraging of a basidi-
omycete, Steccheriumfimbriatum, was shown by
Dowson et al. ( 1988 ) to switch from a slow-dif-
fuse to a fast-effuse growth pattern of its hyphae
when it made contact with a suitable substrate
supplied as a bait. It is, however, not easy to de-
fine an individual in an extensive network of
fungal hyphae, nor to equate the growth and
spread of colonies or hyphae with the dispersal
of populations of motile soil organisms.
Behavioural or other attributes that separate
species in space or time so that they do not com-


pete directly for a common resource are well
known among some communities of larger orga-
nisms, e.g. the species rich communities of ants
that are common in semi-arid environments,
Species diversity ofmicroarthropods is often very
high, as for example 108 species (45 oribatids)
per 250 cm 2 (to a depth of 5 cm) in a Dutch soil
under unmanaged grassland (Siepel and van der
Bund, 1988 ), 8-26 oribatid species per 5 cm 2 (to
a depth of 5 cm) in a forest soil in Finland
(Karppinen, 1958, cited in Siepel, 1994). These
and other high microarthropod densities were
seen by Siepel (1994) to be at odds with the con-
cept that species should minimise niche overlap
if they are to survive interspecific competition,
Siepel concluded that there must be many re-
sources in soil that are separated into discon-
nected patches often on a small scale, as pro-
posed above for microorganisms, or that
potentially competing species of microarthro-
pods might make inefficient use of, or have lim-
ited access to resources, thus explaining their co-
existence. Siepel constructed a simulation model
(Siepel, 1994) to evaluate the balances between
life history traits, efficiency of use of resources,
and tolerance for short-term unpredictable en-
vironmental extremes. With its aid, and on the
basis of his own research on microarthropod
communities, it was demonstrated that the clas-
sical concept of a niche is too restricted, so that
less efficient users of a resource can coexist with
more efficient users of the same resource when
the space containing the resource is distributed
in ephemeral patches, whereas the inferior com-
petitor will be eliminated where the resource is
continuously distributed. Differences in fecund-
ity, the maximum tolerated starvation period,
and relative mobility were among other charac-
teristics shown to be critical in permitting coex-
istence of species with apparently identical
niches. The possibilities for species to share re-
sources are so great that Siepel concluded that
we may wonder why there are so few species
rather than so many species of soil
microarthropods.

Management considerations

Cultivation

The effects of cultivation depend on the depth and
frequency of the cultivation. Tilling to greater depths and
more frequent cultivations have an increased negative
impact on all soil organisms. No-till, ridge tillage, and strip
tillage are the most compatible tillage systems that
physically maintain soil organism habitat and biological
diversity in crop production.

Compaction

Soil compaction reduces the larger pores and pathways,
thus reducing the amount of suitable habitat for soil
organisms. It also can move the soil toward anaerobic
conditions, which change the types and distribution of soil
organisms in the food web. Gaps in the food web induce
nutrient deficiencies to plants and reduce root growth.

Pest control

Pesticides that kill insects also kill the organisms carried
by them. If important organisms die, consider replacing
them. Plant-damaging organisms usually increase when
beneficial soil organisms decrease. Beneficial predator
organisms serve to check and balance various pest
species.
Herbicides and foliar insecticides applied at recommended
rates have a small impact on soil organisms.
Fungicides and fumigants have a much greater impact on
soil organisms.

Fertility

Fertility and nutrient balances in the soil promote biological
diversity. Typically, carbon is the limiting resource to
biological activity. Plant residue, compost, and manure
provide carbon. Compost also provides a mix of organisms,
so the compost should be matched to the cropping
system.

Cover crops and crop rotations

The type of crops that are used as cover or in crop
rotations can affect the mix of organisms that are in the
soil. They can assist in the control of plant pests or serve
as hosts to increase the number of pests. Different species
and cultivars of crops may have different effects on pests.
However, the organisms and their relation to the crop are
presently not clearly understood.

Crop residue management

Mixing crop residue into the soil generally destroys fungal
hyphae and favors the growth of bacteria. Since bacteria
hold less carbon than fungi, mixing often releases a large
amount of carbon as carbon dioxide (CO2). The net result
is loss of organic matter from the soil.

When crop residue is left on the soil surface, primary
decomposition is by arthropod shredding and fungal
decomposition. The hyphae of fungi can extend from
below the soil surface to the surface litter and connect the
nitrogen in the soil to the carbon at the surface. Fungi
maintain a high C:N ratio and hold carbon in the soil. The
net result is toward building the carbon and organic
matter level of the soil. In cropping systems that return
residue, macro-organisms are extremely important.
Manage the soil to increase their diversity and numbers.

References and Suggested Reading

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and T. Persson (Editors), Soil Organisms as Components
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Dowson, C.G., Rayner, A.D.M. and Boddy, L., 1988. Forag-
ing patterns of Phallus impudicus, Phanerochaete laevis, and Steccherinum fimbriatum between discontinuous re-
source unitsinsoil.FEMSMicrobiol. Ecol., 53: 291-298.

Hawksworth, D.L. and Mound, L.A., 1991. Biodiversity da-
tabases: the crucial significance of collections. In: D.L.
Hawksworth (Editor), The Biodiversity of Microorga-
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Jenkinson, D.S. and Ladd, J.N., 1981. Microbial biomass in
soil: measurement and turnover. In: E.A. Paul and J.N
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Karppinen, E., 1958. Uber die Oribatiden (Acar.) der fin- nischen Waldboden. Ann. Zool. Soc. Vanamo., 19:1 - 43.

Lee, K.E., 1959. The earthworm fauna of New Zealand. NZ
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Lee, K.E., 1985. Earthworms: Their Ecology and Relation-
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Lee, K.E., 1994. The functional significance of biodiversity
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Lee, K.E. and Foster, R.C., 1991. Soil fauna and soil struc-
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Lee, K.E. and Pankhurst, C.E., 1992. Soil organisms and sus-
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Levin, S.A., 1991. The mathematics of complex systems. In:
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Lussenhop, J., 1992. Mechanisms ofmicroarthropod-micro-
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