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The cost of soil degradation in England and Wales
Appendix K: Soil biota
Introduction
The structure and processes of terrestrial ecosystems are profoundly dependant upon a functioning
soil biota. The biota is responsible for processing carbon, nutrient cycling, structural genesis and
maintenance, pathogenicity and symbionts. It drives many above ground processes. However, in the
majority of studies the soil biota reflects the pressures and changes in the rest of the ecosystem
arising from human activity, and in only a few cases studies so far, drive or facilitate such changes 1.
Consistent relationships between soil biodiversity and specific soil functions have yet to be
demonstrated2, suggesting that more species do not necessarily provide more services – most likely
due to a high degree of functional redundancy. There have been no single species – exclusive single
function relationships found.
Intense cultivation impacts heavily on the invertebrate population, especially earthworms, and
decreases fungal biomass. Both of these will result in decreases in aggregate stability, increased
susceptibility to erosion – but these losses are captured in the general effects of cultivation – i.e. this
is the mechanism by which losses due to decrease in soil integrity are mediated.
There are few terrestrial ecosystem services which are not dependant on soil biodiversity – either
wholly or in part (Figure 1). Multiple services are provided by single species of soil organisms, which
makes it almost impossible to disentangle the effects of a particular species on a particular function3.
What is more, the delivery of the supporting service underly the delivery of the other services, so that
these other services are dependant on the activities of soil biota, without necessarily exhibiting a
strong relationship with any one group.
Figure 1. Ecosystem services - those in bold are in whole or part dependant on soil
biodiversity
1
Harris, J. (2009). Soil Microbial Communities and Restoration Ecology: Facilitators or Followers? Science 31 July 2009: Vol.
325 no. 5940 pp. 573-574
2
Turbe et al (2010) Soil biodiversity: functions, threats and tools for policy makers. Bio Intelligence Service IRD and NIOO,
Report for European Commission (DG Environment).
3 Turbe et al (2010) Soil biodiversity: functions, threats and tools for policy makers. Bio Intelligence Service IRD and NIOO,
Report for European Commission (DG Environment).
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The Function of the soil biota
Turbe et al (2010)4 have outlined the principal functions of the soil biota in three principal categories,
chemical engineers, biological regulators and ecosystem engineers.
Decomposition – chemical engineers
The soil biota is responsible for the decomposition of organic matter into small molecules which form
the nutrient supply for further plant growth. This is carried out by a number of groups, but fungi and
bacteria are the most important in this regard.
Regulation – biological regulators
Ranging from bacteria to small invertebrates, biological regulators act as predators and pathogens of
plants and other organisms, affecting the distribution and activities of these organisms. They are
extremely important in regulating population size, and the composition of plant communities, and
dampen wild oscillations in populations of soil organisms and vegetation.
Ecosystem engineers
Small invertebrates, from earthworms to ants, and small mammals structure soils and produce the
complexity found in soil environments. Fungi also structure soils by binding together mineral particles
with their hyphae, and many bacteria produce gums which aids this process. The effect of ecosystem
engineers are manifold, from producing pore structure allowing the free movement of liquids and
gases, to isolating organisms and organic matter which regulates the availability of resources to other
organisms.
Factors affecting the soil biological system
Abiotic factors
A number of interacting factors affect the composition and activity of the soil biota. These include
temperature, moisture, pH, salinity, and oxygen availability. Climate regulates the physiology of
organisms, within their range of adaptation although many form resistant structures such as cysts to
avoid periods of drought or low temperatures. The effect of pH is profound in controlling the
composition and activity of the soil biota, by regulating the activity of enzymes, internal physiological
states and the availability of nutrients – outside optimum ranges availability of some elements become
so low or non-existent to the point of starving the biota – other elements, especially metals become so
available that they begin to exert toxic effects.
Biotic factors
Plants can strongly influence the soil biota due to the latter’s dependence of these primary producers
for their energy and carbon needs – and this is most clearly seen in the vicinity of plant roots in the
“rhizosphere” which is the volume immediately surrounding them. Similar plants can be significantly
dependant on the soil biota – the composition of plant assemblages may be regulated by pathogens
and the presence of absence of key symbionts such as N-fixing bacteria and mycorrhizae.
Services provided by soil biodiversity
Soil organic matter cycling and fertility
The soil biota processes organic matter arising from plants (as root exudates and above ground litter)
animals and other organisms. This releases nutrients, builds structure and provides a buffer against
fluctuations in food supply and environmental conditions.
Soil structural formation and maintenance
The formation of soil structure is driven by plant rooting and the activities of ecosystem engineers –
this provides pore spaces and stability of soil structures, particularly aggregates. However, this
4
Turbe et al (2010) Soil biodiversity: functions, threats and tools for policy makers. Bio Intelligence Service IRD and NIOO,
Report for European Commission (DG Environment).
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The cost of soil degradation in England and Wales
structure is constantly being turned over, and with out continued inputs and activities of the soil biota
structure quickly degrades and collapses, increasing susceptibility to eroding forces, particularly wind
and water. Similar compaction caused by grazing and trafficking can only be revered by the activities
of the soil biota – absence of key groups, such as earthworms may make soil structural degradation
irreversible.
Regulation of carbon flux and feedback to climate
Regulation of carbon fluxes has a key role to play in climate feedbacks. Immobilisation of carbon or
its mineralisation (and therefore release into the atmosphere) is dependant upon the interactions
between biota, structure and chemistry of the soil (Figure 2).
C storage
C
Soil
biota
Regulated by
•Structure
•Chemistry
•C input quality & quantity
•Community phenotype
C loss
Figure 2 Control of C storage and loss is regulated by key biotic and abiotic factors, and
mediated by the soil biota.
Regulation of the water cycle
The water relations in the soil are profoundly affected by the soil biota (Figure 3). Microbes, notably
fungi affect water repellency, and after dry spells can impede the penetration of water into the soil
surface. Plant water uptake and evaporation is affected by the relationships between plant roots and
mycorrhizas, and the activities of roots feeding organism, the latter can hamper water uptake.
Generally ecosystem engineers, such as earthworms generate pore spaces, drainage channels,
which are stabilised in conjunction with the activities of fungi producing mycelia, and gums and
mucilages generated by bacteria.
Figure 3. The relationship between soil biota and soil properties with respect to water (redrawn
from Bardgett et al 20015)
5
R. D. Bardgett, J. M. Anderson, V. Behan-Pelletier, L. Brussaard, D. C. Coleman, C. Ettema, A. Moldenke, J. P. Schimel and
D. H. Wall (2001) The Influence of Soil Biodiversity on Hydrological Pathways and the Transfer of Materials between Terrestrial
and Aquatic Ecosystems Ecosystems 4, 421 – 429.
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Decontamination and bioremediation
Many groups of micro-organism are capable of producing molecules and structures capable of
decomposing quite recalcitrant poly-aromatic, cross linked and polymerised compounds such as
lignin. This allows the soil biota to degrade the complex materials produced over the course of
seasonal cycles in response to litter fall. This also allows these groups to attack recalcitrant
xenobiotics compounds produced by human activitiy, such as plastics and contaminants.
Pest control
There are several mechanisms by which soil biota can suppress pest organism. Mycorrhizas can
prevent the infection of plant roots by pathogens. Some fungi will trap pest nematodes and digest
them. Bacterial species produce endotoxins that, when ingested by insects result in death - Bacillus
thuringiensis is a good example of this.
Economic valuation
Attempts to provide a monetary valuation of soil biodiversity are scarce, principally because of the
difficulty in disentangling the effects solely attributable to a single species or group of species on a
particular identifiable service or function – this is due to the complex interactions between soil
organism and the abiotic environment and the emergent properties arising out of this complexity.
Soils cannot exist without a living biota providing all of the supporting services and therefore there is a
danger of double-accounting in considering the biota separately as this underlying service is manifest
through the other services.
This notwithstanding Pimental et al6 did attempt to provide an estimate of the value of ecosystem
services delivered by the soil biota (Table 1).
Table 1. Total economic benefit of services provided by soil biodiversity (modified from
Pimentel et al, 19977)
Service/Activity
World economic benefits
(x US dollars 109 per year adjusted to 2010 prices)
1030
34
122
164
217
271
244
8
2090
Organic matter cycling/waste recycling
Soil formation
Nutrient cycling
Bioremediation
Pest control
Fertility/pollination
Wild food
Biotechnology industry
Total
This translates approximately (assuming only 70% of the terrestrial surface is covered by significant
soil stocks) to £600GBP ha-1 at current exchange rates, and accounting for inflation since 1997. This
total is realised through the variety of services experienced by society, and the cost of degradation of
soil biota is therefore accounted for in the individual services described elsewhere in this report.
Potential relationships between decreasing biodiversity and decline
in service provision
There are a number of potential relationships between biodiversity and ecosystem functions and
therefore ecosystem service delivery (Figure 4). The simplest relationship would be linear – an
increase in biodiversity leads to a steady increase in function. An idiosyncratic relationship occurs
when the addition of further species results in an unpredictable change in a function or service
through competition or predation. Redundancy occurs when beyond a certain diversity no further
6
7
Pimental, D. et al (1997) Economic and environmental benefits of biodiversity. Bioscience 47, 747 – 757.
Pimental, D. et al (1997) Economic and environmental benefits of biodiversity. Bioscience 47, 747 – 757.
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functions are added to the system, as multiple organisms carry out a variety of functions –
furthermore the inflection point, when this has been demonstrated in terrestrial plant communities for
example, tends to be at a very low number, usually around 8 to 10 species. Since biotic communities
in soil are highly diverse it is difficult to imagine a scenario where this low number would be found.
Function or process rate
Redundant
Linear
Idiosyncratic
Species richness
Figure 4. Relationships between species richness and ecosystem process rate or function
(redrawn from Nielsen et al, 2010)
Nielsen et al (2011)8 reviewed studies relating functions related to carbon cycling and biodiversity. In
studies where the system was constrained to have fewer than 10 species, there were generally more
positive relationships between increase in soil biodiversity and C cycling (Table 2). However, in those
studies where more (arguably more realistically) than 10 species were involved the relationships were
for more equivocal, with as many neutral relationships as positive ones, and more negative
relationships than in the studies with fewer than 10 species.
Table 2. Summary of studies where species richness of soil biological groups has been
manipulated and aspects of C-cycling determined and proportions of negative, neutral or
positive relationships detected with 10 or fewer species or more than 10 species (redrawn
from Nielsen et al 20119).
Negative
Relationships in %
Neutral
Positive
4
7
13
4
28
0
0
8
25
7
0
0
15
50
14
100
100
77
25
79
11
13
31
2
57
18
8
13
0
12
18
46
52
50
44
64
46
35
50
44
No. Studies
Communities with < 10 Species
Decomposition
Respiration
Biomass
Other
Total
Communities with > 10 Species
Decomposition
Respiration
Biomass
Other
Total
Further analysis of those studies exhibiting positive relationships between species richness and C
cycling showed that in those studies with fewer than 10 species, the relationships were mainly
idiosyncratic, whereas those with more than 10 species exhibited either redundancy or more often
8
Nielsen, U.N., Ayres, E., Wall, D.H. & Bardgett, R.D. (2011) Soil biodiversity and carbon cycling: a synthesis of studies
examining diversity-function relationships. European Journal of Soil Science, 62, 105-116
9
Nielsen, U.N., Ayres, E., Wall, D.H. & Bardgett, R.D. (2011) Soil biodiversity and carbon cycling: a synthesis of studies
examining diversity-function relationships. European Journal of Soil Science, 62, 105-116
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unknown relationships – demonstrating the inherent difficulty in defending a simple “more diversity
brings increasing function” (Table 3).
Table 3. Summary of the type of positive relationships between soil species richness and Ccycling with 10 or fewer species or more than 10 species (redrawn from Nielsen et al 201110).
No. Studies
Communities with < 10 Species
Decomposition
4
Respiration
7
Biomass
10
Other
1
Total
22
Communities with > 10 Species
Decomposition
5
Respiration
6
Biomass
11
Other
1
Total
23
Linear
Redundant
Idiosyncratic
Unknown
0
1
0
0
1
0
1
0
0
1
4
5
9
1
19
0
0
1
0
1
0
0
0
0
0
0
2
3
0
5
2
0
1
0
3
3
4
7
1
15
It is unlikely that any environmental disturbance would depress species richness to these levels
without the wholesale loss of the soil itself – as occurs during major erosion events.
Susceptibility of the soil biota to physical disturbance
Generally as the size of soil organisms decrease, then their susceptibility to physical disturbance
tends to decrease. In this sense relatively large organisms such as earthworms are readily killed by
cultivation, and civil engineering operations. Fungi, because of their large mycelia structure and the
lack of cross-cell walls in certain species are also susceptible. Bacteria are relatively unaffected by
the direct effects of cultivation but are impacted by consequent changes in soil conditions. For
example, opencast coal operations to form soil stores at the outset of working mine sites leads to the
immediate loss of earthworms and fungi, but the consequent development of anaerobic conditions in
soil stores leads to the majority of the soil biomass.
Impact of human activity
Humanity’s spread and demand for increasing food supply, has lead to a large part of the Earth’s
terrestrial ecosystems becoming converted, dominated by humans 11, through a number of
mechanisms:
 Cultivation
 Monoculture
 Fertilisers
 Pesticides
Direct effects of physical disturbance have been described above, but there are other activities which
may impact on soil biota– such as the pressures on edible fungi through foraging in forests. This may
lead to loss of those symbionts required for the establishment of fastidious plant species.
Implications of the loss of soil biota
It is difficult to imagine situations where biota is lost first, resulting in degradation to the rest of the
system, except in a few particular circumstances. The introduction of the New Zealand flatworm has
led to a loss in aggregate stability in UK pasture soils, which could result in increased susceptibility to
erosion. The elimination of earthworm populations can reduce the water infiltration rate in soil by up
10
Nielsen, U.N., Ayres, E., Wall, D.H. & Bardgett, R.D. (2011) Soil biodiversity and carbon cycling: a synthesis of studies
examining diversity-function relationships. European Journal of Soil Science, 62, 105-116
11
Ellis, E.C., and Ramnkutty, N. (2008) Putting people in the map: anthropogenic biomes of the world. Frontiers in Ecology and
Environment 6, 439 – 447.
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to 93%12. Senepati et al (1999)13 demonstrated a four to five fold increase in plant yield when
earthworms were re-introduced to tea plantations.
There are a number of putative mechanism whereby we could follow the loss of particular groups and
their consequences for ecosystem functions and therefore ecosystem service flows in the functional
groups of the soil biota, however these are heavily interrelated and co-dependant (Table 4).
Table 4. Examples of how losses in biodiversity might impact ecosystem service flows
Functional
group
Chemical
engineers
Biological
regulators
Example
Functional loss
Service loss
Restorative action
Fungi
Yield decline; increased
flood risk
Yield
decrease,
increased erosivity
Retain/add organic matter,
decrease cultivation
Reduce
cultivation,
minimise nutrient addition
Ecosystem
engineers
Earthworms
Decline in aggregate stability,
loss of stable organic matter
Increased susceptibility to
pathogens, nutrient uptake
compromised
Decline in porosity, increase in
compaction and erosivity
Decline in yield, loss of
soil, increased run-off,
increased
cultivation
costs
Decrease cultivation; add
organic matter, increased
herbicide use
Mycorrhizae
Influence of soil type
There is indubitable difference in susceptibility brought about by different soil types. This may be due
to the soil biota having a different “starting point” in different soils, but also some inherent properties
will produce a buffering or amplification of the effects of direct disturbance and indirect pressures. For
example, finer textured soils are more susceptible to physical pressures and the development of
anaerobiosis as a result of the loss of structure.
This means that careful matching of land use to soil type must be achieved to minimise loss in soil
function and associated costs (Figure 5).
Figure 5. mismatch of land use and soil type will lead to increased degradation and costs, loss
in function and hence ecosystem service.
12
Clements, R.O. (1982) Some consequences of large and frequent pesticide applications to grassland. Third Australian
Conference on Grassland Invertebrate Ecology, Adelaide: South Australia Government Printer.
13
Senapati, B.K., Lavelle, P., Giri, S., Pashanasi, B., Alegre, J. Decaëns, T., Jiménez, J.J., Albrecht, A., Blanchart,E., Mahieux,
M., Rousseaux, L., Thomas, R., Panigrahi, P.K. and Venkatachalan, M. (1999) In-soil technologies for tropical ecosystems. In
Earthworm management in tropical agroecosystems, eds P.Lavelle, L. Brussaard and P.F. Hendrix, pp. 199-237. CAB
International, Wallingford.
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Conclusion
Soil biota losses will be greatest where intense cultivation, inorganic fertiliser, biocide use, and
contamination are greatest – but these costs are probably already captured in the costs for erosivity,
flood risk, compaction and yield losses already calculated, e.g. arable, intensive grassland and
contaminated sites. Invasive species will change the composition of the soil biota, with potential
knock-on effects for soil biodiversity and the functions above. It is currently impossible to ascribe
even qualitative relationships between soil biodiversity loss and loss in service values (Turbe et al,
2010). We must also recognise that many aspects of soil biodiversity are central to the fundamental
operation of ecosystems, and economic valuation may not be helpful - and in a number of respects is
dangerous. The soil biota represents critical stocks in ecosystems and is not substitutable.
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