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Assessing the significance of soil erosion
G S Bilotta*, M Grove* and S M Mudd** *School of Environment and Technology, University of Brighton, Cockcroft Building, Lewes Road, Brighton BN2
4GJ
email: [email protected]
**School of GeoSciences, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP
Earth Research Institute, University of California, Santa Barbara, California, USA
revised manuscript received 5 September 2011
In this paper we examine the ecosystem services
that soils provide and we critically evaluate the
current methods through which the erosion of this
resource is assessed. We highlight that erosion is
not only a problem involving the loss of soil; it
involves a loss of both on-site and off-site ecosystem services, which are rarely incorporated into
current assessments of the significance of erosion.
We discuss the need for interdisciplinary research
that addresses current weaknesses in the assessment of the significance of soil erosion.
Soil is a fundamental resource that provides a
number of important ecosystem services. It is the
medium on which we produce 99 per cent of our
food in addition to fodder, fibre, raw materials and
biofuels (FAO 2003). Soils are also a major store of
carbon (C) and regulator of climate, holding two to
three times more C than exists in the atmosphere
(Davidson et al. 2000). Furthermore, soils are also a
regulator of water resources, attenuating hydrological responses and removing contaminants from
percolating water (Haygarth and Ritz 2009). Accelerated erosion of soil by the processes of water erosion (rainsplash, interrill, rill and gully erosion and
soil piping), wind erosion, tillage erosion and soil
loss through crop harvesting represent a serious
threat to this resource and the ecosystem services
that it provides. Indeed, numerous ancient civilisations have declined as a consequence of soil degradation and erosion brought about by unsustainable
human activities (for examples see Montgomery
2008). In addition to the loss of soil ecosystem services, accelerated soil erosion can also cause a loss
of aquatic ecosystem services, associated with the
delivery of excessive quantities of particulate matter into aquatic ecosystems (Bilotta and Brazier
2008). This is one of the most common causes of
water quality impairment globally (Richter et al.
2005).
Recognition of the on-site impacts of accelerated
soil erosion and the growing need for soil conservation began as early as the 1920s (e.g. Bennett and
Chapline 1928), and since this pioneering work
there has been a large amount of erosion research;
a Google Scholar search for articles on ‘soil erosion’
published since 2000 AD yields approximately
287 000 results. This research has been carried out
at various scales and has focused predominantly
on the rates of erosion from different agricultural
land uses. Whilst the soil erosion community is not
currently at a stage where the dynamics of erosion
can be accurately modelled or predicted at every
scale and ⁄ or from every environment and land
use, there is a generally good understanding of the
relative risk of erosion from different land uses.
Indeed, the United States Department for Agriculture (USDA) has published guidance for farmers
and land managers on methods of reducing erosion
risk (USDA-NRCS 2002). The Department for Environment, Food and Rural Affairs (Defra) have published equivalent guidance for agronomists in the
UK (Defra 2005). What is lacking, however, from
both a science and policy perspective, is an understanding of what should be regarded as an acceptable level of erosion for a given environment and
how this should be quantified and assessed.
Conventionally, soil erosion has been quantified
using a variety of geomorphological approaches
including plot studies, field surveys, aerial and
remote sensing surveys, fallout radionuclide (137Cs,
210
Pb) studies, reservoir sedimentation investigations, and riverine suspended sediment monitoring
(Brazier 2004). The rates of soil erosion recorded by
these approaches have traditionally been expressed
Trans Inst Br Geogr NS 2012, doi: 10.1111/j.1475-5661.2011.00497.x
ISSN 0020-2754 2012 The Authors.
Transactions of the Institute of British Geographers 2012 Royal Geographical Society (with the Institute of British Geographers)
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in units of either a mass of soil loss per unit area
per unit time (e.g. kg ha)1 year)1), or in the units
of a depth of soil loss per unit time (e.g. mm
year)1). Assessment of the significance of the
recorded rate of erosion, i.e. whether or not it is
occurring at a significant level, has frequently been
based on comparisons to estimated rates of soil formation; the implicit assumption being that soil erosion is not significant until it exceeds the rate of
soil formation, i.e. until there is a net loss of soil. In
this paper, three major limitations with this
approach are highlighted and recommendations
are made as to how the methods of assessment
could be improved to include the loss of the wider
ecosystem services that soils provide.
The first major limitation with the conventional
geomorphological assessment of the significance of
erosion is that rates of soil formation are currently
poorly constrained. The implications of this are that
our assessments of the significance of soil erosion
rates, i.e. how sustainable they are in a given environment, are highly uncertain even on a simple
mass-balance level of assessment. Inaccurate estimates of the rates of soil formation inevitably lead to
discrepancies between the estimated and real lifespan of the resource. Of particular concern, given
the prevalence and susceptibility to erosion, is the
fact that there is virtually no quantitative information regarding soil formation rates under different
agricultural land uses. Soil formation in the geomorphology community generally refers to the production of physically disturbed and transportable
material from a chemically altered but physically
immobile parent material (e.g. Yoo and Mudd 2008);
the rate of which has been measured at only a limited number of sites and as of yet has never been
measured in an agricultural setting. The most rapid
rate of soil production recorded to date was in the
Oregon Coast Range, at up to 0.27 mm year)1
(Heimsath et al. 2001), which equates to around 3.5 T
ha)1 year)1 (based on a soil density of 1300 kg m)3).
This rate, however, is far exceeded by some of the
rates of agricultural soil erosion that have been
observed globally (e.g. Verheijen et al. 2009).
The second major limitation with the conventional assessment of the significance of erosion is
that it is a threshold based purely on quantity of
soil loss (e.g. kg ha)1 year)1). Soil erosion not only
affects the quantity of soil available, it also affects
the quality of the soil available. Soil quality can be
defined as ‘an account of the ability of soil to provide ecosystem and social services through its
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capacities to perform its functions and respond to
external influences’ (Tóth et al. 2007). These services
are dependent upon the physical, chemical and
biological characteristics of the soil, all of which
may be degraded by erosion regardless of whether
it is occurring at a rate that is higher or lower than
the rate of soil formation. For example, one of the
most important ecosystem services gained from soil
is the provision of food, fodder, fibre, biofuels and
raw materials. Two soil quality parameters that are
of primary interest with regards to this provisioning service are soil texture and nutrient availability.
Slowly eroding soils tend to be finer and specifically have higher clay contents due to increased
exposure to physical and chemical weathering (e.g.
Mudd and Yoo 2010); accelerated erosion reduces
soil residence time leading to soil coarsening and
reduced clay content. This affects ecosystem services because clays help to retain soil moisture (e.g.
Lohse and Dietrich 2005) and clay content is positively correlated with soil C content (Homann et al.
2007), which in turn is positively correlated with
soil nutrient content (e.g. Glendining et al. 2011).
Therefore a decline in soil C content, which can be
brought about by erosion, can reduce soil nutrient
availability and thus reduce the ecosystem’s provisioning service. A study by Lal (2006) suggested
that a loss of 1 Mg soil C ha)1 can result in crop
yield losses of tens to hundreds of kg ha)1. On a
global scale, if these types of effects are ignored
from our assessment of the significance of erosion,
and erosion is presumed to be insignificant purely
because it is occurring at a rate below the estimated rate of soil formation, then this could have
huge implications for the sustainability of global
food supplies. Nevertheless, these types of erosioninduced changes to the physical, chemical and biological properties of the soil are rarely quantified
or incorporated into assessments. The implications
of this are that our assessments do not consider the
loss of many soil ecosystem services. Numerous
institutions are currently involved in defining and
designing metrics to assess soil quality (e.g. Defra
2007; Tóth et al. 2007; Towers et al. 2006; USDA
2011); it is critical that these metrics are incorporated into the assessment of the significance of soil
erosion if the purpose of assessing soil erosion is to
determine the sustainability of land use and the
longevity of the resource and its ecosystem services.
The third major limitation with the conventional
geomorphological assessment of the significance of
Trans Inst Br Geogr NS 2012
ISSN 0020-2754 2012 The Authors.
Transactions of the Institute of British Geographers 2012 Royal Geographical Society (with the Institute of British Geographers)
Boundary Crossings
erosion relates to the fact that soil erosion may
cause significant off-site problems even if it is
occurring at a rate that is lower than the rate of
soil formation. Soil erosion causes off-site problems by delivering excessive levels of particulate
matter into receiving waters. The response of
aquatic organisms to particulate matter is dependent upon a number of factors, including the concentration, the duration and seasonality of
exposure, the particle size ⁄ shape, and the particle
geochemical composition (Alabaster and Lloyd
1982; Bilotta and Brazier 2008; Cordone and Kelley
1961; Newcombe and MacDonald 1991; Owens
et al. 2005; Ryan 1991; Wood and Armitage 1997).
Owing to the complexity of multiple factors in
determining the biological response, the assessment of the significance of a given rate of soil erosion using the conventional geomorphological
expression (e.g. kg ha)1 year)1 or mm year)1) is of
little use in terms of predicting the effects of erosion on aquatic ecosystems. For example, an
annual bulk figure of erosion does not provide
any indication of the frequency and timing of the
fluxes of particulate matter and the conditions
observed in-stream as it is exported from the
catchment. The particulate matter may have
passed through the stream in a single pulse, or in
a series of pulses, or may have been a fairly constant flux; each scenario having differing effects on
the ecology and not necessarily requiring soil erosion to have been in excess of the rate of formation in order to have had a significant impact.
There is a clear need for the conventional methods
of quantifying and expressing rates of erosion to
be adapted to more ecologically meaningful measures so that the off-site effects of erosion, and
thus the sustainability of soil erosion in terms of
effects on water resources and aquatic ecosystem
services, can be incorporated into our assessment
of the significance of erosion.
Conclusions
Soil is a fundamental resource that provides a wide
range of ecosystem services. Accelerated erosion
threatens these services and also threatens the services provided by aquatic ecosystems. Whilst
research over the past several decades has provided us with a generally good understanding of
the relative risk ⁄ rates of erosion under different
agricultural land uses, there remains a poor understanding of the significance of these rates in terms
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of the sustainability of the soil resource and the
ecosystem services that it can provide. In this
paper it is argued that it is crucial that our assessment of the significance of erosion should be based
on evidence from boundary-crossing science that
goes beyond the conventional geomorphological
perspective and is more reflective of both the
on-site ecosystem services and the off-site ecosystem services that are threatened by this process.
Specifically, there is a need for more advanced estimates ⁄ measurements of soil formation, particularly
under different agricultural land uses. These estimates ⁄ measurements should be derived from a
combination of long-term monitoring experiments
and novel applications of emerging techniques
such as those developed by Dosseto et al. (2008).
Until soil formation rates are better constrained our
estimates of the significance of soil erosion, even at
a simple mass-balance level, are highly uncertain
and could result in large under- or over-estimations
of the lifespan of the resource. There is also a need
to develop a harmonised suite of metrics for soil
quality, so that the effects of erosion on the physical (e.g. texture, structure, aggregate stability),
chemical (e.g. C ⁄ N ⁄ P content) and biological (e.g.
microbial and invertebrate biodiversity) properties
of the soil, and the associated ecosystem services,
can be assessed. This will require interdisciplinary
research and collaboration between agronomists,
biologists, chemists, ecologists, geomorphologists
and soil scientists as well as social scientists and
economists. Finally there is also a need to understand better the particulate conditions that are tolerable by aquatic communities in contrasting
environments, so that appropriate sediment erosion
and ⁄ or delivery metrics can be devised. These
should move on from annual loads and average
concentrations, towards more ecologically meaningful metrics such as the particulate concentration–
frequency curve approach developed by Bilotta et
al. (2010). It will not be until the research gaps
highlighted in this paper are addressed, that we
are truly able to assess the significance of soil erosion in terms of the sustainability of this fundamental resource and the ecosystem services that it
provides.
Acknowledgements
This article arises, in part, from research co-funded
by the Engineering and Physical Sciences Research
Council (EPSRC) and Aquaread Ltd. The authors
Trans Inst Br Geogr NS 2012
ISSN 0020-2754 2012 The Authors.
Transactions of the Institute of British Geographers 2012 Royal Geographical Society (with the Institute of British Geographers)
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are grateful to the anonymous peer-reviewers for
their comments and feedback on the draft manuscript.
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Trans Inst Br Geogr NS 2012
ISSN 0020-2754 2012 The Authors.
Transactions of the Institute of British Geographers 2012 Royal Geographical Society (with the Institute of British Geographers)