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ransactions of the Institute of British Geographers Boundary Crossings 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) 2 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 Boundary Crossings 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 3 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) 4 are grateful to the anonymous peer-reviewers for their comments and feedback on the draft manuscript. 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