A perspective on the role of lowland, agricultural grasslands in contributing to erosion and water quality problems in the UK

Abstract

The recognition of soil erosion by water as an environmental problem in the UK did not occur until the early 1970s, when authors such as Evans (1971) challenged the perception that climate, topography and vegetation cover all contributed to ensure relatively low (in global terms) erosion rates (Hudson, 1967; Douglas, 1970; Fournier, 1972). Subsequent authors, including Boardman (2002), recognize Evans’ work as a ‘wake-up call’, which has prompted a wide and varied body of research into soil erosion in the UK over the last 35 years. A review of this literature (see Boardman and Evans, 1994; Brazier, 2004; Evans, 2005, for comprehensive examples), however, demonstrates that almost all of this work relates to water erosion processes on lowland arable land or upland areas, with a generally implicit assumption that lowland grassland is devoid of on-site erosion processes and therefore does not contribute (or contributes minimally) to off-site erosion problems. Recently, however, there has been a shift in emphasis from the on-site impacts of soil erosion (surface lowering, loss of soil productivity) towards off-site impacts, driven by the demands of the EU Water Framework Directive and the need to ensure water quality (Neal and Jarvie, 2005). This shift means that the contribution of lowland grassland environments to the sediment and nutrient budget of UK river catchments needs to be re-evaluated, as it has been almost completely overlooked, despite the fact that intensively managed, lowland grasslands, typified by intensive dairy systems, occupy 29% of the land area of England and Wales (Defra, 2005). Historically, there has been clear justification for the focus of erosion-related work to be on the more susceptible land use types, such as row crops or winter cereals (Robinson and Boardman, 1988; Walling and Quine, 1990; Evans, 1993); more recently, a limited but growing literature supports the need to reassess our understanding of grassland systems as contributors of sediment and associated nutrients to waterbodies (e.g. Harrod and Theurer, 2002; Heathwaite et al., 1990; Heathwaite and Dils, 2000; Hooda et al., 2001; Preedy et al., 2001; Theurer et al., 1998). In focussing research efforts to further the understanding of grasslands and the role that they play within the wider environment in the UK, a number of issues need to be considered. To begin with, there is a fundamental problem with the distinction that is often drawn between mineral and organic matter within the soil, which may be particularly important in affecting estimates of sediment delivery from grassland environments. Existing definitions of soil loss consider the organic matter component of the eroded soil as separate from the mineral component. In practical terms, organic material is often removed, normally through the process of ashing, to calculate a mass of (mineral) erosion and a mass of organic matter (O.M.) leaving the hillslope or catchment (Walling and Webb, 1987). Research has shown that the organic fraction of material contained within runoff is important in the transport of sorbed contaminants such as phosphorus (P) (Chardon and Oenema, 1995; Haygarth et al., 1998b; Heathwaite et al., 2005; Hens and Merckx, 2002) and so it should be included in estimates of the rate of soil erosion from grassland. In addition, grassland soils have high organic matter contents due to the high surface cover of the grass sward, which may lead to underestimates of soil erosion if these are not taken into account. It is unlikely that this approach would increase erosion rates to in excess of the ‘tolerable’ threshold of 1 m ha a suggested by Morgan (1980). However, perhaps more importantly, the existing approach will lead to underestimates in the water quality problems brought about by the