Integrated tool for risk assessment in agricultural management of soil erosion and losses of phosphorus and nitrogen
Introduction
For many decades, control of emissions of nutrients from agricultural sources has been regarded as one of the most urgent environmental protection issues related to bodies of water. For example, a recent study demonstrated that farming is responsible for 50% of the nitrogen (N) and 30% of the phosphorus (P) in anthropogenic nutrient contributions to Norwegian rivers (Selvik et al., 2006). Over the past 20–30 years, extensive efforts have been made in Norway to reduce diffuse pollution from agricultural areas, and the North Sea Declaration has been an important driving force in that context. General methods to mitigate nutrient losses from agricultural areas in the country have included limitations on numbers of livestock and mandatory nutrient management planning. More targeted strategies have included measures such as the subsidies that were introduced in 1991 to promote changes in farming practices like reduced soil tillage done to reduce erosion. These subsidies were initially given for all areas managed by reduced tillage. However, after a few years they were differentiated according to the erosion risk (Lundekvam et al., 2003), so that high-risk areas received greater subsidies. This was done because some parts of the landscape contribute more to losses than others. In the United States, Gburek and Sharpley (1998) found that a small fraction of the catchment area contributed a large proportion of the nutrient load. More explicitly, these critical source areas are characterised by having a substantial potential to release nutrients into surface or subsurface runoff in conjunction with hydrologic activity and connectivity with streams (Needelman et al., 2001). Considering intensively tile-drained agricultural soils, as are common in most of Northern Europe, there is extensive connectivity between fields and streams, and the critical source areas are generally more widespread than is the case for non-tile-drained soils (Heathwaite et al., 2003). Despite that, Ulén et al. (2001) observed that four of 15 fields contributed to 74% of the total P loss from a tile-drained catchment in Sweden. Accordingly, also for tile-drained areas, ranking of fields in relation to their risk of nutrient losses and soil erosion can lead to a more targeted mitigation strategy. Furthermore, Strauss et al. (2007) showed that focus on high-risk areas provided a more cost-efficient mitigation scheme compared to general application of such methods.
There are a variety of models to estimate the risk of nutrient losses and erosion, ranging from complex process-based approaches to simple indicators like soil-surface balances. For decision-makers and scientists faced with a specific water management problem, it is essential to choose methods that can yield realistic results and at the same time are economically feasible. The methods must also take into account data availability, which is often the limiting factor. Moreover, managers want techniques that are based on a scientific framework but are nonetheless simple to use. Together, these requirements mean that input data should be readily available but still cover all essential source and transport factors, and it has been suggested that the index approach can take these considerations into account (Sharpley et al., 2001, Heathwaite et al., 2003).
The index concept was first developed in the United States as a tool to rank fields according to their risk of P losses (Lemunyon and Gilbert, 1993). Other index tools have subsequently been devised for P and to some extent also for N, but only a few studies have addressed an integrated strategy for evaluating the risk of losses of both P and N (Heathwaite et al., 2000, McDowell et al., 2002). McDowell et al. (2002) suggested that assessment of the risk of P loss should be based on hydrologically active areas, whereas corresponding assessment of N should focus on efficiency of use in crop production. The objective of the present study was to initiate development of an integrated assessment tool to rank fields according to their risk of erosion, P and N losses. Such a tool should be based on easily accessible input data and also be suitable for practical use by managers at the field scale within a catchment.
Section snippets
Methodological approach
The purpose of our integrated risk assessment tool was to estimate the combined risk of erosion and P and N losses from fields to surface water by defining indices for each of these variables. Conceptually, the separate risks for these variables differ because they concern the following aspects: mobilisation and transport for erosion; sources, mobilisation, and transport for P losses; mainly the input–output balance in agricultural plant production for N losses. The catchment approach was
Case study area
The Skuterud catchment in south-eastern Norway was selected as the object of the case study. This catchment is part of the Norwegian Agricultural Environmental Monitoring Programme (JOVA), and it has a total area of 4.5 km2, of which approximately 61% (2.7 km2) is agricultural land and the remaining parts are forest, inhabited areas, bogs, and roads. There are 51 farm fields within the catchment (Fig. 1), and the land is used primarily for cereal production. All fields are systematically tile
Erosion index
The characteristic feature of the fields with the highest erosion index is that they were subjected either to autumn tillage only or to autumn tillage followed by sowing of winter cereals (Fig. 1, Fig. 2). Autumn ploughing and other tillage methods before winter wheat have been shown to increase erosion in several studies (e.g., Grønsten et al., 2007, Lundekvam, 2007). In addition to soil management, the erosion index is influenced by natural factors such as soil type and slope. In all, 15
Concluding remarks
Overall, the present study has illustrated the potential of applying a simple assessment methodology based on readily available input data as a means of ranking farm fields according to their joint risk of losing P and N. This integrated tool is intended to aid selection of appropriate areas for implementation of methods to reduce nutrient losses. Farmers or watershed managers who are trying to improve water quality need to be able to determine the combined effects of a mitigation method on P
Acknowledgements
The authors thank the Norwegian Research Council for contributing to this investigation, which was funded through the INTRA project (No.159255/s30). Patricia Ödman is acknowledged for the language revision.
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