ReviewIndicators of nutrients transport from agricultural catchments under temperate climate: A review
Introduction
The transfer of energy and matter within complex spatial structures is a central topic in landscape ecology, which provides useful information for functional water management and land use planning. In this context, comprehensive studies exist on the nutrient cycles in rural catchments (Duxbury and Peverly, 1978, Vitousek et al., 1997, Neal and Heathwaite, 2005), with a major focus on human impacts such as land use intensification (White et al., 1981), increased fertilisation (Miller, 1979), afforestation (Bormann and Likens, 1979) and stream channelisation (Yarbro et al., 1984). The catchment is the basic territorial unit in nutrient studies. The two main preconditions for a chemical element to be transported in a catchment are the availability of material and energy, i.e. flow. Both, especially the latter, are controlled by landscape (complex spatial) factors (Gergel et al., 2002). The geochemical concept of elementary landscapes identifies areas of uniform soil cover and vegetation (Perelman, 1975). The elements are typically organised along hillslopes as mesotopographic sequences (Pärn et al., 2010). Particularly sensitive elements (e.g., hill slope hollows) serve as conduits for nutrient fluxes, whereas others act as barriers or sinks (e.g., riparian strips for down slope flow) (Gold et al., 2001). Ecologists have identified environmental impacts of chemical loads and mitigate these using freshwater wetlands and riparian buffers (Whigham et al., 1988, Verhoeven et al., 2008). These may retain and recycle nutrients. Soil biota and vegetation control these recycling processes, e.g., plant uptake related to ecosystem photosynthesis (Mulholland et al., 2008), and denitrifying microorganisms connected to ecosystem respiration (Burt and Pinay, 2005).
In order to estimate the N and P loss risk, current research combines indicators into forecasting panels most commonly known as index models. Most catchment index models are calculated as a sum of field nutrient models (Delgado et al., 2006, Weld et al., 2007). However, since data on factors is unavailable or uneasily applicable at field scale, researchers have adapted N and P index models to be calculated at catchment scale sensu stricto as well (Hughes et al., 2005, Jordan and Smith, 2005, Andersen and Kronvang, 2006, Environmental Agency, 2011). Transport factors represent the loss of nutrients and may include erosion, leaching, runoff, connectivity to the water-body, vegetation, and soil. For catchment N and P index models, the transport factors are ranked from very low to very high in terms of loss ratings. Weighting factors are usually assigned to the particular factor when calculating the overall estimate or ranking. Suitable weightings and loss rates can be defined by the user or, alternatively, values can be adapted from literature values (Drewry et al., 2011). Although many studies have addressed catchment-scale nutrient losses, most index-type models do not address fluxes within a landscape framework. This paper aims to compare the individual indicators currently included in the catchment-scale nutrient index models. The other objectives of this work were to review and analyse the literature on indicators and factors influencing N and P transport from upland source areas through complex landscapes, including hydric riparian zones, to surface water, and to evaluate the magnitude of N and P fluxes.
Section snippets
Methods
The main sources of literature for this study were based on studies indexed by Mander and Mauring (1994) for the sources published before 1995, and by the ISI Web of Science for the period 1995–2011. We used the data to build a conceptual diagram of N and P transport in agricultural landscapes under temperate climate. We used the landscape framework of Polynov (Perelman, 1975), which considers upland source (autonomous), transit, and riparian (superaquatic) landscape elements. N and P, lost
Catchment N and P indicators
Table 1, Table 2 present the common factors for N and P commonly included in index models. The following paragraphs discuss these nutrient export indicators.
Conclusions
Nutrient loss index models use factors of contributing distance, connectivity, catchment soil properties, and erosion as indicators. Soil chemical factors determine N more, while P losses are mainly dependent on overland flow conduits and barriers. Literature has also established riparian buffers as critical nutrient sinks. Especially riparian vegetation accumulates great N and P amounts, while it is usually just a temporary sink. Riparian soil is a smaller but a more permanent store, whereas
Acknowledgements
This study was supported by the Ministry of Education and Science of Estonia (grant SF0180127s08), the Estonian Science Foundation (grant 7527), a grant EE0012 from Iceland, Liechtenstein and Norway through the EEA Financial Mechanism and the Norwegian Financial Mechanism, and a grant through the IAEA Coordinated Research Project on Strategic Placement and Area-Wide Evaluation of Water Conservation Zones in Agricultural Catchments for Biomass Production, Water Quality and Food Security
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