The role of mobilisation and delivery processes on contrasting dissolved nitrogen and phosphorus exports in groundwater fed catchments
Graphical abstract
Introduction
Land-to-water transfer of nitrogen (N) and phosphorus (P) is a major concern worldwide as excessive concentrations of these two nutrients cause eutrophication in freshwater and marine ecosystems (Conley et al., 2009). Diffuse emissions from agricultural origin can represent a significant contribution to annual N and P loads in rivers (Dupas et al., 2015a).
Mitigation measures to decrease N and P emissions from agricultural landscapes must rely on underpinning research to understand mobilisation and delivery mechanisms in order to be effective and several mechanisms have been identified (Lloyd et al., 2016, Mellander et al., 2012, Outram et al., 2016). It is generally accepted that N is prone to vertical leaching from the soil, through the unsaturated zone down to the saturated zone. It can then be transferred to surface waters, mainly as nitrate, via groundwater (Legout et al., 2007, Mellander et al., 2014, Molenat et al., 2008). Nitrate mobilised below the rooting zone may be subject to denitrification, which generally takes place within anoxic groundwaters, riparian wetlands and the hyporheic zone (Anderson et al., 2014, McAleer et al., 2017, Oehler et al., 2007). Mobilised nitrate may also have long time lags between mobilisation and emergence, due to the potentially long transit time of water in the unsaturated and saturated zone (Fovet et al., 2015, Hrachowitz et al., 2010). Phosphorus has a higher adsorption affinity with the soil compared to nitrate; as a result it is less soluble and is less prone to leaching. A general understanding is that P is typically transferred to surface waters via surface pathways, either in soluble or particulate form (Sharpley et al., 2008). However, several studies have highlighted the possibility of soluble P leaching into shallow groundwater and subsequent lateral transfer to surface water as a dominant mechanism in groundwater-fed catchments (e.g. Dupas et al., 2015c, Haygarth et al., 1998, Holman et al., 2010, Mellander et al., 2016, van der Salm et al., 2011).
Nitrate and soluble P leaching is controlled by several factors, some of which are manageable and some of which are inherent properties of soils and climate. Nitrate leaching is controlled by i) the balance between fertiliser inputs and crop uptake on an annual basis; ii) soil mineralisation, which increases when the soil C:N ratio is low and when favourable moisture and temperature conditions are met (Rodrigo et al., 1997); iii) temporal mismatches between crop uptake capacity and high nitrate concentration in the soil combined with high drainage potential (particularly in autumn, Dupas et al., 2015d). In temperate regions, grasslands have been shown to have a better capacity to take up N from the soil throughout the year, particularly in the autumn period, compared to cropland (McDowell et al., 2014, Moreau et al., 2012), although urine patches in grazed grassland can be hotspots of nitrate leaching.
Soluble P leaching is controlled by i) the soil P content, as determined by soil P tests; ii) soil chemistry, particularly the abundance of iron (Fe) and aluminium (Al) oxides, which are important adsorption sites in acidic soils (Daly et al., 2015, Schoumans and Chardon, 2015); iii) temporal variation of soil pH, redox state (Henderson et al., 2012) and drying-rewetting or freezing-thawing cycles (Blackwell et al., 2010); iv) organic matter (OM) content, which may control the formation of Fe-bound organic P colloids that are more mobile than truly soluble P (Granger et al., 2007), and which influences the concentration of dissolved organic matter (DOM) that can compete with P for adsorption sites (Kang et al., 2009); v) land use, particularly the presence of grassland, which is often associated with high OM content in soil and which may release root exudates that stimulate the microbial biomass and bring P into solution (Roberts et al., 2013). Grassland may also favour preferential flow paths through macropores, enhancing nitrate and soluble P transport (Djodjic et al., 2004, Gachter et al., 1998).
Several studies comparing the export behaviour of N and P in agricultural catchments along the Atlantic seaboard of Europe have highlighted the crucial role played by dominant flow pathways on export patterns (e.g. Lloyd et al., 2016, Mellander et al., 2012, Outram et al., 2016). In general, well-drained catchments are prone to nitrate leaching and subsequent transfer via groundwater, whereas poorly-drained catchments can transfer large masses of P via surface flow pathways (Jordan et al., 2012, McDowell et al., 2014). However, few studies have compared catchments with similar dominant flow pathways to investigate the role of mobilisation and delivery mechanisms at this small catchment scale.
The main aim of this paper, therefore, was to investigate mobilisation processes (i.e. leaching) as an important role in the transfer continuum of nitrate-N and soluble P in groundwater-fed catchments. This was undertaken by monitoring the nutrient transfer continuum, from source to mobilisation and from mobilisation to delivery (Haygarth et al., 2005), in two intensively studied agricultural catchments (with similar groundwater fed systems) on the Atlantic seaboard of Europe. A detailed survey of N and P input and content in soil was used to characterise the sources, nitrate-N and soluble P concentration in shallow groundwater was used to characterise the amount of nutrient being mobilised below the rooting zone and comparison of groundwater and stream nitrate-N and soluble P concentration was used to characterise delivery. The specific objectives were to i) investigate the respective influence of mobilisation and delivery mechanisms on contrasting nitrate-N and soluble P concentrations and N:P ratios in the two catchments and ii) analyse the respective controls of soil properties, climate and land use on nitrate-N and soluble P mobilisation. We use this analysis to explore some implications for the future trajectory (i.e. evolution under changing external forces) of diffusion pollution in these and similar catchment types under land use and climate change scenarios.
Section snippets
Study areas
This investigation took place in two intensively farmed catchments in the pedo-climatic zone of the Atlantic seaboard of Europe, in Western France and South-western Ireland (Fig. 1).
The French catchment, Kervidy-Naizin, belongs to the AgrHyS environmental research observatory, where the impacts of agriculture and climate change on water quality are studied (Aubert et al., 2013). Kervidy-Naizin is a 5 km2 catchment drained by a stream of second Strahler order. Climate is temperate oceanic, with
Temporal variability in the hydroclimate, nitrogen and phosphorus loads
The Kervidy-Naizin and the Timoleague catchments appeared to be affected by similar seasonal weather patterns but different inter-annual weather patterns. Both catchments were characterised by mild temperatures and the occurrence of rainfall throughout the year (Table 1; Fig. 2 a and b); mean annual temperature was slightly but significantly higher in Kervidy-Naizin (11.3 ± 0.5 °C versus 10.1 ± 0.5 °C, p < 0.05) but the annual cumulated rainfall was not significantly different in both catchments (938 ±
Conclusion
In two groundwater-driven and intensively farmed catchments on the Atlantic seaboard of Europe (Western France and Southwest Ireland), mobilisation processes appeared to play a dominant role in the contrasting export of nitrate-N and soluble P from land to the receiving streams. Nitrate-N leaching was mainly controlled by mineralisation and the timing of plant uptake: grasslands have a longer growth period and better utilisation of N than cropland, hence grasslands reduced nitrate-N leaching.
Acknowledgement
Long-term monitoring in the Kervidy-Naizin catchment is supported by ‘ORE AgrHyS’ (http://www6.inra.fr/ore_agrhys_eng). Monitoring in the Timoleague catchment is a part of the ‘Agricultural Catchments Programme’ (http://www.teagasc.ie/agcatchments/) funded by the Irish Department of Agriculture, Food and the Marine. The authors would like to thank all those who helped with the field and lab work.
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