NUTRIHORT: Nutrient management, nutrient legislation and innovative techniques in intensive horticulture
September 16-18, 2013, Ghent, Belgium
Number of accepted abstract: 74
The phosphorus cycle in North-West European agricultural soils
Amery Fien1,Vandecasteele Bart1
1 Institute for Agricultural and Fisheries Research (ILVO)
Plant Sciences Unit - Crop Husbandry and Environment research area
Burg. van Gansberghelaan 109, Merelbeke,Belgium
Abstract:Phosphorus (P) concentrations in NW European surface waters generally exceed ecological thresholds. The Nitrates Directive and the Water Framework Directive haveaskedEU Member States to address this problem. The resulting monitoring networks and P concentration standards differ widely between Member States. Because agriculture is considered to have a large impact on the surface water P concentrations, the diffuse P losses from soils with high P contents are targeted. Attempts have been made to reduce the soil P concentration under the threshold above which P losses increase exponentially. However, agro-economic interests should also be taken into account. A target zone of soil P concentrations with optimum crop yields and limited P losses can be defined. In order to reach or stay in this target zone,fertilisation recommendations should be adapted to include the risk for P losses. Soil P application standards are ideally differentiated to soil P concentrations. In this way, soils with large P concentrations should receive less P in order to reduce the P losses and soils with small P concentrations can receive more P to ensure satisfactory crop yields.
Keywords:surface water, P losses, P standards
Introduction
Algal blooms, the result of excess phosphorus (P) in the water, decrease the ecological status of surface waters in Europe. BecauseP is the limiting nutrient for primary production (Correll, 1999), actions to decrease P concentrations in surface water are being taken under the European Nitrates Directive and the Water Framework Directive. It is unclear, however,whether the main proposed action, i.e.reducing P fertilisation bylowering agricultural P application standards, is an efficient tool to reduce P concentrations in surface water. Moreover, questions have arisen in relation to the impact of reduced phosphorus application on crops and soil. A literature study on P processes in soil and plant and on legislation aims to shed light on these uncertainties and propose guidelines for efficient measures to reduce P concentrations in surface waters.
Materials and Methods
A literature study was performed on the P cycle in agricultural soils. The focus of the literature study was on Flanders (Belgium), but other European countries were also considered.
Results and Discussion
Phosphorus in surface waters
Phosphorus concentrations in many European surface waters are above ecological thresholds, resulting in algal blooms (Sharpley and Rekolainen, 1997). The Water Framework Directive of the European Union (2000) aims to reach a good ecological status or potential for the surface waters by 2015. Member States are requested to define ecological reference conditions and ecological classifications (very good, good, moderate, poor and bad) for each surface water type present in the country. Relationships between chemical/physical conditions and ecological status can then result in P concentrations related to the ecological class boundaries. The P concentration for the good/moderate boundary then becomes the P standard for that specific surface water type. In an attempt to harmonise the P standards in the different Member States, a non legally-binding document,‘Guidance Document’(Anon., 2009)was published. An intercalibration exercise between the Member States was also performed between 2004-2007. Despite theseinitiatives, P standards between Member States differ and are difficult to compare given the differences in kind of concentration (total versus ortho phosphorus concentration), statistics (mean, median or 90th percentile), water type, time of measurement (summer or year mean), etc.Claussen et al. (2012) did attempt a comparison in spite of these differences(Figure 1).Phosphorus standards for surface waters in Flanders are not present in the study of Claussen et al. (2012) but are listed in Table 1 (Anon., 2010).
a)
b)
Figure 1Phosphorus standards (moderate/good boundary) for total phosphorus concentration in Europe for (a) rivers and (b) lakes. Arrangement from left to right is not necessarily in order from good to bad becausedifferences can be attributed to different water types, climate, statistical interpretation, etc. (from Claussen et al. (2012)).
Table 1Phosphorus standards for surface waters in Flanders (Anon., 2010)
Parameter / Water type / P standard (mg/l)Total P concentration (summer meana) / Rivers / 0.14
Lakes / 0.03-0.11
Ortho P concentration (year mean) / Large rivers, brackish waters, estuarial waters / 0.14
Small and very large rivers / 0.12
Brooks / 0.10
Brooks in the Campine / 0.07
Lakes / /
a between April 1st and September 30th
Not only P concentration standards, but also the monitoring networks varybetween the Member States. Guidelines for the monitoring required for the Water Framework Directive are listed in a non-legally binding Guidance Document (Anon., 2003). Monitoring is also required for the Nitrates Directive, but guidelines for the monitoring of the effect of the action programmes are not present in the Draft Monitoring Guidelines (Fraters et al., 2011). As a result, monitoring networks differ widely. Densities of the networks are largest in Belgium and smallest in Finland (Table 2).
Table 2Densities of ground and surface water monitoring networks for the Nitrates Directive in European countries (van Grinsven et al., 2012)
Groundwater(points/1000 km²) / Surface water
(points/1000 km²)
Belgium / 99 / 38
Germany / 3 / 1
Denmark / 34 / 5
France / 5 / 3
Ireland / 1 / 3
The Netherlands / 33 / 13
United Kingdom / 13 / 33
Finland / 0.2 / 0.4
average / 13.7 / 7.4
The phosphorus standards are generally exceeded in a lot of European countries. Recent evaluations reveal that 60% of the measuring points in the monitoring network of the Nitrates Directive in Flanders have average phosphorus concentrations above the P standard (Vlaamse Landmaatschappij, 2012). This percentage is 57% for The Netherlands (Klein et al., 2012). Agriculture is considered to be a large contributor to the P pressure on surface water (Sharpley and Rekolainen, 1997). The estimated share of agriculture in the phosphorus load of surface water in Flanders is 44%(Van Steertegem, 2013). Fifty-four percent of the P load of surface water in the Netherlands originates from soil leaching and runoff (van der Bolt, 2012). In order to reduce the surface water P concentrations to meet the Water Framework Directive goals, agriculture seems to be the main target sector for P measures.
Phosphorus in agricultural soils
Due to historically large P fertiliser additions, P levels in agricultural soils are high in northwestern Europe. Phosphorus losses from soils (runoff and leaching) are considered to increase with increasing soil P concentrations (Tunney et al., 1997; Rubaek et al., 2010). Often a threshold soil P concentration is observed above which P losses increase exponentially (McDowell and Sharpley, 2001) (dashed line in Figure 2). Although it is recognised that transport and hydrological factors are also important for soil P losses (Haygarth and Jarvis, 1999), the high soil P concentrations seem to be the main target for reducing the large impact of agriculture on surface water P concentrations.
But not only soil P losses are influenced by soil P concentrations. Crop yield and quality are also largely affected by the available soil P concentrations (solid line in Figure 2). Farmers are reluctant to decrease P fertilisation because they fear that the resulting smaller soil P concentrations will induce smaller crop yields. In fact, none or only minimal crop yield reductions are expected within ten years if P inputs are smaller than P export numbers, given the large soil P reserves in northwestern Europe (Jungk et al., 1993; Gallet et al., 2003). Effects occur faster for grasslands, especially for plant P concentrations (Ehlert et al., 2008). Whether effects on crop yield and quality occur largely depends on the P input-export difference and the initial soil P concentration (position on the solid line in Figure 2).
Figure 2Crop yield (solid line) and soil P loss (dashed line) vary with increasing soil P concentration. A target zone with optimum yields and limited P losses is defined.
A target zone is defined (Figure 2) to assure optimum crop yields and minimal soil P losses. Grassland studies in Ireland (Tunney, 2002) suggest that threshold values for rapidly increasing P losses are at the upper end of the range for optimum grassland production. This small target zone is therefore the optimal soil P concentration range from both an agro-economical and an ecological point of view. Soils with P concentrations smaller than the target zone (left side of Figure 2) have sub-optimum yields and can increase the soil P concentration by P fertilisation with limited ecological risk. Soils with P concentrations larger than the target zone (right side of Figure 2) can reduce the P losses by P fertilisation reduction, i.e. smaller input than export, without risks for crop yield reductions. Soils within the target zone ideally stay in the target zone by balancing P inputs and exports (equilibrium fertilisation).
Reaching the target zone for soil P concentration: fertilisation advice andapplication standards
The target zone defined in Figure 2 is the optimum for agriculture and environment considered together. Fertilisation advice systems mainly consider agricultural interests, and the target zone recommended by fertilisation advisers therefore does not always equal the target zone defined in Figure 2. Target zones for P fertilisation advice differ widely across Europe (Jordan-Meille et al., 2012). For instance, the phosphorus concentration range for the fertilisation target zone in Flanders is 2-3 times higher than for the fertilisation target zones of other European countries using the same soil P test (ammonium lactate extraction). Moreover, recommended fertilisation P doses for the fertilisation target zone in Flanders (Maes et al., 2012) are generally larger than the P doses exported by the crops (Ehlert et al., 2009; Anon., 2011) resulting in steadily increasing soil P concentrations. This can be visualised by a movement to the right on the curves in Figure 2.
Some European countries have P fertilisation standards whose aim is to reduce or limit soil P concentrations (Table 3). In Sweden and Slovenia, only standards for organic P fertilisation exist. However, most countries do not haveany P application standards.
Table 3Phosphorus application standards in European countries
Country (region) / Differentiation / P fertilisation standard (kg P2O5/ha/yr)Belgium (Flanders) / Crop / 65-95 (55-90)a
The Netherlands / Soil and agricultural systemb / 55-120
Ireland / Soil and crop / 0-286
Sweden / - / 50c
Slovenia / - / 120c
France (Brittany) / Type of farm / 183-218
a First numbers: for 2013-2014. Numbers between brackets: proposed standards from 2017 on. Standard for phosphate saturated soils: 40 kg P2O5/ha/yr; b Agricultural system: grassland or arable crops; c Standards only for organic P fertilisation.
The P application standards in Flanders are differentiated to the cultivated crop. Since the amount of P exported by the crop depends on the kind of crop (Ehlert et al., 2009), crop differentiation of the application standards is necessary to attain equilibrium conditions. In fact, P application standards in Flanders are slightly smaller than crop export doses resulting in slightly negative soil P balances (Anon., 2011). This results in a very slow movement to the left in Figure 2, regardless of the initial soil P concentration. Soils with excessive soil P concentrations will therefore maintain high soil P concentrations for many years (not environmentally optimal), while soils with sub-optimum P concentrations will remain to the left-hand side of the target zone in Figure 2 (not agro-economically optimal). In contrast, P application standards in the Netherlands are differentiated to the soil P concentration. Soils with large P concentrations have smaller P application standards than soils with small P concentrations. This results in a movement to the right for soils located on the left-hand side of the target zone in Figure 2, and a movement to the left for soils located on the right-hand side of the target zone. Standards in the Netherlands therefore better account for the agro-economical and environmental issues than the standards in Flanders do. Standards in Ireland are differentiated both to crop and soil P concentration. The application standards in Ireland can be much larger than in the Netherlands and in Flanders, but soils with excessive P contents can be restricted to no P fertilisation at all (Table 3).
Despite the attempts to reduce P fertilisation, phosphorus concentrations in surface waters remain high. Models predict only small decreases in surface water P concentrations due to fertilisation reduction (van der Bolt et al., 2008; Groenendijk et al., 2012). The question thus ariseswhether alternative measures (e.g. water bodies and drainage management and chemical and hydrological measures)would not be more cost-efficient to reach the Water Framework Directive goals (Schoumans and Kruijne, 1995; van der Bolt et al., 2008). It is also questioned whether point sources are not the main contributors of P loads towards water bodies (Jarvie et al., 2006; Greene et al., 2011). The conclusion that in many cases even after several decades of P nutrient mitigation no improvement in water quality is observed, calls for a more holistic approach of eutrophication management (Jarvie et al., 2013).
Conclusion
Actions need to be taken to reduce the high phosphorus concentrations in European surface waters. The Nitrates Directive and the Water Framework Directive introduced monitoring networks and P concentration standards in Europe, but large differences between Member States exist. It appears that diffuse P losses from agricultural soils with large P contentshave large impacts on surface waters. To increase the chance of success, measures to reduce this impact should take both agro-economical and environmental interests into account. Phosphorus fertilisation advice systems should therefore also consider the risk for P losses, and P application standards are preferentially differentiated for the soil P content in order to reach the target zone for optimum crop yield and limited P losses.
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