Filtration procedures influence environmental monitoring of orthophosphate

Phosphate is becoming a main water quality determining factor in regions with intensive animal husbandry. Environmental quality limits to control eutrophication in freshwater are commonly based on orthophosphate, which is measured by a colorimetric assay as molybdate reactive P (MRP) following ISO protocols. Surprisingly, sample filtration is not specified in these protocols, and filtration procedures vary among certified laboratories using these protocols. It is well established that MRP includes both free orthophosphate and phosphate associated with colloidal material such as Fe and Al oxyhydroxides, while only a minor part of the organic P is included. The aims of this study were to identify the effect of filtration on MRP in environmental samples, and to determine how filtration procedures in different certified laboratories affect reported data. In a round-robin test, ten waters were collected from streams in Flanders (Belgium) and sent to certified laboratories. The coefficients of variation in MRP results among the certified laboratory ranged 13-115 %. The same waters were subjected to size fractionation in our laboratory using filtration (paper filter, 0.45 μm, 0.1 μm) and dialysis (12-14 kDa). The MRP concentrations decreased with decreasing size fractionation cut-off. The MRP concentrations in dialysates (14 kDa), which approximated the free orthophosphate, were 3 – 80 % (mean 38%) of the corresponding values in unprocessed samples. This percentage decreased with increasing Fe concentrations, suggesting that the non-dialyzable P was mostly bound to Fe-rich particles and colloids. The MRP concentrations in the 0.10 μm membrane filtered solution only marginally exceeded the free orthophosphate concentrations, indicating that this filtration may be a pragmatic choice to identify the free orthophosphate in freshwater. In summary, the current protocols for environmental monitoring of orthophosphate yield widely variable results due to P bound to Fe and Al oxyhydroxide particles and colloids. This highlights the importance of more stringent protocols for environmental monitoring of orthophosphate.status: publishe

A soil P-test to predict the threshold for yield decrease in a depletion scenario

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Freshwater phosphate limits are based on a poorly standardized molybdate reactive P method

Phosphate is becoming a main water quality determining factor in regions with intensive animal husbandry. The freshwater limits in Flanders are based on phosphate determined by the molybdate reactive P (MRP) method and the certified laboratories have to adhere to ISO protocols (ISO 15681-2:2003 or ISO 15923-1:2013). Surprisingly, filtration is not specified in these guidelines and sample preparation procedures range from unfiltered to 0.45 µm filtration in these certified laboratories. It is well established that MRP includes both free orthophosphate and phosphate associated with colloidal material such as Fe and Al oxyhydroxides while organic P is only weakly included. The aim of this study was to compare analytical results among certified laboratories and to identify filtration effects on the analytical results. In a round-robin test, ten waters were collected from streams in Flanders and sent to certified laboratories. In addition, the same waters were tested in our laboratory assessing effects of filtration (paper filter, 0.45 µm, 0.1 µm) and dialysis (12-14 kDa) on MRP. The MRP concentrations in the water samples decreased gradually by filtration over progressively smaller membrane pore sizes and after dialysis. Filtration over a 0.45 µm membrane filter reduced MRP concentrations to 74-84% of MRP in unfiltered waters with low Fe (2.5 mg Fe/L). In the round-robin test, the coefficient of variation of MRP among certified laboratories ranged from 4-71%. We postulate that this variation can be reduced by more stringent laboratory guidelines.status: publishe

Iron colloids reduce the bioavailability of phosphorus to the green alga Raphidocelis subcapitata

Phosphorus (P) is a limiting nutrient in many aquatic systems. The bioavailability of P in natural waters strongly depends on its speciation. In this study, structural properties of iron colloids were determined and related to their effect on P sorption and P bioavailability. The freshwater green alga Raphidocelis subcapitata was exposed to media spiked with radiolabelled 33-PO4, and the uptake of 33-P was monitored for 1 h. The media contained various concentrations of synthetic iron colloids with a size between 10 kDa and 0.45 μm. The iron colloids were stabilised by natural organic matter. EXAFS spectroscopy showed that these colloids predominantly consisted of ferrihydrite with small amounts of organically complexed Fe. In colloid-free treatments, the P uptake flux by the algae obeyed Michaelis-Menten kinetics. In the presence of iron colloids at 9 or 90 μM Fe, corresponding to molar P:Fe ratios between 0.02 and 0.17, the truly dissolved P (<10 kDa) was between 4 and 60% of the total dissolved P (<0.45 μm). These colloids reduced the P uptake flux by R. subcapitata compared to colloid-free treatments at the same total dissolved P concentration. However, the P uptake flux from colloid containing solutions equalled that from colloid-free ones when expressed as truly dissolved P. This demonstrates that colloidal P did not contribute to the P uptake flux. It is concluded that, on the short term, phosphate adsorbed to ferrihydrite colloids is not available to the green alga R. subcapitata.publisher: Elsevier articletitle: Iron colloids reduce the bioavailability of phosphorus to the green alga Raphidocelis subcapitata journaltitle: Water Research articlelink: http://dx.doi.org/10.1016/j.watres.2014.04.010 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved.status: publishe

The bioavailability of colloidal phosphorus to freshwater algae

The eutrophication of freshwaters is a major environmental concern in developed countries and is often attributed to excessive P fertilizer application. However, the eutrophication risk depends strongly on P bioavailability, which in turn depends on P speciation. Colloidal P species, e.g. P associated with colloidal Fe and Al oxyhydroxides, are included in routine colorimetric measurements of the available P fraction as “molybdate reactive P”, but the availability of this colloidal P fraction remains questionable. The aim of this study was to address the bioavailability of colloidal P in a well-defined model system. Growth and P uptake by a freshwater green alga (Raphidocelis subcapitata) were measured in synthetic solutions with or without colloidal Fe oxyhydroxides. Short term (1 hour) uptake experiments using radiotracers show that algal P uptake decreases with increasing colloidal P fraction. The P uptake rate is related to the free orthophosphate fraction (quantified by 10 kDa ultrafiltration), i.e. colloidal P does not contribute to the actual P uptake. Growth experiments on the longer term (up to 14 days) under P-limited conditions reveal that colloidal P contributes partially, but not completely, to algal growth. This is likely a result of desorption when free orthophosphate is taken up and becomes depleted. This potential P bioavailability correlates to the “labile P pool”, which is quantified by dialysis of the test solution against ferrihydrite as infinite P sink. It is concluded that colloidal P is only partially bioavailable, and that the eutrophication risk in freshwaters may be overestimated if P is measured as “molybdate reactive P”.status: publishe

Which soil P test reflects best the phosphorus availability for leaching?

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Phosphorus in deep soil layers: an inventory of Belgian soils

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Evidence-based sustainable phosphorus use in agriculture in Flanders (Belgium)

Phosphorus (P) is an important nutrient for agriculture but excess P application to soils can contribute to eutrophication of surface waters. Phosphorus fertilisation recommendations rarely take environmental concerns into account (Jordan-Meille et al., 2012). Soil P tests used for soil P content measurements mostly build on chemical extractions for which empirical (not mechanistic) relations with crop responses are determined. Recently, the Flemish Land Agency (government of Flanders, Belgium) initiated a research project to stimulate sustainable phosphorus use in agriculture. Three research institutes (Institute for Agricultural and Fisheries Research (ILVO), Soil Service of Belgium and KU Leuven) started this four-year project at the beginning of 2015. In the first phase of the project, soil P tests are evaluated in order to select the (combination of) soil P test(s) that best reflect both (i) the P availability for plants and (ii) the risk of P losses towards surface waters at a relatively low cost. Several common and new tests (ammonium lactate extraction, 0.01 M CaCl2 extraction, oxalate extraction, Olsen extraction, etc) are compared in a pot experiment in a depletion scenario and on soil samples from 14 long term fertilisation field trials in NW Europe with reported yield differences due to soil P differences. Suitability of the test is assessed by the correlation with crop yield and the relative width of the 95% confidence interval of the critical soil P content, i.e. the soil P content corresponding with 95% relative yield. Not only single soil P tests but also combinations are evaluated. According to van Rotterdam-Los (2010), especially the combination of a soil P test reflecting P intensity (directly available P) and one reflecting P quantity (P available in the long term) shows promise for describing the behaviour and availability of P. The same tests are also evaluated for correlations with soil P losses by performing soil column leaching experiments under unsaturated conditions. The selected test(s) will be used in the second and third parts of the research project. In this second part, the soil P contents at which yields are optimal and losses still small, will be defined as the target zone. In the third part, we will derive soil P fertilisation advice in order to reach or remain in the target zone from sorption/desorption experiments and field trials. The outline of this project and its first results for sustainable P use will be presented and discussed.status: publishe

Internal Loading and Redox Cycling of Sediment Iron Explain Reactive Phosphorus Concentrations in Lowland Rivers

The phosphate quality standards in the lowland rivers of Flanders (northern Belgium) are exceeded in over 80% of the sampling sites. The factors affecting the molybdate reactive P (MRP) in these waters were analyzed using the data of the past decade (>200 000 observations). The average MRP concentration in summer exceeds that winter by factor 3. This seasonal trend is opposite to that of the dissolved oxygen (DO) and nitrate concentrations. The negative correlations between MRP and DO is marked (r = -0.89). The MRP concentrations are geographically unrelated to erosion sensitive areas, to point-source P-emissions or to riverbed sediment P concentration. Instead, MRP concentrations significantly increase with increasing sediment P/Fe concentration ratio (p 0.4. In contrast, no such release was found from sediments with lower P/Fe irrespective of temperature and DO treatments. This study suggests that internal loading of the legacy P in the sediments explains the MRP concentrations which are most pronounced at low DO concentrations and in regions where the P/Fe ratio in sediment is large.status: publishe

The phosphate desorption rate in soil limits phosphorus bioavailability to crops

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