Article pubs.acs.org/JAFC
Colloidal-Bound Polyphosphates and Organic Phosphates Are Bioavailable: A Nutrient Solution Study Jessica Bollyn,*,† Joran Faes,† Andreas Fritzsche,‡ and Erik Smolders† †
Department of Earth and Environmental Sciences, KU Leuven, Kasteelpark Arenberg 20 bus 2459, 3001 Leuven, Belgium Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, D-07749 Jena, Germany
‡
S Supporting Information *
ABSTRACT: Colloidal forms of Fe(III) minerals can be stabilized in solution by coatings of organic or poly-phosphate (P), which reduce the zeta-potential. This opens up a route toward the development of nanoforms of P fertilizers. However, it is unclear if such P forms are bioavailable. To address this question, spinach (Spinacia oleracea) was grown in nutrient solutions, at equal total P, using three different forms of P (orthophosphate = Pi; hexametaphosphate = HMP; myo-inositol hexaphosphate = IHP), free or bound to goethite/ferrihydrite colloids. After 10 days, P uptake was determined with a dose−response curve using colloid-free Pi as a reference treatment. The Pi concentration generating equal P uptake as in colloidal P treatments was used to calculate the relative bioavailability of colloidal P (RBAcolloid). The RBAcolloid was about 60% for Pi-loaded goethite, stabilized with natural organic matter. For HMP/IHP-Pi-loaded colloids, RBAcolloid ranged between 10 and 50%, in line with their higher sorption strength. In conclusion, colloidal organic P or poly-P can stabilize Fe(III) colloids in solution and can contribute to plant-available P. Soil experiments are required to assess their potential as nanofertilizers. KEYWORDS: iron oxyhydroxide colloids, colloidal stability, nanofertilizer, polyphosphate, organic phosphate
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INTRODUCTION Phosphorus (P) is a nonrenewable, limited resource that must be used efficiently to keep up agriculture.1 Resource recycling, more efficient fertilizer use, and increased efficiency in plant P uptake from soil are needed, requiring an improved understanding of all processes of the (soil) P cycle. Phosphorus occurs as different species in the soil, i.e., inorganic P and organic P (Po), and both may appear as truly dissolved, ionic species as well as adsorbed on minerals. Only truly dissolved orthophosphate ions in soil solution (Pi, mainly as H2PO4− and HPO42−) are directly available for plant uptake, though they often constitute less than 1% of the total soil P content. The majority of P is either sorbed on soil minerals, such as Al- and Fe-oxyhydroxides, or precipitated as phosphate minerals, e.g., in Ca-phosphates. Organic P can comprise 20−80% of the total phosphorus, depending on land use and soil type, and consists of a wide variety of compounds.2 The general concept in plant nutrition is that Pi is absorbed by plant roots from solution.3 In P-deficient soils, colloids can constitute the majority of the mobile P fraction. Colloids are defined as particles with sizes between 1 and 1000 nm in at least one dimension,4 which enables them to diffuse through the ambient phase due to omnidirectional Brownian motion instead of settling down due to unidirectional gravity. The prevailing idea is that such colloidsorganic macromolecules, inorganic minerals, and organo−mineral compositesare not directly available for plant uptake. Thus, P in the soil solution should be diagnosed using appropriate analytical techniques that quantify specifically the plant-available, truly dissolved Pi species. However, this view has been challenged in recent studies. Santner et al.5 observed for the first time that P uptake by plant roots in nutrient solution is limited by P diffusion toward the root surface; this diffusive barrier was overcome by © XXXX American Chemical Society
buffering the solution Pi with P-loaded Al2O3 nanoparticles (NPs). These authors concluded that the P sorbed on the NPs is released near the roots, where the truly dissolved P is close to zero, and hence demonstrated that colloidal P can be a mobile vector of Pi, which is released from its colloidal carrier before direct absorption into the root. Another recent study6 showed that natural colloidal P in a nonfiltered soil−water extract increased plant P uptake compared to the filtered, colloid-free solution. A similar result was found for a freshwater alga using P sorbed on Fe-oxyhydroxide colloids under P-limiting conditions.7,8 All solution studies with plants has so far been carried out for uptake rates below 48 h, which is likely related to the inability to ensure colloidal stability. Recently, engineered nanomaterials have been studied increasingly to improve P fertilization. In studies by Raliya et al.,9,10 zinc oxide NPs increased the activity of P-mobilizing enzymes, such as phytase, thus mobilizing native P and increasing P uptake by mung bean. Also addition of Fe3O4 NPs was reported to improve P availability to Lactuca sativa (lettuce).11 Both setups, however, relied on the mobilization of native P pools and did not function as a fertilizer themselves. To function as a nanofertilizer, the stability of P-loaded mineral colloids is a key factor for their contribution to bioavailability. We recently showed that the colloidal stability of P-loaded Fe-oxyhydroxide nanoparticles (P-FeOx-NPs) under environmental conditions was largely explained by their zetapotential: the addition of polyphosphates (Pp) or fulvates was needed to obtain stable colloidal suspensions.12 The P-FeOxReceived: Revised: Accepted: Published: A
March 31, 2017 July 11, 2017 July 21, 2017 July 21, 2017 DOI: 10.1021/acs.jafc.7b01483 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
Journal of Agricultural and Food Chemistry
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NPs stabilized with myo-inositol hexaphosphate (IHP, also known as phytic acid) or hexametaphosphate (HMP), a circular Pp, proved to be the most stable under environmental conditions. This was attributed to their specific binding mechanism: IHP sorption on goethite proceeds through four of the six phosphate groups, with two groups remaining in solution.13 These multiple free phosphate groups lead to a larger negative surface charge compared to a surface loaded with Pi, which leads to more interparticle repulsion and thus a longer residence time in solution of IHP-coated colloids than Pi-coated colloids. A similar mechanism was speculated for Ppcoated colloids.12 Desorption from the mineral surfaces will be more restricted for these compounds compared to P i. Therefore, it remains unresolved if Po or Pp loaded to mineral colloids are bioavailable P forms. One study showed that after several desorption cycles less than 5% P desorbed for IHP in contrast to almost 20% for Pi sorbed to goethite.14 Similar findings emerged from a recent study by Ruyter-Hooley et al.,15 with limited desorption of IHP (
