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Risk assessment of metallic nanoparticles (NPs) is critically affected by the concern that toxicity goes beyond that of the metallic ion. The present study addressed this concern for soils with silver nanoparticles (AgNPs) using the Ag-sensitive nitrification assay. Three agricultural soils (A, B, and C) were spiked with equivalent doses of either AgNP (diameter = 13 nm) or AgNO 3 . Soil solution was isolated and monitored over 97 d with due attention to accurate Ag fractionation at low (∼10 μg L -1 ) Ag concentrations. Truly dissolved (<1 kDa) Ag in the AgNO 3 -amended soils decreased with reaction half-lives of 4 to 22 d depending on the soil, denoting important Ag-aging reactions. In contrast, truly dissolved Ag in AgNP-amended soils first increased by dissolution and subsequently decreased by aging, the concentration never exceeding that in the AgNO 3 -amended soils. The half-lives of AgNP transformation-dissolution were approximately 4 d (soils A and B) and 36 d (soil C). The Ag toxic thresholds (10% effect concentrations, milligrams of Ag per kilogram of soil) of nitrification, evaluated at 21 or 35 d after spiking, were similar between the 2 Ag forms (soils A and B) but were factors of 3 to 8 lower for AgNO 3 than for AgNP (soil C), largely corroborating dissolution differences. This fate and bioassay showed that AgNPs are not more toxic than AgNO 3 at equivalent total soil Ag concentrations and that differences in Ag dissolution at least partially explain toxicity differences between the forms and among soils. Environ Toxicol Chem 2018;37:2123-2131. © 2018 SETAC., (© 2018 SETAC.)

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 = P i ; 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 P i as a reference treatment. The P i concentration generating equal P uptake as in colloidal P treatments was used to calculate the relative bioavailability of colloidal P (RBA colloid ). The RBA colloid was about 60% for P i -loaded goethite, stabilized with natural organic matter. For HMP/IHP-P i -loaded colloids, RBA colloid 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.

Species diversity is commonly hypothesized to result from trade-offs for different limiting resources, providing separate niches for coexisting species 1-4 . As soil nutrients occur in multiple chemical forms, plant differences in acquisition of the same element derived from different compounds may represent unique niche dimensions 5,6 . Because plant productivity of ecosystems is often limited by phosphorus 7 , and because plants have evolved diverse adaptations to acquire soil phosphorus 6,8 , a promising yet untested hypothesis is phosphorus resource partitioning 6,9,10 . Here, we provided two different chemical forms of phosphorus to sown grassland mesocosms to investigate phosphorus acquisition of eight plant species that are common in European grasslands, and to identify subsequent patterns of plant abundance. For the first time, we show that the relative abundance of grassland plant species can be influenced by soil phosphorus forms, as higher abundance was linked to higher acquisition of a specific form of phosphorus. These results were supported by a subsequent isotope dilution experiment using intact grassland sods that were treated with different inorganic or organic phosphorus forms. Here, 5 out of 14 species showed greater phosphorus acquisition in the inorganic phosphorus treatment, and 4 in the organic phosphorus treatments. Furthermore, for the species used in both experiments we found similar acquisition patterns. Our results support the hypothesis of phosphorus resource partitioning and may provide a new mechanistic framework to explain high plant diversity in phosphorus-poor ecosystems 6,11-13 . As world biodiversity hotspots are almost invariably related to phosphorus limitation 8,11,12 , our results may thus also be key to understanding biodiversity loss in an era of ever-increasing nutrient enrichment 14 .

Iron oxyhydroxide nanoparticles (Fe-NPs) are natural vectors of phosphate (PO 4 ) in the environment. Their mobility is determined by colloidal stability, which is affected by surface composition. This might be manipulated in engineered NPs for environmental or agricultural applications. Here, the stability of PO 4 -Fe-NPs (HFO/goethite) was determined across contrasting environmental conditions (pH, Ca concentration) and by using fulvates (FA) and polyphosphates (poly-P's) as coatings. The PO 4 -Fe-NPs are unstable at Ca concentrations above 0.1 mM. Addition of FA and some poly-P's significantly improved stability. Zeta potential explained colloidal stability across treatments; surface charge was calculated with surface complexation models and explained for phytic acid (PA) and hexametaphosphate (HMP) by a partial (1-4 of the 6 PO 4 units) adsorption to the surface, while the remaining PO 4 units stayed in solution. This study suggests that Ca concentration mainly affects the mobility of natural or engineered PO 4 -Fe-NPs and that HMP is a promising agent for increasing colloidal stability.