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Nanotechnology may be used to design more performant copper agrochemicals. Assessing the benefits and risks of the novel nano-products requires a good understanding of their fate in soil relative to other forms of copper. This study compares nine forms of Cu including nano and conventional Cu-based formulations, as well as their salt or bulk equivalents. Readily available Cu was extracted with 0.01 M CaCl2 over a period of 29 days in three soils with contrasting properties. For eight out of nine formulations, available Cu decreased over time indicating that complexation of Cu with soil phases was the process determining the availability of Cu. Freshly suspended CuO nanoparticles were the only form where available Cu increased over time, indicating that dissolution was limiting availability. The complexation/dissolution kinetics were mainly determined by the soil pH, whereas both soil pH and OC% determined the equilibrium concentration of Cu in the soil solution, in line with results from equilibrium modelling. Overall, the fraction of readily available Cu was very low in all cases, reaching

The vital element phosphorus (P) invokes two extremes in the environment; (i) scarcity, as a non-renewable resource and as a poorly bioavailable limiting nutrient for plants, and (ii) excess, as cause of eutrophication in surface waters. To tackle both these problems, the inter-relationship between the P and the iron (Fe) cycle is widely discussed with a special interest in the ferrous iron-phosphate mineral vivianite (Fe(II)3(PO4)2*8H2O). Vivianite forms naturally in sub-/anoxic environments with high Fe(II) and PO4 concentrations, and is a sink to the dissolved P concentration. On the other hand, vivianite has been proposed as a P source through application as a slow-release Fe-P fertilizer prepared from recycled P. However, vivianite is a metastable mineral under oxic conditions; it readily oxidizes, notably changing color from white to dark blue/purple. This transformation changes the properties of the mineral (surface), and thus its suitability as a fertilizer.We investigated the oxidation and dissolution of vivianite under different environmental conditions with the aim of developing a mechanistic and kinetic model that relates the oxidation process with dissolution rates. Moreover, the effect of secondary mineral precipitation on the ‘net’ availability of P and Fe for soil organisms was also studied. Quantifying dissolution rates and secondary mineral formation under environmentally relevant conditions provides the fundamental knowledge needed to assess the suitability of vivianite as Fe and P fertilizer. This information is also paramount to the idea of a circular economy concept: starting with the reduction of P loads of (waste) waters and using the byproduct vivianite as P source for fertilization.

Phytosiderophores (PS) are natural chelating agents, exuded by graminaceous plants (grasses) for the purpose of Fe acquisition (Strategy II). They can form soluble Fe complexes with soil-Fe that can be readily taken up. PS are exuded in a diurnal pulse release, and with the start of PS release a “window of iron uptake” opens. In the present study we examined how this window is constrained in time and concentration by biogeochemical processes. For this purpose, a series of interaction experiments was done with a calcareous clay soil and the phytosiderophore 2′-deoxymugineic acid (DMA), in which metal and DMA speciation were examined as a function of time and DMA concentration. Various kinetically and thermodynamically controlled processes affected the size of the window of Fe uptake. Adsorption lowered, but did not prevent Fe mobilization by DMA. Microbial activity depleted DMA from solution, but not on time scales jeopardizing Strategy II Fe acquisition. Complexation of competing metals played an important role in constraining the window of Fe uptake, particularly at environmentally relevant PS concentrations. Our study provides a conceptual model that takes into account the chemical kinetics involved with PS-mediated Fe acquisition. The model can help to explain how success or failure of PS-mediated Fe acquisition depends on environmental conditions.