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Colloids may facilitate the transport of trace elements and nutrients like phosphate in soil. In this study, we characterized soil colloids (<0.45 μm), extracted from four agricultural soils by Na-bicarbonate and Na-pyrophosphate, by two complementary analytical techniques; asymmetric flow field-flow fractionation (AF4) and X-ray absorption spectroscopy (XAS). The combined results from AF4 and XAS show that colloidal Fe is present as (i) free Fe-(hydr)oxide nanoparticles, (ii) Fe-(hydr)oxides associated with clay minerals, and (iii) Fe in clay minerals. Free Fe-(hydr)oxide nanoparticles, which can be as small as 2-5 nm, are extracted with Na-pyrophosphate but not with Na-bicarbonate, except for one soil. In contrast, Fe-(hydr)oxides associated with clay minerals are dispersed by both extractants. XAS results show that the speciation of Fe in the colloidal fractions closely resembles the speciation of Fe in the bulk soil, indicating that dispersion of colloidal Fe from the studied soils was rather unselective. In one Fe-rich soil, colloidal Fe was dominantly dispersed in the form of free Fe-(hydr)oxide nanoparticles. In the other three soils, dispersed Fe-(hydr)oxides were dominantly associated with clay minerals, suggesting that their dispersion as free nanoparticles was inhibited by strong attachment. However, in these soils, Fe-(hydr)oxides can be dispersed as oxide-clay associations and may as such facilitate the transport of trace elements.

Phosphorus transport from agricultural land contributes to eutrophication of surface waters. Pipe drain and trench waters from a grassland field on a heavy clay soil in the Netherlands were sampled before and after manure application. Phosphorus speciation was analyzed by physicochemical P fractionation, and the colloidal P fraction in the dissolved fraction (<0.45 μm) was analyzed by asymmetric flow field-flow fractionation (AF4) coupled to high-resolution inductively coupled plasma-mass spectrometry and ultraviolet diode array detector. When no manure was applied for almost 7 mo, total P (TP) concentrations were low (<21 μmol L), and TP was almost evenly distributed among dissolved reactive P (DRP), dissolved unreactive P (DUP), and particulate P (PP). Total P concentrations increased by a factor of 60 and 4 when rainfall followed shortly after application of cattle slurry or its solid fraction, respectively. Under these conditions, DRP contributed 50% or more to TP. The P speciation within the DUP and PP fractions varied among the different sampling times. Phosphorus associated with dissolved organic matter, probably via cation bridging, comprised a small fraction of DUP at all sampling times. Colloidal P coeluted with clay particles when P application was withheld for almost 7 mo and after application of the solid cattle slurry fraction. At these sampling times, PP correlated well with particulate Fe, Al, and Si, indicating that P is associated with colloidal clay particles. After cattle slurry application, part of DUP was probably present as phospholipids. Physicochemical fractionation combined with AF4 analysis is a promising tool to unravel the speciation of colloidal P in environmental water samples.

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Abstract:Peat is used as rooting medium in greenhouse horticulture. Biochar is a sustainable alternative for the use of peat, which will reduce peat derived carbon dioxide emissions. Biochar in potting soil mixtures allegedly increases water storage, nutrient supply, microbial life and disease suppression but this depends on feedstock and the production process. The aim of this paper is to find combinations of feedstock and production circumstances which will deliver biochars with value for the horticultural end user. Low-temperature (600 • C-750 • C) gasification was used for combined energy and biochar generation. Biochars produced were screened in laboratory tests and selected biochars were used in plant experiments. Tests included dry bulk density, total pore space, specific surface area, phytotoxicity, pH, EC, moisture characteristics and microbial stability. We conclude that biochars from nutrient-rich feedstocks are too saline and too alkaline to be applied in horticultural rooting media. Biochars from less nutrient-rich feedstocks can be conveniently neutralized by mixing with acid peat. The influence of production parameters on specific surface area, pH, total pore space and toxicity is discussed. Biochar mildly improved the survival of beneficial micro-organisms in a mix with peat. Overall, wood biochar can replace at least 20% v/v of peat in potting soils without affecting plant growth.