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.
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.
Summary Phosphate (PO4) and organic matter (OM) compete for adsorption to metal (hydr)oxides. Our objective was to quantify the effect of OM on PO4 solubility in forest and arable soil by desorption experiments and surface complexation (SC) modelling. We sampled different types of soil along an age gradient (≈50–2500 years) and from different depths (0–80 cm). The soil types are calcareous and cover a range of soil organic carbon (SOC) contents (5.6–43.5 g kg−1), PO4 contents (0.2–5.9 mmol kg−1) and water‐soluble PO4 concentrations (0.03–13.4 µm). Assuming that PO4 concentrations are controlled by desorption, PO4 concentrations were expected to correlate with the PO4 loading on metal‐(hydr)oxide surfaces. However, we show that the PO4 loading alone is a poor predictor of PO4 solubility because its solubility increases with increasing SOC content. These data were explained by SC modelling, which shows a decrease in the apparent adsorption affinity of PO4 with increasing OM loading on to the metal (hydr)oxides. As a consequence, if the competition with OM is disregarded in SC modelling, it results in underestimation of the PO4 concentration by several orders of magnitude. For forest soil, predicted OM loadings increase slightly with increasing soil age. For arable soil, however, OM loadings were much smaller, which we explain by the replacement of PO4 with OM. Overall, adsorption interactions strongly affect PO4 solubility and levels of OM and PO4 stabilization in soil.
Chemical phosphorus recovery from animal manure and digestate; Laboratory and pilot experiments.Wageningen, Wageningen Environmental Research, Report 2849. 114 pp.; 43 fig.; 37 tab.; 41 ref.Since a number of decades the Netherlands faces manure surpluses, because the amount of phosphate produced in animal manure is larger than the amount that can be applied on agricultural land according to the application limits. This surplus needs to be exported from the Dutch agricultural sector. The costs of manure export are however high, and therefore new, simple and cheap innovative techniques are needed to recover phosphate from fresh or digested manure (digestate). The relative small amount of concentrated recovered P can be transported more economically for use outside the Netherlands or can be utilized as secondary P resource for industries, e.g. for the production of fertiliser. We report laboratory and pilot experiments that were carried out in search for cheap and innovative techniques of phosphorus recovery. The main goal of the experiments was to reduce the P-content in pig slurry or digested pig slurry by 25% to 75%. If this goal can be reached with -recovery, in principle all manure including its valuable components (organic matter and other nutrients) can be applied sustainably on agricultural land in The Netherlands. The results show that both on laboratory scale and pilot scale 25-75% of P can be technically recovered with the so called 'acid-base approach'. From a sustainability point of view the impact is positives in terms of energy savings, CO 2 -savings, phosphorus savings and also nitrogen savings. The economical perspective is also positive but will strongly depend on the disposal prices of treated pig slurry with a reduced P content and its value as substitute for mineral N fertiliser and value in terms of organic soil improver on the manure market.Keywords: phosphate, manure, digestate, intensive livestock production, phosphate recovery, pig slurry, manure treatment, manure separation. • Acquisition, duplication and transmission of this publication is permitted with clear acknowledgement of the source.• Acquisition, duplication and transmission is not permitted for commercial purposes and/or monetary gain.• Acquisition, duplication and transmission is not permitted of any parts of this publication for which the copyrights clearly rest with other parties and/or are reserved. New innovative approaches are needed to close the phosphorus (P) cycle in The Netherlands because the agricultural sector currently produces about 25% more P, in the form of manure, than can be applied on land. One promising solution to deal with the surplus amount of P in manure is to recover the surplus of P in the form of a mineral P product for use as a P fertiliser (for export) or as a raw P feedstock for the P fertiliser industries. This idea for 'closing the manure P cycle in The Netherlands' has been circulating for a while, but was never thought well over until in 2013 a Public Private Partnership (PPP) was set up with t...
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