The global phosphorus crisis provided impetus to develop fertilisers with better P use efficiency. We tested layered double hydroxides (LDHs) as slow release fertilisers with superior performance to fertilise strongly P fixing soils. Mg-Al LDHs with varying M 2+ /M 3+ ratios were synthesised as NO 3 forms and were exchanged with HPO 4 2-. XRD and XANES spectroscopy confirmed the identity of the phosphate exchanged LDH. Decreasing the M 2+ /M 3+ ratio, i.e. increasing the anion exchange capacity, increased the selectivity of P adsorption due to the increasing charge density of the LDH layers. The fertilisation efficiency of phosphate exchanged LDH (Mg/Al ratio of 2) was compared to that of a soluble P fertiliser in two P deficient soils, an acid weathered and a calcareous soil. The P-use efficiency of P-LDH in the acid soil was up to 4.5 times higher than that of soluble P. This was likely related to a liming effect of the LDH. In the calcareous soil, the P-use efficiency at low doses was only 20 % above that of soluble P, whereas it was lower at high doses. These overall encouraging results warrant further studies on the boundary conditions under which P-LDHs may outperform traditional fertilisers. KeywordsLayered double hydroxide (LDH), ion exchange, slow release fertiliser, phosphorus use efficiency, x-ray diffraction (XRD), x-ray absorption near edge structure (XANES) P use efficiency between P-LDH fertiliser and different commercial P fertilisers, including slow release fertilisers, the boundary conditions of the P-LDH use can be better determined. AcknowledgementsM.E. thanks IWT for a PhD fellowship and S.B. the Onderzoeksfonds KU Leuven for a the postdoctoral mandate. We are grateful to KU Leuven for support in the interdepartmental PB3 project. Also acknowledged are Ann Kristin Eriksson (SLU, Sweden) Dean Hesterberg (North Carolina State University, US) and Steve Hillier (James Hutton Institute, Aberdeen, UK) for compiling the XANES reference spectra. Wantana Klysubun and the staff at BL-8 at SLRI, Thailand are thanked for support and organization of the beamline. Associated contentIn the supporting information, additional details of the material stability, the XANES measurement, the anion geometry, the desorption kinetics and the pot trial are provided.
A common technique to quantitatively estimate P speciation in soil samples is to apply linear combination fitting (LCF) to normalized P K-edge X-ray absorption near-edge structure (XANES) spectra. Despite the rapid growth of such applications, the uncertainties of the fitted weights are still poorly known. Further, there are few reports to what extent the LCF standards represent unique end-members. Here, the co-variance between 34 standards was determined and their significance for LCF was discussed. We present a probabilistic approach for refining the calculation of LCF weights based on Latin hypercube sampling of normalized XANES spectra, where the contributions of energy calibration and normalization to fit uncertainty were considered. Many of the LCF standards, particularly within the same standard groups, were strongly correlated. This supports an approach in which the LCF standards are grouped. Moreover, adsorbed phytates and monetite were well described by other standards, which puts into question their use as end-members in LCF. Use of the probabilistic method resulted in uncertainties ranging from 2 to 11 percentage units. Uncertainties in the calibrated energy were important for the LCF weights, particularly for organic P, which changed with up to 2.7 percentage units per 0.01 eV error in energy. These results highlight the necessity of careful energy calibration and the use of frequent calibration checks. The probabilistic approach, in which at least 100 spectral variants are analyzed, improves our ability to identify the most likely P compounds present in a soil sample, and a procedure for this is suggested in the paper.
Anaerobic conditions mobilise phosphorus (P) in soils and sediments. The role of anaerobic microsites in well-drained soil on P migration is unknown. This study aimed to identify mechanisms that control field-scale vertical P mobility as affected by organic fertilisers that may trigger variable redox conditions. Soils were sampled at different depths in a well-drained Luvisol after 19-years of application of organic fertilisers. The concentrations of P and manganese (Mn) in 0.45-µm-filtered extracts (10 -3 M CaCl2) of field-moist soil samples were strongly correlated (r = + 0.95) and both peaked in and below the compacted plough pan; suggesting that reductive processes mobilise P. Waterlogged soil incubations confirmed that anaerobic respiration comobilises Mn and P and that this leads to the release of colloidal P and iron (Fe). The long-term applications of farmyard manure and immature compost enhanced the concentrations of Mn, Fe, and aluminium (Al) in the soil solution of subsurface samples, whereas less such effect was found under the application of more stable organic fertilisers. Farmyard manure application significantly enhanced soil P stocks below the plough layer despite a small P input. Overall, multiple lines of evidence confirm that anaerobic respiration, sparked by labile organic matter, mobilises P in this seemingly well-drained soil.
Summary The kinetics of orthophosphate (PO4) sorption in soil has far reaching consequences on its long‐term fate. Traditionally, a distinction is made between fast, reversible adsorption and slow fixation. The kinetics are commonly described by compartmental models (CMs) assuming kinetically distinct homogeneous pools (e.g. a rapid‐ and slow‐sorbing pool), with phenomenological equations or with complex diffusion‐based models. Alternatively, this process can be described by assuming frequency distributions of both adsorption and desorption rate constants and, thereby, enabling better description of experimental data with fewer adjustable parameters. Here, we developed such a rate constant distribution (RCD) model and compared it with CMs and other commonly used rate equations. Batch 33PO4 sorption was measured in agitated suspensions between 2 minutes and 20 days after spiking in 13 contrasting types of soil and two iron oxyhydroxides. Overall, the RCD model, with three adjustable parameters, describes the data better than the other models tested. The so‐called slow reactions, denoted as the factor change in soluble 33PO4 between 1 and 20 days after spiking, were described better by the RCD model and ranged from 1.0 (i.e. no change) to 6.9. The extent of slow reactions increased with the increase in the fraction of poorly crystalline iron in the soil (r = 0.69; P = 0.0088). Equilibrium was elusive up to 20 days for PO4 sorption on ferrihydrite and on soil samples with a large fraction of poorly crystalline iron oxyhydroxides. The RCD model code is available as freeware from the first author. Highlights A new rate constant distribution (RCD) model for PO4 sorption kinetics was developed. The RCD model offers a simple approach to take surface heterogeneity into account. The RCD model fitted PO4 sorption kinetics better than commonly used equations. Equilibrium was elusive for PO4 sorption on ferrihydrite and soil with poorly crystalline iron.
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