Unravelling the biogeochemistry that affects the mobility of phosphorus in agricultural soil

The intensification of agriculture has been associated with excessive addition of phosphorus (P) fertilisers to agricultural soils. The excess P in soil is a legacy for the environment since P slowly migrates downward by leaching towards the groundwater. Once P reaches surface water, eutrophication is induced and a limiting natural resource is lost. The current understanding of the leaching of P states that P leaching is slow and that P is strongly retarded by the sorption of the orthophosphate (PO4) anions on iron (Fe) and aluminium (Al) oxyhydroxides in soil or by precipitation as calcium phosphates in neutral or calcareous soils. Phosphorus only becomes highly mobile once the sorption sites are saturated. The current understanding predicts that the mobility of P is related to the Degree of Phosphate Saturation of the Fe and Al oxyhydroxides (DPS) and that there is very limited P migration in most European soils because the sorption saturation is only rarely reached. However, this prediction is contradicted by observations of significant subsurface P accumulation in fertilised soils and loss of P via drainage pipes, the major reasons being unclear. The general objective of this thesis was, therefore, to understand and quantify the biogeochemical processes that affect the migration of P in agricultural soils. The underlying idea was that the current DPS-model needs to be validated for more contrasting soil types (since it was established for noncalcareous sandy soils from the Netherlands) and that three factors, not yet embedded in the current concept required attention i.e. 1) sorption kinetics, 2) colloid-facilitated P transport and 3) effects of anaerobic redox reactions on P migration. First, the sorption of PO4 on Fe and Al oxyhydroxides was confirmed to control leachate P concentrations from 120 unsaturated columns, filled with agricultural soils with contrasting properties, including pH-neutral soils (Chapter two). Leachate P concentrations ranged from 0.7 to 240 µM and increased as the ratio of P to Fe and Al in acid oxalate soil extracts (i.e. DPS) increased and as the PO4-distribution-coefficient (determined by a radiotracer experiment) decreased. This shows that the DPS-model can be extended to pH-neutral soils as well. Surprisingly, leachate P increased with increasing leachate Fe and Al concentrations. Surface complexation modelling was done to describe PO4 sorption to ferrihydrite (using the CD-MUSIC model). This yielded a reasonable description of leachate P concentrations, but only when reactive PO4 was described from isotopically exchangeable PO4, when organic matter was included as the main competing adsorbate and when mobile colloidal ferrihydrite was included. The model revealed that PO4-binding by colloids enhanced leachate PO4 concentrations up to a factor 50 (in comparison to the leachate PO4 concentrations in absence of colloids), mostly at small DPS and at small calcium (Ca2+) concentration in solution, likely because a high Ca2+ concentration causes flocculation of the organic-matter stabilised Fe and Al colloids. A second deviation from the current model was revealed in a Luvisol after 19-years of application of different organic fertilisers where P migrated more than expected from the DPS concept (Chapter four). Anaerobic respiration mobilised P and colloids, even in the unsaturated zone of this upland soil. Extracts (10-3 M calcium chloride, CaCl2) of field-moist soil, sampled at distinct depths, and in situ wick samplers indicated a markedly strong correlation between soluble (< 0.45 µm) P and manganese (Mn) and both peaked in and below the compacted plough pan, where a high water-filled porosity and a high organic carbon (OC) content coincided. Waterlogged soil incubations confirmed that anaerobic respiration co-mobilises Mn and colloidal P. The long-term applications of farmyard manure and an immature compost enhanced soluble Mn, Fe, and Al in the subsurface with respect to the mineral N treatment, 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 net positive soil P balance. Overall, multiple lines of evidence confirm that anaerobic respiration, sparked by labile organic matter, mobilises P in this seemingly well-drained soil. Additional soil cores (57 in total) were sampled down to 160 cm depth at contrasting locations to identify the controlling soil properties of redox-related mobilisation of P and its role on P migration (Chapter five). Field-moist soil extracts (10-3 M CaCl2) showed that reductive dissolution of Mn and Fe oxyhydroxides generally occurred and this was enhanced at low pH and a high OC content. This process enhanced soluble (< 0.45 µm) Fe(III) and Al concentrations, that are most likely colloidal particles, which enhanced, in turn, soluble P concentrations. The depth distributions of P in these 57 soil cores were empirically fitted with a sigmoidal curve to identify the soil factors that explain actual P migration. In fertilised plots, P enrichments down to 70 cm depth were detected. The depth distributions showed much more P dispersion than what reasonably can be estimated by a transport model under nonlinear equilibrium sorption. The slopes of the DPS-depth sigmoidal lines were more spread out, i.e. more disperse, with increasing OC content and decreasing pH of the topsoils, suggesting that anaerobic-induced P mobilisation explains the large dispersion. Thirdly, the potential disequilibrium of PO4 sorption creates another possible source of error for models assuming local equilibrium sorption (e.g. the CD-MUSIC model; Chapter three). A new rate constant distribution (RCD) model, that assumes frequency distributions of both adsorption and desorption rate constants, was developed and compared with other kinetic models. Batch radiolabeled PO4 (33PO4) sorption was measured in agitated suspensions between two minutes and 20 days after spiking in thirteen 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 one and 20 days after spiking, were described better by the RCD model and ranged from 1.0 (i.e. no change) to 6.9. Orthophosphate sorption on ferrihydrite and on some soil samples with a high ratio of poorly crystalline iron oxyhydroxides to total iron did not cease within 20 days. Thus, the lack of sorption equilibrium may be a source of error for equilibrium models. Taken together, this study confirms that the interaction between PO4 and Fe and Al oxyhydroxides, i.e. the DPS, is crucial to understand the mobility of P in fertilised soils. However, this interaction is more dynamic than previously modelled and thought; 1) the PO4 sorbing Fe and Al oxyhydroxides can detach from the soil's solid phase as mobile colloids and mediate vertical PO4 transport; 2) PO4 sorption on Fe(III) and Al oxyhydroxides is not instantaneous and 3) reductive dissolution of Mn and Fe oxyhydroxides triggers the mobilisation of P, even in the unsaturated zone of agricultural soil. Models that succeed at incorporating these dynamic interactions will offer an improved understanding of the fate of legacy P in agricultural soil.status: publishe

Soil organic matter increases antimonate mobility in soil: an Sb(OH)6 sorption and modelling study

© 2019 Elsevier Ltd The role of organic matter (OM) in antimonate (further denoted as Sb(OH) 6 ) mobility in soil is unclear. The objective of this study was to evaluate Sb(OH) 6 –OM interaction. Antimonate solid:liquid distribution coefficients (K D ) were measured at low Sb concentrations in soil samples with a natural gradient in soil organic carbon (OC) that were collected from different depths of up to 3 m in two excavated soil profiles and in a subset of four soil samples with experimentally increased OM concentration from addition of Suwannee River OM. The K D values were related to soil properties by multiple linear regression and described with the CD–MUSIC model of ferrihydrite. The K D values ranged from 12 to 2800 L kg −1 and decreased strongly with increasing OC concentrations, when normalized to the amount of iron (Fe) and aluminium (Al) in acid oxalate extracts (r = −0.69; p < 0.0001). Experimentally increasing OC by ∼1.5 g kg −1 increased soluble Sb and decreased Sb(OH) 6 K D values by up to a factor of 8. The multiple regression model reveals that sorption of Sb(OH) 6 to Fe and Al hydroxides decreases with increasing pH and increasing dissolved organic carbon concentration. This effect could be explained with geochemical modelling by the competitive and electrostatic effects of adsorbed humic substances on Sb(OH) 6 surface complexation to the reactive surface sites of the Fe and Al hydroxides. Finally, both models could predict the in situ pore water Sb concentrations of unspiked samples, with a RMSE of 0.35 for the regression model and 0.43 for the geochemical model on the log 10 Sb concentrations. For these predictions, the 0.1 M Na 2 HPO 4 –extractable Sb concentration was measured and used to estimate the reversibly sorbed Sb pool. This study shows that increasing soil OM increases Sb(OH) 6 mobility at low soil Sb concentration, likely due to competitive sorption on Fe and Al hydroxides and a process based, geochemical model was calibrated to describe Sb(OH) 6 mobility in soil.status: Published onlin

Invited: Colloids Mediate Phosphate Leaching in Agricultural Soil: Observations and CD-MUSIC Modelling.

Phosphate (PO4) leaching through the soil profile is a pathway by which waterbodies are enriched with phosphorus (P) in agricutural areas. Colloidal-mediated PO4 leaching has been revealed before but it is unclear which factors contribute. Here, 123 columns, prepared from 40 different agricultural soils in Flanders (Belgium) and Feucherolles (France), were leached with artificial rainwater under unsaturated condition. Steady state P leachate concentrations ([P]) ranged from 0.001 to 0.235 mM P, out of which 113 exceeded 0.0045 mM P, i.e. the Flemish environmental limit for surface waters. The iron concentrations ([Fe]) in the leachates ranged from below detection limit to 1.4 mM. Moreover, aluminium concentrations ([Al]) in the filtrates correlated strongly with [Fe]. Both [Fe] and [Al] sharply increased with decreasing calcium concentrations ([Ca]), with largest concentrations below 1 mM Ca. The leachate [P] correlated remarkably strong with [Fe + Al] (r = +0.68 on log-log relationship) for soils with low P saturation (i.e. POx / 0.5(FeOx+AlOx) < 0.30, as measured in a soil oxalate extract). Multisite surface complexation modelling, with the CD-MUSIC model, predicted PO4 leaching by including effects of organic matter anions and Ca on PO4 sorption to particulate Fe- and Al(oxy)hydroxides and to colloids, with leachate Fe and Al as proxies for ferrihydrite colloids. The inclusion of the colloids improved predicted PO4 leaching (RMSE of the 10log transformed [P] = 0.47). The fraction of predicted colloid-bound PO4 increased with increasing [Fe + Al] (p < 0.001), yielding about a doubling of PO4 mobility at 0.3 mM [Fe + Al]. Up to 96 % of PO4, leached at steady state, was modelled to be colloid-bound. This study shows that high solution Ca, rather than high pH constraints colloidal PO4 transport.status: publishe

Investigation on the control of phosphate leaching by sorption and colloidal transport: Column studies and multi-surface complexation modelling

© 2018 Elsevier Ltd Surface complexation modelling (SCM) is a powerful tool to estimate speciation and fate of solutes in soil, provided sufficient model validation. This study aims to describe phosphate (PO4) leaching with SCM. The leachate phosphorus concentrations ([P]) of 120 unsaturated columns of contrasting agricultural soils were measured and modelled. Leachate [P] ranged 0.7–240 μM. Leachate [P] increased as the ratio of P to iron and aluminium ([Formula presented]) in acid oxalate soil extracts increased and as leachate Fe and Al concentrations ([Al + Fe]) increased. SCM was used to describe PO4 sorption to ferrihydrite (CD-MUSIC model). This yielded adequate description of leachate [P] (RMSElog10 = 0.39), but only when reactive PO4 was described from isotopically exchangeable PO4, when organic matter was included as the main competing adsorbate and when mobile colloidal ferrihydrite was included. The model reveals that colloidal PO4 transport enhanced leachate PO4 concentrations up to a factor 50 at small soil P content and small calcium (Ca2+) concentration in solution, as a large Ca2+ concentration enhances colloidal stability. This modelling approach explained that long-term application of organic fertilisers with higher Ca content reduced P leaching, likely due to the effect of Ca2+ on colloidal stability. A two-parameter empirical Langmuir model, based on soil Fe and Al oxyhydroxides, fitted data better than any SCM, suggesting that the empirical model might be advocated for application at large scale. This study revealed the power of SCM to better understand colloidal transport of P in soil.status: publishe

Which soil P test reflects best the phosphorus availability for leaching?

status: publishe

A Probabilistic Approach to Phosphorus Speciation of Soils Using P K-edge XANES Spectroscopy with Linear Combination Fitting

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 Respiration in the Unsaturated Zone of Agricultural Soil Mobilizes Phosphorus and Manganese

Anaerobic conditions mobilize 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 fertilizers that may trigger variable redox conditions. Soils were sampled at different depths in a well-drained Luvisol after 19 years of application of organic fertilizers. 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 mobilize P. Waterlogged soil incubations confirmed that anaerobic respiration comobilizes 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 aluminum (Al) in the soil solution of subsurface samples, whereas less such effect was found under the application of more stable organic fertilizers. 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, mobilizes P in this seemingly well-drained soil.status: publishe

Phosphate-Exchanged Mg–Al Layered Double Hydroxides: A New Slow Release Phosphate Fertilizer

The global phosphorus crisis provided impetus to develop fertilizers with better P use efficiency. We tested layered double hydroxides (LDHs) as slow release fertilizers with superior performance to fertilize strongly P-fixing soils. Mg–Al LDHs with varying M²⁺/M³⁺ ratios were synthesized as NO₃^– forms and were exchanged with HPO₄^2–. XRD and XANES spectroscopy confirmed the identity of the phosphate-exchanged LDH. Decreasing the M²⁺/M³⁺ 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 fertilization efficiency of the phosphate-exchanged LDH (Mg/Al ratio of 2) was compared to that of a soluble P fertilizer in two P-deficient soils, an acid weathered soil and a calcareous soil. The P use efficiency of the 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 fertilizers

Soil phosphorus tests compared on established European long-term trials: which test is the winner?

status: publishe