Because of the high sorption affinity of phosphorus (P) for the soil solid phase, mitigation options to reduce diffuse P transfer usually focus on trapping particulate P delivered via surface flow ...paths. Therefore, placing riparian buffers between croplands and watercourses has been promoted worldwide, sometimes in wetland areas. To investigate the risk of P-accumulating riparian wetlands (RWs) releasing dissolved P into streams, we monitored molybdate-reactive P (MRP) in the soil pore water of two RWs in an agricultural watershed. Two main mechanisms released MRP under the control of groundwater dynamics. First, soil rewetting after the dry summer period was associated with the presence of a pool of mobile P, limited in size. Its mobilization started under water saturated conditions caused by a rise in groundwater. Second, anoxic conditions at the end of winter caused reductive dissolution of Fe (hydr)oxides along with a release of MRP. Comparison of sites revealed that the first MRP release occurred only in RWs with P-enriched soils, whereas the second was observed even in RWs with low soil P status. Seasonal variations in stream MRP concentrations were similar to concentrations in RW soils. Hence, RWs can act as a key component of the P transfer continuum in agricultural landscapes by converting particulate P from croplands into MRP transferred to streams.
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•Linking groundwater dynamics and P remobilization mechanisms in riparian wetlands.•Cell lysis and reductive dissolution of Fe (hydr)oxides release P into soil pore water.•P release mechanisms in wetlands produce a clearly discernable signal in the stream.
In agricultural landscapes, establishment of vegetated buffer zones in riparian wetlands (RWs) is promoted to decrease phosphorus (P) emissions because RWs can trap particulate P from upslope fields. ...However, long-term accumulation of P risks the release of dissolved P, since the unstable hydrological conditions in these zones may mobilize accumulated particulate P by transforming it into a mobile dissolved P species. This study evaluates how hydroclimate variability, topography and soil properties interact and influence this mobilization, using a three-year dataset of molybdate-reactive dissolved P (MRDP) and total dissolved P (TDP) concentrations in soil water from two RWs located in an agricultural catchment in western France (Kervidy-Naizin), along with stream P concentrations. Two main drivers of seasonal dissolved P release were identified: i) soil rewetting during water-table rise after dry periods and ii) reductive dissolution of soil Fe (hydr)oxides during prolonged water saturation periods. These mechanisms were shown to vary greatly in space (according to topography) and time (according to intra- and interannual hydroclimate variability). The concentration and speciation of the released dissolved P also varied spatially depending on soil chemistry and local topography. Comparison of sites revealed a similar correlation between soil P speciation (percentage of organic P ranging from 35–70%) and the concentration and speciation of the released P (MRDP from <0.10 to 0.40mgl−1; percentage of MRDP in TDP from 25–70%). These differences propagated to stream water, suggesting that the two RWs investigated were the main sources of dissolved P to streams. RWs can be critical areas due to their ability to biogeochemically transform the accumulated P in these zones into highly mobile and highly bioavailable dissolved P forms. Hydroclimate variability, local topography and soil chemistry must be considered to decrease the risk of remobilizing legacy soil P when establishing riparian buffer zones in agricultural landscapes.
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•P release dynamics were investigated in natural soil solutions for three years.•Soil P status and topography control spatial variations of dissolved P.•Hydroclimate and topography control intra-/interannual variations of dissolved P.•Retention processes occurred during dissolved P transport from soil to stream.
Researches have proved that agricultural phosphorus (P) loss contributes significantly to surface water eutrophication. Various soil test P (STP) methods have been developed to assess the P loss risk ...from agricultural soils. In the intensively-cultivated Brittany region of Western France, hydromorphic soils in wetland domains exhibit high risks of leaching and transferring dissolved P - the most bio-available form of P - to surface waters. It remains unclear whether STP conventionally developed for well-drained soils can accurately predict the risk of dissolved P release from these hydromorphic soils. In this study, we measured the dissolved reactive P (DRP) concentrations in soil solutions sampled in situ from 26 hydromorphic soils in the Brittany region and examined their relationship with several STPs available on the corresponding soils, such as the degree of soil P saturation, the equilibrium soil P concentration, or the soil Olsen P, Dyer P, and water extractable P contents. DRP concentrations ranged from 0.01 to 0.310 mg P l−1 (mean = 0.075 mg P l−1), highlighting the potential of hydromorphic soils as hotspots for DRP release in agricultural landscapes. Correlations between DRP concentrations and STPs were relatively weak (0.09 < r2 < 0.64), indicating that conventional STPs are generally unable to accurately predict the DRP release risks in hydromorphic soils. Tentatively, Olsen P showed promises as a useful risk indicator, with a relatively high r2 value of 0.6 and wide inclusion in the current STP database, especially in the Brittany region. Nevertheless, this hypothesis requires further evaluation with additional data. This study confirms the high risk of dissolved P release from hydromorphic soils in agricultural wetland domains and emphasizes the need for developing specific risk assessment tools to these hydromorphic soils.
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•High dissolved reactive P concentrations in natural wetland soil waters were found•Conventional soil P tests fail to accurately predict the dissolved P release risk•The dissolved P release potential is dependent on soil hydromorphic class•It is imperative to take measures to avoid the accumulation of P in wetlands
Several studies dedicated to the aquatic geochemistry of rare earth elements (REEs) have displayed a wide topography-related spatial variability in the REE signatures of shallow groundwater. The aim ...of this study was to understand the processes leading to this specific REE signature, notably with regard to the size of the Ce anomaly. Soils were sampled in order to encompass the expected topographic variability in the organic carbon (OC)/Fe(Mn) ratio. Leaching experiments were performed with the uppermost horizons of the soil. The REE patterns that developed in the soil leaching solution were similar to the REE patterns for the shallow groundwater collected in this catchment. The negative Ce anomaly evolves in a similar manner according to the topography. This spatial variation is strongly correlated with the soil OC/Fe ratio. For a low OC/Fe ratio, the negative Ce anomaly amplitude in the soil solution is large, whereas a high OC/Fe ratio generates a small or insignificant Ce anomaly. Reductive dissolution experiments using soil with low OC/Fe ratios demonstrated that the REE pattern for soil Fe oxyhydroxides exhibited a positive Ce anomaly and HREE enrichment, indicating a preferential association of these elements with Fe-oxyhydroxides. The rare earth element signature observed in the shallow groundwater is affected by Fe oxyhydroxides present in the upper soil horizons. In contrast, in soil with a high OC/Fe ratio, the REE pattern obtained under reducing conditions did not exhibit any Ce anomaly, suggesting that in the bottomland, the REE signature is affected by the OC content in the uppermost soil. This study highlights the impact of organic matter on the Fe pedofeatures, which control the development of a negative Ce anomaly in shallow groundwater.
•The dissolved soil-REE patterns were similar to those of shallow groundwaters.•REE signature observed in shallow groundwaters is affected by soil-Fe oxyhydroxides.•Fe/OC ratio of soil controls the development of the Ce anomaly in shallow groundwaters.•Topography-related REE pattern is controlled by the upper soil horizons.
The Humic Ion Binding Model VI (Model VI) – previously used to model the equilibrium binding of rare earth elements (REE) by humic acid (HA) – was modified to account for differences in the REE ...constant patterns of the HA carboxylic and phenolic groups, and introduced into PHREEQC to calculate the REE speciation on the HA binding sites. The modifications were shown to greatly improve the modeling. They allow for the first time to both satisfactorily and simultaneously model a large set of multi-REE experimental data with the same set of equations and parameters. The use of PHREEQC shows that the light rare earth elements (LREE) and heavy rare earth elements (HREE) do not bind to HA by the same functional groups. The LREE are preferentially bound to carboxylic groups, whereas the HREE are preferentially bound to carboxy–phenolic and phenolic groups. This binding differentiation might lead to a fractionation of REE–HA patterns when competition between REE and other metals occur during complexation. A survey of the available data shows that competition with Al
3+ could lead to the development of HREE-depleted HA patterns. This new model should improve the hydrochemical modeling of the REE since PHREEQC takes into account chemical reactions such as mineral dissolution/precipitation equilibrium and redox reactions, but also models kinetically controlled reactions and one-dimensional transport.
In wetland soils, several soil phases such as Fe(III)–oxyhydroxides, organic matter (OM) or mixed Fe–OM particles can host trace metals which can be subsequently released during soil reduction. ...Anoxic and oxic wetland soil incubation experiments, combined with analyses of soil solutions sampled from a natural wetland during a reduction event, are used to test the possibility that rare earth elements (REE) could be used as a tool to identify the soil phases contributing to trace metal solubilization. Significant amounts of trace metals (Cu, Cr, Co, Ni and Pb) and REE are released during anoxic incubation of the wetland soil, concomitantly with the build-up of high concentrations of Fe(II) and dissolved organic matter (DOM). Rare earth element patterns obtained in the soil solution exhibit a middle rare earth elements (MREE) downward concavity. The REE pattern obtained from field samples yields the same feature as developed in the soil solution from an oxic incubation experiment at pH 7 designed to promote soil OM desorption. By contrast, significantly different REE patterns are obtained in incubation experiments designed to promote chemical reduction of Fe–oxyhydroxides in soils. The REE pattern displays a continuous REE enrichment from La to Lu. These distinct and recognizable REE signatures allow us to conclude that (i) soil organic matter is the main source of REE and trace metals during wetland soil reduction; (ii) Fe(II) is provided by the reduction of amorphous Fe(III) nanoparticles embedded within the organic matter, which do not bind REE or other trace metals in significant proportions (REE and trace elements being preferentially complexed to organic matter); and finally (iii) REE provide a reliable and powerful tool, suitable for identifying trace metal sources during wetland soil reduction.
► Soil organic matter is the main source of REE and trace metals during wetland soil reduction. > Fe(II) is provided by the reduction of amorphous Fe(III) nanoparticles embedded within organic matter in wetland soil. ► REE provide a reliable and powerful tool, suitable for identifying trace metal sources during wetland soil reduction.
This study investigates the combined effects of land management and hydrology on the temporal dynamics of dissolved organic matter (DOM) quantity and composition in stream water and groundwaters in ...an agricultural watershed. We assessed dissolved organic carbon (DOC) concentrations, DOM UV–Vis absorbance, and DOM fluorescence in groundwater under cultivated upland, riparian grassland, and riparian woodland land covers, as well as in the stream water at the watershed outlet and livestock-impacted runoff. During one year, stream water and groundwater were monitored weekly to biweekly, complemented by sub-hourly stream sampling during seven storm events. Results showed that: (1) groundwater DOC concentration was lower in cultivated upland (6.4 ± 5.6 mg l⁻¹) than in riparian grassland and woodland (22.4 ± 13.7 mg l⁻¹ and 17.2 ± 9.9 mg l⁻¹, respectively). (2) The proportion of microbially processed compounds decreased in the order upland cropland > riparian grassland > riparian woodland. (3) Principal component analysis (PCA) of groundwater DOM revealed a change in composition indicating that low-aromaticity microbially processed compounds were preferentially exported to the stream. (4) PCA of stream DOM indicated that seasonal increases in groundwater elevation expanded the contributing source areas, thereby increasing the connectivity between upland croplands and the stream, which amplified the effects of cultivation on fluvial DOM during the winter. (5) Storm events occurring after manure application in spring produced hot moments of manure-derived protein-like DOM transport to streams. Together, these results suggest that cultivated uplands in agricultural lands using animal manure as fertilizer may leach more DOM than vegetative buffers.
The Stockholm Humic Model (SHM) and Humic Ion-Binding Models V and VI were compared for their ability to predict the role of dissolved organic matter (DOM) in the speciation of rare earth elements ...(REE) in natural waters. Unlike Models V and VI, SHM is part of a speciation code that also allows us to consider dissolution/precipitation, sorption/desorption and oxidation/reduction reactions. In this context, it is particularly interesting to test the performance of SHM. The REE specific equilibrium constants required by the speciation models were estimated using linear free-energy relationships (LFER) between the first hydrolysis constants and the stability constants for REE complexation with lactic and acetic acid. Three datasets were used for the purpose of comparison: (i) World Average River Water (Dissolved Organic Carbon (DOC)
=
5
mg
L
−1), previously investigated using Model V, was reinvestigated using SHM and Model VI; (ii) two natural organic-rich waters (DOC
=
18–24
mg
L
−1), whose REE speciation has already been determined with both Model V and ultrafiltration studies, were also reinvestigated using SHM and Model VI; finally, (iii) new ultrafiltration experiments were carried out on samples of circumneutral-pH (pH 6.2–7.1), organic-rich (DOC
=
7–20
mg
L
−1) groundwaters from the Kervidy-Naizin and Petit-Hermitage catchments, western France. The results were then compared with speciation predictions provided by Model VI and SHM, successively. When applied to World Average River Water, both Model VI and SHM yield comparable results, confirming the earlier finding that a large fraction of the dissolved REE in rivers occurs as organic complexes This implies that the two models are equally valid for calculating REE speciation in low-DOC waters at circumneutral-pH. The two models also successfully reproduced ultrafiltration results obtained for DOC-rich acidic groundwaters and river waters. By contrast, the two models yielded different results when compared to newly obtained ultrafiltration results for DOC-rich (DOC
>
7
mg
L
−1) groundwaters at circumneutral-pH, with Model VI predictions being closer to the ultrafiltration data than SHM. Sensitivity analysis indicates that the “active DOM parameter” (i.e., the proportion of DOC that can effectively complex with REE) is a key parameter for both Model VI and SHM. However, a survey of ultrafiltration results allows the “active DOM parameter” to be precisely determined for the newly ultrafiltered waters studied here. Thus, the observed discrepancy between SHM predictions and ultrafiltration results cannot be explained by the use of inappropriate “active DOM parameter” values in this model. Save this unexplained discrepancy, the results presented in this study demonstrate that both Model VI and SHM can provide reliable estimates of REE speciation in organic-rich waters. However, it is essential to know the proportion of DOM that can actively complex REE before running these two speciation models.
Organic or inorganic colloids play a major role in the mobilization of trace elements in soils and waters. Environmental physicochemical parameters (pH, redox potential, temperature, pressure, ionic ...strength, etc.) are the controlling factors of the colloidal mobilization. This study was dedicated to follow the colloid-mediated mobilization of trace elements through time at the soil/water interface by means of an experimental approach. Soil column experiments were carried out using percolating synthetic solutions. The percolated solutions were ultrafiltrated with various decreasing cutoff thresholds to separate the different colloidal phases in which the dissolved organic carbon and trace element concentrations were measured. The major results which stem from this study are the following: (i) The data can be divided into different groups of organic compounds (microbial metabolites, fulvic acids, humic acids) with regard to their respective aromaticity and molecular weight. (ii) Three groups of elements can be distinguished based on their relationships with the colloidal phases: the first one corresponds to the so-called “truly” dissolved group (Li, B, K, Na, Rb, Si, Mg, Sr, Ca, Mn, Ba, and V). The second one can be considered as an intermediate group (Cu, Cd, Co, and Ni), while the third group gathers Al, Cr, U, Mo, Pb, Ti, Th, Fe, and rare earth elements (REE) carried by the organic colloidal pool. (iii) The data demonstrate that the fulvic acids seem to be a major organic carrier phase for trace elements such as Cu, Cd, Co, and Ni. By contrast, the trace elements belonging to the so-called colloidal pool were mostly mobilized by humic acids containing iron nanoparticles. Lead, Ti, and U were mobilized by iron nanoparticles bound to these humic acids. Thus, humic substances allowed directly or indirectly a colloidal transport of many insoluble trace elements either by binding trace elements or by stabilizing a ferric carrier phase. (iv) Finally, the results demonstrated also that REE were mostly mobilized by humic substances. The REE normalized patterns showed a middle REE downward concavity. Therefore, as previously shown elsewhere humic substances are a major control of REE speciation and REE fractionation patterns as well since the humic substance/metal ratio was the key parameter controlling the REE pattern shape.
The elemental distribution in the colloidal phase higher than 2 kDa is represented at the end of the column-mediated soil leaching experiment. Three different groups can be identified.
The effect of metal loading on the binding of rare earth elements (REE) to humic acid (HA) was studied by combining ultrafiltration and Inductively Coupled Plasma Mass Spectrometry techniques. REE–HA ...complexation experiments were performed at pH 3 for REE/C molar ratios ranging from ca 4
×
10
−4 to 2.7
×
10
−2. Results show that the relative amount of REE bound to HA strongly increases with decreasing REE/C. A middle-REE (MREE) downward concavity is shown by patterns at high metal loading, whereas patterns at low metal loading display a regular increase from La to Lu. Humic Ion Model VI modelling are close to the experimental data variations, provided that (i) the ΔLK
2 parameter (i.e. the Model VI parameter taken into account the presence of strong but low density binding sites) is allowed to increase regularly from La to Lu (from 1.1 to 2.1) and (ii) the published log
K
MA values (i.e. the REE–HA binding constants specific to Model VI) are slightly modified, in particular with respect to heavy REE. Modelling approach provided evidence that
log
K
d
REE
patterns with varying REE/C likely arises because REE binding to HA occurs through two types of binding sites in different density: (i) a few strong sites that preferentially complex the heavy REE and thus control the
log
K
d
REE
atterns at low REE/C; (ii) a larger amount of weaker binding sites that preferentially complex the middle-REE and thus control the
log
K
d
REE
pattern at high REE/C. Hence, metal loading exerts a major effect on HA-mediated REE binding, which could explain the diversity of published conditional constants for REE binding with HA. A literature survey suggests that the few strong sites activated at low REE/C could be multidentate carboxylic sites, or perhaps N-, or P-functional groups. Finally, an examination of the literature field data proposed that the described loading effect could account for much of the variation in REE patterns observed in natural organic-rich waters (DOC
>
5
mg
L
−1 and 4
⩽
pH
⩽
7).
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