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For soils and sediments, which are complex and heterogeneous, root-triggered processes, bioturbation of benthic communities, and local decomposition of reactive organic materials can greatly increase their biogeochemical heterogeneity. In practice, common monitoring methods can only be used to determine the centimeter- to millimeter-scale heterogeneity from vertical or horizontal one-dimensional profiles. However, significant heterogeneity exists in both soils and sediments on the millimeter to sub-millimeter scales. Therefore, in-situ and high-resolution sampling and imaging methods are essential to capture the spatial and temporal heterogeneity of soils and sediments. This article reviews micro-scale sampling methods in soil and sediments, including diffusive equilibration in thin films (DET), diffusive gradient in thin films (DGT), planar optodes (PO), nanoparticle- and microparticle-based luminescence imaging, and soil zymography. Besides, we introduce their imaging principles and approaches in detail, and discuss some problems and solutions during imaging. Micro-scale sampling and imaging methods have given us many insights into the biogeochemical processes in soils and sediments, but the full potential of these methods has not been exhausted yet. Finally, the potential directions for the development of micro-scale sampling methods in the study of complex biogeochemical processes in heterogeneous soils and sediments are prospected. [ABSTRACT FROM AUTHOR]

Purpose: To study the effects of snail (Bellamya aeruginosa) bioturbation on phosphorus (P) mobility in sediment and the relevant mechanisms behind them. Materials and method: A microcosm simulation experiment was carried out at millimeter resolution with an 89 days culture of homogeneous sediments. High-resolution dialysis (HR-Peeper) and diffusive gradient in thin films (DGT) techniques were utilized to simultaneously acquire the vertical distribution of soluble and labile P/iron (Fe) in bioturbation sediments. Results and discussion: Snail bioturbation significantly enhanced soluble reactive P (SRP) and labile P concentrations, with the maximum increment to 306% and 1133% of the control sediments within the influence depths from 0 to − 30 mm and to − 20 mm, respectively. The P fluxes released from the sediment–water interfaces in snail treatments were correspondingly enhanced with the maximum increments to 218% of the control. Also, snail bioturbation significantly enhanced the soluble Fe(II) and labile Fe concentrations by maximum increment of 218% and 199% of the control within 0 to −24 mm and − 19 mm, respectively. Conclusions: The high correlation between soluble/labile P and Fe suggested that P release from the sediment under snail bioturbation was due to the reduction and dissolution of Fe(III) oxyhydroxide. [ABSTRACT FROM AUTHOR]

Abstract Zinc (Zn) contamination in lake zones dominated by algae and macrophytes in Lake Taihu was analyzed through diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) methods. It was found that in both zones Zn contamination varied by season. In July and October, dissolved Zn was present in high concentrations, and in July, high concentrations of labile Zn were found in sediments. In July, reductive dissolutions of Fe/Mn oxides likely played a key role in the release of Zn, which was confirmed by both zones having the lowest percentage of the reducible fraction of Zn in July. Complexation of dissolved organic matter (DOM) with Zn may be responsible for the observed increase in the dissolved Zn concentration in October. This conclusion was supported by noting that October had the highest percentages of Zn-DOM complexes (25.3% and 34.4%) in the algae- and macrophytes-dominated zones, respectively. However, in January, low dissolved and labile Zn contents were observed in sediments in the two zones, suggesting that the decrease of Zn in sediments was caused by the adsorption of Fe/Mn oxides. Graphical abstract Unlabelled Image Highlights • Zinc mobility showed a seasonal variation in both algae- and macrophyte-dominated zones. • In summer, Fe/Mn redox reduction increased Zn mobility in two different zones. • In autumn, DOM complexation increased Zn mobility in two different zones. • In winter, the Zn mobility was decreased by Fe/Mn oxide absorption in two different zones. [ABSTRACT FROM AUTHOR]

Abstract Dredging is used worldwide to remove polluted sediments from water bodies. However, the dredging efficacy remains hard to identify. Here, we studied the efficacy of dredging engineering as a means to remove Cu, Cd, and Pb from polluted lake sediments, after six years of completion. Dissolved metals and DGT-labile metals were quantified in the non-dredged and post-dredged sediments by high-resolution dialysis (HR-Peeper) and diffusive gradients (DGT) in thin films techniques. April and July measurements showed that dredging was effectively remediate the polluted sediments. The dissolved Pb, Cd, and Cu contents decreased up to 30%, 44%, and 26%, and the DGT-labile contents decreased up to 51%, 27%, and 33% compared with the contents in the non-dredged zone. Dredging was thus proven efficient in decreasing the labile metal fractions, increasing the capacity of available solids to bind metals, and slowing the leaching of metals from available solids in the post-dredged sediments. In October and January, the dredging efficacy was counteracted by the decomposition of algae, which increased the dissolved and DGT-labile metal concentrations in the post-dredged zone. Graphical abstract Unlabelled Image Highlights • Dredging efficacy varied by season after six years of completion. • Dredging was effective in controlling heavy metals pollution in April and July. • Efficacy results from increased inert with slower kinetic response in sediments. • Dredging negligibly affected metal mobility in October and January. • Algal degradation affected dredging efficacy by releasing additional amounts of metals. [ABSTRACT FROM AUTHOR]

Applications of aluminium (Al) salt or lanthanum (La) modified bentonite (LMB) have become popular methodologies for immobilizing phosphorus (P) in eutrophic lakes. The presence of humic substances, has been shown to inhibit this form of treatment due to the complexation with La/Al. However, the effects of other dissolved organic matter (DOM), especially that derived from phytoplankton (the dominant source in eutrophic lakes) are unknown. In this study, the interaction with La/Al of Suwannee River Standard Humic Acid Standard II (SRHA) and algae-derived DOM (ADOM) were investigated and compared. Differed to SRHA which was dominated by polyphenol-like component (76.8%, C1-SRHA), majority in ADOM were protein-like substance, including 41.9% tryptophan-like component (C2-ADOM) and 21.0% tyrosine-like component (C3-ADOM). Two reactions of complexation and coprecipitation were observed between SRHA/ADOM and La/Al. Complexation dominated at low metal inputs less than 10 μM and coprecipitation was the main reaction at higher metal inputs. For ADOM, the tryptophan-like component (C2-ADOM) was the important component to react with metal. The reaction rate for C2-ADOM with La were about two-third of that for C1-SRHA, indicating that the influence of C2-ADOM was significant during the P immobilization by La/Al-based treatment in eutrophic lakes. The P removal data in the presence of ADOM confirmed the significant inhibition of ADOM. In addition, based on the composition of coprecipitates and relatively biodegradable character of tryptophan-like substances (C2-ADOM), the coprecipitation of ADOM was assumed to reduce the stability of precipitated P in eutrophic lakes. The release of P from the potential biodegradation of the coprecipitates and thus the possible decline of the performance of P immobilization by La/Al-based treatments is an important work in the future. Image 1 • Complexation and coprecipitation occurred between SRHA/ADOM and La/Al. • A tryptophan-like component of ADOM complexed strongly with La. • The tryptophan-like component has strong ability to coprecipitate with La and Al. • The behaviors of PARAFAC components explained the inhibitory effect of SRHA/ADOM on P removal. • The stability of coprecipitates of DOM, Al/La and P should be noted. [ABSTRACT FROM AUTHOR]

Abstract This study examined lead (Pb) pollution in algae- and macrophytes-dominated sediments, using diffusive gradient in thin films (DGT) and dialysis (HR-Peeper) techniques. Lead pollution varied by season in the two different ecotype sediments. In the algae-dominated zone, the highest concentrations of DGT-labile Pb and dissolved Pb occurred in April and July, respectively. The reductive dissolution of Fe/Mn oxides was identified as an important driver for Pb releases in April and July. This was supported by the decrease of the reducible fraction of Pb in sediments during those sampling periods. Furthermore, dissolved organic matter (DOM) complexation with Pb in sediments also significantly increased the dissolved Pb concentrations in July. The Pb-DOM complexes accounted for 95% of the total chemical species of Pb in pore water, calculated by Visual MINTEQ 3.1 model. Low concentrations of labile and dissolved Pb were observed in October and January; these resulted from the formation of Pb-sulfide precipitates and adsorption by Fe/Mn oxides. It was supported by the high rate of Pb(HS) 2 precipitation (saturation index > 0), at 36%, in October samples and the high reducible fraction of Pb in sediments in January samples. In the macrophytes-dominated region, there was a decrease of labile and dissolved Pb concentrations in April and July. It is likely because of the uptake of Pb by submerged macrophyte roots and the Fe/Mn plaques in the root surface. High concentrations of labile and dissolved Pb were observed in October and January, likely resulting from the DOM complexation with Pb in sediments. This was supported by the fact that the Pb-DOM complexes accounted for 90% and 87% of the total chemical species of Pb in October and January, respectively. Graphical abstract Unlabelled Image Highlights • Pb mobility differed in sediments in the algae- and macrophyte-dominated areas. • Fe/Mn redox reduction, DOM complexation increased Pb mobility in algae-dominated area in spring, summer. • Sulfide, Fe/Mn oxide absorption decreased Pb mobility in algae-dominated area in fall, winter. • Root, plaque accumulation decreased Pb mobility in macrophyte-dominated area in spring, summer. • DOM complexation increased Pb mobility in fall and winter in macrophyte-dominated area. [ABSTRACT FROM AUTHOR]

Abstract Studies of dredging effectiveness, especially the ones that last for several years, are scarce. In this study, we evaluated effectiveness of dredging performed for six years on controlling cobalt (Co), zinc (Zn), and nickel (Ni) contamination of sediments. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) methods were applied to analyze the non-dredged and post-dredging sediments. The soluble and DGT-labile Co and Ni content declined by 22% and 44% (soluble) and by 16% and 26% (labile) in April, July and October in the post-dredging region. In contrast, their concentrations increased by 105% and 9% (soluble) and 322% and 27% (labile) in January. These changes in the dredging effects were caused by the corresponding changes in the reducible and residual fractions of Co and the residual fraction of Ni in sediments in the dredged site, respectively. Soluble and DGT-labile Zn decreased on average by 23% and 29% in July and October and increased on average by 151% and 52% in April and January in the post-dredging region. The different Zn mobility in the post-dredging region was controlled by the reducible fraction of Zn. The results revealed positive influence of dredging engineering in summer, autumn and/or spring and negative one in winter. Therefore, an accurate assessment of dredging effectiveness should take its seasonal variation into consideration. Graphical abstract Image 1 Highlights • Dredging effects change during the different seasons as reflected by soluble and labile concentrations. • Dredging was most effective in April, July and October and had a negative effect in January. • Efficacy of Co was affected by the reducible and residual fractions in sediments. • Efficacies of Zn and Ni were respectively affected by reducible and residual fractions. [ABSTRACT FROM AUTHOR]

Abstract Iron (Fe) and manganese (Mn) reactions have been regarded as the primary factors responsible for the mobilization of phosphorus (P) in lake sediments, although their individual roles are hard to distinguish. In this study, in situ mobilization of P, Fe and Mn in sediments was assessed by high resolution spatio-temporal sampling of their labile forms using diffusive gradient in thin films (DGT) and suction device (Rhizon) techniques. It was found that the monthly concentration distributions showed greater agreement and better correlation coefficients between labile P and labile Fe, than those between labile P and labile Mn, implying that Fe plays a key role in controlling P release in sediments. Furthermore, better correlations were observed between hourly changes in concentrations of soluble reactive phosphorus (SRP) and soluble Fe(II), than those between SRP and soluble Mn. Changes were observed under simulated anaerobic incubation conditions, suggesting that P release was caused by the reductive dissolution of Fe oxides. This was supported by the lack of influences on P release from reductive dissolution of Mn oxides in the sediment-water interface and top sediment layers under the anaerobic incubations. In simulated algal bloom experiments, positive correlations and consistent changes were observed between SRP and soluble Fe(II) concentrations, but not between SRP and soluble Mn concentrations. This further demonstrated the Fe-dependent and Mn-independent release of P in sediments. Therefore, Fe redox reactions have a high impact on P mobilization in sediments, while Mn redox reactions appear to have negligible influences. Graphical abstract Image 1 Highlights • Monthly distribution of labile P concentration correlated with labile Fe concentration in sediments. • Hourly change of SRP correlated with soluble Fe(II) concentrations under anoxic conditions. • Phosphorus was not released during the reductive dissolution of Mn oxides. • Consistent changes in P and Fe but not in Mn occurred in algal bloom sediments. • Fe but not Mn redox reactions should control mobilization of P in sediments. Evidence that Fe redox reactions play a key role in controlling P mobilization in sediments, while Mn redox reactions do not have a significant influence. [ABSTRACT FROM AUTHOR]

Abstract This study assessed the effectiveness of dredging in controlling arsenic (As), selenium (Se), and antimony (Sb) contamination in sediments, by examining contaminant concentrations in sediments six years after dredging was completed. High-resolution diffusive gradients in thin films (DGT) and dialysis (HR-Peeper) techniques were used to monitor the concentrations of DGT-labile metalloids and soluble metalloids in sediments, respectively. Results revealed that dredging effectively remediated metalloid contamination in sediments only in April, July and/or January. Compared to non-dredged sediments, the concentrations of soluble and DGT-labile As, Se, and Sb in dredged sediments decreased on average by 42%, 52%, and 43% (soluble), and 54%, 50%, and 53% (DGT), respectively. The effectiveness of the dredging was primarily due to the transformation of metalloids from labile to inert fractions, which increased the ability of the sediments to retain the metalloids, and the slowed rate of resupplied metalloids from available solid pools. In contrast, negligible/negative effects of dredging were seen in October, and the concentrations of soluble and DGT-labile metalloids even increased in some profiles of dredged sediments. This was mainly caused by a release of the metalloids from algal degradation, which may offset the dredging effectiveness. Graphical abstract Image 1 Highlights • Dredging effects varied by season, reflected by metalloid concentrations. • Dredging was most effective in April, July and/or January. • Efficacy due to higher fraction of inert metalloids with slower kinetic response. • Dredging had negligible and negative effects in October. • Algal decomposition and metalloid release suppressed dredging efficacy. [ABSTRACT FROM AUTHOR]

Little is known about the effects of bacterial community on iron (Fe) and phosphorus (P) cycles in sediments under different primary producer habitats in different seasons. Lake Taihu has both the cyanobacteria- and macrophyte-dominated lake zones. In this work, the abundance and structure of bacterial community was investigated using qPCR and 16S rRNA gene high throughput sequencing, respectively. Compared with the sediments in the cyanobacteria-dominated lake zone, sediments in the macrophyte-dominated lake zone had higher TP, TOC and TN contents but lower DO and Eh values. Dissolved reactive P, dissolved Fe, and their molar ratios (Fe/P) were lower in the sediments of the cyanobacteria-dominated lake zone than those in the macrophyte-dominated lake zone. Consistent with this was the significantly lower abundance of total and typical Fe redox transforming bacteria in the sediment of the cyanobacteria-dominated lake zone than those in the macrophyte-dominated lake zone. Correlation analyses also revealed positive influence of abundances of total bacteria and typical Fe reducing bacteria on dissolved Fe and Fe/P ratio. The results showed that, in the cyanobacteria-dominated open water zone, Acidimicrobiaceae was capable of Fe metabolism, contributing to higher P flux in summer. In the cyanobacteria-dominated bay, Sva0081 sediment group and Desulfobulbaceae could transform sulfate to sulfide, which resulted in the reduction of Fe (III), while in the macrophyte-dominated zones, Clostridium sensu stricto 1 could couple oxidation of organic carbon with the reduction of Fe (III). The present study adds new knowledge linking the bacterial communities with the physicochemical cycles of Fe and P in sediments under different primary producer habitats. [ABSTRACT FROM AUTHOR]

It is proposed that the internal loading of phosphorus (P) from sediments plays an important role in seasonal nitrogen (N) limitation for harmful algal blooms (HABs), although there is a lack of experimental evidence. In this study, an eutrophic bay from the large and shallow Lake Taihu was studied for investigating the contribution of internal P to N limitation over one-year field sampling (February 2016 to January 2017). A prebloom-bloom period was identified from February to August according to the increase in Chl a concentration in the water column, during which the ratio of total N to total P (TN/TP) exponentially decreased with month from 43.4 to 7.4. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) analysis showed large variations in the vertical distribution of mobile P (SRP and DGT-labile P) in sediments, resulting in the SRP diffusion flux at the sediment-water interface ranging from − 0.01 to 6.76 mg/m 2 /d (minus sign denotes downward flux). Significant and linear correlations existed between SRP and soluble Fe(II) concentrations in pore water, reflecting that the spatial-temporal variation in mobile P was controlled by microbe-mediated Fe redox cycling. Mass estimation showed that the cumulative flux of SRP from sediments accounted for 54% of the increase in TP observed in the water column during the prebloom-bloom period. These findings are supported by the significantly negative correlation ( p < 0.01) observed between sediment SRP flux and water column TN/TP during the same period. Overall, these results provide solid evidence for the major role of internal P loading in causing N limitation during the prebloom-bloom period. [ABSTRACT FROM AUTHOR]

Algal growth causes a drastic change in aquatic conditions over a diel cycle, which may induce sensitive feedback systems in sediments, causing P release. In this study, a microcosm experiment was performed using a suction sampler (Rhizon) to observe changes in soluble reactive phosphorus (SRP) and soluble Fe(II) concentrations in the top 20 mm sediment layer on a 3-h time interval, at different phases of harmful algal bloom (HAB) development. The results showed that the algal blooms prevailed up to 15 days after incubation, after which the process of bloom collapse proceeded until the 70th day. The concentrations of pore-water soluble Fe(II) and SRP increased throughout the incubation period. Compared to day 1, maximum increases of 214% in soluble Fe(II) and 387% in SRP were observed at night during the bloom and collapse periods, respectively. The diffusive fluxes of Fe and P at the sediment-water interface (SWI) generally corresponded to their changes in concentrations. Hourly fluctuation in soluble Fe(II) and SRP concentrations were observed with two distinct concentration peaks occurred at 21:00 p.m. and 06:00 a.m. (or 03:00 a.m.), respectively. These findings suggest that Fe-P coupling mechanisms are responsible for the release of P from sediments. During the collapse period, soluble Fe(II) concentrations were suppressed by the increase of labile S(-II) at night. Meanwhile, SRP concentrations were decoupled from Fe cycling with small fluctuations (<11% RSD) on an hourly timescale, and the decomposition of algae was a dominant source contributing to the release of P from sediments. These results significantly improved the understanding of processes and mechanisms behind the stimulated release of P from sediments during HABs. [ABSTRACT FROM AUTHOR]

The effectiveness of sediment dredging for the control of internal phosphorus (P) loading, was investigated seasonally in the eutrophic Lake Taihu. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to measure the concentrations of soluble Fe(II) and soluble reactive P (SRP) as well as DGT-labile Fe/P in the non-dredging and post-dredging sediments. The P resupply kinetics from sediment solids were interpreted using DGT Induced Fluxes in Sediments (DIFS) modeling. The results showed no obvious improvement in water and sediment quality after dredging for 6 years, due to their geographical proximity (a line distance of approximately 9 km). However, dredging significantly decreased the concentrations of soluble Fe(II)/SRP and DGT-labile Fe/P in sediments, with effects varying at different depths below the sediment-water interface; More pronounced effects appeared in January and April. The diffusive flux of pore water SRP from sediments decreased from 0.746, 4.08 and 0.353 mg/m 2 /d to 0.174, 1.58 and 0.048 mg/m 2 /d in April, July and January, respectively. DIFS modeling indicated that the P retention capability of sediment solids was improved in April in post-dredging site. Positive correlations between pore water soluble Fe(II) and SRP as well as between DGT-labile Fe and P, reflect the key role of Fe redox cycling in regulating dredging effectiveness. This effect is especially important in winter and spring, while in summer and autumn, the decomposition of algae promoted the release of P from sediments and suppressed dredging effectiveness. Overall, the high-resolution HR-Peeper and DGT measurements indicated a successful control of internal P loading by dredging, and the post-dredging effectiveness was suppressed by algal bloom. [ABSTRACT FROM AUTHOR]

The release of arsenic (As) from sediments poses a risk to human health especially at high pH levels. Despite this, the distribution and kinetic response of mobile As remains unclear under varying pH conditions. In this study, a microcosm incubation experiment was performed, using sediment cores in combination with dialysis (Peeper) and thin film diffusive gradients (DGT), to investigate the distributions of mobile As (soluble As in pore water and DGT-labile As) at high vertical resolutions (2-4 mm). Results show that the concentrations of soluble As present in the water column increased 1.5-fold with an increase in pH from 5.4 to 11.2. Both soluble As in pore water and DGT-labile As exhibited stable low-level distributions in the uppermost layer beneath the sediment-water interface, followed by increasing concentration distribution with decreasing layers to middle depths. The mean concentrations of mobile As species increased with increased water pH in both sediment profiles and with upward diffusion gradients, showing a 0.2-fold increase of soluble As in the top 20-mm layer and a 0.6-fold increase in deeper 20-52-mm layers, while DGT-labile As showed a 1.0- and 1.1-fold increase in these two layers, respectively. Modeling of DGT-induced flux in sediments (DIFS) showed that the desorption rate constant increased more rapidly than the absorption rate constant, resulting in the increased availability of solid As pools, therefore resupplying the soluble As in pore water from sediments. [ABSTRACT FROM AUTHOR]

Excessive phosphorus (P) input plays an important role on the release of heavy metals in sediments under the eutrophic environment. In this study, a microcosm experiment with 40-day incubation using homogenized sediments was performed to investigate this aspect at a millimeter resolution. Diffusive gradient in thin films (DGT) and dialysis (Peeper) techniques were employed to simultaneously measure labile and dissolved P, Pb, Cd, Zn, Co and Ni in sediments at a millimeter scale, respectively. The results showed that an increase of water P from 0.02 mg L − 1 to 0.20 and 2.4 mg L − 1 generally led to intensified decrease of DGT-labile metals from the 10th to 20th days after the onset of incubation. The decrease in dissolved metals in pore water also appeared on the 20th day. The degree of decrease in the five metals was in the order of Pb > Cd > Zn > Co > Ni, which has a negative correlation with the solubility constants ( K sp ) of each metal-P precipitate. This indicated that the negative effect was caused by the precipitation of metal P. On the 40th day during incubation, the concentrations of DGT-labile metals had different increasing pattern compared to those on the 20th day. The extent of metal recovery was positively correlated with the change of desorption rate constant ( k − 1 ) and negatively correlated with the change in the characteristic time ( T c ) to reach equilibrium from DGT perturbation derived from DGT-induced fluxes in sediments (DIFS) modeling. This suggested that the recovery of metal lability from elevated water P was a result of the increased releases of metals from sediment solids. [ABSTRACT FROM AUTHOR]

This study investigated the effects of tubificid worm bioturbation on the lability of phosphorus (P) in microcosm sediments. High-resolution dialysis (HR-Peeper) and two types of diffusive gradients in thin films (DGT) (Zr-oxide DGT and ZrO-Chelex DGT) were used to measure soluble P and Fe, and labile P and Fe at a millimeter spatial scale. The worm bioturbation promoted P release (up to 511% of the control) to the overlying water on the 6th day, but it was reduced compared to the control (up to 171% of the control) from the 22nd day to the 102nd day because of the adsorption by Fe(III) oxyhydroxides. The worm bioturbation reduced the pore water soluble P concentration up to 48% and the DGT-labile P concentration up to 29% of the control from a sediment depth of −10 mm to approximately −130 mm before the 22nd day of incubation due to worm ingestion of sediment particles. Two-dimensional measurements of DGT-labile P also showed a much lower concentration of labile P around the worm burrow. This effect disappeared on the 53rd and 102nd day. However, the soluble P and DGT-labile P decreased again up to 41% and 38%, compared to the control from the sediment depth of −20 mm and −10 mm to approximately −130 mm, respectively, on the 152nd day of incubation due to the adsorption by Fe(III) oxyhydroxides. Soluble Fe(II) and DGT-labile Fe did not show significant changes from the worm bioturbation on the 6th day, but decreased up to 31% and 47% of the control after the 6th day. The results that worm ingestion of sediment particles is a significant driver of soluble and labile P reduction in the sediments before the 22nd day. After that, soluble and labile P reduction was attributed to P adsorption by Fe(III) oxyhydroxides. [ABSTRACT FROM AUTHOR]

The effects of bivalve ( Corbicula fluminea ) bioturbation on the lability of phosphorus (P) in sediments were investigated. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were employed to obtain soluble and labile P/Fe profiles at a vertical resolution of 2 and 1 mm, respectively. The bivalve bioturbation increased the concentrations of soluble reactive P (SRP) in pore water and DGT-labile P up to 116% and 833% of the control within the sediment depths from the sediment water interface (SWI) to − 64 mm and − 44 mm, respectively. The sediments with bioturbation had a smaller distribution coefficient than the control (1964 vs. 3010 cm 3 g − 1 ), reflecting a weaker ability in retaining P. Meanwhile, the sediments with bioturbation had a greater ratio of the concentration of DGT-labile P to that of SRP (0.20 vs. 0.03), demonstrating a stronger ability to resupply pore water SRP by the sediment solids when they are affected by the bioturbation. The DGT-induced fluxes in sediments (DIFS) modeling further showed a much shorter response time (277.9 vs. 18,670 s) and a much higher rate (0.192 vs. 0.002 day − 1 ) of the solids in release of P with the bioturbation. Correspondingly, the flux of P to the overlying water from the bioturbation treatment increased up to 157% of the control. The bivalve bioturbation significantly increased the concentrations of soluble Fe(II) and DGT-labile Fe up to 84% and 334% of the control from the SWI to − 46 mm, respectively. The SRP and DGT-labile P were highly correlated with respective soluble and DGT-labile Fe. It was concluded that the release of P from the sediments with bioturbation to the pore water and the overlying water was promoted by the reductive dissolution of easily reducible Fe(oxyhydr)oxides due to the depletion of oxygen in the top sediments from bivalve respiration. [ABSTRACT FROM AUTHOR]

The effects of chironomid larvae bioturbation on the lability of phosphorus (P) in sediments were investigated through sediment incubation for 140 days. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were applied to obtain soluble and labile P/Fe profiles at a millimeter resolution, respectively. The larvae bioturbation decreased concentrations of soluble/labile P and Fe by up to over half of the control at the sediment depths of influence up to 70 and 90 mm respectively. These effects continued over 116 days and disappeared on the 140th days due to eclosion of chironomid larvae. Labile P was highly correlated with labile Fe, while a weak correlation was observed between soluble P and soluble Fe. It was concluded that Fe(II) oxidation and its enhanced adsorption were the major mechanisms responsible for the decreases of soluble and labile P. [ABSTRACT FROM AUTHOR]

Lake eutrophication and algal blooms may result in the mortality of macrozoobenthos. However, it is still not clear how macrozoobenthos decomposition affect phosphorus (P) mobility in sediments. High-resolution dialysis (HR-Peeper) and the diffusive gradients in thin films (DGT) technique were used in this study to assess the dissolved organic matter (DOM), dissolved/DGT-labile iron (Fe), P, and sulfur (S(–II)) profiles at a millimeter resolution. The decomposition of Bellamya aeruginosa significantly increased the internal loading of sediments P. The Fe(III) and sulfate were reduced under anaerobic conditions and promoted P desorption from sediments. This was supported by the significant increase in DGT-labile S(–II) and dissolved/DGT-labile P, Fe(II) and the significant positive correlation between Fe and P on day 8. The simultaneous increase in DOM and soluble reactive phosphorus (SRP) and the significant positive relationship between these factors were observed during the decomposition of B. aeruginosa. This suggested that complexation of DOM with metals may promotes the release of P from sediments. [Display omitted] • Decomposition of benthic organisms significantly increased sediment P levels. • Reduction of Fe(III) and sulfate promoted P release from sediments. • DOM complexes with metals reduced sediment P immobilization. During macrozoobenthos decomposition, the reduction and dissolution of Fe(III) oxides and the complexation of DOM with metal ions caused the massive release of P from sediments and increased the risk of lake eutrophication. [ABSTRACT FROM AUTHOR]

Abstract To study the mechanisms of chromium (Cr) mobilization in sediments of lakes with different ecotypes, seasonal sampling was performed in the macrophyte-dominated East Taihu (MDET) and cyanobacteria-dominated Meiliang Bay (CDMB) in Lake Taihu. Concentrations of labile Cr(VI) and dissolved Cr were assessed using diffusive gradients in thin films (DGT) and high-resolution dialysis passive sampling devices, respectively. Results indicated that in pore water the dissolved Cr concentrations and in sediments total Cr and Cr fractions concentrations (dissolved, exchangeable and carbonate fraction (F1), Fe-Mn oxide fraction (F2), organic/sulfide fraction (F3)) were lower in MDET than in CDMB. However, the highly toxic DGT-labile Cr(VI) concentrations were higher in MDET sediments than in CDMB sediments. In the two lake zones, the seasonal variations in concentrations of dissolved Cr and DGT-labile Cr(VI) were significant, while total Cr and Cr fractions showed negligible changes (except F1 fraction). In summer, the high mobility of dissolved Cr (MDET: 103.42 ± 10.82 μg/L; CDMB: 108.99 ± 4.24 μg/L) were mainly caused by dissolved organic matter complexing with Cr(III). In winter the high mobility of dissolved Cr (MDET: 100.27 ± 22.04 μg/L; CDMB: 102.01 ± 8.81 μg/L) and DGT-labile Cr(VI) (MDET: 28.26 ± 3.73 μg/L; CDLZ: 25.82 ± 2.26 μg/L) were primarily caused by the oxidization of Cr (III) by Mn(III/IV) oxides. This study establishes the mechanisms for seasonal variation of Cr mobilization in different lake ecological zones, highlighting the urgent need for remediation of Cr pollution, especially in macrophyte-dominated lake zones. Graphical abstract Unlabelled Image Highlights • The dissolved Cr level was higher in Meiliang Bay than in East Taihu. • The DGT-labile Cr(VI) level was higher in East Taihu than in Meiliang Bay. • The high Cr mobility in summer was caused by DOC complexation with Cr(III). • The Cr(III) oxidization by Mn(III/IV) caused the high Cr mobility in winter. [ABSTRACT FROM AUTHOR]

There are few studies about the processes and mechanisms for aquatic plants to take up phosphorus (P) in wetland soils and sediments. Direct observation of P mobilization in rhizosphere is lacking. In this study, high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were used to capture the small-scale changes of soluble reactive P (SRP) and soluble Fe, and labile P in the rhizosphere of Vallisneria natans ( V. natans ), respectively. The results showed 5.92- and 3.12-fold enrichments of P and Fe in the Fe plaques formed on the root surfaces, respectively, in comparison with the P and Fe concentrations in the non-rhizosphere sediments. Moreover, simultaneous releases of P and Fe appeared in rhizosphere and the SRP concentration showed up to 114-fold increases compared to the non-rhizosphere sediments. Five kinds of low-molecular weight organic acids (LMWOAs) were detected in the root exudates; oxalic acid accounted for 87.5% of the total. Extraction of Fe and P in the Fe plaques was greatly enhanced by root exudates compared to deionized water, and oxalic acid contributed to 67% and 75% of the total extracted Fe and P, respectively. The coupling processes of Fe plaque enrichment of P and oxalic acid complexation of Fe(III) led to significantly enhanced P acquisition in the rhizosphere of V. natans . [ABSTRACT FROM AUTHOR]

Sediments have a heterogeneous distribution of labile redox-sensitive elements due to a drastic downward transition from oxic to anoxic condition as a result of organic matter degradation. Characterization of the heterogeneous nature of sediments is vital for understanding of small-scale biogeochemical processes. However, there are limited reports on the related specialized methodology. In this study, the monthly distributions of labile phosphorus (P), a redox-sensitive limiting nutrient, were measured in the eutrophic Lake Taihu by Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) on a two-dimensional (2D) submillimeter level. Geographical information system (GIS) techniques were used to visualize the labile P distribution at such a micro-scale, showing that the DGT-labile P was low in winter and high in summer. Spatial analysis methods, including semivariogram and Moran’s I, were used to quantify the spatial variation of DGT-labile P. The distribution of DGT-labile P had clear submillimeter-scale spatial patterns with significant spatial autocorrelation during the whole year and displayed seasonal changes. High values of labile P with strong spatial variation were observed in summer, while low values of labile P with relatively uniform spatial patterns were detected in winter, demonstrating the strong influences of temperature on the mobility and spatial distribution of P in sediment profiles. [ABSTRACT FROM AUTHOR]

The release of phosphorus (P) from sediments may have significant influences on the concentration of water P and the trophic status of lakes. This study investigated the effects of temperature on the mobilization of P in sediments and its release to the overlying water in an incubation experiment and in the field of a shallow Lake Taihu (China). The ZrO-Chelex diffusive gradients in thin films (ZrO-Chelex DGT) and dialysis sampler (Peeper) were used to simultaneously measure labile P/Fe and soluble P/Fe in sediments at vertical resolutions of 2 mm and 4 mm, respectively. The results showed that the mean DGT-labile P and soluble reactive P (SRP) concentrations at 25 °C increased by 279% and 125% in comparison with those at 7 °C in the microcosm experiment, respectively, while in the field the proportions became 460% and 189% respectively. The microbial activity in sediments also increased with the rising temperature. Moreover, positive correlations (p < 0.001) existed between DGT-labile P and labile Fe and between SRP and soluble Fe under different temperature, implying that the mobilization of P and its release to the overlying water was dominantly controlled by Fe redox cycling. This study provided strong evidence that the rising temperature enhanced the microbial reduction of P-hosted Fe (oxyhydr)oxide, while an on-site accumulation and degradation of algal material in the sediment surface may lead to a greater effect on P mobilization. [ABSTRACT FROM AUTHOR]

There is a great heterogeneity in the distribution of mobile phosphorus (P) in natural sediments, while the assessment of P immobilization by amendment agents has mostly neglected this feature. In this study, the effects of aluminum sulfate (ALS) on P immobilization were investigated via a 60-day microcosm experiment using sampled sediment cores. The high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) techniques were first employed to establish the profiles of soluble reactive P (SRP) and DGT-labile P in aluminum-amended sediments at 2 and 1 mm resolutions, respectively. Both concentrations of two mobile P forms decreased with increasing ALS dosages. The static layers with extremely low P concentrations (≤0.060 mg L −1 for SRP and ≤0.024 mg L −1 for DGT-labile P) were observed in the upper 6–16 mm sediment layers at 6–15 ALS/P mobile treatments (mobile P is the sum of NH 4 Cl-P, BD-P, and NaOH-nrP; mol mol −1 ). The static layer was followed by an active layer in which the upward release potential ( R AL ) decreased from 33.4 to 21.9 for SRP and from 92.2 to 51.0 for DGT-labile P, respectively. As the formation of the static layer is a key to sustaining P immobilization in sediments, the minimal dosage of 9 ALS/P mobile is required for ALS capping. Modeling with DGT Induced Fluxes in Sediments (DIFS) showed a greater increase in adsorption rate constant ( k 1 , maximum 7.2-fold) compared to adsorption rate constant ( k 1 , maximum 2.2-fold), demonstrating that the release of P from sediment solids became increasingly difficult after ALS amendment. [ABSTRACT FROM AUTHOR]

Geo-engineering is recognized as a promising technique to reduce the release of phosphorus (P) from sediments to water. To investigate the efficiency of lanthanum modified bentonite (LMB), in this study, a microcosm experiment with LMB capping of sediment cores, was performed up to 110 days. High-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) were employed to measure soluble reactive P (SRP) and labile P in overlying water-sediment profiles, respectively, at spatial resolutions from millimeter to submillimeter scale. Results showed that the concentrations of SRP and DGT-labile P decreased with treatment time. A top static layer, with low concentration and small variation (<30% RSD) of SRP and labile P, was formed on the 20th day after capping. The mean concentrations of SRP and DGT-labile P in this layer were stabilized at 5% of those before amendment on the 45th day in the 10 mm top layer, with the immobilization of P in the sediments achieving an ideal phase. Meanwhile, the upward diffusive gradients of SRP and DGT-labile P below the static layer were persistently relaxed, demonstrating a decrease in the release potential of P to the static layer. This was caused by the increasing difficulty of P release from labile solid P pool with time, which was reflected by the increases in distribution coefficient of P between the solid pool and pore water ( K d ) and the adsorption rate constant ( k 1 ). It is suggested that the formation of the static layer is the key to efficiently retain the capping effect of LMB. [ABSTRACT FROM AUTHOR]

A millimeter-scale investigation is key to the understanding of the competitive effects of phosphate(P) on arsenic(As) mobility in sediments by taking the great biogeochemical heterogeneity of the sediments into consideration. In this study, a microcosm experiment was performed in this aspect using high-resolution dialysis and diffusive gradients in thin films (DGT) to simultaneously measure dissolved and labile P, As, and iron (Fe) in sediments, respectively. With the increase of P content in water from 0.02 mg L −1 to 0.20 and 2.4 mg L −1 , consistent release of As from sediments was observed. The concentrations of DGT-labile As increased significantly especially in the upper sediment layer (up to 12 times of the 0.02 mg P L −1 treatment) due to the competition of phosphate, which corresponded well to the increase in DGT-labile P. There was limited increase in dissolved P and As contents due to the buffering provided by sediment solids, while the concentrations of both dissolved and DGT-labile Fe in sediments decreased. A stoichiometric calculation showed that 47% and 8% of the added P were removed through Fe(II) precipitation for the 0.20 and 2.4 mg P L −1 treatments, respectively, which greatly suppressed the release of As induced by P competition for the 0.20 mg P L −1 treatment. The DGT-induced fluxes in sediments (DIFS) modeling showed an increase in solid resupply to pore water As from elevation of water P through the increases of the desorption rate constant from 5.4 to 31( × 10 −7 ) s −1 and the sorption rate constant from 1.8 to 22( × 10 −4 ) s −1 . [ABSTRACT FROM AUTHOR]