Spatially varying relationships of soil Se concentration and rice Se concentration in Guangxi, China: A geographically weighted regression approach.

In recent years, the biogeochemical behavior and environmental impact of Selenium (Se) on soil-plant systems have received widespread attention, and traditional statistical methods reveal generally positive correlations between rice Se and soil Se. However, that initial positive relationship may have been obscured by local external factors. Using local scale data from the geochemical evaluation of land quality project, this work employed geographically weighted regression (GWR) to examine the spatial variation of rice Se (as the dependent variable) and soil Se (as the independent variable) in Guangxi. Strong and weak correlation coefficients occur between rice Se and soil Se, thereby indicating that their relationships are spatially varying. Guangxi is characterized by significantly positive correlations in most areas, with weak correlations mostly found in the south-western and central-eastern regions. Areas with weak correlation can be divided into two patterns: high soil Se with low rice Se and high rice Se with low soil Se. The unique patterns are correlated with distinct natural factors, particularly the abundance of Fe-rich soils in the carbonate area; by contrast, sandstone areas in central Guangxi may have been affected by anthropogenic activities. To reveal the spatially varying relationships at the local scale, we employed GWR, an effective tool that allowed us to identify the association between environmental variables and influencing factors and explore spatially varying relationships between them. This study breaks through the existing understanding that soil Se is completely positively correlated with rice Se for the first time, and concludes that their correlation is spatially variable, providing an effective approach for the study of complex relationships.

Identification of the spatial patterns and controlling factors of Se in soil and rice in Guangxi through hot spot analysis.

Selenium (Se) is essential to human health, anti-cancer, possessing antioxidant, and antiviral properties. In this study, the spatial patterns of rice Se and their varying relationship with soil Se on a regional scale were studied using hot spot analysis for the agricultural soils in Guangxi. According to the hot and cold spot maps, rice Se correlates positively with soil Se in Guangxi agricultural soils. High rice Se accompanies high soil Se in the central part of Guangxi (e.g., Liuzhou, Laibin), and low rice Se is in line with low soil Se in the western part (e.g., Baise). However, the hot spot analysis maps indicate that southwestern Guangxi exhibits a special characteristic of low rice Se with high soil Se (e.g., Chongzuo). This special pattern is strongly associated with the high concentrations of Fe2O3 (ferromanganese nodules) in the carbonate rock area. The hot spot analysis proves useful in revealing the spatial patterns of rice Se in Guangxi and identifying the hidden patterns.

Influences of geological factors on the distribution of uranium in drinking water limescale in the junction zone of the East European Platform and the Southern Urals.

An assessment of uranium contents and distribution in drinking water limescale has been conducted in the Republic of Bashkortostan (RB), Russia. A total of 515 limescale samples from 262 settlements of the RB were analyzed. The spread of U concentration values in limescale samples ranged from 0.01 to 61.0 µg/g. Elevated U concentrations in the West of the RB corresponded with the horsts of the granite-gneiss crystalline basement of the South-Tatar Dome and their Eastern slopes, the areas with the Lower Permian red beds and the oil and gas fields. The U migration from the granite-gneiss basement is attributed to the tectonic factor and hydrocarbons movement. Elevated concentrations of U within the South of the RB are associated primarily with the deposits of the Southern Ural brown coal basin. The Bashkir Trans-Urals anomalies are mainly associated with Lower Paleozoic eclogite complex, Devonian and Carboniferous volcanic-sedimentary, carbonate, intrusive formations, as well as the Jurassic cover of terrigenous marine sediments. The negative anomalies of the spatial distribution of U are located in the area of the Ufimian Plateau mainly composed of limestone.

Research progress and application prospect of anaerobic biological phosphorus removal.

Anaerobic biological phosphorus removal has proposed a new direction for the removal of phosphorus from wastewater, and the discovery of phosphate reduction makes people have a more comprehensive understanding of microbial phosphorus cycling. Here, from the perspective of thermodynamics, the bioreduction reaction of phosphate was analyzed and its mechanism was discussed. The research progress of phosphate reduction and the application prospects of anaerobic biological phosphorus removal from wastewater were introduced, pointing out the situation and guiding the further research in this field.

Phosphorus mobilization in lake sediments: Experimental evidence of strong control by iron and negligible influences of manganese redox reactions.

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.

Mechanisms driving phosphorus release during algal blooms based on hourly changes in iron and phosphorus concentrations in sediments.

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.

Synergistic adsorption of phosphorus by iron in lanthanum modified bentonite (Phoslock®): New insight into sediment phosphorus immobilization.

Iron redox cycle plays a primary role in controlling the mobility of P in sediments. It is crucial to better understand how lanthanum (La) modified bentonite (LMB, Phoslock®), an increasingly employed capping agent, immobilizes P from sediments by altering Fe redox-coupled P cycling. Batch adsorption experiments found that LMB effectively adsorbed Fe(II) with a capacity of 8.51 mg g-1. Fe(II)-preloaded LMB effectively retained P during a 518-hour equilibration, while up to 16.7% of adsorbed P was release-sensitive in LMB without Fe(II) preloading. A 60-day incubation experiment was performed using sediment cores, with an LMB amendment dosage of up to 200 LMB/Pmob (w/w, Pmob denotes the amount of mobile P in the surface 40 mm sediment layer). The concentrations of pore water soluble reactive P (SRP) and labile P were measured by high resolution dialysis (HR-Peeper) and by diffusive gradient in thin films (DGT), respectively, at a vertical millimeter scale. They stratified into static layers with extremely low concentration distribution in the top 16-22 mm sediments (mean SRP ≤ 0.28 mg L-1 and mean DGT-labile P ≤ 0.051 mg L-1) and active layers with decreased upward diffusion potential (≤5.85 for SRP and ≤12.7 for DGT-labile P) below the static layer, when the applied dosage reached 60 LMB/Pmob. The LMB amendment reduced the pore water Fe and DGT-labile Fe in sediments, while considerable amounts of Fe and Fe-bound P existed in the LMB binding layer (25% of the total P in 200 LMB/Pmob treatment). These findings show that the adsorption of Fe by LMB plays a significant role in the stabilization of LMB-bound P, possibly by adsorbing release-sensitive P initially bound to the rhabdophane surface. LMB adsorbed Fe and P were not released until the redox potential decreased to extremely reductive conditions (-150 mV to -300 mV), possibly due to the re-adsorption of Fe and P by LMB. This study reveals synergistic effects of Fe adsorption and provides new insight into the immobilization mechanisms of P by LMB application.

Internal phosphorus loading from sediments causes seasonal nitrogen limitation for harmful algal blooms.

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 Chla 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 6.76mg/m2/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.

Submillimeter-scale heterogeneity of labile phosphorus in sediments characterized by diffusive gradients in thin films and spatial analysis.

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.

Direct evidence for the enhanced acquisition of phosphorus in the rhizosphere of aquatic plants: A case study on Vallisneria natans.

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.

A millimeter-scale observation of the competitive effect of phosphate on promotion of arsenic mobilization in sediments.

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.

Fine-scale bioturbation effects of tubificid worm (Limnodrilus hoffmeisteri) on the lability of phosphorus in sediments.

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.

Intensification of phosphorus cycling in China since the 1600s.

Phosphorus (P) is an essential nutrient for living systems with emerging sustainability challenges related to supply uncertainty and aquatic eutrophication. However, its long-term temporal dynamics and subsequent effects on freshwater ecosystems are still unclear. Here, we quantify the P pathways across China over the past four centuries with a life cycle process-balanced model and evaluate the concomitant potential for eutrophication with a spatial resolution of 5 arc-minutes in 2012. We find that P cycling in China has been artificially intensified during this period to sustain the increasing population and its demand for animal protein-based diets, with continuous accumulations in inland waters and lands. In the past decade, China's international trade of P involves net exports of P chemicals and net imports of downstream crops, specifically soybeans from the United States, Brazil, and Argentina. The contribution of crop products to per capita food P demand, namely, the P directly consumed by humans, declined from over 98% before the 1950s to 76% in 2012, even though there was little change in per capita food P demand. Anthropogenic P losses to freshwater and their eutrophication potential clustered in wealthy coastal regions with dense populations. We estimate that Chinese P reserve depletion could be postponed for over 20 y by more efficient life cycle P management. Our results highlight the importance of closing the P cycle to achieve the cobenefits of P resource conservation and eutrophication mitigation in the world's most rapidly developing economy.

Kinetics of phosphorus release from sediments and its relationship with iron speciation influenced by the mussel (Corbicula fluminea) bioturbation.

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 1mm, 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.

Iron-coupled inactivation of phosphorus in sediments by macrozoobenthos (chironomid larvae) bioturbation: Evidences from high-resolution dynamic measurements.

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.

In situ, high-resolution imaging of labile phosphorus in sediments of a large eutrophic lake.

Understanding the labile status of phosphorus (P) in sediments is crucial for managing a eutrophic lake, but it is hindered by lacking in situ data particularly on a catchment scale. In this study, we for the first time characterized in situ labile P in sediments with the Zr-oxide diffusive gradients in thin films (Zr-oxide DGT) technique at a two-dimensional (2D), submillimeter resolution in a large eutrophic lake (Lake Taihu, China, with an area of 2338 km(2)). The concentration of DGT-labile P in the sediment profiles showed strong variation mostly ranging from 0.01 to 0.35 mg L(-1) with a considerable number of hotspots. The horizontal heterogeneity index of labile P varied from 0.04 to 4.5. High values appeared at the depths of 0-30 mm, likely reflecting an active layer of labile P under the sediment-water interface (SWI). Concentration gradients of labile P were observed from the high-resolution 1D DGT profiles in both the sediment and overlying water layers close to the SWI. The apparent diffusion flux of P across the SWI was calculated between -21 and 65 ng cm(-2) d(-1), which showed that the sediments tended to be a source and sink of overlying water P in the algal- and macrophyte-dominated regions, respectively. The DGT-labile P in the 0-30 mm active layer showed a better correlation with overlying water P than the labile P measured by ex situ chemical extraction methods. It implies that in situ, high-resolution profiling of labile P with DGT is a more reliable approach and will significantly extend our ability in in situ monitoring of the labile status of P in sediments in the field.

Bioavailability assessment of phosphorus and metals in soils and sediments: a review of diffusive gradients in thin films (DGT).

This paper provides an overview of the principle and latest development of the diffusive gradients in thin films (DGT) technology and its applications in environmental studies with a focus on bioavailability assessment of phosphorus and metals in sediments and soils. Compared with conventional methods, DGT, as a passive sampling method, has significant advantages: in situ measurement, time averaged concentrations and high spatial resolution. The in situ measurement avoids artificial influences including contamination of samples and sample treatment which may change the forms of chemicals. The time averaged concentration reflects representative measurement over a period of time. The high-resolution information captures the biogeochemical heterogeneity of elements of interest distributed in microenvironments, such as in the rhizosphere and the vicinity of the sediment-water interface. Moreover, DGT is a dynamic technique which simultaneously considers the diffusion of solutes and their kinetic resupply from the solid phases. All the advantages of DGT significantly promote the collection of "true" information of the bioavailable or labile forms of chemicals in the environment. DGT provides potential for applications in agriculture, environmental monitoring and the mining industry. However, the applications are still at the early testing stage. Further studies are needed to properly interpret the DGT-measured results under complex environmental conditions, and standard procedures and guideline values based on DGT are required to pave the way for its routine applications in environmental monitoring.

Speciation of organic phosphorus in a sediment profile of Lake Taihu. II. Molecular species and their depth attenuation.

The understanding of organic phosphorus (P) dynamics in sediments requires information on their species at the molecular level, but such information in sediment profiles is scarce. A sediment profile was selected from a large eutrophic lake, Lake Taihu (China), and organic P species in the sediments were detected using solution phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR) following extraction of the sediments with a mixture of 0.25 mol/L NaOH and 50 mmol/L EDTA (NaOH-EDTA) solution. The results showed that P in the NaOH-EDTA extracts was mainly composed of orthophosphate, orthophosphate monoesters, phospholipids, DNA, and pyrophosphate. Concentrations of the major organic P compound groups and pyrophosphate showed a decreasing trend with the increase of depth. Their half-life times varied from 3 to 27 years, following the order of orthophosphate monoesters > phospholipids > or = DNA > pyrophosphate. Principal component analysis revealed that the detected organic P species had binding phases similar to those of humic acid-associated organic P (NaOH-NRP(HA)), a labile organic P pool that tends to transform to recalcitrant organic P pools as the early diagenetic processes proceed. This demonstrated that the depth attenuation of the organic P species could be partly attributed to their increasing immobilization by the sediment solids, while their degradation rates should be significantly lower than what were suggested in previous studies.

Diffusive gradients in thin films technique equipped with a mixed binding gel for simultaneous measurements of dissolved reactive phosphorus and dissolved iron.

Developing a technique to track the release of phosphorus (P) and iron (Fe) simultaneously in sediments would be very useful in deepening our understanding of the internal loading process of P coupled with Fe cycling in aquatic systems. In this study, a new technique was established to measure simultaneously the dissolved reactive P (DRP) and dissolved Fe primarily released from sediment solids based on the diffusive gradients in thin films (DGT) theory. A mixed binding gel (ZrO-Chelex gel) used for assembling DGT was developed for simultaneous uptake of DRP and dissolved Fe(II) using amorphous zirconium hydroxide (Zr-oxide) and Chelex-100 resin as binding agents. Simultaneous measurements of DRP and dissolved Fe(II) with the ZrO-Chelex DGT were validated in solution and were independent of solution pH and ionic strength in normal environments. The capacities of the ZrO-Chelex DGT for measurements of DRP and dissolved Fe(II) were 90 µg P cm(-2) and 75 µg Fe cm(-2), with the latter being greater than that (45 µg Fe cm(-2)) observed with the Chelex-100 resin DGT commonly used in DGT measurements of metals. Microcosm experiments further confirmed the feasibility of the ZrO-Chelex DGT for simultaneous measurement of P and Fe in sediments, with a higher concentration of Fe being measured due to this method's higher capacity compared with the Chelex-100 resin DGT.

Speciation of organic phosphorus in a sediment profile of Lake Taihu. I: Chemical forms and their transformation.

Organic phosphorus (nonreactive P, NRP) is a major component of P in sediments, but information about its chemical forms and dynamic transformation is limited. The chemical forms and dynamic behaviors of NRP in a sediment profile from Lake Taihu, a freshwater and eutrophic lake in China, were investigated. Five forms of NRP in the sediments were extracted based on a chemical fractionation technique, including easily labile NRP (NaHCO3-NRP), reactive metal oxide-bound NRP (HCl-NRP), humic acid-associated NRP (NaOH-NRP(HA)), fulvic acid-associated NRP (NaOH-NRP(FA)) and residual NRP (Res-TP). There were notable transformations with increasing sediment depth from the labile NaHCO3-NRP and NaOH-NRP pools to the recalcitrant HCl-NRP and Res-TP pools, which caused the NRP to become increasingly recalcitrant as the early diagenetic processes proceeded. Further analyses showed that the relative changes in contents of organic matter and reactive Fe oxides in the sediment profile triggered a competition for binding NRP fractions and led to the transformation of NRP. The results highlighted the importance of abiotic processes in regulating the diagenesis of organic P and its stability in sediments.