Metal/metalloid and phosphorus characteristics in porewater associated with manganese geochemistry: A case study in the Jiulong River Estuary, China

Arsenic mobilization across the sediment-water interface of the Three Gorges Reservoir as a function of water depth using DGT and HR-Peepers, a preliminary study

The artificial regulation of the Three Gorges Reservoir (TGR) creates large water level fluctuation zones (WLFZ) that may change the behavior of metals and metalloid in sediment, particularly redox sensitive elements. Mobilization of As, Fe and Mn across the sediment-water interface (SWI) in the TGR as a function of different water depth (periodically and permanently submerged sediments, respectively) was in situ determined by diffusive gradients in thin films (DGT) and high-resolution dialysis technique (HR-Peeper), respectively. The results showed that the mobilization of As was significantly affected by Fe/Mn especially Mn, across the SWI. Duo to the oxic-anoxic transitional state in near bottom water, the reduced Fe and Mn in sediment pore water could be oxidized and precipitated again, leading to the co-precipitation of As with Fe/Mn oxides (hydroxides). Consequently, concentrations of As, Fe and Mn in labile phases and pore water were generally low across the SWI, then they sharply increased at a few centimeters below the SWI. Considering different water depth, various trends were found in labile phase, whereas concentrations of As, Fe and Mn in pore water in permanently submerged sediments were significantly higher than those in periodically submerged sediments. The dry-re-wetting alternation processes in the WLFZ may play vital roles in the resupply capacity of sediments as it was found that periodically submerged sediments with longer re-wetting time had higher Fe/Mn resupply capacity than those with shorter re-wetting times and permanently submerged sediments.

Coastal degradation regulates the availability and diffusion kinetics of phosphorus at the sediment-water interface: Mechanisms and environmental implications

Coastal wetlands have experienced considerable loss and degradation globally. However, how coastal degradation regulates sediment phosphorus (P) transformation and its underlying mechanisms remain largely unknown in subtropical coastal ecosystems. This study conducted seasonal field measurements using high-resolution diffusive gradient in thin films (DGT) and dialysis (Peeper) techniques, as well as a DGT-induced fluxes in sediments (DIFS) model, to evaluate the mobilization and diffusion of P along a degradation gradient ranging from pristine wetlands to moderately and severely degraded sites. We observed that sediment P is diminished by coastal degradation, and severely degraded sites exhibit a decline in the concentration of available P, despite the presence of distinct seasonal patterns. High-resolution data based on DGT/Peeper analysis revealed that labile P and soluble reactive P (SRP) concentrations varied from 0.0006 mg L-1 to 0.084 mg L-1 (mean 0.0147 mg L-1) and from 0.0128 mg L-1 to 0.1677 mg L-1 (mean 0.0536 mg L-1), respectively. Coastal degradation had a substantial impact on increasing SRP and labile P concentrations, particularly at severely degraded sites. Although severely degraded wetlands appeared to be P sinks (negative P flux at these sites), we did also observe positive diffusive flux in October, indicating that coastal degradation may accelerate the diffusion and remobilization of sediment P into overlying water. The simulations of the DIFS model provided compelling proof of the high resupply capacity of sediment P at severely degraded sites, as supported by the increased R and k-1 values but decreased Tc values. Taken together, these results suggest coastal degradation reduces the sediment P pool, primarily attributed to the strong remobilization of P from the sediment to porewater and overlying water by enhancing the resupply capability and diffusion kinetics. This acceleration induces nutrient loss which adversely impacts the water quality of the surrounding ecosystem. To reduce the adverse effects of coastal degradation, it is essential to adopt a combination of conservation, restoration, and management efforts designed to mitigate the risk of internal P loading and release, and ultimately maintain a regional nutrient balance.

Bio-accessibility and mobilization dynamics of soil vanadium during a 48-year vegetation restoration in a vanadium titano-magnetite tailings reservoir

Bio-accessibility of vanadium (V) in soils determines the effectiveness of vegetation restoration in the vanadium titano-magnetite tailings reservoirs because of persistent V toxicity, yet the variations in the bio-accessibility and mobilization of V in the soils with vegetation restoration remain elusive. Here, the bio-accessibility and mobilization of V in the soil-water interface were investigated along a 48-year vegetation restoration chronosequence in the Majiatian tailings reservoir using the diffusive gradients in thin films technique (DGT) and DGT-induced flux model. We found a low concentration of DGT-extracted V along the vegetation restoration chronosequence and the V fraction was dominated by the residual form, indicating a low V bio-accessibility in the soils. The bio-accessibility of V increased along the chronosequence because of the increased V resupply from solid phase, especially from the organic V fraction and the clay bound V. Low supply coefficient (R = 0.25) revealed a limited release of V from solid phase to soil solution. The kinetic resupply processes of V and its key regulating parameters were stage-specific during the vegetation restoration. The pool size of labile V in the soils determined the rapid V supply at the early and late stages, while the low desorption rate of V from the solid to liquid phase regulated the slow supply regime at the middle stage. The results of the present study highlight the importance of the long-term monitoring of soil V mobilization in the tailings reservoir because of the increased bio-accessibility and the dynamic supply of V during the vegetation restoration.

Nutrients in overlying water affect the environmental behavior of heavy metals in coastal sediments

Excessive nutrients in aquatic ecosystems are the main driving factors for eutrophication and water quality deterioration. However, the influence of nutrients in overlying water on sediment heavy metals is not well understood. In this study, the effects of nitrate nitrogen (NO3-N) addition and phosphate addition in the overlying water on the environmental behaviors of chromium (Cr), copper (Cu), and cadmium (Cd) in coastal river sediments were investigated. Fresh estuary sediments and synthetic saltwater were used in microcosm studies conducted for 13 d. To determine the biological effect, unsterilized and sterilized treatments were considered. The results showed that the diffusion of Cr and Cu was inhibited in the unsterilized treatments with increased NO3-N. However, under the NO3-N sterilized treatments, Cr and Cu concentrations in the overlying water increased. This was mostly related to changes in the microbial regulation of dissolved organic carbon and pH in the unsterilized treatments. Further, in the unsterilized treatments, NO3-N addition considerably increased the concentrations of the acid-soluble (Cr, Cu, and Cd increased by 5%-8%, 29%-41%, and 31%-42%, respectively) and oxidizable (Cr, Cu, and Cd increased by 10%, 5%, and 14%, respectively) fractions. Additionally, compared with that in the unsterilized treatments, Cu and Cd concentrations in P-3 treatments decreased by 7% and 63%, respectively. By producing stable metal ions, microorganisms reduced the amount of unstable heavy metals in the sediment and heavy metal concentration in the overlying water, by considerably enhancing the binding ability of phosphate and heavy metal ions. This study provides a theoretical basis for investigating the coupling mechanisms between heavy metals and nutrients.

Spatiotemporal response of water quality in fragmented mangroves to anthropogenic activities and recommendations for restoration

Mangroves have received substantial attention for their pivotal role as ecological barriers between land and sea, owing to their capacity to effectively capture considerable quantities of terrestrial pollutants. Mangrove fragmentation has been a widespread global trend. There is limited information on the water quality status of these small scattered mangrove patches in coastal sub-developed areas, coupled with a paucity of efficient and intuitive assessment methodologies. To address this gap, the Water Quality Index (WQI) was introduced to evaluate the spatiotemporal characteristics of mangrove water quality. The major sources of pollution and anthropogenic activities that affect mangrove water quality were identified. The results revealed an average WQI value of 44.1 ± 13.3 for mangrove patches, consistently indicating a "low" water quality classification throughout all seasons. Both the size and natural conditions impact the water quality of mangroves. The large artificial patch (WQI: 56.4 ± 7.61) and the natural patch (WQI: 46.6 ± 13.6) exhibited relatively superior water quality, while the WQI value of a size-equivalent artificial patch compared with the natural patch is 38.6 ± 11.8. Aquaculture was the primary human activity that adversely affected the water quality of mangroves, and the potential sources of pollution were rainfall runoff and river discharge. These findings elucidate the unfavorable water quality characteristics and dominant pollution of fragmented mangroves, and validate the applicability of the WQI method for long-term evaluation of the water quality in mangrove patches. This study provides a basis for decision-making in water quality assessment and management of coastal wetlands and marine ecosystems. Scientific guidance to the management for mangrove protection and restoration was offered, such as regulating aquaculture activities, controlling non-point source pollution, implementing mangrove reforestation by using native species in historical mangrove sites.

Arsenic migration at the sediment-water interface of anthropogenically polluted Lake Yangzong, Southwest China

The lability and controlling factors of arsenic (As) at the sediment-water interface (SWI) are crucial for understanding As behaviors and fates in As-contaminated areas. In this study, we combined high-resolution (5 mm) sampling using diffusive gradients in thin films (DGT) and equilibrium dialysis sampling (HR-Peeper), sequential extraction (BCR), fluorescence signatures, and fluorescence excitation-emission matrices (EEMs)-parallel factor analysis (PARAFAC) to explore the complex mechanisms of As migration in a typical artificially polluted lake, Lake Yangzong (YZ). The study results showed that a high proportion of the reactive As fractions in sediments can resupply pore water in soluble forms during the change from the dry season (winter, oxidizing period) to the rainy season (summer, reductive period). In dry season, the copresence of Fe oxide-As and organic matter (OM)-As complexes was related to the high dissolved As concentration in pore water and limited exchange between the pore water and overlying water. In the rainy season, with the change in redox conditions, the reduction of Fe-Mn oxides and OM degradation by microorganisms resulted in As deposition and exchange with the overlying water. Partial least squares path modelling (PLS-PM) indicated that OM affected the redox and As migration processes through degradation. Based on comprehensive analyses of the As, Fe, Mn, S and OM levels at the SWI, we suggest that the complexation and desorption of dissolved organic matter and Fe oxides play an important role in As cycling. Our findings shed new light on the cascading drivers of As migration and OM features in seasonal lakes and constitute a valuable reference for scenarios with similar conditions.

Stimulation of oxalate root exudate in arsenic speciation and fluctuation with phosphate and iron in anoxic mangrove sediment

Mutual transformations of rhizospheric arsenic (As) in pollution-prone mangrove sediments affected by root exudate oxalate were simulated. This study focuses on the effect of oxalate on As release, mobilization, and phase speciation associated with P and Fe was examined under anoxic conditions in time-dependent changes. Results showed that oxalate addition significantly facilitated As-Fe-P release from As-contaminated mangrove sediments. Sediment As formed the adsorptive and the carbonate-binding fractionations, facilitating the re-adsorption processes. Solution As and As⁵⁺ correlated with NaOH-P positively but with NaHCO₃-P and HCl-P negatively. Dominant Fe³⁺ (>84 %) from the amorphous Fe regulated suspension changes and then time-dependent co-precipitation with As and P. Sediment P formed strong complexes with Fe oxides and could be substituted for As via STEM analysis. Oxalate ligand exchange, competitive adsorption of oxalate, and Fe-reduced dissolution are confirmed to involve, allowing for an insight As/P/Fe mobilization and fate in mangrove wetland.

A new DGT technique based on nano-sized Mg2Al layered double hydroxides with DTPA for sampling of eight anionic and cationic metals

In this work, a new resin gel incorporated with layered double hydroxide nanoparticles modified with diethylenetriaminepentaacetic acid is developed for application in diffusive gradients in thin-film devices (abbreviated as LDHs DGT) to monitor eight anions and cations (such as Fe, Mn, Co, Ni, Cu, Cd, Pb, and As) in natural waters and soils. The accumulated anions and cations were quantitatively recovered by one-step elution using 0.5 mol·L^-1 HNO₃ with an optimized elution time of 30 min. The performance of the LDHs DGT was independent of solution pH (5-8) and ionic strengths (5-100 mmol·L^-1). The capacities of the LDHs DGT for Mn(II), Fe(II), Co(II), Ni(II), Cu(II), As(V), Cd(II), and Pb(II) individually are determined to be 202.9, 363.6, 246.9, 88.8, 99.5, 75.3, 159.8, and 671.7 μg·cm^-2. During the field deployments in a nature river, LDHs DGT measured concentrations of cations and anions were almost like those measured by the traditional sampling method (except Fe(II), Cd(II), and Co(II)). In addition, bioavailable Cd measured by LDHs DGT correlated well with Cd in rice grains (R² = 0.55), indicating that LDHs DGT is a reliable tool for assessing the risk of Cd.

Evaluating the release risk of potentially toxic elements from sediments in the New Zhuzhao River Estuary of Nansi Lake, using high-resolution technology and sequential extraction

Potentially toxic elements (PTEs) re-release from sediment is an essential process in the sediment-water interface (SWI), especially for the influent river estuary as an important accumulation site. In this study, the diffusive gradient in thin films (DGT), high-resolution dialysis (HR-peeper) technique, and BCR sequential extraction were employed to evaluate the release risk of PTEs (As, Cu, Pb, Zn, Cd) in the New Zhuzhao River Estuary of Nansi Lake. Results showed that Cd existed primarily in the non-residual fraction (accounting for 59.87%), and the residual fractions of As, Cu, Pb, and Zn accounted for a greater proportion (12.65 to 33.07%). The mobility of Cd was the highest with a risk assessment code of 33.53% reaching the medium risk category. The resupply capacity calculated by C_DGT/C_Dis showed that As was the largest, with an average value of 0.43, indicating the strongest release capacity of As from the sediment to pore water. Furthermore, the diffusive fluxes using DGT and HR-peeper showed that As possesses a much higher potential to release upward overlying water than other elements.

Response mechanism of microorganisms to the inhibition of endogenous pollution release by calcium peroxide

To further explore the response mechanism of microorganisms to the synchronous control of nitrogen and phosphorus release from sediments by CaO₂, the spatiotemporal changes in the physical, chemical and biological indicators of the overlying water, interstitial water and sediments in each reactor were measured in the experiment. The experiment results showed that CaO₂ could increase the ammonia monooxygenase activity, nitrite oxidase activity and Nitrospira abundance in the sediment near its dosing position, and enhanced the activities of nitrate reductase and nitrite reductase at a certain distance from the dosing position, thereby promoting nitrogen removal in sediments through the alternating process of nitrification and denitrification. At the same time, the increase of alkaline phosphatase activity and Saccharimonadales abundance in the test groups accelerated the hydrolysis of organic phosphorus, and the P immobilization in sediments was realized through the subsequent precipitation reaction of Ca²⁺ and PO₄^3- under alkaline conditions. In addition, the enhanced activities of dehydrogenase and catalase ensured that CaO₂ would not cause great killing effect on microorganisms when improving the hypoxic conditions and inhibiting endogenous release. As a result, the dissolved product of CaO₂ such as Ca(OH)₂ and H₂O₂ reduced the nutrients concentration and killed the algae, which kept the algae density and chlorophyll a concentration at a low level throughout the test groups. Therefore, this study systematically clarified the microbial mechanism of CaO₂ synchronously controlling the release of nitrogen and phosphorus from sediments, which provided a new idea for the remediation of endogenous pollution in the water system.

Potential pollution assessment of labile trace metals in Xixi River estuary sediments in Xiamen, China

The release of trace metals caused by industrial effluents and anthropogenic activities has been recorded in the Xixi River estuary, southern China. However, a thorough understanding of the behavior of trace heavy metals in Xixi River sediments is lacking. A total of 12 sediment cores were collected in June and December in the upper estuary section and mouth of the estuary. Here, an in situ high-resolution sampling technique, namely, diffusive gradients in thin films (DGT), was employed to acquire profiles of trace element concentrations and the release of bioavailable metals from sediments in different seasons. A three-step Community Bureau of Reference (BCR) sequential extraction method was used to explore the chemical speciation of trace metals in different seasons and to thereby assess the release potential of trace elements in sediments. The BCR sequential extraction results showed that the trace metals Fe, Mn, Co and Pb were mainly in the residual fraction, which rarely influences living organisms. The total mobile fractions (F1 + F2 + F3) of all trace metals were higher in winter than in summer, suggesting that accumulation occurred from summer to winter. DGT measurements showed that the intensity of sulfate reduction was higher in summer than in winter because of the high temperatures and high organic matter in summer. The intensity of sulfate and Mn(III/IV) reduction increased from the upper estuary section to the lower estuary. Fe(III) reduction decreased in summer but increased slowly in winter. The Pearson correlation results showed that the release of DGT-labile Co in pore water was related to Mn(III/IV) reduction, while the release of DGT-labile Pb was basically not controlled by the Fe-Mn-S redox transition. Abnormally high DGT-labile Pb concentrations were observed at the sampling station (XR3) closest to the estuary in winter, which might have been caused by the high Pb content in the local micro-sediments.

Partitioning behavior and ecological risk of arsenic and antimony in the sediment-porewater profile system in the Three Gorges Reservoir, China

Arsenic and antimony are widely distributed toxic metalloids in aquatic environments. However, their partitioning behaviors in the sediment profile remain not well understood. Here, partitioning behaviors, diffusive fluxes, as well as the ecological risks of As and Sb in the sediment-porewater profile system in the tributaries of the Three Gorges Reservoir (TGR) were investigated. As and Sb showed markedly different spatial variations in the longitudinal profiles of both porewater and sediment samples. Specifically, the concentration of As showed an accumulation trend with depth, while that of Sb showed a relatively complicated trend. Further, As showed lower sediment-porewater partitioning coefficient (K_d) values, suggesting that it had a relatively lower sediment affinity and a higher mobility than Sb. Its residual fraction (30%-60%) was also lower than that of Sb. This phenomenon could be attributed to the chemical fractions of the trace metals and the pH value of the sediments. Furthermore, the K_d values corresponding to As were influenced by both the residual fraction (r = 0.338, p < 0.05) and the exchangeable fraction (r = -0.643, p < 0.01), while those corresponding to Sb were only influenced by pH. Additionally, even though these two trace metals showed low ecological and mobility risks, the diffusive fluxes at the sediment-water interface suggested that the sediment acted as a source of As and a sink for Sb relative to the overlying water. This study indicated that As and Sb had different partitioning behaviors and release risks in the sediment-porewater profile system, enhanced the understanding the transport and fate of As and Sb in the aquatic environment.

Biogeochemical behavior of P in the soil and porewater of a low-salinity estuarine wetland: Availability, diffusion kinetics, and mobilization mechanism

Estuarine wetlands, which typically store large amounts of phosphorus (P), are experiencing increased salinity as well as changed environmental factors caused by rising sea levels. In this study, the seasonal dynamics of P speciation, availability, and biogeochemical couplings with iron (Fe)-sulfur (S) in soil and porewater were measured in a low-salinity estuarine wetland using in situ high-resolution diffusive gradients in thin films (DGT) and dialysis (HR-Peeper) techniques. The diffusion kinetics and resupply capacity of P from the soil phase to solution were simulated using a DGT-induced fluxes in soils (DIFS) model. The transition from freshwater to brackish wetlands reduced soil P pools and shifted to more recalcitrant speciation. The concentration of DGT-labile P across the soil-water profiles ranged from 0.002 to 0.039 (mean: 0.015) mg L^-1, which increased with increasing salinity in both the field and mesocosm experiments. The distributions of labile and soluble P showed high heterogeneity across the profiles, and there were some sharp peak values below the soil-water interface (SWI), which significantly increased the concentration and lability of P. The strong coupling between labile P and Fe (S) provided direct evidence for the coexistence of iron reduction (IR) and sulfate reduction (SR) in the estuary, while IR might predominate in P mobilization in the brackish environment because of higher labile Fe concentrations and stronger Fe-P couplings. The diffusion fluxes of P were positive at both sites, demonstrating that the kinetics of P were from the soils to the overlying water. Higher R and k_-1 values fitted in the DIFS model implied that a stronger resupply capacity and desorption rate and thus faster remobilization kinetics of P occurred with increasing salinity. Our findings indicated that increased salinity (even at low levels) can alter the desorption rate and resupply capacity of soil P in estuarine wetlands and accelerate P remobilization and release by regulating the IR and SR processes, thereby leading to the deterioration of water quality.

Effects of manganese, iron and sulfur geochemistry on arsenic migration in the estuarine sediment of a small river in Xiamen, Southeast China

The geochemistry of iron (Fe), manganese (Mn) and sulfur (S) and their effects on arsenic (As) mobility in the mudflats of small river estuaries remain unclear. Here, diffusive gradient in thin films (DGT) and high-resolution dialysis (HR-Peeper) techniques combined with a sequential extraction procedure (BCR) were employed to investigate As, Fe, Mn and S geochemistry in the mudflat of the Jiuxi River estuary, Southeast China. Grain size analysis indicated that fine-grained particles were likely to be deposited in the estuarine intertidal zone and coastal area. DGT and HR-Peeper results revealed that in the estuary and coastal area, the dissolved As in sediment in summer was controlled by Mn geochemistry, which includes not only the release of As through Mn/Fe reduction but also the stabilization of dissolved As in pore water. This stabilization of dissolved As may due to the formation of As-Mn-OM complexes. In winter, the significant positive correlations between DGT-Fe, DGT-Mn, DGT-As and DGT-S indicated that sulfate reduction was the start of As mobilization in sediment in winter. In both the estuary and the coastal area, the easily reducible Fe, Mn and As contents in intertidal sediment were higher than those in the subtidal zone. Combined with the As flux across the sediment-overlying water interface (SWI), these phenomena suggested that As in subtidal sediment diffused into overlying water and that As in overlying water tended to accumulate in the intertidal sediment. The total organic carbon content (TOC) and DGT results in the lower reach, estuary and coastal areas indicated that organic matter is the controlling factor of Fe/Mn reduction, sulfate reduction and As mobilization. The BCR test results showed higher reactive fraction contents of Fe, Mn and As in winter sediment, which threaten the overlying water quality.

Microbial controls on heavy metals and nutrients simultaneous release in a seasonally stratified reservoir

The eutrophication of reservoirs can change the physicochemical parameters of water, thus affecting the migration and transformation of heavy metals. At present, there is insufficient research on the coupling mechanisms between nutrients and heavy metals, especially between heavy metals in suspended particles. In this paper, spatial and temporal distribution characteristics of nutrients dissolved heavy metals, and heavy metals in suspended particles were analyzed in a seasonally stratified reservoir. Combined with the nitrogen and phosphorus biogeochemical process, the coupling mechanisms between heavy metals and nutrients were discussed. The results showed that the Aha Reservoir had temperature and dissolved oxygen stratification in April and July. The reduction and dissolution of Fe and Mn oxide/hydroxide and the resuspension of sediments might result in a simultaneous increase in the concentrations of nutrients, dissolved heavy metals and heavy metals in suspended particles in hypolimnion in July and October. In the presence of dissimilatory iron-reducing bacteria (DRIB), the dissolution of iron-bound phosphorus in sediments and suspended particulate matter (SPM) might lead to the simultaneous release of iron and phosphorus into the water. The dissolution of metal sulfides in the sediments and SPM under the action of dissimilatory nitrate reduction to ammonium (DNRA) bacteria might lead to the simultaneous release of ammonia nitrogen and heavy metals into the water. Due to the coupling between nitrogen and phosphorus and heavy metals, seasonal stratified reservoir may face the risk of periodic simultaneous pollution of eutrophication and heavy metals in summer and autumn. This research provides theoretical support for the treatment of heavy metal and eutrophication combined pollution in karst areas.

Factors controlling phosphorus mobility in nearshore aquifers adjacent to large lakes

Internal P stores in offshore lakebed sediments play an important role in lake nutrient dynamics. While P stores in nearshore aquifer sediments may also be important for nutrient dynamics, it is unclear whether P accumulates in these sediments, and if so, what factors control P accumulation and its potential later release from the sediments to nearshore waters. This knowledge gap was addressed by conducting field investigations at seven nearshore sites located along the shores of Lake Erie, Lake Huron and Lake Ontario, Canada, with more detailed dissolved and sediment phase characterization completed for two nearshore sites. PO₄ concentrations were observed to be higher (>50 μg/L) in the more reducing nearshore aquifers compared to more oxidizing nearshore aquifers (<20 μg/L), despite similar total solid phase P concentrations at the sites. PO₄ mobility in the nearshore aquifers was found to be closely linked to redox-driven Fe cycling. In the more reducing aquifers, dissolved PO₄ was highest near the redox boundary present in the shallow sediments where oxic infiltrating surface water mixes with reducing groundwater. In the more oxidizing aquifers, solid phase characterization indicated that PO₄ is sequestered to Fe oxide mineral phases throughout the nearshore aquifer which explains the low dissolved PO₄. While pH was not found to be important for PO₄ mobility at the study sites, batch laboratory experiments indicate that increased infiltration of more alkaline surface water into nearshore aquifers may promote PO₄ release from the sediments. The study findings demonstrate that while internal P storage mechanisms in nearshore aquifer sediments may currently be limiting P loads to lakes, it is possible that P stores that build up over time may result in increased P loads to lakes in the future.

Macro- and Micronutrient Cycling and Crucial Linkages to Geochemical Processes in Mangrove Ecosystems

High mangrove productivity is sustained by rapid utilization, high retention efficiency and maximum storage of nutrients in leaves, roots, and soils. Rapid microbial transformations and high mineralization efficiencies in tandem with physiological mechanisms conserve scarce nutrients. Macronutrient cycling is interlinked with micronutrient cycling; all nutrient cycles are linked closely to geochemical transformation processes. Mangroves can be N-, P-, Fe-, and Cu-limited; additions of Zn and Mo stimulate early growth until levels above pristine porewater concentrations induce toxicity. Limited nutrient availability is caused by sorption and retention onto iron oxides, clays, and sulfide minerals. Little N is exported as immobilization is the largest transformation process. Mn and S affect N metabolism and photosynthesis via early diagenesis and P availability is coupled to Fe-S redox oscillations. Fe is involved in nitrification, denitrification and anammox, and Mo is involved in NO3− reduction and N2-fixation. Soil Mg, K, Mn, Zn and Ni pool sizes decrease as mangrove primary productivity increases, suggesting increasing uptake and more rapid turnover than in less productive forests. Mangroves may be major contributors to oceanic Mn and Mo cycles, delivering 7.4–12.1 Gmol Mn a−1 to the ocean, which is greater than global riverine input. The global Mo import rate by mangroves corresponds to 15–120% of Mo supply to the oceanic Mo budget.

Distribution characteristics and release mechanisms of Pb in surface sediments in different aquatic environments

As a trace heavy metal, lead (Pb) has many anthropogenic applications but also produces many environmental pollution problems because of its high toxicity. In this study, we combined two in situ high-resolution sampling techniques - high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) - with the DGT-induced fluxes in sediment (DIFS) model to explore the mechanism of Pb release and resupply between sediments and pore water in the lower reaches and estuary of the Jiuxi River and the adjacent coast. An analysis of the chemical forms of Pb in the sediments showed that the content of the acid-extractable fraction (F1) was higher at the coastal site than at the other sampling sites, which indicates that Pb in the coastal sediments had greater activity and was more likely to cause Pb pollution. The apparent diffusion fluxes of Pb across the sediment-water interface (SWI) in the lower reaches, estuary and coastal zone are negative, and the absolute value of Pb flux in the estuary is several times higher than that in the other two stations, indicating a strong downward Pb diffusion trend, which may be due to water pollution caused by the nearby sewage outlet. As an insensitive element to redox, Pb did not exhibit an obvious correlation with Fe. In particular, the high Pb concentration and strong downward diffusion trend of the overlying water in the estuary caused the significant negative correlation between Pb and Fe. The calculated results of the DIFS model show that the reduced layer in the intertidal zone along the coast has the highest R value, the highest desorption rate (k_-1) and the shortest response time (T_c), indicating that sediment particles in the coastal intertidal zone supply Pb to the pore water at the fastest rate; consequently, Pb pollution in the coastal zone is worthy of further attention.