DGT-induced Fluxes In Sediments Research Articles

Mobile phosphorus stratification in sediments by aluminum immobilization

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/Pmobile treatments (mobile P is the sum of NH4Cl-P, BD-P, and NaOH-nrP; mol mol−1). The static layer was followed by an active layer in which the upward release potential (RAL) 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/Pmobile is required for ALS capping. Modeling with DGT Induced Fluxes in Sediments (DIFS) showed a greater increase in adsorption rate constant (k1, maximum 7.2-fold) compared to adsorption rate constant (k1, maximum 2.2-fold), demonstrating that the release of P from sediment solids became increasingly difficult after ALS amendment.

The partitioning of P in soil determines the fluxes and deliveries of labile P in soil solution

Phosphorus (P) is an increasingly limited resource for food production, which necessitates studying soil factors controlling rates of P release to soil solution. Phosphorus use efficiency is among other, affected by the plants, the plant root induced displacement of P equilibrium in soil during uptake and the following rates of replenishment from geochemically active P stores in soil. Here, soil P partitioning and flux data collected from a 50year field trial, based on application of differing P masses, was investigated using a diffusive gradients in thin films (DGT) technique and “DGT-induced fluxes in sediments” (DIFS) model. Partitioning of P in soil was accomplished by deploying DGT at increasing contact times (6h to 120h). DGT induced P fluxes increased with soil P, and they were highest the first hours after installation. A Langmuir adsorption approach to determine Kd concealed accumulated P labile stores, whereas Kd estimations from ammonium lactate (AL) extractions did not. The estimated fluxes in the latter situation stabilized after 24h deployment, and they were more clearly reflecting the long term P treatments. The two Kd approaches showed the importance of considering inherent P-stores for calculating fluxes and deliveries of labile P available for plants during P-uptake. The use of DGT and DIFS in combination enabled good estimations of fluxes. Such fluxes may be used to estimate, In Situ, critical soil solution P concentrations available for plants during growth, in different soil types.

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.

An investigation of the effects of elevated phosphorus in water on the release of heavy metals in sediments at a high resolution

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 0.02mgL−1 to 0.20 and 2.4mgL−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 (Ksp) 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 (Tc) 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.

A Holistic Approach to Understanding the Desorption of Phosphorus in Soils.

The mobility and resupply of inorganic phosphorus (P) from the solid phase were studied in 32 soils from the UK. The combined use of diffusive gradients in thin films (DGT), diffusive equilibration in thin films (DET) and the "DGT-induced fluxes in sediments" model (DIFS) were adapted to explore the basic principles of solid-to-solution P desorption kinetics in previously unattainable detail. On average across soil types, the response time (Tc) was 3.6 h, the desorption rate constant (k-1) was 0.0046 h(-1), and the desorption rate was 4.71 nmol l(-1) s(-1). While the relative DGT-induced inorganic P flux responses in the first hour is mainly a function of soil water retention and % Corg, at longer times it is a function of the P resupply from the soil solid phase. Desorption rates and resupply from solid phase were fundamentally influenced by P status as reflected by their high correlation with P concentration in FeO strips, Olsen, NaOH-EDTA and water extracts. Soil pH and particle size distribution showed no significant correlation with the evaluated mobility and resupply parameters. The DGT and DET techniques, along with the DIFS model, were considered accurate and practical tools for studying parameters related to soil P desorption kinetics.

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 −64mm and −44mm, respectively. The sediments with bioturbation had a smaller distribution coefficient than the control (1964 vs. 3010 cm3g−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. 18,670s) and a much higher rate (0.192 vs. 0.002day−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 −46mm, 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.