Use Of Diffusive Gradients In Thin-films Research Articles

Plant responses to nitrate and ammonium availability in Australian soils as measured by diffusive gradients in thin-films (DGT) and KCl extraction

Determining soil nitrogen (N) availability is essential in agriculture to minimise over-application, maximise growers’ returns and reduce potential environmental consequences. The present study assesses soil mineral N (nitrate-N and ammonium-N) using the diffusive gradients in thin-films (DGT) technique against the conventional potassium chloride (KCl) extraction. The DGT technique has demonstrated reliable predictability for plant-available P, Cu and Zn. However, the use of DGT to quantify soil N bioavailability is underreported and N measurements made with DGT have not been compared to plant growth responses or N uptake. A pot trial using wheat was performed to determine the suitability of the DGT technique to predict N plant uptake and plant biomass. Four contrasting soil types from South Australia were used, and four rates of N were applied to the soil. DGT devices and KCl extraction were used at sowing to measure soil mineral N. These data were then compared with plant relative yield (YR) and N uptake after harvesting the plants. Soil mineral N, as measured by both the DGT and KCl extraction techniques, demonstrated a significant positive correlation with YR, with an R2 value of 0.6; however, DGT-N extracted comparatively more nitrate (NO3–, >87 % of CN) than KCl-N (65 % of EN). Mineral N and NO3– extracted by both DGT and KCl significantly correlated with plant N uptake albeit this correlation was stronger for KCl (R2 = 0.8) than DGT (R2 = 0.6). The same parameters also positively and significantly correlated with YR, however in this case, both correlations were similar and only modest (R2 < 0.6 in all cases). These results are explained in terms of the differences between the pools of N accessed by these techniques and limitations related to soil N dynamics. In conclusion, KCl showed similar or better predictive ability for N uptake and yield response (YR) in wheat compared to DGT across four Australian soils. Given its low cost and ease of application, KCl presents a competitive advantage in this study.

Evaluation of the diffusive gradients in thin-films (DGT) technique for measuring nitrate and ammonium in soil

Environmental context Nitrate (NO3−) and ammonium (NH4+) are the most important soil nitrogen forms for plant growth. However, conventional extraction techniques may introduce artefacts affecting the measurement of plant-available N concentrations following sampling and sample preparation processes. This is the first study of the DGT technique being used to measure NO3-N and NH4-N in a wide range of soils, compared with conventional KCl extraction, and examined different factors that contribute to the plant-availability of these ions in soils. The knowledge would help to optimise soil nitrogen management practices, increase economic benefits and reduce environmental impacts. Rationale The availability of soil nitrogen for plant uptake can be affected by numerous soil factors such as soil texture, moisture and organic matter content, temperature and microbial activity. Conventional extraction techniques may affect the measurement of plant-available N concentrations following sampling and sample preparation processes, including drying, sieving, homogenising, freezing and thawing. The diffusive gradients in thin-films (DGT) technique can overcome some limitations of the conventional extraction techniques and has been used to successfully estimate the plant-available fractions of nutrients, such as P, K, Zn, Cu and Mn in soils. Therefore, it is important to evaluate the use of DGT for measuring NO3− and NH4+ in a wide variety of soils and examine the factors that contribute to the plant-availability of these ions in soils. Methodology The experiment evaluated the ability of the DGT technique to measure NO3-N and NH4-N in soils using binding layers containing A520E anion exchange resin or Microlite® PrCH cation exchange resin, respectively. The DGT results were compared to those from conventional KCl extraction. Results The A520E- and PrCH-DGTs showed good detection limits for NO3-N (6.90 µg L−1) and NH4-N (6.23 µg L−1) and were able to measure potentially available NO3-N and NH4-N in unfertilised soils. The mass of NO3-N and NH4-N that accumulated on the DGT device increased linearly across soil concentrations ranging from 5 to 300 mg kg−1 NO3-N (depending on soil type) and 5–300 mg kg−1 NH4-N; which is equivalent to fertiliser rates of 75–450 kg ha−1 N. DGTs were used to measure potentially available NO3-N and NH4-N in ten soils with various physical and chemical properties. The DGT results were compared with conventional KCl extraction used to determine soil mineral N. DGT and KCl extraction measured values were significantly correlated with each other for NO3-N (R2 = 0.53; P-value &lt; 0.001), but the relationship between the two measurements was weaker for NH4-N (R2 = 0.20, P-value = 0.045). Discussion The results suggest that the two methods sample different N pools in the soils, with DGT targeting the NO3-N and NH4-N that are available in soil pore water and attached to labile solid phases.

A new approach to improve the accuracy of DGT (Diffusive Gradients in Thin-films) measurements in monitoring wells

The Diffusive Gradients in Thin-films (DGT) technique represents an ideal tool for monitoring water quality of inorganic species in systems with a high flow such as rivers, streams, lakes and seas. However, in low-flow systems (non-turbulent waters), the influence of a diffusive boundary layer (DBL) formed on the surface of the DGT device has been observed, which can lead to erroneous measurements by DGT. Therefore, the use of DGT in wells for groundwater monitoring is still very limited until now.In this sense, the present study evaluates the applicability of the DGT technique in non-turbulent and low-flow water systems. We propose a new way to calculate the DBL with the objective to carry out a robust DGT analysis in environmental monitoring wells. For this purpose, DGT devices with different diffusive gel thicknesses were deployed in an experimental set-up simulating a groundwater monitoring well. A DBL thickness (for each element) was calculated from the slopes of the linear regressions between the DGT accumulated mass of metal and the deployment time (4, 8, 12, 24 and 48 h) for each of the two diffusive gel thicknesses. The mean DBL thickness (averaging the individual DBL thicknesses calculated from the slopes) was 0.06 cm. The concentrations of the analysed elements were corrected with this DBL with the result that the metal concentrations measured by DGT improved and were highly approximated to their actual total values in this non-complexing medium.

Predicting Trace Metal Exposure in Aquatic Ecosystems: Evaluating DGT as a Biomonitoring Tool

Exposure to trace metals is a leading factor associated with the development of non-communicable diseases. Aquatic ecosystems are not only effective agents of trace metal dispersion but are also at risk of contaminant enrichment. The use of sentinel organism as indicators of water quality is one method to evaluate potential human and ecological risk. An alternative or companion approach is to employ chemical techniques that mimic biotic accumulation/responses by aquatic fauna. The Diffusive Gradients-in-Thin-Films (DGT) technique has been widely used in bioavailability studies in water, soil and sediment. Many efforts have been made trying to understand the relationships between DGT labile determinations of trace elements and its biouptake by different organisms. However, the relationship between DGT labile determinations and the biological accumulation/response in aquatic animals is still not clear. The present work aims to improve the understanding of this relationship by summarizing the works using DGT devices and accumulation/response by animals in aquatic media. Several papers studying nineteen different elements in aquatic media are revised and discussed. The papers were separated and discussed in four categories: DGT and Biotic response, DGT and Bioaccumulation, DGT and Biomonitoring, and DGT and Bioturbation. DGT is expected to correlate well with the biological accumulation/responses in kinetically controlled situations. In biomonitoring studies, the use of chemical (DGT) and biological approaches gives complementary and high valuable information. The use of DGT and biological approaches should be considered in future studies about bioavailability and might improve the comprehension about uptake of trace metals by the aquatic fauna.

Use of diffusive gradients in thin-films for studies of chemical speciation and bioavailability

Environmental context The health of aquatic organisms depends on the distribution of the dissolved forms of chemical components (speciation) and their rates of interaction (dynamics). This review documents and explains progress made using the dynamic technique of diffusive gradients in thin-films (DGT) to meet these challenges of measuring directly chemical speciation and associated dynamics in natural waters. The relevance of these measurements to uptake by biota of chemical forms in soils, sediments and water is discussed with reference to this expanding literature. Abstract This review assesses progress in studies of chemical speciation using diffusive gradients in thin-films (DGT) by examining the contributions made by key publications in the last 20 years. The theoretical appreciation of the dynamic solution components measured by DGT has provided an understanding of how DGT measures most metal complexes, but excludes most colloids. These findings strengthen the use of DGT as a monitoring tool and provide a framework for using DGT to obtain in situ kinetic information. Generally, the capabilities of DGT as an in situ perturbation and measurement tool have yet to be fully exploited. Studies that have used DGT to investigate processes relevant to bioavailability have blossomed in the last 10 years, especially for soils, as DGT mimics the diffusion limiting uptake conditions that, under some conditions, characterise uptake by plants. As relationships between element accumulated by DGT and in plants depend on the plant species, soils studied, and the element and its chemical form, DGT is not an infallible predictive tool. Rather its strength comes from providing information on the labile species in the system, whether water, soil or sediment. Recent studies have shown good relationships between measurements of metals in periphyton and by DGT, and unified dose response curves have been obtained for biota in sediments when they are based on DGT measurements. Both these cases suggest that alternative approaches to the established ‘free ion’ approach may be fruitful in these media and illustrate the growing use of DGT to investigate environmental chemical processes.

In situ measurement of solution concentrations and fluxes of sulfonamides and trimethoprim antibiotics in soils using o-DGT

Techniques, such as Diffusive Gradients in Thin-films (DGT), which either minimally disturb the soil or perturb it in a controlled way are most likely to provide information relevant to toxicity. Herein, we report the first use of DGT for organics (o-DGT) in soil systems to gain insight into the mobility and lability of four antibiotics—sulfamethoxazole (SMX), sulfamethazine (SMZ), and sulfadimethoxine (SDM), trimethoprim (TMP) in soil. In experiments where the same known amount of antibiotics were spiked into the soil, which was then further modified with NaOH, NaCl or dissolved organic matter, directly measured soil solution concentrations (Csoln) of these antibiotics were in the order: SMX>SMZ≈SDM>TMP. The R values (ratio of concentrations measured by o-DGT and directly in solution) were 0.56, 0.41, 0.40 and 0.28, respectively, indicating that the removal of these antibiotics from the solution can be to some extent resupplied by release from the solid phase. The nonlinearity of the relationship between o-DGT fluxes and the reciprocal of diffusive layer thickness (Δg) also suggested that soil solution concentrations were only partially sustained by the solid phase. The potential fluxes of these antibiotics in this soil were 5.4, 3.6, 2.4, and 1.2pg/cm2/s for SMX, SMZ, SDM, and TMP, respectively. o-DGT is a promising tool for understanding the fate and behaviour of polar organic chemicals in soil, and it potentially provides an in situ approach for assessing their bioavailability.

In situ trace metal speciation in lake surface waters using DGT, dialysis, and filtration.

In situ measurements of Fe and Mn by dialysis and diffusive gradients in thin-films (DGT) in 5 lakes (pH 4.7-7.5, ionic strength 0.3-5 mmol l(-1)) and Cu and Zn in an acidic and circumneutral lake were compared to results from on site filtration. For the most acidic lake (pH 4.7) all measurements agreed, indicating an absence of colloids and negligible complexation by organic matter. There was little difference in the Mn concentrations measured by the three techniques for any lake, consistent with it being free from complexation. Zn measured by dialysis in circumneutral water was only slightly higher than DGT measurements, appropriate to only partial complexation. Substantial differences between dialysis and DGT for Cu were consistent with complexation by fulvic and humic substances, though not to the extent predicted by the speciation code WHAM. To achieve a good fit it was necessary to adjust the pK for Cu-fulvic binding from 0.8 to 1.3 and to assume that fulvic substances dominated. The presence of low molecular weight strong binding ligands would also be consistent with the data. Differences between the three measurement methods were greatest for Fe, attributable to the presence of large oxyhydroxide colloids, organic complexation and low molecular weight, reactive hydrolysis products. Fe and Mn concentrations measured by DGT on samples returned to the laboratory were much lower than in situ concentrations, illustrating the need for in situ measurements. While use of two in situ techniques provided useful information on the speciation of these natural waters, further refinements are required for unambiguous characterization of the solution. The use of DGT with a more restricted gel that excludes complexes with humic substances should provide complementary information to in situ dialysis.