Copper contamination affects the biogeochemical cycling of nitrogen in freshwater sediment mesocosms

Trace elements can have a wide variety of effects on microbial populations and their function in the aquatic environment. However, specific impacts on chemical and biological processes are often difficult to unravel, due to the wide variety of chemical species involved and interactions between different elemental cycles. A replicated mesocosm experiment was used to test the effect of increasing copper concentrations, i.e., from 6 mg kg‾¹ to 30 and 120 mg kg‾¹, on nitrogen cycling in a freshwater sediment under laboratory conditions. Nitrous oxide emissions from the treated sediments were measured over three consecutive 24 h periods. This was followed by measurements of iron, manganese, copper and mineral nitrogen species (nitrate and ammonium) mobilisation in the sediments using the diffusive gradients in thin films (DGT) and diffusive equilibria in thin films (DET) techniques and sequential extractions. Increasing copper concentrations are shown to have resulted in significantly reduced nitrate formation near the sediment–water interface and increased nitrous oxide emissions from the sediment overall. The concomitant mobilisation and sequestration of iron with ammonium in the sediment with the highest Cu treatment strongly imply links between the biogeochemical cycles of the two elements. Modest Cu contamination was shown to affect the nitrogen cycle in the tested freshwater sediment, which suggests that even relatively small loads of the metal in fresh watercourses can exert an influence on nutrient loads and greenhouse gas emissions from these environments

Temporal Changes in Cd Sorption and Plant Bioavailability in Compost-Amended Soils

The application of Cd-contaminated phosphate fertiliser has enriched concentrations of this non-essential element in many agricultural soils. Consequently, concentrations of the metal in some agricultural products exceed the Maximum Limit in foods. Composts can reduce the transfer of Cd from soil to plants; however, it is unclear how long this beneficial effect endures. We aimed to determine temporal changes of phytoavailable Cd in two market garden soils (an Allophanic Orthic Granular Soil and a Recent Silt Loam). Soils were amended with either municipal green waste compost or sawdust and animal waste compost at a rate of 2.5% w/w under three incubation regimes: at 19 °C, at 30 °C, and at 30 °C with additional N added as urea at 0.6 g urea/kg soil added over 1 year. Each replicate was sampled after 1, 5, 9, 13, 21, 31, and 49 weeks, and phytoavailable Cd was estimated through 0.05 M Ca(NO3)2 extraction. Seed potato (Solanum tuberosum), ‘Nadine’ variety, was grown in the Pukekohe Allophanic Orthic Granular Soil, freshly amended with municipal compost and the same soil aged for one year. The concentration of Cd in all samples was analysed using an ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometer). The C concentration in the soil—compost mixtures decreased over the year, with the greatest decreases occurring in the soils incubated at 30 °C with added N. Unexpectedly, the concentration of Ca(NO3)2-extractable Cd in the compost-amended soils did not increase over time and in some cases even decreased. This was confirmed through a pot experiment, which showed the Cd concentration in potato was reduced by 50% in both the freshly amended soil and the amended soil aged for one year. Cadmium immobilisation in soils might be due to both the sorption of Cd by organic matter and the occlusion of sorbed Cd by oxy-hydroxides of iron and aluminium. Over 49 weeks, soluble Cd does not increase as organic matter oxidises. The application of municipal compost to soil will reduce both plant Cd solubility and plant Cd uptake for at least one year in the soils tested

Heavy metals in suburban gardens and the implications of land-use change following a major earthquake

Numerous studies have shown that urban soils can contain elevated concentrations of heavy metals (HMs). Christchurch, New Zealand, is a relatively young city (150 years old) with a population of 390,000. Most soils in Christchurch are sub-urban, with food production in residential gardens a popular activity. Earthquakes in 2010 and 2011 have resulted in the re-zoning of 630 ha of Christchurch, with suggestions that some of this land could be used for community gardens. We aimed to determine the HM concentrations in a selection of suburban gardens in Christchurch as well as in soils identified as being at risk of HM contamination due to hazardous former land uses or nearby activities. Heavy metal concentrations in suburban Christchurch garden soils were higher than normal background soil concentrations. Some 46% of the urban garden samples had Pb concentrations higher than the residential land use national standard of 210 mg kg⁻¹, with the most contaminated soil containing 2615 mg kg⁻¹ Pb. Concentrations of As and Zn exceeded the residential land use national standards (20 mg kg⁻¹ As and 400 mg kg⁻¹ Zn) in 20% of the soils. Older neighbourhoods had significantly higher soil HM concentrations than younger neighbourhoods. Neighbourhoods developed pre-1950s had a mean Pb concentration of 282 mg kg⁻¹ in their garden soils. Soil HM concentrations should be key criteria when determining the future land use of former residential areas that have been demolished because of the earthquakes in 2010 and 2011. Redeveloping these areas as parklands or forests would result in less human HM exposure than agriculture or community gardens where food is produced and bare soil is exposed

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

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 < 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

Temperature effects on dislocation core energies in silicon and germanium

Temperature effects on the energetics of the 90-degree partial dislocation in silicon and germanium are investigated, using non-equilibrium methods to estimate free energies, coupled with Monte Carlo simulations. Atomic interactions are described by Tersoff and EDIP interatomic potentials. Our results indicate that the vibrational entropy has the effect of increasing the difference in free energy between the two possible reconstructions of the 90-degree partial, namely, the single-period and the double-period geometries. This effect further increases the energetic stability of the double-period reconstruction at high temperatures. The results also indicate that anharmonic effects may play an important role in determining the structural properties of these defects in the high-temperature regime.Comment: 8 pages in two-column physical-review format with six figure

Copper contamination affects the biogeochemical cycling of nitrogen in freshwater sediment mesocosms

The effects of trace elements on microbial populations and function can be wide-ranging and difficult to disentangle, hence reports of specific impacts on processes in elemental biogeochemical cycles are rare. This study used a replicated laboratory mesocosm experiment that simulated a flowing watercourse to test the effect of increasing copper concentrations from 6 mg/kg to 30 and 120 mg/kg on nitrogen cycling in a freshwater sediment. After a seven-week equilibration period, the release of nitrous oxide from the treated sediments was measured over three consecutive 24 h periods. This was followed by measurements of copper (Cu), iron, manganese and nitrogen species (nitrate and ammonium) mobilisation in the sediments using diffusive gradients in thin films (DGT), diffusive equilibrium in thin-films (DET) and sequential extractions. Copper is shown to have resulted in significantly reduced nitrate formation near the sediment-water interface and increased nitrous oxide emissions. The concomitant mobilisation and sequestration of iron with ammonium in the sediment with the highest Cu treatment strongly implies links between the biogeochemical cycles of the two elements. This research concludes that modest Cu contamination can affect the nitrogen cycle in freshwater sediments, which may have further implications on water quality and greenhouse gas emissions from freshwater environments

Theoretical comparison of how soil processes affect uptake of metals by diffusive gradients in thinfilms and plants.

The theoretical basis for using measurements of metal uptake by the technique of diffusive gradients in thinfilms (DGT) to mimic processes in soils that affect uptake of metals by plants is examined. The uptake of metals by plants and DGT were compared conceptually and quantitatively by using the classic Barber model of plant uptake and the DIFS (DGT-induced fluxes in soils) model of uptake by DGT. For most metals and plants considered, uptake fluxes were similar to those induced by DGT using the most common gel layer thicknesses of 0.2 to 2mm. Consequently DGT perturbs the chemical equilibrium of metals in the soil solution and between soil solution and solid phase, to a similar extent to plants, and therefore induces a similar balance in supply by diffusion and by release from the solid phase. DIFS was used to show that desorption kinetics, which are not considered by the plant uptake model, are likely important for uptake when the capacity of the soil solid phase is large. Model calculations showed that mass flow into a plant root would only contribute appreciably to the total flux of metal under circumstances when the solid phase reservoir of metal was very low. Generally, however, DGT is likely to emulate supply processes from the soil that govern uptake of metal by plants. Exceptions are likely to be found in poorly buffered soils (typically sandy and/or low pH), and at very high concentrations of metals in soil solution, such that the soil solution concentration at the plant root interface is higher than the Michaelis-Menten constant (Km). Abbreviations: DGT, diffusive gradients in thinfilms • DIFS, DGT-induced fluxes in soils • Kd, solid-solution phase partitioning coefficient • Tc, time needed for the partitioning components of Kd to reach 63% of their equilibrium values, assuming the solution concentration is initially zero • Km, Michaelis-Menten constant • Imax, maximum flux of ions into a root • CE, effective concentratio

An evaluation of DGT performance using a dynamic numerical model.

A numerical model of the transport and dynamics of metal complexes in the resin and gel layers of a DGT (diffusive gradients in thin films) device was developed and used to investigate how the chelating resin and metal-ligand complexes in solution affect metal uptake. Decreasing the stability constant or concentration of the binding resin increases the competition for free metal ions by ligands in solution, lowering the rate of mass uptake. Such effects would be rarely observed for moderately or strongly binding resins (K > 1012), including Chelex, which out-compete labile ligands in solution. With weakly binding resins, strongly bound solution complexes can diffuse into the resin layer before a measurable amount of dissociation occurs, such that concentrations of bound metal at the rear and front surfaces of the resin layer are equal. With more strongly binding resins, metal mainly binds to the front surface of the resin. Only complexes with the largest binding constants penetrate the gel layer containing Chelex, but their lack of lability means that the DGT sensitivity to the complex is, in any case, very low. The slow diffusion of complexes, such as those of fulvic acids, which increases the time required to establish steady state, compromises the use of the simple DGT equation. Errors are negligible for 24 h deployments, when diffusive layer thicknesses are less than 1 mm, but 3 day deployments are required to ensure accuracy with 2.4 mm thick layers. The extent to which the commonly used equation, that accounts for the concentration and diffusion of metal-complex species, overestimates DGT uptake if the rate of dissociation is slow, was estimated

Environmental and edaphic factors affecting soil cadmium uptake by spinach, potatoes, onion and wheat

The relative ease with which cadmium (Cd) in agricultural soils can transfer to crop plants can pose a potential health risk to consumers. However, efforts to predict and mitigate these risks are often confounded by the various factors that influence metal accumulation in the edible plant parts. The aim of this work was to identify key drivers that determine Cd concentrations in spinach leaves, potato tubers, onion bulbs and wheat grain grown in commercial horticultural operations across New Zealand (NZ). Paired soil and plant samples (n = 147) were collected from farms across different NZ growing regions. Cadmium concentrations in the edible parts were measured and four different tests were used to examine the potential bioavailability of soil Cd: pseudo-total and porewater concentrations, 0.05 M Ca(NO₃)₂-extraction and diffusive gradients in thin-films (DGT). Information on a range of soil and climatic variables was also collected. The methods' ability to represent Cd concentrations in the plant parts was assessed through single and multiple regression analysis that considered the different variables and the farm locations. Soil Cd concentrations determined by the different tests were positively related to plant concentrations and there were clear regional differences between these relationships. The Ca(NO₃)₂ extraction predicted over 76% of the variability in Cd concentrations in onion bulbs and spinach leaves, while DGT and porewater Cd provided the best estimates for potato tubers and wheat grains, respectively, once regional differences were considered, along with certain environmental and soil variables. The results show that certain soil and environmental factors can be a key influence for determining Cd accumulation in the edible parts of some plants and that regional differences are important for modulating the extent to which this occurs. These effects should be considered when trying to mitigate the potential risks arising from Cd in agricultural soils

How many samples? Soil variability affects confidence in the use of common agroecosystem soil indicators

There is a need for accurate and easily-measured indicators suitable for characterising and monitoring agroecosystem multi-functionality. This is particularly true in intensively-farmed landscapes where it is of interest to quantify the role of small, woody vegetation features in providing ecosystem services such as carbon sequestration. However, soil variability introduced by natural and management processes can interact with sampling designs to result in inappropriate sampling intensities and high levels of uncertainty in measured indicators. This can have consequences for upscaling of ecosystem quantities and decision making. Here, we present results from a pilot study aimed at quantifying and understanding variation in ten common indicators of soil condition and function, within shelterbelt and adjacent field soils, at four dryland sheep farms in Canterbury, New Zealand. Our results demonstrate a high level of spatially-structured soil variability, driven by (1) the effects of woody vegetation on shelterbelt soils relative to field soils, (2) differences in underlying soil types among sites, and (3) possible effects of grazing animals within fields. This soil variability had clear knock-on impacts for appropriate sampling effort, depending on the soil indicator in question, the original soil sampling density, and whether the aim was to estimate population mean values or to detect differences among sites with confidence. On the whole, confidence in soil indicator estimates was highest for soil condition indicators (pH, soil moisture, bulk density), variable for carbon quantities, depending on the measure used, and lowest for soil biological process indicators (tea bag index decomposition rate, bait lamina probe micro-invertebrate activity, and dehydrogenase enzyme activity); estimation confidence was also mostly lower for shelterbelt soils due to the effect of woody roots and inputs on soil variability. Based on our results, we present indicative sample size requirements to estimate population means for these different soil indicators. Ultimately, we advocate for the use of pilot studies, such as the one presented here, to facilitate understanding of variability in soil function indicators within different agroecosystems, and how this variability is partitioned spatially within and among vegetated features