Carsten Brackhage

Metal/metalloid accumulation/remobilization during aquatic litter decomposition in freshwater: A review

The focus of this article is to combine two main areas of research activities in freshwater ecosystems: the effect of inorganic pollutants on freshwater ecosystems and litter decomposition as a fundamental ecological process in streams. The decomposition of plant litter in aquatic systems as a main energy source in running water ecosystems proceeds in three distinct temporal stages of leaching, conditioning and fragmentation. During these stages metals and metalloids may be fixed by litter, its decay products and the associated organisms. The global-scale problem of contaminated freshwater ecosystems by metals and metalloids has led to many investigations on the acute and chronic toxicity of these elements to plants and animals as well as the impact on animal activity under laboratory conditions. Where sorption properties and accumulation/remobilization potential of metals in sediments and attached microorganisms are quite well understood, the combination of both research areas concerning the impact of higher trophic levels on the modification of sediment sorption conditions and the influence of metal/metalloid pollution on decomposition of plant litter mediated by decomposer community, as well as the effect of high metal load during litter decay on organism health under field conditions, has still to be elucidated. So far it was found that microbes and invertebrate shredder (species of the genera Gammarus and Asellus) have a significant influence on metal fixation on litter. Not many studies focus on the impact of other functional groups affecting litter decay (e.g. grazer and collectors) or other main processes in freshwater ecosystems like bioturbation (e.g. Tubifex, Chironomus) on metal fixation/release.

Silicon supply modifies C:N:P stoichiometry and growth of Phragmites australis

Silicon is a non-essential element for plant growth. Nevertheless, it affects plant stress resistance and in some plants, such as grasses, it may substitute carbon (C) compounds in cell walls, thereby influencing C allocation patterns and biomass production. How variation in silicon supply over a narrow range affects nitrogen (N) and phosphorus (P) uptake by plants has also been investigated in some detail. However, little is known about effects on the stoichiometric relationships between C, N and P when silicon supply varies over a broader range. Here, we assessed the effect of silicon on aboveground biomass production and C:N:P stoichiometry of common reed, Phragmites australis, in a pot experiment in which three widely differing levels of silicon were supplied. Scanning electron microscopy (SEM) showed that elevated silicon supply promoted silica deposition in the epidermis of Phragmites leaves. This resulted in altered N:P ratios, whereas C:N ratios changed only slightly. Plant growth was slightly (but not significantly) enhanced at intermediate silicon supply levels but significantly decreased at high levels. These findings point to the potential of silicon to impact plant growth and elemental stoichiometry and, by extension, to affect biogeochemical cycles in ecosystems dominated by Phragmites and other grasses and sedges.

Silica uptake from nanoparticles and silica condensation state in different tissues of Phragmites australis

Silicon is described as beneficial for grasses by enhancing yield and fitness via a considerable contribution to pathogen, drought, and pest resistance. Silicic acid is the predominant form for uptake and transport within the plant and will precipitate in leaves. But it is unknown whether polymeric nanosilicon compounds in its synthetic form, with an increasing concentration in aquatic environments, can be suitable for plant nutrition. Therefore, we investigated the uptake, transport, and deposition of silicic acid/silica within plants using synthetic nanosilica. Our results show a significant difference in silicon (Si) content within the different tissues of Phragmites australis. The nanosilica had been dissolved prior to the uptake by plants. The chemical form of Si during uptake was not traceable. A significant enhancement in the condensation state of the silica was found from root to leaves especially from culm to leaf tips visible by the increasing content of Q(4)-groups in the NMR spectra. We conclude that synthetic nanosilica has the same quality as source for the beneficial element Si like natural silica. Since the condensation state is described to control silica solubility, we suggest that different condensation states within the plant may result in different remobilization of silicon during decomposition of the plant material.

Silicon Availability Affects the Stoichiometry and Content of Calcium and Micro Nutrients in the Leaves of Common Reed

PurposeAlthough silicon is not an essential element in sensu stricto for plant growth, it affects plant stress resistance and may affect the composition of cell wall compounds, especially of grasses. Where silicon availability alters the stoichiometry of macro nutrients in grasses, data on the interaction with calcium and micro nutrients are rare and hence are focussed on in this study.MethodsThe effect of silicon availability on calcium and micro nutrient content of the leaf blades of common reed, Phragmites australis, were assessed in a pot experiment with three levels of silicon supply.ResultsCalcium and micro nutrient concentrations and stoichiometry in leaf blades is altered by changing silicon availability during plant growth. In addition, Scanning Electron Microscopy (SEM) reveals that elevated silicon supply promotes silica deposition and changes the element content of micro and macro nutrients in the near epidermis tissue of P. australis leaves.ConclusionSilicon availability has a major impact on calcium and micro nutrient content and stoichiometry in grasses. This in turn may considerably affect the nutrient cycling in grass dominated ecosystems.

UV-screening of grasses by plant silica layer?

UV-screening by terrestrial plants is a crucial trait since colonization of terrestrial environments has started. In general, it is enabled by phenolic substances. Especially for grasses it remains unclear why plants grown under the absence of UV-B-radiation exhibit nonetheless a high UV-B-screening potential. But this may be explained by the UV-screening effect of the silicon double layer. It was shown for seedlings of soybeans (Glycine max L.) and wheat (Triticum aestivum L.) that enhanced silicon supply reduces stress induced by UV-radiation. Even more important is a direct correlation between silicon content in the epidermis near area (intercellular spaces) and the absorption of UV-radiation in this area shown in other papers. The silicon double layer may act like a glass layer and decreases the transmission of UV-radiation at the epidermis near area. In summary, the absorbance/reflection of ultraviolet radiation is dependent on the characteristics of the epidermis near area of leaves, particularly the occurrence (qualitatively and quantitatively) of phenolic substances and/or a silicon double layer in this area. Consequently, UV-screening by plant silicon double layer should get more attention in future research with emphasis on effects of UV-radiation on plant physiology.

Readily available phosphorous and nitrogen counteract for arsenic uptake and distribution in wheat ( Triticum aestivum L.)

Elevated arsenic content in food crops pose a serious human health risk. Apart from rice wheat being another main food crop is possibly cultivated on contaminated sites. But for wheat uptake mechanisms are not entirely understood especially with regard to nutrient fertilization and different moisture regimes taking into account heavy rainfall events due to climate change. Here we show that especially higher P-fertilization under changing redox conditions may enhance arsenic uptake. This counteracts with higher N-fertilization reducing arsenic transfer and translocation into aboveground plant parts for both higher P-fertilization and reducing soil conditions. Arsenic speciation did not change in grain but for leaves P-fertilization together with reducing conditions increased the As(V) content compared to other arsenic species. Our results indicate important dependencies of nutrient fertilization, moisture conditions and substrate type on As accumulation of wheat as one of the most important crop plants worldwide with implications for agricultural practices.

Silica decouples fungal growth and litter decomposition without changing responses to climate warming and N enrichment

Ongoing global changes, such as climate warming and increasing supply of reactive nitrogen (N), are expected to affect essential ecosystem processes such as the decomposition of plant litter. Determining the influence of environmental heterogeneity on the magnitude of these effects remains an important task, with silicon (Si) availability being a notable component of this heterogeneity, especially for grasses. We conducted an outdoor enclosure experiment to test if increased Si supply to a widespread foundation species (Phragmites australis) alters the effect of climate warming and excess N supply on litter decomposition by curbing fungal decomposers. Consistent with expectations, Si supply during plant growth reduced fungal biomass in decomposing leaf blades by 50%, an effect that was doubled by excess external N supply. These strong impacts, however, did not directly translate to reduced litter decomposition or associated changes in nutrient dynamics. Instead, plant tissue-specific effects determined th...

Spectral changes in the leaves of barley plant due to phytoremediation of metals—results from a pot study

Abstract This research studied the changes in leaf reflectance spectra (350–2500 nm) due to metal phytoextraction into barley plants grown in metal-spiked soils (3 levels of Cd, Pb, As and their metal-mixture treatments). Growth of barley was adversely affected due to 100 mg As kg-1 and metal-mixture (10 Cd+150 Pb+100 As; mg kg-1) treatments. Metal phytoextraction were in order of: root>straw≥leaves >grains. Results of reflectance spectra of leaves show the influence of As-treatment only, causing spectral changes in visible and infrared domains mostly, as apparent from the significant correlation between leaf-As and leaf-spectra. Chlorophyll and water stress indices and band depths analyses showed significant correlations to leaf-As, and can be used to distinguish metal-stressed plants. Finally, regression models demonstrate the potential use of hyperspectral reflectance data to monitor plant health during phytoremediation process and to estimate leaf-As in barley, particularly in this study.

Limited transfer of uranium to higher trophic levels by Gammarus pulex L. in contaminated environments

In contrast to the classification of most invertebrate shredders being sensitive to uranium, a G. pulex L. population with reproduction was found in a stream at a former uranium mining site with uranium concentrations of 150 microg l(-1) in water and up to 2000 mg kg(-1) DW(-1) (dry weight) in litter born organic sediments. The survival of G. pulex, collected from a site without uranium contamination, was tested in a laboratory microcosm experiment using synthetic uranium-contaminated water and uranium-contaminated but nutrient rich food, simulating physicochemical conditions of water from former uranium mining sites. The results reveal that there are no significant differences in survival rate between individuals exposed and those not exposed to uranium. The uptake of uranium by G. pulex in environments with concentrations in food of 1152 mg kg(-1) in DM (dry mass, organically bound) and in water of 63.9 microg L(-1) is very low (4.48(1.93-8.46) mg kg(-1) in DM). The accumulation of uranium in these invertebrates was verified to be via two pathways: body surface and food. A relevant amount of uranium adsorbs to the body surface where it can readily be desorbed.

Long-term differences in transfer and accumulation of potentially toxic trace elements and radionuclides in trees on uranium mining dumps (Erzgebirge, Germany)

Seasonal and long-term variation of radionuclide and heavy metal content in different compartments of trees were investigated on uranium mining dumps in the Erzgebirge, Germany. Seasonal increase in concentration could be substantiated partially for lead and uranium in leaves/needles but for no other element and tree part although variation was high. A major fraction of variation was attributed to the micro-site within each sampling plot. Long-term monitoring comprising a 5-year period showed no differences in radionuclide and heavy metal content.