Mobilization of Zn upon waterlogging riparian Spodosols is related to reductive dissolution of Fe minerals

Contaminated riparian soils can release metals to surface water. Periodic waterlogging affects metal mobility but the processes and soil factors governing net trends are not well understood. Experiments were combined with geochemical modelling to identify processes explaining the dynamics of zinc (Zn) in contaminated soils following waterlogging. Samples were collected from 12 Spodosols near streams in a metal‐contaminated area and four similar but uncontaminated soils were sampled in a reference area. Air‐dried samples were submerged and incubated under N₂. The soil redox potential decreased from 470 mV initially to approximately 30 mV over 2 months. The pore‐water Zn concentrations surprisingly increased over the same period by, on average, a factor of 18 (range 0.6–80; immobilization in one soil only) despite an increase in pH of 1.8 units, on average. Dissolved organic matter (DOM) in the soil solution increased during waterlogging but the observed increase in Zn solubility could not be explained by increased complexation with DOM, because the estimated Zn²⁺ activity also increased by a factor of 18 on average (range 0.2–82). Speciation modelling suggests that Zn mobilization during waterlogging results from Fe²⁺ displacing sorbed Zn²⁺ from particulate organic matter and from dissolution of Zn‐bearing Fe/Mn oxyhydroxides. This hypothesis is supported by the significant positive correlation (r = 0.87, n = 13) between the factor change in pore‐water Zn concentration and the ratio of dithionite‐extractable Fe to organic carbon content. These results show that Fe dynamics are important for predicting the fate of trace metals in anoxic soils.     

Grape contribution to wine aroma: production of hexyl acetate, octyl acetate, and benzyl acetate during yeast fermentation is dependent upon precursors in the must

Wine is a complex consumer product produced predominately by the action of yeast upon grape juice musts. Model must systems have proven ideal for studies of the effects of fermentation conditions on the production of certain wine volatiles. To identify grape-derived precursors to acetate esters, model fermentation systems were developed by spiking precursors into model must at different concentrations. Solid-phase microextraction-gas chromatgraphy mass spectrometry analysis of the fermented wines showed that a variety of grape-derived aliphatic alcohols and aldehydes are precursors to acetate esters. The C6 compounds hexan-1-ol, hexenal, (E)-2-hexen-1-ol, and (E)-2-hexenal are all precursors to hexyl acetate, and octanol and benzyl alcohol are precursors to octyl acetate and benzyl acetate, respectively. In these cases, the postfermentation concentration of an acetate ester increased proportionally with the prefermentation concentration of the respective precursor in the model must. Determining viticultural or winemaking methods to alter the prefermentation concentration of precursor compounds or change the precursor-to-acetate ester ratio will have implications upon the final flavor and aroma of wines.     

Response of shoot and root growth to supply of different nitrogen forms is not related to carbohydrate and nitrogen status of tobacco plants

In the present study, we investigated effects of homogeneous or localized supply of different nitrogen (N) forms on shoot and root growth of tobacco. While homogeneous supply of NH₄⁺ and N deprivation inhibited shoot growth compared with application of NO₃^—, the N form had no significant effect on root growth. In contrast, in a split-root experiment, application of NH₄⁺ or N deprivation in one half of the root system repressed root growth compared with the other part of the root, which was supplied with NO₃^—. However, shoot growth was not affected by localized NH₄⁺ application or local N deprivation. Inhibitory effects on shoot and root growth by variations of N supply could not be related to limitations in N or C status of the plants or to NH₄⁺ toxicity. A possible involvement of NO₃^— as a signal compound including of phytohormones is discussed.