Temporal changes in photoreactivity of dissolved organic carbon and implications for aquatic carbon fluxes from peatlands

Aquatic systems draining peatland catchments receive a high loading of dissolved organic carbon (DOC) from the surrounding terrestrial environment. Whilst photo-processing is known to be an important process in the transformation of aquatic DOC, the drivers of temporal variability in this pathway are less well understood. In this study, 8 h laboratory irradiation experiments were conducted on water samples collected from two contrasting peatland aquatic systems in Scotland: a peatland stream and a reservoir in a catchment with high percentage peat cover. Samples were collected monthly at both sites from May 2014 to May 2015 and from the stream system during two rainfall events. DOC concentrations, absorbance properties and fluorescence characteristics were measured to investigate characteristics of the photochemically labile fraction of DOC. CO2 and CO produced by irradiation were also measured to determine gaseous photoproduction and intrinsic sample photoreactivity. Significant variation was seen in the photoreactivity of DOC between the two systems, with total irradiation-induced changes typically 2 orders of magnitude greater at the high-DOC stream site. This is attributed to longer water residence times in the reservoir rendering a higher proportion of the DOC recalcitrant to photo-processing. During the experimental irradiation, 7 % of DOC in the stream water samples was photochemically reactive and direct conversion to CO2 accounted for 46 % of the measured DOC loss. Rainfall events were identified as important in replenishing photoreactive material in the stream, with lignin phenol data indicating mobilisation of fresh DOC derived from woody vegetation in the upper catchment. This study shows that peatland catchments produce significant volumes of aromatic DOC and that photoreactivity of this DOC is greatest in headwater streams; however, an improved understanding of water residence times and DOC input–output along the source to sea aquatic pathway is required to determine the fate of peatland carbon

Soil bacteria override speciation effects on zinc phytotoxicity in zinc-contaminated soils

The effects of zinc (Zn) speciation on plant growth in Zn-contaminated soil in the presence of bacteria are unknown but are critical to our understanding of metal biodynamics in the rhizosphere where bacteria are abundant. A 6-week pot experiment investigated the effects of two plant growth promoting bacteria (PGPB), <i>Rhizobium leguminosarum</i> and <i>Pseudomonas brassicacearum</i>, on Zn accumulation and speciation in <i>Brassica juncea</i> grown in soil amended with 600 mg kg^–1 elemental Zn as three Zn species: soluble ZnSO₄ and nanoparticles of ZnO and ZnS. Measures of plant growth were higher across all Zn treatments inoculated with PGPB compared to uninoculated controls, but Zn species effects were not significant. Transmission electron microscopy identified dense particles in the epidermis and intracellular spaces in roots, suggesting Zn uptake in both dissolved and particulate forms. X-ray absorption near-edge structure (XANES) analysis of roots revealed differences in Zn speciation between treatments. Uninoculated plants exposed to ZnSO₄ contained Zn predominantly in the form of Zn phytate (35%) and Zn polygalacturonate (30%), whereas Zn cysteine (57%) and Zn polygalacturonate (37%) dominated in roots exposed to ZnO nanoparticles. Inoculation with PGPB increased (>50%) the proportion of Zn cysteine under all Zn treatments, suggesting Zn coordination with cysteine as the predominant mechanism of Zn toxicity reduction by PGPB. Using this approach, we show, for the first time, that although speciation is important, the presence of rhizospheric bacteria completely overrides speciation effects such that most of the Zn in plant tissue exists as complexes other than the original form

Mixed planting with a leguminous plant outperforms bacteria in promoting growth of a metal remediating plant through histidine synthesis

<p>The effectiveness of plant growth promoting bacteria (PGPB) in improving metal phytoremediation is still limited by stunted plant growth under high soil metal concentrations. Meanwhile, mixed planting with leguminous plants is known to improve yield in nutrient deficient soils but the use of a metal tolerant legume to enhance metal tolerance of a phytoremediator has not been explored. We compared the use of <i>Pseudomonas brassicacearum, Rhizobium leguminosarum</i>, and the metal tolerant leguminous plant<i> Vicia sativa</i> to promote the growth of <i>Brassica juncea</i> in soil contaminated with 400 mg Zn kg^–1, and used synchrotron based microfocus X-ray absorption spectroscopy to probe Zn speciation in plant roots.<i> B. juncea </i>grew better when planted with <i>V. sativa</i> than when inoculated with PGPB. By combining PGPB with mixed planting,<i> B. juncea</i> recovered full growth while also achieving soil remediation efficiency of >75%, the maximum ever demonstrated for <i>B. juncea.</i> μXANES analysis of <i>V. sativa</i> suggested possible root exudation of the Zn chelates histidine and cysteine were responsible for reducing Zn toxicity. We propose the exploration of a legume-assisted-phytoremediation system as a more effective alternative to PGPB for Zn bioremediation.</p

Wind farm development on peatlands increases fluvial macronutrient loading

Wind farms can help to mitigate increasing atmospheric carbon (C) emissions. However, disturbance caused by wind farm development must not have lasting deleterious impacts on landscape C sequestration. To understand the effects of wind farm development on peatlands, we monitored streamwater at Europe’s second largest onshore wind farm (539 MW), Whitelee, Scotland, for 31 months. Using nested catchment sampling to understand impacts on water quality, increasing macronutrient concentrations and exports were associated with wind farm development, particularly forest-felling and borrow pits. Low/poor water quality occurred in small headwater catchments most disturbed by development. At the site exit, dissolved organic C and soluble reactive phosphorus (SRP) concentrations increased during construction, though [SRP] recovery occurred within 2 years. Since C was lost and streamwater quality negatively affected, we propose future good practice measures for wind farm development, including limiting total disturbance within individual catchments and locating borrow pits, where deemed necessary, off site avoiding peatlands