Biogeochemical processes in the active layer and permafrost of a high Arctic fjord valley

Warming of ground is causing microbial decomposition of previously frozen sedimentary organic carbon in Arctic permafrost. However, the heterogeneity of the permafrost landscape and its hydrological processes result in different biogeochemical processes across relatively small scales, with implications for predicting the timing and magnitude of permafrost carbon emissions. The biogeochemical processes of iron- and sulfate-reduction produce carbon dioxide and suppress methanogenesis. Hence, in this study, the biogeochemical processes occurring in the active layer and permafrost of a high Arctic fjord valley in Svalbard are identified from the geochemical and stable isotope analysis of aqueous and particulate fractions in sediment cores collected from ice-wedge polygons with contrasting water content. In the drier polygons, only a small concentration of organic carbon (<5.40 dry weight%) has accumulated. Sediment cores from these drier polygons have aqueous and solid phase chemistries that imply sulfide oxidation coupled to carbonate and silicate dissolution, leading to high concentrations of aqueous iron and sulfate in the pore water profiles. These results are corroborated by δ34S and δ18O values of sulfate in active layer pore waters, which indicate the oxidative weathering of sedimentary pyrite utilising either oxygen or ferric iron as oxidising agents. Conversely, in the sediments of the consistently water-saturated polygons, which contain a high content of organic carbon (up to 45 dry weight%), the formation of pyrite and siderite occurred via the reduction of iron and sulfate. δ34S and δ18O values of sulfate in active layer pore waters from these water-saturated polygons display a strong positive correlation (R2 = 0.98), supporting the importance of sulfate reduction in removing sulfate from the pore water. The significant contrast in the dominant biogeochemical processes between the water-saturated and drier polygons indicates that small-scale hydrological variability between polygons induces large differences in the concentration of organic carbon and in the cycling of iron and sulfur, with ramifications for the decomposition pathway of organic carbon in permafrost environments

Pathways of degradation in rangelands in Northern Tanzania show their loss of resistance, but potential for recovery

Semiarid rangelands are identified as at high risk of degradation due to anthropogenic pressure and climate change. Through tracking timelines of degradation we aimed to identify whether degradation results from a loss of resistance to environmental shocks, or loss of recovery, both of which are important prerequisites for restoration. Here we combined extensive field surveys with remote sensing data to explore whether long-term changes in grazing potential demonstrate loss of resistance (ability to maintain function despite pressure) or loss of recovery (ability to recover following shocks). To monitor degradation, we created a bare ground index: a measure of grazeable vegetation cover visible in satellite imagery, allowing for machine learning based image classification. We found that locations that ended up the most degraded tended to decline in condition more during years of widespread degradation but maintained their recovery potential. These results suggest that resilience in rangelands is lost through declines in resistance, rather than loss of recovery potential. We show that the long-term rate of degradation correlates negatively with rainfall and positively with human population and livestock density, and conclude that sensitive land and grazing management could enable restoration of degraded landscapes, given their retained ability to recover

Virus-induced changes in root volatiles attract soil nematode vectors to infected plants

Plant-derived volatiles mediate interactions among plants, pathogenic viruses, and viral vectors. These volatile-dependent mechanisms have not been previously demonstrated belowground, despite their likely significant role in soil ecology and agricultural pest impacts. We investigated how the plant virus, tobacco rattle virus (TRV), attracts soil nematode vectors to infected plants. We infected Nicotiana benthamiana with TRV and compared root growth relative to that of uninfected plants. We tested whether TRV-infected N. benthamiana was more attractive to nematodes 7 d post infection and identified a compound critical to attraction. We also infected N. benthamiana with mutated TRV strains to identify virus genes involved in vector nematode attraction. Virus titre and associated impacts on root morphology were greatest 7 d post infection. Tobacco rattle virus infection enhanced 2-ethyl-1-hexanol production. Nematode chemotaxis and 2-ethyl-1-hexanol production correlated strongly with viral load. Uninfected plants were more attractive to nematodes after the addition of 2-ethyl-1-hexanol than were untreated plants. Mutation of TRV RNA2-encoded genes reduced the production of 2-ethyl-1-hexanol and nematode attraction. For the first time, this demonstrates that virus-driven alterations in root volatile emissions lead to increased chemotaxis of the virus's nematode vector, a finding with implications for sustainable management of both nematodes and viral pathogens in agricultural systems

Biogeochemistry of low‐ and high‐centered ice‐wedge polygons in wetlands in Svalbard

Arctic wetlands are a globally significant store of soil organic carbon. They are often characterized by ice-wedge polygons, which are diagnostic of lowland permafrost, and which greatly influence wetland hydrology and biogeochemistry during summer. The degradation of ice-wedge polygons, which can occur in response to climate change or local disturbance, has poorly understood consequences for biogeochemical processes. We therefore used geochemical analyses from the active layer and top permafrost to identify and compare the dominant biogeochemical processes in high-centered (degraded) and low-centered (pristine) polygons situated in the raised beach sediments and valley-infill sediments of Adventdalen, Central Svalbard. We found similar organic-rich sediments in both cases (up to 38 dry wt.%), but while low-centered polygons were water-saturated, their high-centered counterparts had a relatively dry active layer. Consequently, low-centered polygons showed evidence of iron and sulfate reduction leading to the precipitation of pyrite and siderite, whilst the high-centered polygons demonstrated more oxidizing conditions, with decreased iron oxidation and low preservation of iron and sulfate reduction products in the sediments. This study thus demonstrates the profound effect of ice-wedge polygon degradation on the redox chemistry of the host sediment and porewater, namely more oxidizing conditions, a decrease in iron reduction, and a decrease in the preservation of iron and sulfate reduction products

Mass Spectrometry

International audienceFor twenty years or so now, mass spectrometry has been used to get exact measurements of the mass of biological molecules such as proteins, nucleic acids,oligosaccharides, and so on. Over the past ten years, this technology has followed the trend toward miniaturisation and the samples required can be much smaller. In particular, the nanoelectrospray source (online or by needle) allow one to work at flow rates of a few tens of nanolitres/min. There are many applications, both in the field of proteomics and in the analysis of protein structure, dynamics, and interactions. Combining this source with nanoHPLC, complex mixtures only available in small quantities can be separated and analysed online. There are also some advantages over conventional HPLC, despite a set of constraints related to the small dimensions and low flow rates. Combining capillary electrophoresis with the electrospray source also gives useful results, with its own set of advantages and constraints. Finally, developments are currently underway to combine this source with chips, providing a means of separation and analysis online