Microorganisms in the phylloplane modulate the BVOC emissions of Brassica nigra leaves.

Numerous factors can affect the Biogenic Volatile Organic Compounds (BVOC) emitted by plants. One of these factors is the microbial communities living on leaf surfaces (phylloplane). Bacteria and fungi can use compounds produced and emitted by plants for their own metabolism. Thus, microorganism communities can modulate BVOC emissions and affect interactions between plants and other organisms. The aim of this study was to evaluate the role of microbial communities on BVOC emissions of Brassica nigra leaves. Therefore, we removed bacteria and/or fungi by using bactericide/fungicide treatments in a factorial design experiment with Brassica nigra grown in pots. BVOC emissions were sampled before and after the treatment application. Our results showed that four new compounds (cyclohexanone, cyclohexyl cyanide and two unknown compounds) were emitted after the removal of fungi, whereas no effect was detected in response to the bactericide treatment. This suggests that fungi inhibit or reduce the production of the above mentioned BVOCs from Brassica nigra leaves or use those compounds for their own metabolism. The origin and the roles of the novel compounds emitted requires further investigation.

Uptake of Soil-Derived Carbon into Plants: Implications for Disposal of Nuclear Waste.

Radiocarbon (14C) is potentially significant in terms of release from deep geological disposal of radioactive waste and incorporation into the biosphere. In this study we investigated the transfer of soil-derived C into two plant species by using a novel approach, where the uptake of soil-derived C into newly cultivated plants was studied on 8000-year leftover peat in order to distinguish between soil-derived and atmospheric C. Two-pool isotope mixing model was used to reveal the fraction of soil C in plants. Our results indicated that although the majority of plant C was obtained from atmosphere by photosynthesis, a significant portion (up to 3-5%) of C in plant roots was derived from old soil. We found that uptake of soil C into roots was more pronounced in ectomycorrhizal Scots pine than in endomycorrhizal reed canary grass, but nonetheless, both species showed soil-derived C uptake in their roots. Although plenty of soil-derived C was available in canopy air for reassimilation by photosynthesis, no trace of soil-derived C was detected in aboveground parts, possibly due to the open canopy. The results suggest that the potential for contamination with 14C is higher for roots than for leaves.

Vertical stratification of bacteria and archaea in sediments of a small boreal humic lake.

Although sediments of small boreal humic lakes are important carbon stores and greenhouse gas sources, the composition and structuring mechanisms of their microbial communities have remained understudied. We analyzed the vertical profiles of microbial biomass indicators (PLFAs, DNA and RNA) and the bacterial and archaeal community composition (sequencing of 16S rRNA gene amplicons and qPCR of mcrA) in sediment cores collected from a typical small boreal lake. While microbial biomass decreased with sediment depth, viable microbes (RNA and PLFA) were present all through the profiles. The vertical stratification patterns of the bacterial and archaeal communities resembled those in marine sediments with well-characterized groups (e.g. Methanomicrobia, Proteobacteria, Cyanobacteria, Bacteroidetes) dominating in the surface sediment and being replaced by poorly-known groups (e.g. Bathyarchaeota, Aminicenantes and Caldiserica) in the deeper layers. The results also suggested that, similar to marine systems, the deep bacterial and archaeal communities were predominantly assembled by selective survival of taxa able to persist in the low energy conditions. Methanotrophs were rare, further corroborating the role of these methanogen-rich sediments as important methane emitters. Based on their taxonomy, the deep-dwelling groups were putatively organo-heterotrophic, organo-autotrophic and/or acetogenic and thus may contribute to changes in the lake sediment carbon storage.

Stable carbon isotopic composition of peat columns, subsoil and vegetation on natural and forestry-drained boreal peatlands.

We studied natural and forestry-drained peatlands to examine the effect of over 34 years lowered water table on the δ13C values of vegetation, bulk peat and subsoil. In the seven studied sites, δ13C in the basal peat layer was 1.1 and 1.2 ‰ lower than that of the middle-layer and surface layer, respectively. Furthermore, there was a positive correlation between the δ13C values of the basal and surface peat layers, possibly due to carbon (C) recycling within the peat column. In the same mire complex, natural fen peat δ13C values were lower than those of the nearby bog, possibly due to the dominance of vascular plants on fen and the generally larger share of recycled C in the fens than in the bogs. Furthermore, natural and 51 years previously drained fen and bog, on the opposite sides of a ditch on the same mire complex, showed no significant differences in δ13C values. Plant δ13C values were lower, while δ13C values of subsoil were higher in the drained than in the natural site of the fen.

Heterogeneity of carbon loss and its temperature sensitivity in East-European subarctic tundra soils.

Arctic peatlands store large stocks of organic carbon which are vulnerable to the climate change but their fate is uncertain. There is increasing evidence that a part of it will be lost as a result of faster microbial mineralization. We studied the vulnerability of 3500-5900 years old bare peat uplifted from permafrost layers by cryogenic processes to the surface of an arctic peat plateau. We aimed to find biotic and abiotic drivers of CLOSS from old peat and compare them with those of adjacent, young vegetated soils of the peat plateau and mineral tundra. The soils were incubated in laboratory at three temperatures (4°C, 12°C and 20°C) and two oxygen levels (aerobic, anaerobic). CLOSS was monitored and soil parameters (organic carbon quality, nutrient availability, microbial activity, biomass and stoichiometry, and extracellular oxidative and hydrolytic enzyme pools) were determined. We found that CLOSS from the old peat was constrained by low microbial biomass representing only 0.22% of organic carbon. CLOSS was only slightly reduced by the absence of oxygen and exponentially increased with temperature, showing the same temperature sensitivity under both aerobic and anaerobic conditions. We conclude that carbon in the old bare peat is stabilized by a combination of physical, chemical and biological controls including soil compaction, organic carbon quality, low microbial biomass and the absence of plants.