Soil metatranscriptome demonstrates a shift in C, N, and S metabolisms of a grassland ecosystem in response to elevated atmospheric CO2.

Soil organisms play an important role in the equilibrium and cycling of nutrients. Because elevated CO2 (eCO2) affects plant metabolism, including rhizodeposition, it directly impacts the soil microbiome and microbial processes. Therefore, eCO2 directly influences the cycling of different elements in terrestrial ecosystems. Hence, possible changes in the cycles of carbon (C), nitrogen (N), and sulfur (S) were analyzed, alongside the assessment of changes in the composition and structure of the soil microbiome through a functional metatranscriptomics approach (cDNA from mRNA) from soil samples taken at the Giessen free-air CO2 enrichment (Gi-FACE) experiment. Results showed changes in the expression of C cycle genes under eCO2 with an increase in the transcript abundance for carbohydrate and amino acid uptake, and degradation, alongside an increase in the transcript abundance for cellulose, chitin, and lignin degradation and prokaryotic carbon fixation. In addition, N cycle changes included a decrease in the transcript abundance of N2O reductase, involved in the last step of the denitrification process, which explains the increase of N2O emissions in the Gi-FACE. Also, a shift in nitrate ( NO 3 - ) metabolism occurred, with an increase in transcript abundance for the dissimilatory NO 3 - reduction to ammonium ( NH 4 + ) (DNRA) pathway. S metabolism showed increased transcripts for sulfate ( SO 4 2 - ) assimilation under eCO2 conditions. Furthermore, soil bacteriome, mycobiome, and virome significantly differed between ambient and elevated CO2 conditions. The results exhibited the effects of eCO2 on the transcript abundance of C, N, and S cycles, and the soil microbiome. This finding showed a direct connection between eCO2 and the increased greenhouse gas emission, as well as the importance of soil nutrient availability to maintain the balance of soil ecosystems.

Elevated Atmospheric CO2 Modifies Mostly the Metabolic Active Rhizosphere Soil Microbiome in the Giessen FACE Experiment.

Elevated levels of atmospheric CO2 lead to the increase of plant photosynthetic rates, carbon inputs into soil and root exudation. In this work, the effects of rising atmospheric CO2 levels on the metabolic active soil microbiome have been investigated at the Giessen free-air CO2 enrichment (Gi-FACE) experiment on a permanent grassland site near Giessen, Germany. The aim was to assess the effects of increased C supply into the soil, due to elevated CO2, on the active soil microbiome composition. RNA extraction and 16S rRNA (cDNA) metabarcoding sequencing were performed from bulk and rhizosphere soils, and the obtained data were processed for a compositional data analysis calculating diversity indices and differential abundance analyses. The structure of the metabolic active microbiome in the rhizospheric soil showed a clear separation between elevated and ambient CO2 (p = 0.002); increased atmospheric CO2 concentration exerted a significant influence on the microbiomes differentiation (p = 0.01). In contrast, elevated CO2 had no major influence on the structure of the bulk soil microbiome (p = 0.097). Differential abundance results demonstrated that 42 bacterial genera were stimulated under elevated CO2. The RNA-based metabarcoding approach used in this research showed that the ongoing atmospheric CO2 increase of climate change will significantly shift the microbiome structure in the rhizosphere.

Effects of short-term drying and irrigation on electron flow in mesocosms of a northern bog and an alpine fen.

Methane emissions and element mobility in wetlands are controlled by soil moisture and redox conditions. We manipulated soil moisture by weekly drying and irrigation of mesocosms of peat from a bog and iron and sulfur rich fen. Water table changed more strongly in the decomposed fen peat ( approximately 11 cm) than in the fibric bog peat ( approximately 5 cm), where impacts on redox processes were larger due to larger change in air filled porosity. Methanogenesis was partly decoupled from acetogenesis and acetate accumulated up to 5.6 mmol L(-1) in the fen peat after sulfate was depleted. Irrigation and drying led to rapid redox-cycles with sulfate, hydrogen sulfide, nitrate, and methane being produced and consumed on the scale of days, contributing substantially to the total electron flow and suggesting short-term resilience of the microbial community to intermittent aeration. Anaerobic CO2 production was partly balanced by methanogenesis (0-34%), acetate fermentation (0-86%), and sulfate reduction (1-30%) in the bog peat. In the fen peat unknown electron acceptors and aerenchymatic oxygen influx apparently drove respiration. The results suggest that regular rainfall and subsequent drying may lead to local oxidation-reduction cycles that substantially influence electron flow in electron acceptor poor wetlands.