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

The impact of chromium toxicity on the yield and quality of rice grains produced under ambient and elevated levels of CO2

Rice is a highly valuable crop consumed all over the world. Soil pollution, more specifically chromium (Cr), decreases rice yield and quality. Future climate CO₂ (eCO₂) is known to affect the growth and yield of crops as well as the quality parameters associated with human health. However, the detailed physiological and biochemical responses induced by Cr in rice grains produced under eCO₂ have not been deeply studied. Cr (200 and 400 mg Cr⁶⁺/Kg soil) inhibited rice yield and photosynthesis in Sakha 106, but to less extend in Giza 181 rice cultivar. Elevated CO₂ reduced Cr accumulation and, consequently, recovered the negative impact of the higher Cr dose, mainly in Sakha 106. This could be explained by improved photosynthesis which was consistent with increased carbohydrate level and metabolism (starch synthases and amylase). Moreover, these increases provided a route for the biosynthesis of organic, amino and fatty acids. At grain quality level, eCO₂ differentially mitigated Cr stress-induced reductions in minerals (e.g., P, Mg and Ca), proteins (prolamin, globulin, albumin, glutelin), unsaturated fatty acids (e.g., C20:2 and C24:1) and antioxidants (phenolics and total antioxidant capacity) in both cultivars. This study provided insights into the physiological and biochemical bases of eCO₂-induced grain yield and quality of Cr-stressed rice.

Elevated atmospheric CO2 concentrations caused a shift of the metabolically active microbiome in vineyard soil

BACKGROUND

Elevated carbon dioxide concentrations (eCO₂), one of the main causes of climate change, have several consequences for both vine and cover crops in vineyards and potentially also for the soil microbiome. Hence soil samples were taken from a vineyard free-air CO₂ enrichment (VineyardFACE) study in Geisenheim and examined for possible changes in the soil active bacterial composition (cDNA of 16S rRNA) using a metabarcoding approach. Soil samples were taken from the areas between the rows of vines with and without cover cropping from plots exposed to either eCO₂ or ambient CO₂ (aCO₂).

RESULTS

Diversity indices and redundancy analysis (RDA) demonstrated that eCO₂ changed the active soil bacterial diversity in grapevine soil with cover crops (p-value 0.007). In contrast, the bacterial composition in bare soil was unaffected. In addition, the microbial soil respiration (p-values 0.04-0.003) and the ammonium concentration (p-value 0.003) were significantly different in the samples where cover crops were present and exposed to eCO₂. Moreover, under eCO₂ conditions, qPCR results showed a significant decrease in 16S rRNA copy numbers and transcripts for enzymes involved in N₂ fixation and NO₂^- reduction were observed using qPCR. Co-occurrence analysis revealed a shift in the number, strength, and patterns of microbial interactions under eCO₂ conditions, mainly represented by a reduction in the number of interacting ASVs and the number of interactions.

CONCLUSIONS

The results of this study demonstrate that eCO₂ concentrations changed the active soil bacterial composition, which could have future influence on both soil properties and wine quality.

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 (NO3-) metabolism occurred, with an increase in transcript abundance for the dissimilatory NO3- reduction to ammonium (NH4+) (DNRA) pathway. S metabolism showed increased transcripts for sulfate (SO42-) 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.