Climate change goes underground: effects of elevated atmospheric CO2 on microbial community structure and activities in the rhizosphere.

General concern about climate change has led to growing interest in the responses of terrestrial ecosystems to elevated concentrations of CO2 in the atmosphere. Experimentation during the last two to three decades using a large variety of approaches has provided sufficient information to conclude that enrichment of atmospheric CO2 may have severe impact on terrestrial ecosystems. This impact is mainly due to the changes in the organic C dynamics as a result of the effects of elevated CO2 on the primary source of organic C in soil, i.e., plant photosynthesis. As the majority of life in soil is heterotrophic and dependent on the input of plant-derived organic C, the activity and functioning of soil organisms will greatly be influenced by changes in the atmospheric CO2 concentration. In this review, we examine the current state of the art with respect to effects of elevated atmospheric CO2 on soil microbial communities, with a focus on microbial community structure. On the basis of the existing information, we conclude that the main effects of elevated atmospheric CO2 on soil microbiota occur via plant metabolism and root secretion, especially in C3 plants, thereby directly affecting the mycorrhizal, bacterial, and fungal communities in the close vicinity of the root. There is little or no direct effect on the microbial community of the bulk soil. In particular, we have explored the impact of these changes on rhizosphere interactions and ecosystem processes, including food web interactions

Phospholipid fatty acid composition of microorganisms in pine forest soils of Central Siberia

The phospholipid fatty acid (PLFA) composition of microorganisms in podzolic soils of pine forests was studied in Central Siberia. The live microbial biomass in the 1-m mineral soil layer was found to gradually decrease with depth. Although the biomass decreased by half in the lower horizons, its content remained sufficiently high (12–14 nmol FAs/g soil). The coefficient of similarity in the fatty acid composition between the soils of forest and open (unforested) sites was 0.85. The coefficient of fatty acid richness in the mineral soil layer decreased with depth, while differences in fatty acid profiles increased

Bacteria dominate ammonia oxidation in soils used for outdoor cattle overwintering.

In areas used for cattle overwintering detrimental effects normally associated with grazing are intensified. Among the alterations observed, increases on the N availability and soil pH may highly influence structure of ammonia oxidizing microbes and thus influence nitrification pattern in soil. To evaluate this assumption, we assessed the abundance and diversity of ammonia oxidizing bacteria (AOB) and archaea (AOA) in three sites with different degrees of animal impact (severe, moderate or no impact) of an overwintering pasture by means of qPCR and T-RFLP of amoA genes. In areas where no animal impact could be identified AOA was dominating over AOB. However, AOB abundance increased as the degree of animal impact enhances, becoming most dominant in the severely impacted site. Interestingly, the diversity of AOB was the highest in the severely impacted area, where AOA diversity was the lowest. Obviously the pressure imposed by altered environmental conditions created by cattle husbandry lead to the selection of AOB and AOA populations, adapted to alkaline pH and higher ammonia concentration

Overwintering management on upland pasture causes shifts in an abundance of denitrifying microbial communities, their activity and N₂O-reducing ability.

Pasture soils used for cattle overwintering may represent significant sources of N2O emissions from soils. Therefore, the long-term effect of cattle overwintering on the abundance and activity of a denitrifying community was explored. The study was performed at a cattle overwintering area in South Bohemia (Czech Republic), where three sites differing in the degree of animal impact were selected: severely impacted (SI) and moderately impacted (MI), as well as a control site with no impact (NI). N2O flux measurement and soil sampling were performed in spring and fall of 2005. The activity was measured in terms of potential denitrification activity. Bacterial nirK, nirS and nosZ genes were used as functional markers of the denitrifying communities; abundance was analyzed using a real-time PCR assay. Surprisingly, in situ N2O emissions were the highest in spring at MI and significantly differed from those at SI and NI, while in autumn, rates of emissions generally decreased. In contrast potential denitrification rates were highest at SI, followed by MI. and the lowest at NI. An overall significant shift in N2O/N-2 molar ratio was shown in cattle impacted sites. The highest abundance of all genes measured at both sampling times was found at site SI, whereas at site MI increased numbers were observed only in spring. Our results indicate a strong influence of cattle on the abundance as well as the activity of microbes involved in denitrification

Effects of cattle husbandry on abundance and activity of methanogenic archaea in upland soils.

In the present study, we tested the hypothesis that animal treading associated with a high input of organic matter would favour methanogenesis in soils used as overwintering pasture. Hence, methane emissions and methanogen populations were examined at sections with different degree of cattle impact in a Farm in South Bohemia, Czech Republic. In spring, methane emission positively corresponded to the gradient of animal impact. Applying phospholipid etherlipid analysis, the highest archaeal biomass was found in section severe impact (SI), followed by moderate impact (MI) and no impact. The same trend was observed for the methanogens as showed by real-time quantitative PCR analyses of methyl coenzyme M reductase (mcrA) genes. The detection of monounsaturated isoprenoid side chain hydrocarbons (i20:1) indicated the presence of acetoclastic methanogens in the cattle-impacted sites. This result was corroborated by the phylogenetic analysis of mcrA gene sequences obtained from section SI, which showed that 33% of the analysed clones belonged to the genus Methanosarcina. The majority of the sequenced clones (41%) showed close affiliations with uncultured rumen archaeons. This leads to the assumption that a substantial part of the methanogenic community in plot SI derived from the grazing cattle itself. Compared to the spring sampling, in autumn, a significant reduction in archaeal biomass and number of copies of mcrA genes was observed mainly for section MI. It can be concluded that after 5 months without cattle impact, the severely impact section maintained its methane production potential, whereas the methane production potential under moderate impact returned to background values

Microbial characteristics of soils on a latitudinal transect in Siberia

Soil microbial properties were studied from localities on a transect along the Yenisei River, Central Siberia. The 1000 km-long transect, from 56degreesN to 68degreesN, passed through tundra, taiga and pine forest characteristic of Northern Russia. Soil microbial properties were characterized by dehydrogenase activity, microbial biomass, composition of microbial community (PLFAs), respiration rates, denitrification and N mineralization rates. Relationships between vegetation, latitude, soil quality (pH, texture), soil organic carbon (SOC) and the microbial properties were examined using multivariate analysis. In addition, the temperature responses of microbial growth (net growth rate) and activity (soil respiration rate) were tested by laboratory experiments. The major conclusions of the study are as follows:1. Multivariate analysis of the data revealed significant differences in microbial activity. SOC clay content was positively related to clay content. Soil texture and SOC exhibited the dominant effect on soil microbial parameters, while the vegetation and climatic effects (expressed as a function of latitude) were weaker but still significant. The effect of vegetation cover is linked to SOC quality, which can control soil microbial activity.2. When compared to fine-textured soils, coarse-textured soils have (i) proportionally more SOC bound in microbial biomass, which might result in higher susceptibility of SOC transformation to fluctuation of environmental factors, and (ii) low mineralization potential, but with a substantial part of the consumed C being transformed to microbial products.3. The soil microbial community from the northernmost study region located within the permafrost zone appears to be adapted to cold conditions. As a result, microbial net growth rate became negative when temperature rose above 5 degreesC and C mineralization then exceeded C accumulation.</p

Anaerobic oxidation of methane in grassland soils used for cattle husbandry.

While the importance of anaerobic methane oxidation has been reported for marine ecosystems, the role of this process in soils is still questionable. Grasslands used as pastures for cattle-overwintering show an increase in anaerobic soil micro-sites caused by animal treading and excrement deposition. Therefore anaerobic potential methane oxidation activity of severely impacted soil from a cattle winter pasture was investigated in an incubation experiment under anaerobic conditions using 13C-labeled methane. We were able to detect a high microbial activity utilizing CH4 as nutrient source shown by the respiration of 13CO2. Measurements of possible terminal electron acceptors for anaerobic oxidation of methane were carried out. Soil sulfate concentrations were too low to explain the oxidation of the amount of methane added, but enough nitrate and iron(III) were detected. However, only nitrate was consumed during the experiment. 13C-PLFA analyses clearly showed the utilization of CH4 as nutrient source mainly by organisms harbouring 16:1&omega;7 PLFAs. These lipids were found in Gram-negative microorganisms and anaerobes. The fact that these lipids are also typical for type I methanotrophs, known as aerobic methane oxidizers, might indicate a link between aerobic and anaerobic methane oxidation

Aspects dynamiques des phases précoces de la formation d'étoiles en amas

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF