Short-Term Uptake of 15N by a Grass and Soil Micro-Organisms after Long-Term Exposure to Elevated CO2

This study examines the effect of elevated CO2 on short-term partitioning of inorganic N between a grass and soil micro-organisms. 15N-labelled NH4+ was injected in the soil of mesocosms of Holcus lanatus (L.) that had been grown for more than 15months at ambient or elevated CO2 in reconstituted grassland soil. After 48 h, the percentage recovery of added 15N was increased in soil microbial biomass N at elevated CO2, was unchanged in total plant N and was decreased in soil extractable N. However, plant N content and microbial biomass N were not significantly affected by elevated CO2. These results and literature data from plant-microbial 15N partitioning experiments at elevated CO2 suggest that the mechanisms controlling the effects of CO2 on short- vs. long-term N uptake and turnover differ. In particular, short-term immobilisation of added N by soil micro-organisms at elevated CO2 does not appear to lead to long-term increases in N in soil microbial biomass. In addition, the increased soil microbial C:N ratios that we observed at elevated CO2 suggest that long-term exposure to CO2 alters either the functioning or structure of these microbial communitie

Responses of soil nitrite-oxidizers to global environmental changes

Background/Question/Methods
There is increasing evidence that global change can alter the structure of plant
communities with large impacts on the functioning of terrestrial ecosystems. However, little is known about the impact of global change on soil microbial communities. In particular, the response of soil nitrite-oxidizers microorganisms that mediate the second step of nitrification, a key process of the nitrogen cycle, has never been investigated.
Here, we examined the effects of four main global environmental changes on the activity, the abundance and the diversity of soil nitrite-oxidizers in an annual grassland ecosystem as part of the Jasper Ridge Global Change Experiment (CA, USA). This experiment includes four treatments - CO2, temperature, precipitation and nitrogen - with two levels per treatment (ambient and elevated, with elevated treatment based on prediction for the end of the century), and all of their factorial combinations. We measured potential nitrite oxidation, the abundance of soil Nitrobacter- and Nitrospiralike nitrite-oxidizers (using quantitative PCR targeting nxrA and 16S rRNA gene, respectively) and the diversity of soil Nitrobacter-like nitrite-oxidizers (using cloning sequencing targeting the nxrA gene) in each treatment combination at the end of the 7th and 8th growing seasons under treatments. Furthermore, we analyzed to what extent changes in the activity of the soil nitrite-oxidizers result from changes in their abundance or diversity.

Results/Conclusions
Simulated global environmental changes significantly altered the activity, as well as the abundance and the diversity of soil nitrite-oxidizers. Potential nitrite oxidation decreased with increased precipitation and increased with elevated CO2 when combined with added nitrogen or precipitation. The abundance of soil Nitrobacter-like nitrite-oxidizers also decreased with increased precipitation and increased with elevated levels of CO2 and nitrogen. In contrast, the abundance of soil Nitrospira-like nitrite-oxidizers increased with enhanced precipitation and decreased with elevated levels of CO2 and temperature. Finally, the structure of the soil Nitrobacter-like nitrite-oxidizers was significantly altered by the treatments. Consistent with results reported by Attard et al. (2010) for agroecosystems, we found that changes in potential rates of nitrite oxidation in response to treatments were partly explained by changes in the abundance of soil Nitrobacter-like nitrite-oxidizers, but not by changes in the abundance of soil Nitrospiralike nitrite-oxidizers, suggesting that Nitrobacter-like nitrite-oxidizers were the main functional players of the soil nitrite-oxidizing microbial community.
Our study provides evidence that global change could alter the abundance and diversity of soil nitrite-oxidizers, with potential impacts for soil nitrogen cycling.

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Global Change Could Amplify Fire Effects on Soil Greenhouse Gas Emissions

Background: Little is known about the combined impacts of global environmental changes and ecological disturbances on ecosystem functioning, even though such combined impacts might play critical roles in shaping ecosystem processes that can in turn feed back to climate change, such as soil emissions of greenhouse gases.[br/] Methodology/Principal Findings: We took advantage of an accidental, low-severity wildfire that burned part of a long-term global change experiment to investigate the interactive effects of a fire disturbance and increases in CO(2) concentration, precipitation and nitrogen supply on soil nitrous oxide (N(2)O) emissions in a grassland ecosystem. We examined the responses of soil N(2)O emissions, as well as the responses of the two main microbial processes contributing to soil N(2)O production - nitrification and denitrification - and of their main drivers. We show that the fire disturbance greatly increased soil N(2)O emissions over a three-year period, and that elevated CO(2) and enhanced nitrogen supply amplified fire effects on soil N(2)O emissions: emissions increased by a factor of two with fire alone and by a factor of six under the combined influence of fire, elevated CO(2) and nitrogen. We also provide evidence that this response was caused by increased microbial denitrification, resulting from increased soil moisture and soil carbon and nitrogen availability in the burned and fertilized plots. [br/] Conclusions/Significance: Our results indicate that the combined effects of fire and global environmental changes can exceed their effects in isolation, thereby creating unexpected feedbacks to soil greenhouse gas emissions. These findings highlight the need to further explore the impacts of ecological disturbances on ecosystem functioning in the context of global change if we wish to be able to model future soil greenhouse gas emissions with greater confidence

Nouvelles stratégies thérapeutiques médicamenteuses dans la démence de type Alzheimer

TOULOUSE3-BU Santé-Centrale (315552105) / SudocSudocFranceF

Impact des changements globaux sur le cycle de l'azote

Au cours de ma thèse, j ai cherché à évaluer l impact des changements globaux sur le cycle de l azote (N) et plus particulièrement sur la nitrification et la dénitrification. Les effets des changements globaux ont été étudiés à travers deux dispositifs : une expérience en serre dans laquelle des monocultures de Dactylis glomerata ont été soumises à une élévation de la teneur en CO2 couplée à une fertilisation azotée et une expérience in situ dans laquelle des parcelles de prairie annuelle ont été soumises aux effets individuels et combinés d une augmentation de la teneur en CO2, de la température, des précipitations et de la déposition d N. De plus, les effets interactifs d un feu accidentel et des traitements ont été explorés. Un résultat intéressant est l absence d effet des traitements sur la nitrification brute, malgré les effets marqués des traitements azotés sur la nitrification potentielle. Les réponses des deux étapes de la nitrification ont été également contrastées : la nitritation a été augmentée par la déposition d N, tandis que la nitratation a été réduite par le traitement précipitation. Des modifications de l abondance et de la structure des communautés impliquées pourraient être à l origine de ces réponses. La dénitrification potentielle a été augmentée dans les parcelles supplémentées en N et en eau, probablement du fait d une augmentation de la disponibilité en substrats et d une diminution de la teneur en oxygène du sol. Enfin, un des résultats les plus impressionnants de cette étude a été l impact très fort du feu sur les émissions de N2O, un puissant gaz à effet de serre, et ce particulièrement dans les parcelles sous fort CO2 et forte déposition d N.In my PhD thesis, I investigated the impact of global change on nitrogen (N) cycle and more specifically on nitrification and denitrification. Impacts of global change were assessed through two experimental designs: a greenhouse experiment in which monocultures of Dactylis glomerata were grown under the interactive effects of elevated CO2 and N supply, and an in situ experiment in which grassland plots were exposed to elevated CO2, temperature, precipitation, N deposition and all their combinations. In addition, the interactive effects of a wildfire and treatments were explored. An interesting result is the non-responsiveness of gross nitrification to global change treatments despite the large effects of N treatments on potential nitrification. The two steps of nitrification also showed contrasting responses to global change treatments: ammonia oxidation increased under high N deposition, while nitrite oxidation decreased in the elevated precipitation treatment. Changes in the abundance and structure of the microbial communities involved could be responsible for these responses. Potential denitrification increased under high N and high precipitation conditions, probably because of higher N availability and lower soil oxygen content. Finally, one of the most striking results of this study was the large impact of fire on soil emissions of N2O, a potent greenhouse gas, especially in the elevated CO2 and increased N deposition plots.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Short-term uptake of 15N by a grass and soil micro-organisms after long-term exposure to elevated CO2

ISSN:0032-079XISSN:1573-503

In situ fate of mineral N in the tree-soil-microorganism system before and after budburst in 20-year-old Quercus petraea (Matt.) Liebl.

Aims: We simultaneously quantified the fate of soil absorbed mineral nitrogen in different tree compartments along with nitrogen immobilization by microorganisms during spring in 20-year-old oak trees. Methods: A soil-applied(15)N solution was traced in situ into the fine roots, medium roots, xylem, phloem, branches, leaves, extractable soil, and microbial biomass before and after budburst, until LAI maximum was reached. Results: During the three weeks following the labeling, around half of the(15)N applied was incorporated into the microbial biomass while the leafless trees absorbed less than 10% of the(15)N. Before and after budburst, the microbial compartment was the main pool of(15)N, and yet the soil-absorbed(15)N still significantly contributed to the leaf nitrogen pool at budburst. This contribution in leaves sharply increased in the days following budburst. Conclusion: The potential competition for mineral nitrogen between trees and the soil microbial biomass is strong during spring. The dependence of the leaf nitrogen pool to internal or external stocks of nitrogen at budburst could be conditioned by environmental conditions

Several components of global change alter nitrifying and denitrifying activities in an annual grassland

International audienceThe effects of global change on below-ground processes of the nitrogen (N) cycle have repercussions for plant communities, productivity and trace gas effluxes. However, the interacting effects of different components of global change on nitrification or denitrification have rarely been studied in situ. 2. We measured responses of nitrifying enzyme activity (NEA) and denitrifying enzyme activity (DEA) to over 4 years of exposure to several components of global change and their interaction (increased atmospheric CO2 concentration, temperature, precipitation and N addition) at peak biomass period in an annual grassland ecosystem. In order to provide insight into the mechanisms controlling the response of NEA and DEA to global change, we examined the relationships between these activities and soil moisture, microbial biomass C and N, and soil extractable N. 3. Across all treatment combinations, NEA was decreased by elevated CO2 and increased by N addition. While elevated CO2 had no effect on NEA when not combined with other treatments, it suppressed the positive effect of N addition on NEA in all the treatments that included N addition. We found a significant CO2-N interaction for DEA, with a positive effect of elevated CO2 on DEA only in the treatments that included N addition, suggesting that N limitation of denitrifiers may have occurred in our system. Soil water content, extractable N concentrations and their interaction explained 74% of the variation in DEA. 4. Our results show that the potentially large and interacting effects of different components of global change should be considered in predicting below-ground N responses of Mediterranean grasslands to future climate changes