Philip L. Staddon

Active root-inhabiting microbes identified by rapid incorporation of plant-derived carbon into RNA

Plant roots harbor a large diversity of microorganisms that have an essential role in ecosystem functioning. To better understand the level of intimacy of root-inhabiting microbes such as arbuscular mycorrhizal fungi and bacteria, we provided 13CO2 to plants at atmospheric concentration during a 5-h pulse. We expected microbes dependent on a carbon flux from their host plant to become rapidly labeled. We showed that a wide variety of microbes occurred in roots, mostly previously unknown. Strikingly, the greatest part of this unsuspected diversity corresponded to active primary consumers. We found 17 bacterial phylotypes co-occurring within roots of a single plant, including five potentially new phylotypes. Fourteen phylotypes were heavily labeled with the 13C. Eight were phylogenetically close to Burkholderiales, which encompass known symbionts; the others were potentially new bacterial root symbionts. By analyzing unlabeled and 13C-enriched RNAs, we demonstrated differential activity in C consumption among these root-inhabiting microbes. Arbuscular mycorrhizal fungal RNAs were heavily labeled, confirming the high carbon flux from the plant to the fungal compartment, but some of the fungi present appeared to be much more active than others. The results presented here reveal the possibility of uncharacterized root symbioses.

Does elevated atmospheric carbon dioxide affect arbuscular mycorrhizas

It is well established that an increase in the concentration of atmospheric CO(2) stimulates plant growth. Recently, many researchers have concluded that elevated CO(2) concentrations also stimulate mycorrhizal colonization. However, new evidence suggests that the observed CO(2) effects on arbuscular mycorrhizal fungi are indirect and are a result of faster plant growth at higher CO(2) concentrations. Potential changes to species assemblages of mycorrhizal fungi could affect soil carbon storage and, consequently, the feedback effects of terrestrial soil-vegetation systems on global environmental change.

Connectivity, non-random extinction and ecosystem function in experimental metacommunities.

The spatial insurance hypothesis indicates that connectivity is an important attribute of natural ecosystems that sustains both biodiversity and ecosystem function. We tested the hypothesis by measuring the impact of manipulating connectivity in experimental metacommunties of a natural and diverse microecosystem. Isolation led to the extinction of large-bodied apex predators, subsequently followed by increases in prey species abundance. This trophic cascade was associated with significantly altered carbon and nitrogen fluxes in fragmented treatments. The ecosystem impacts were characteristic of a function debt because they persisted for several generations after the initial loss of connectivity. Local extinctions and disruption of ecosystem processes were mitigated, and even reversed, by the presence of corridors in the connected metacommunities, although these beneficial effects were unexpectedly delayed. We hypothesized that corridors maintained grazer movement between fragments, which enhanced microbial activity, and decomposition in comparison to isolated fragments. Our results indicate that knowledge of habitat connectivity and spatial processes is essential to understand the magnitude and timing of ecosystem perturbation in fragmented landscapes.

Mycorrhizas and global environmental change: research at different scales

Global environmental change (GEC), in particular rising atmospheric CO2 concentration and temperature, will affect most ecosystems. The varied responses of plants to these aspects of GEC are well documented. As with other key below-ground components of terrestrial ecosystems, the response of the ubiquitous mycorrhizal fungal root symbionts has received limited attention. Most of the research on the effects of GEC on mycorrhizal fungi has been pot-based with a few field (especially monoculture) studies. A major question that arises in all these studies is whether the GEC effects on the mycorrhizal fungi are independent of the effects on their plant hosts. We evaluate the current knowledge on the effects of elevated CO2 and increased temperature on mycorrhizal fungi and focus on the few available field examples. The value of using long-term and large-scale field experiments is emphasised. We conclude that the laboratory evidence to date shows that the effect of elevated CO2 on mycorrhizal fungi is dependent on plant growth and that temperature effects seen in the past might have reflected a similar dependence. Therefore, how temperature directly affects mycorrhizal fungi remains unknown. In natural ecosystems, we predict that GEC effects on mycorrhizal fungal communities will be strongly mediated by the effects on plant communities to the extent that community level interactions will prove to be the key mechanism for determining GEC-induced changes in mycorrhizal fungal communities.

Earthworm extraction by electroshocking does not affect canopy CO2 exchange, root respiration, mycorrhizal fungal abundance or mycorrhizal fungal vitality

Abstract Electroshocking has been used to manipulate earthworm populations in agroecosystems. However, data on the effects of electroshocking on vegetation are lacking. Here we report on a field experiment with the aim to validate electroshocking as a means of manipulating earthworms without otherwise affecting the soil–plant system. We showed that there was no effect of electroshocking on canopy CO 2 exchange, root respiration or mycorrhizal fungal abundance and vitality (i.e. the proportion of mycorrhizal fungal structure which was alive). The potential for using electroshocking to manipulate earthworm populations in the field and test ecological hypotheses relating to the role of soil biodiversity in soil processes is discussed.

Insights into mycorrhizal colonisation at elevated CO2 : a simple carbon partitioning model

A simulation model was used to investigate the effect of an increased rate of plant photosynthesis at enhanced atmospheric CO2 concentration on a non-leguminous plant-mycorrhizal fungus association. The model allowed the user to modify carbon allocation patterns at three levels: (1) within the plant (shoot–root), (2) between the plant and the mycorrhizal fungus and (3) within the mycorrhizal fungus (intraradical–extraradical structures). Belowground (root and fungus) carbon losses via respiration (and turnover) could also be manipulated. The specific objectives were to investigate the dynamic nature of the potential effects of elevated CO2 on mycorrhizal colonisation and to elucidate some of the various mechanisms by which these effects may be negated. Many of the simulations showed that time (i.e. plant age) had a more significant effect on the observed stimulation of mycorrhizal colonisation by elevated CO2 than changes in carbon allocation patterns or belowground carbon losses. There were two main mechanisms which negated a stimulatory effect of elevated CO2 on internal mycorrhizal colonisation: an increased mycorrhizal carbon allocation to the external hyphal network and an increased rate of mycorrhizal respiration. The results are discussed in relation to real experiments. The need for studies consisting of multiple harvests is emphasised, as is the use of allometric analysis. Implications at the ecosystem level are discussed and key areas for future research are presented.

A decade of free‐air CO2 enrichment increased the carbon throughput in a grass‐clover ecosystem but did not drastically change carbon allocation patterns

The response of the soil carbon cycle to increasing atmospheric CO2 concentration has far reaching consequences for the ecosystem carbon balance under future climatic conditions. We report on work carried out in the Swiss free‐air CO2 enrichment (FACE) experiment, where we used in situ 13CO2 labelling to determine whether elevated CO2 (+230 μL L−1) concentration changes the fate of recently assimilated carbon in the soil microbial community. Elevated CO2 (eCO2) concentration had an overall positive effect on microbial abundance (P < 0·001) with the gram‐negative bacteria showing significantly increased quantities. Gram‐negative bacteria and saprotrophic fungi tended to utilize a higher amount of recently assimilated carbon under eCO2. Arbuscular mycorrhizal fungi (AMF) utilized plant‐assimilated carbon within 1 day after the 13CO2 pulse and 13C uptake patterns in AMF suggest that carbon transfer is faster under eCO2 concentration than under ambient CO2 (aCO2). Additionally, the respiration of recently assimilated carbon was significantly higher under eCO2 than aCO2 concentration. Our data suggest that elevated atmospheric CO2 concentration accelerated and increased the utilization of recently assimilated carbon by the microbial community without changing the microbial community composition drastically. We conclude that a higher standing soil microbial biomass under eCO2 concentration was the key cause for the higher carbon flow through the plant–soil system. Carbon utilization by microbial functional groups was only little affected by a decade of CO2 enrichment.