by Makari Krause
Ecosystems provide a multitude of services to humans, but one that will continue to grow in importance as climate change progresses is terrestrial carbon storage. In their paper, Bardgett et al. (2013) develop a framework for understanding the multiple mechanisms through which climate indirectly impacts the carbon cycle. These mechanisms are broken into three categories: individual responses, community reordering, and species immigration and loss. Individual responses only include changes to individual organisms without any alteration of the larger communities in which they live. While the individual responses occur in the short term, in the long term (years to decades) observable changes will occur within entire communities. This community reordering involves changes in the abundance of certain species but not the complete extinction of species or the invasion of new species. If the time period is extended even further there will be shifts in species resulting in the invasion of new species and the extinction of old ones. Each of these responses alters interactions between soil and plant communities and has associated implications for the global carbon cycle.D. Bardgett et al. realizing that there was no comprehensive list of mechanisms which influenced plant-soil interactions and no way to organize the known mechanisms, decided to present a framework for understanding the interactions between plant and soil communities and how those interactions will adjust under changing climate conditions. Their report is sub-divided into three different sections, each of which deals with a different time window and the associated effects of climate change in that time window.
Individual responses only include changes to individual organisms without any alteration of communities. Plant and soil communities are tightly coupled and an alteration of plant behavior due to climate change can directly upset the interactions between plants and soil communities. As an example Bardgett et al. examine the many effects that increasing global temperatures can have on plant-soil interactions. Warmer temperatures can increase plant growth and root exudation and thereby increase the carbon flux into the soil. Increased exudation also allows for increased mineralization of nitrogen leading to an increase in the available nitrogen and increased plant growth. While it would seem that warmer temperatures would facilitate carbon storage, there are other effects that might produce a different result.
An increase in carbon in the soil due to elevated CO2 levels may lead to nitrogen immobilization and a decrease in usable nitrogen for plant growth thereby decreasing carbon transfer to the soil. Climate change can also directly change the behavior of below ground organisms. Warmer temperatures can increase soil respiration but it is unknown whether this increase is permanent or whether respiration acclimates to increased temperatures. While respiration releases CO2, increased respiration also increase in the rate of nitrogen fixation, which would indirectly lead to increased plant growth and a decrease in atmospheric CO2.
In addition to changing the inputs to the system, climate change can also disrupt the existing coupled interactions between plant and soil communities. In the short term most of this disruption will result from a change in plant phenology such as altered growing and flowering seasons. A change in the growing season of the plant community and a lack of the corresponding change in the soil community can disrupt interactions and have significant impacts. Bardgett et al. note that some high altitude ecosystems that are highly nitrogen limited have complex ways of partitioning nitrogen according to the growing season and a change in either the growing season of plants or of soil microbes could quickly throw the ecosystem out of balance and lead to decreased growth due to nutrient limitations.
Over the long term, years to decades, the short term phenological changes described above will result in community reordering. Community reordering does not mean the extinction of specific species or the invasion of new species but rather changes in the abundance of existing species. Changes in the abundance of species in the plant community will affect the amount of carbon entering the soil. One example of community reordering affecting carbon cycling and soil communities is that reduced precipitation and warming selects for deeper rooting plants. These deeper rooting plants, now making up a larger percentage of the population, increase below ground carbon flux and soil carbon stability.
While there is not conclusive evidence, some studies have postulated that below ground communities may be impacted by climate change much more heavily and quickly than their above ground counterparts. While the mechanism for this is unknown it is probably related to generation length and resilience of soil microbes as compared to plants. Regardless of the mechanism, if the change in below ground communities occurs without a corresponding change in plant communities then a decoupling event could occur with the same consequences as described above. The reverse may also happen where the above ground communities change more quickly than their below ground counterparts.
Species Immigration and Losses
Bardgett et al. then extend their time period even further and consider the consequence of individual responses and community reordering on a population in the long run. The long run effects are the extinction of existing species and the invasion of previously absent species. These changes may lead to completely new interactions between species both above and below ground and also loss of previously existing interactions. Climate change shifts the ranges of certain species moving them into new areas and out of old ones. If species aren’t able to adapt quickly enough they may be completely pushed out of their niche and go extinct. These shifts in species will undoubtedly decouple plant and soil communities and have an impact on the global climate cycle. One example cited by Bardgett et al. is that plant litter has been found to decompose more rapidly in its home environment because of the close association that develops between the decomposer community and the plant species. If a certain plants move into new ranges and their complimentary decomposers do not, it could change the rate at which CO2 is released from dead plant matter. Just as plants might move out of the range of their specific decomposers they might also move out of range of their specific pathogens. Plants are often better at dispersing than their counterpart pathogens and, in response to climate change, might expand their range to areas where they no longer suffer from pathogens. This would lead to increased plant growth in those areas and would have significant implications for carbon sequestration. On a similar note, Bardgett et al. bring up the point that some plants have important mutualists that might not disperse with them into their new ranges. This decoupling of plants from their mutualists might limit the plants ability to thrive in new environments and result in decreased plant growth, thereby inhibiting carbon storage.
While it is clear that there is potential for climate change to impact plant-soil interactions across all time frames and sample sizes, many of these interactions are not well studied and the effects of disrupting these interactions are unclear. While this paper does not provide the answers to these questions it provides a framework on which to base future research.
Bardgett, R. D., Manning, P., Morrien, E., & Vries, F. T., 2013. Hierarchical responses of plant–soil interactions to climate change: consequences for the global carbon cycle. Journal of Ecology 101(2), 334–343.