| Global climate change, nutrient deposition, changes in
plant community composition, increases in atmospheric CO2
concentrations, and other disturbances all have the potential to alter
important ecosystem properties such as nutrient availability to both
plants and soil microbes, soil organic matter (SOM) decomposition
rates, and the quantity and quality of carbon (C) inputs to the soil.
However, in many cases we don’t understand the mechanisms
underlying important ecosystem processes well enough to predict the
effects of disturbances. Since soil microorganisms mediate C and
nutrient fluxes, we need a better understanding of their role in
regulating biogeochemical processes in order to predict how ecosystems
will respond to changes. In an effort to improve our understanding of
how ecosystems function and predict their responses to disturbances, my
goal is to gain insight into the controls on soil nutrient dynamics and
SOM decomposition by linking the ecology of soil microorganisms to
ecosystem processes.
My previous work studying decomposition has
stimulated my interest in the role of microbial extra-cellular enzymes,
which breakdown organic polymers into molecules small enough for
microbes to take up. Since polymer breakdown is the rate-limiting step
in decomposition, extra-cellular enzymes link microbes and their
ecology to larger scale ecosystem processes. Knowing how microbes
handle the nutrient and energy tradeoffs in exo-enzyme production may
yield insight into ecosystem responses to disturbances - i.e. if
nutrient levels increase due to external inputs, do rates of SOM
breakdown decrease because soil microbes no longer require SOM bound
nutrients, or do they increase because microbes can invest more in
N-rich enzymes? Can we predict the response of microbes to increased
nutrient availability by determining whether nutrients or C limits
them? Are shifts in the activities of the major classes of soil enzymes
accompanied by significant changes in the composition of the microbial
community? Answering these questions is important for our understanding
of how nutrient and C availability regulate the composition of the
microbial community, decomposition, soil CO2 efflux, and nutrient
cycling.
I am also interested in how the role of soil
organic N changes with increasing N availability, as our overall
understanding of the importance of organic N in different ecosystems is
still extremely limited. For example, what proportion of dissolved
organic N (DON), beyond amino acids, is biologically available to
plants and microbes? Does the importance of DON to soil microbes and
plants change across an N availability gradient?
The idea that the importance of different N forms in the soil changes with N availability has influenced the direction of my research into N deposition.
We are beginning to understand the effects of elevated N levels on
plant growth and soil N loss, but many questions remain. How does the
form of deposited N affect its fate? Do microbes immobilize all of this
N and then release the excess? How does higher N availability change
microbial community and SOM decomposition dynamics? Is the availability
of deposited N affected by abiotic reactions?
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| The question of how patterns of soil nutrient
availability and SOM decomposition change with climate warming also
interests me. Warmer temperatures have the potential to change the
timing of nutrient availability in the soil and feedback to both
microbial and plant communities. If nutrient availability increases
when plants are not active, will nutrient loss rates from the soil
increase? Will this promote plant invasions?
By answering questions such as these, we can
improve our understanding of the direct effects of disturbances on
important ecosystem processes, gain insight into the microbial
mechanisms that control these processes, and use this understanding to
refine our predictions of how terrestrial ecosystems will respond to
the disturbances they increasingly face.
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