We have a number of research projects active within the lab on which we either lead or collaborate. Our role in these projects is typically focused on experimental manipulation or observation in field settings to simulate/follow global change impacts on terrestrial ecosystems. We test the consequences of these manipulations on ecosystem processes such as soil carbon dynamics and respiration. Typically we evaluate these impacts through application of stable carbon and nitrogen isotope techniques, and through controlled, lab-studies to evaluate competing mechanistic hypotheses. Below is a description of a number of the main projects in which we are involved.

Linking microbial community structure and processes across a land-use gradient

Collaborators: Noah Fierer, Chris Lauber, Dan Richter, Mac Callahan, Stuart Grandy

Funding: Andrew W. Mellon Foundation

The role that soils play in mediating global biogeochemical processes is a significant area of uncertainty in ecosystem ecology. One of the main reasons for this uncertainty is that we do not understand how belowground microbial community structure is linked to soil processes. Building upon established theory in soil microbial ecology and ecosystem ecology, we predict that the structure of belowground microbial communities will be a key driver of carbon and nutrient dynamics in terrestrial ecosystems. We propose to test and develop the established theories by combining state-of-the-art DNA-based techniques for microbial community analysis together with stable isotope tracer techniques. By doing so, we expect to advance our conceptual and practical understanding of the fundamental linkages between soil microbial community structure and ecosystem-level carbon and nutrient dynamics.

Consequences of non-random tree species loss on litter decay, nutrient dynamics, carbon cycling and soil decomposer communities

Collaborators: Mark Hunter, John Kominoski, Dave Coleman, Cathy Pringle, Ron Hendrick, Jim Vose, Brian Kloeppel, Jennifer Knoepp

Funding: National Science Foundation

Numerous studies have assessed the role of species diversity in ecosystem functioning through experimental designs that simulate random species loss (or gain). However, we are well aware that under global change (e.g., elevated CO2, increasing temperature, invasive species, nitrogen deposition) species are differentially sensitive to pressures exerted upon the ecosystems in which they live. We are interested in determining the consequences of non-random, tree species loss on the functioning of forested systems and, more generally, whether the effects of species loss can be predicted from what we know about single species (additive effects) or whether they are determined by species interactions (non-additivity). These questions are being explored through numerous approaches at the Coweeta Long-Term Ecological Research site in the southern Appalachians. These include: (1) full-factorial litterbag study of decomposition of the litter from four dominant tree species; (2) removal of rhododendron and girdling of red oak to simulate sudden oak death (an invasive pathogen); (3) girdling of hemlock to simulate hemlock woolly adelgid (invasive pathogen).

Heterotrophic soil respiration in warming experiments: partitioning contributions from labile and recalcitrant soil organic carbon

Collaborators: Jerry Melillo, Jim Reynolds, Jackie Mohan, Kathleen Treseder, Matt Wallenstein

Funding: Department of Energy

Of all global change factors, temperature increase may elicit particularly immediate and pronounced effects on soil microbial communities. These communities are key regulators of the mineralization of soil organic carbon (SOC), which is a major contributor to soil respiration, a process that constitutes the second largest pathway in the global carbon cycle. Shifts in soil microbial communities may be intimately related to disruptions of the carbon dynamics of terrestrial ecosystems. To test this idea, we are making use of an experimental opportunity provided by two soil-warming studies in the Harvard Forest. The first study was initiated in 1991. The immediate impact of warming was a stimulation of soil CO2 efflux. This stimulation progressively declined until rates in warmed plots were equivalent to those of controls (this took about 10 years). In the second study, warming began in 2003. Together, the studies represent temporally-distinct phases in the response of a temperate forest to elevated soil temperatures. We are testing two hypotheses: (1) The initial, short-lived augmentation of soil CO2 efflux in response to warming is a result of rapid depletion of highly temperature-sensitive labile SOC pools; (2) Once this labile pool is depleted, soil CO2 efflux in warmed plots is maintained at control rates through increased mineralization of less temperature-sensitive recalcitrant SOC pools. In testing these hypotheses we are learning about not only how soil carbon stocks are likely to respond to increasing global temperatures, but also the physiology, ecology and structure of soil microbial communities and thus the likely role they will play in influencing future atmospheric CO2 concentrations.

Are microbial communities functionally equivalent or dissimilar across space and time?

Collaborators: Noah Fierer, Chris Lauber

Soil microbial communities are some of the most diverse on Earth but we often treat them as a single, functionally redundant entity. In this work we are testing whether or not microbial communities from distinct habitats, or similar habitats in different locations, are functionally equivalent or functionally dissimilar. We are investigating these hypotheses using microcosms constructed of different litter resources that have been inoculated with microbial communities collected across space and/or time. To determine if communities function differently we measure a number of functional parameters (e.g., carbon mineralization) and examine the microbial community using molecular approaches. If microbial communities are found to be functionally dissimilar then we must begin to rethink how biogeochemical models are designed, and which processes determine the biogeographical distribution of microbial groups.

Impacts of an invasive plant species on soil processes

Collaborators: John Maerz, Jayna DeVore, Jen Fraterrigo, Monica Turner

Invasive species are recognized to have major impacts on biodiversity but our understanding of their impacts on ecosystem biogeochemical cycling is still rudimentary. Here we examine how stiltgrass (Microstegium vimineum) impacts carbon and nitrogen cycling, and microbial community structure in southeastern U.S., deciduous forests. We are using the invasions as a study system to evaluate hypotheses relating to plant-microbial nitrogen partitioning and the influence of soil community structure on this and soil carbon dynamics. Work in this area is vital if we are to understand the biogeochemical and community consequences of species invasion in terrestrial systems.

Adaptive responses of soil microorganisms to altered resource availability

Collaborators: Mat Goddard

Microbial populations may have rapid generation times, providing ample opportunity for evolutionary adaptation to changed conditions in relatively short periods of times. Given that microorganisms play a central role in global biogeochemical processes, such as the carbon and nitrogen cycles, whether they adapt, how rapidly and, if so, what the functional consequences are, is important to elucidate. Using simulation systems we are evaluating how microorganisms adapt to alter resource availability and, if so, what this may mean for ecosystem carbon and nitrogen dynamics.