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Research Interests:

Ecosystem modification is an unavoidable consequence of human activity. The proliferation of direct (e.g., land use change) and indirect (e.g., nitrogen, N, deposition) anthropogenic modifiers requires understanding the drivers and effects of these modifiers in virtually all ecosystems.

My research focuses on the responses of ecosystem properties and processes to natural and anthropogenic changes and how such changes may feed back to further influence global, ecosystem, or community level changes.  Because predicting and managing the effects of natural and anthropogenic changes depends on accurately depicting basic ecosystem processes, I combine field and laboratory experimental approaches with quantitative methods to better understand fundamental processes like decomposition.

Current & Past Research:

My research investigates how multiple global change vectors interact to influence nitrogen (N) pools and fluxes, carbon (C) cycling and  sequestration, and plant communities, and strives to better understand basic ecosystem processes. 

Global Change and Carbon

A fundamental unanswered question in ecosystem ecology is how multiple factors of global change will interact to influence belowground C storage.  I conducted several field, laboratory and analytical C studies within the Biodiversity, CO2, and Nitrogen (BioCON) experiment at Cedar Creek Ecosystem Science Reserve in Central Minnesota.

First, using a mass balance approach, I estimated the amount of C allocated belowground by plants each year (Adair et al. 2009). I found that increasing diversity and N increased plants’ belowground C allocation by increasing root biomass. Elevated CO2
(eCO2) also increased belowground C allocation,
 BioCON Experiment (image from http://www.cedarcreek.umn.edu/highres/)
but via increases in non-root biomass. Second, I mentored an undergraduate student on a project investigating how these treatments impact labile to recalcitrant soil C pools. Again, increasing diversity increased soil C via root biomass, but eCO2 and N increased soil C independently of root biomass (Reid et al. in prep). Third, I am using physical fractionation and isotopic measurements to investigate the role of physical protection of C in this experiment (e.g., in soil aggregates).

I also conducted a study to elucidate the mechanism(s) behind the consistent positive effect of e
CO2 on soil respiration. This effect may be the result of increased availability of C (via higher photosynthesis at eCO2) or soil water (via lower stomatal conductance at eCO2) for roots and microbes. Elevated CO2 did increase soil moisture and soil respiration, but CO2-induced changes in soil moisture had essentially no effect on soil respiration. These results suggest that increased substrate availability is the primary driver of eCO2-induced increases in soil respiration, and provide further support for increased rates of C cycling at eCO2.

Currently, I am beginning to use BioCON data to ask if increasing atmospheric
CO2, diversity or N increases the temperature sensitivity of soil respiration. This research is critical to determining whether ecosystem models are overestimating the ability of terrestrial ecosystems to act as C sinks.

These results will provide process level information on belowground C storage in the face of interacting global changes.

Understanding Global Decomposition

My most recent project, funded by the National Center for Ecological Analysis and Synthesis (NCEAS),
combines large data sets and model selection techniques to discover the level of complexity that is needed to model decomposition at large spatial and temporal scales.  In previous work, I used information-theoretic methods and a large North American data set to compare numerous decomposition models. I found that a surprisingly simple model - based only on initial litter chemistry and basic climate data - explained 70% of the variation in long-term global decomposition (Adair et al. 2008). Currently, I am investigating whether incorporating microbial populations and/or processes will explain the remaining variation.

After compiling a large-scale, long-term litter decomposition database (see the figure below for the sites I've included so far), I will compare two sets of models:  the first varies only in how microbial activity is modeled; the second compares the best model(s) from the first set to published models.  A model comparison using Bayesian statistical techniques and this extensive data set will allow me to evaluate how much of this complex process must be explicitly modeled to accurately describe and predict decomposition at global and regional scales.

Sites included (thus far) in my decomposition model analysis
Combining experimental approaches to understanding the biogeochemical effects of abiotic or biotic global changes with data assimilation, analysis, and model comparison techniques will provide information on some major, unresolved issues in ecosystem ecology including the fate of litter and soil C in the face of global change.  Such an approach will advance not only basic science, but also provide crucial information for management and policy formation.

Photodegradation of Litter

Accurately modeling ecosystem C balance depends on a thorough understanding of decomposition. Previously, I found a simple model accurately described litter decomposition in many ecosystems, but this model performed poorly in arid systems. The primary candidate for explaining the failure of this and other biotic decomposition models in arid environments is photodegradation, the abiotic decomposition of plant litter by solar radiation.  I am currently adding this formerly ignored process to the traditional models of biotic decomposition found in Adair et al. (2008) and the DAYCENTURY
ecosystem model. This work will enable accurate predictions of C loss from drylands, which make up 40% of terrestrial land surface.

Setting up UV shelters at Cedar Creek (photo by B. Keeler)

Causes and Consequences of Exotic Species Invasion

My dissertation research focused on a different environmental problem that has its roots in human activities:  the invasion of exotic species into unmanaged, wild ecosystems.  I investigated the interactions between exotic species and resource availability, which is one of the factors that might modulate the community responses to plant invasion. I used field studies and experiments combined with model selection and comparison techniques to

(1) define the success of exotic plant invasion in Colorado’s foothills,
(2) determine how Bromus tectorum, an invasive annual grass, responded to native plant mortality and soil resource availability, and
(3) investigate the biogeochemical consequences B. tectorum invasion.

Bromus tectorum invasion increased with water and N additions but unexpectedly decreased with native plant mortality (Adair et al. 2008).  A post-invasion field study found that pools of N were larger beneath B. tectorum than adjacent uninvaded perennial grassland communities (Adair et al. 2010). This suggests that high available N promotes cheatgrass invasion and that, once established, cheatgrass-induced increases available N may further increase its success.
Measuring microbial biomass by chloroform fumigation

This research has important management implications, indicating that activities that increase available N (e.g., agricultural runoff, N deposition) may increase community invasiblility. I also found that the biogeochemical effects of this invasion were linked to concurrent changes in plant community phenology. This result suggests that changes in phenology (e.g., due to exotic species invasion or climate changes) may substantially affect ecosystem properties and processes.

River Regulation and Nitrogen Cycling

My M.S. research investigating nitrogen (N) accumulation and cycling in semi-arid riparian ecosystems suggested that regulating river flows (i.e., with dams and controlled releases) may fundamentally alter patterns of riparian N availability by doubling rates of N cycling and availability in young, low-lying riparian areas (Adair et al. 2002). River regulation may therefore have unanticipated consequences on ecosystem properties and processes such as plant establishment (Adair and Binkley 2002), plant diversity (Uowolo et al. 2005), invasion and trace gas emissions.

Unregulated Yampa River in NW Colorado

Banner photo: Bouteloua gracilis, photo by S.E. Hobbie