Previous Research

How do arctic shrubs affect soil microbial activity and soil carbon cycling?

Arctic ecosystems store a substantial fraction of terrestrial biosphere carbon, and they are rapidly warming. Rising temperatures have triggered the expansion of several shrub species in arctic tundra, but it is unclear how shrubs interact with microbes to alter the cycling of soil carbon.

PhD student Carly Phillips led a project on shrub-microbial interactions on the north slope of Alaska. Carly studied how the expansion of Betula, Alnus and Salix is changing microbial activity and the cycling of carbon in soils. Carly's research combined field and lab-based approaches to isolate the potential mechanisms responsible for shrub effects on the ecosystem.

Funding: NSF DDIG

How will increased drought affect southern Appalachian forests?

The southeastern U.S. is experiencing an increased frequency of drought events during the growing season. How will drought affect plant competition and ecosystem processes in early successional forests of the southern Appalachians? Of particular importance is the fate of Robinia pseudoacacia, an N2-fixing species that contributes to ecosystem resiliency by supplying new N to the ecosystem.

In a greenhouse experiment we reduced soil moisture and monitored plant growth and water exchange by four early successional species. We found that moderate drought enhanced both N2 fixation and the competitive ability of Robinia pseudoacacia. This work was published in Oecologia.

But this work led to another question: How will Robinia pseudoacacia respond to more frequent or more severe drought events? We answered this with two studies led by PhD student Jeff Minucci. First, Jeff conducted a field experiment in the Cowee valley, NC where he studied how rainfall reduction in the growing season influences tree growth and competition of R. pseudoacacia and three non-fixing tree species. He found that increasing soil dryness was negatively associated with the growth of R. pseudoacacia. His work suggests that drought has the potential to indirectly reduce forest growth and recovery.

Second, Jeff conducted a greenhouse experiment with R. pseudoacacia to understand the impact of the frequency and duration of drought events on N2 fixation and other physiological processes. He found that R. pseudoacacia growth is resistant to increased drought frequency because it employs two strategies – drought tolerance or drought avoidance followed by compensation. His work was published in New Phytologist.

Collaborators: Chelcy Miniat and Robert Teskey

Funding: USDA FS SRS

Nutrient limitation on asymbiotic nitrogen fixation in tropical forests

Nitrogen fixation is a critical process in tropical ecosystems but we poorly understand what constrains it. Phosphorus has been considered the primary element that restricts nitrogen-fixers in nature, and tropical forests in particular. we demonstrated that molybdenum, a trace metal and component of the nitrogenase enzyme, can limit free-living, nitrogen-fixing bacteria in tropical soils. This work was published in Nature Geoscience and exposed the significance of an underappreciated trace metal in the tropical nitrogen cycle.

But, why and when do molybdenum and phosphorus limitation arise? To answer this question, we conducted field experiments across lowland tropical forests of Panama and assays of soil chemistry in the lab. We found that free-living nitrogen-fixing bacteria are constrained by the interaction of phosphorus and molybdenum in soils at two scales: within local soil layers and across landscape gradients in soil phosphorus. This work was published in PLOS ONE and provides a mechanistic framework for the nutrient limitation of free-living nitrogen-fixing bacteria.

Collaborators: Alex Barron, J.P. Bellenger, Lars Hedin, Anne Kraepiel and Joe Wright.

Plant litter chemistry and mycorrhizal fungi produce a nitrogen feedback

Have mycorrhizal fungi co-evolved strategies with their host plants to regulate nutrient cycles for their own coupled benefit? We found evidence that an evergreen shrub, Rhododendron maximum, makes soil nitrogen less available to roots of forest trees that associate with AM and ECM fungi, compared to its own roots, which associate with ericoid mycorrhizal fungi. This research is one of the first in-situ demonstrations of a plant-soil-nutrient feedback that can facilitate niche partitioning, plant competition and regulation of the nitrogen cycle in terrestrial ecosystems. This work was published in Journal of Ecology was featured on the Ecological Society of America’s blog. Nina was awarded the 2009 Harper Prize from the British Ecological Society for this work.

Collaborator: Ronald Hendrick