A balancing act : fire and nitrogen fixation in the longleaf pine ecosystem
Fire is a critical force in maintaining the structure and diversity of longleaf pine ecosystems, but it also removes a substantial amount of nitrogen (N), which can limit plant growth. N fixation can replenish N losses, but whether and how fixation balances fire-induced N losses in frequently-burned longleaf pine has remained unknown. We quantified N fixation and estimated N loss from fire across plots of longleaf pine at Fort Benning and Eglin Air Force Base. We found that fixation was insufficient to balance N losses from fire. Progressive N loss from the ecosystem may signal a decline in resiliency. An alternative possibility is that longleaf pine ecosystems have accumulated excess N as a result of land-use change and N deposition. In this case, fire may be a relief mechanism for excess N, critical for returning the ecosystem to its N-poor state. This work is published in Ecology.
As a part of this project, MS student Mike Ament examined how N fixation by the diverse community of herbaceous legumes associated with longleaf pine is regulated by soil nutrients. He found that phosphorus limits growth and fixation rates of all legumes, but there are substantial differences in fixation rates across species. His work was published in Oecologia.
Does nitrogen fixation facilitate biomass gains in tropical forests?
Wet tropical forests are linchpins in the global carbon cycle, removing more atmospheric CO2 annually than any other terrestrial biome. The extent to which soil nitrogen acts to constrain this carbon sink is unclear. We are analyzing long-term data and conducting research in Trinidadian forests where Pentaclethra macroloba, a leguminous N-fixing tree, accounts for ~30% of the stems in the forest. We are quantifying N fixation and analyzing stem growth across permanent forest plots to understand how N fixation is regulated at the tree level and whether fixed N at the plot level is facilitating a long-term pattern in biomass gain among non-fixing trees. Our work is published in Scientific Reports.
Mycorrhizal fungi as drivers and modulators of ecosystem processes
The distribution of mycorrhizal associations across biomes parallels a distinct gradient of soil carbon and nitrogen stocks, which raises the question of whether mycorrhizal traits confer ecosystem properties. We examined how arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) tree seedlings and their fungi differ in strategies for nitrogen acquisition using isotopically-labeled organic matter in lab mesocosms. We found that AM and ECM strategies for N can manifest in differences in soil carbon pools and soil C:N. Our work is published in Ecology.
To test this idea across temperate forests, we analyzed FIA data to determine if forest mycorrhizal identity explained patterns in soil carbon and nitrogen, after accounting for potential effects of climate, phylogeny and leaf traits. We found that increasing ECM dominance was associated with higher soil C:N, which was explained by differences in soil N rather than soil C. This work was published in Journal of Ecology, along with other studies on mycorrhizal fungi and ecosystems in a Special Feature. This work also motivated an oral session for the ESA 2016 meeting, and a meeting report for New Phytologist.
In a related study, MS student Melanie Taylor sought to understand if mycorrhizal fungi have indirect effects on organic matter decomposition. She crossed soils derived from four AM and four ECM tree species with leaf and root litter originating from these species and monitored soil CO2 efflux in a laboratory microcosm experiment. Matching the mycorrhizal identity of litter and soil resulted in a difference in total respiration, such that combinations of AM litters with AM soils lost more C than did combinations of ECM litters with ECM soils. Her work was published in Journal of Ecology.
Funding: UGA Faculty Research Grant, NSF: CWT LTER
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. One of Carly's studies is published in Pedobiologia.
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. This work is published in Ecology.
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.
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.
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