Research in my lab is diverse, working across scales, taxa, and ecosystems. Historically, we have explored how perturbations like drought, elevated CO2, and habitat fragmentation can influence above- and belowground carbon and nitrogen cycling. These efforts have been funded by the National Science Foundation, NASA, and the Department of Energy. A current, primary focus is the influence of climate change, historic land cover, and post-disturbance system dynamics on interactions between soil microorganisms and soil organic matter, nutrient and carbon flows through microbial populations and communities, and microbial production of CO2 and N2O. We explore these and related questions in temperate and boreal forests and mid-western grasslands. Our work relies on stable isotopes and the isotopic fingerprints imparted by biological and chemical fluxes on ecosystem pools of carbon and nitrogen, both in soils and tree-rings. Much of our isotope data is developed at KU’s W.M. Keck Paleoenvironmental and Environmental Stable Isotope Laboratory, or via cavity ring down spectroscopy. We also employ data describing microbial communities and their activities, developed at the KU Center for Metagenomic Microbial Community Analysis.
On-going research in the lab includes:
The Calhoun Critical Zone Observatory
A team of investigators from multiple universities and led by Duke University’s Dr. Daniel Richter is developing a NSF-funded Critical Zone Observatory at the Calhoun Experimental Forest in Union, South Carolina. We study the extent to which former land use continues to influence current ecosystem functioning, integrating both human and natural forcings. The Billings lab leads KU’s contribution to the project. We focus on biogeochemical legacies of disturbance evident in Calhoun’s recovering forests by contrasting subsoil carbon and nutrient fluxes in recently disturbed landscapes, recovering forests, and old-growth reference sites. The work puts current soil carbon and nutrient fluxes into a context of centuries-old ecological change and contemporary climate change. Work with Dr. Pam Sullivan, also at KU, focuses on understanding how disturbance aboveground can influence subsoil processes. Our efforts are relevant around the globe, including in eastern Kansas, given the intensity of agricultural land use, associated accelerated erosion, and the frequency of abandonment in many regions.
This work is funded by NSF grant EAR-1331846.
Temperature sensitivity of soil organic matter decomposition from enzymes to microbial communities
In collaboration with Dr. Ford Ballantyne at the University of Georgia and Dr. Susan Ziegler at Memorial University, this project explores how the stoichiometry of available resources, activation energies of substrate decay, and adaptive microbial physiology influence the temperature responses of substrate transformation into CO2. We have quantified temperature sensitivities of decay for three key substrate-exo-enzyme reactions that are as close to intrinsic values as is feasible. This work, recently published in Soil Biology and Biochemistry, permits investigators to quantify the extent to which apparent temperature sensitivities of decay deviate from intrinsic values. A related paper, also published in Soil Biology and Biochemistry, assesses these dynamics at multiple pH. We have developed the concept of the C:N flow ratio, which is the ratio of C and N atoms liberated from macromolecules upon decay, and highlighted its use for understanding how changing temperature regimes may alter microbial resource availability. This work is published in Global Change Biology. We also explore how exo-enzyme age and temperature interact to drive substrate decay rates and associated flows of liberated C and N, and how the C use efficiency and C isotopic budgets of microbes growing at a known rate can vary. These avenues of research will ultimately will be useful for predicting CO2 release from soils in a warmer world. We are currently developing a theoretical framework based on these ideas, useful for understanding how microbial allocation of substrate-derived resources responds to multiple substrate landscapes.
This work is funded by NSF grant DEB-0950095.
The influence of warming on soil organic carbon decomposition and formation in boreal forests
Given the large reservoir of soil organic carbon compounds in boreal forests, it is important to learn about the microbial processes governing their transformations. In collaboration with scientists in the Canadian Forest Service and Dr. Susan Ziegler at Memorial University of Newfoundland, we are exploring the transformations of carboniferous compounds in soil profiles along a climate transect of balsam fir forests in Newfoundland. We work along the Newfoundland Labrador Boreal Ecosystem Latitudinal Transect (NL-BELT), seeking to understand how climate influences the formation and transformations of soil organic carbon compounds. Publications from this work are available in Soil Biology and Biochemistry, Biogeochemistry (Biogeochemistry Letters), and Journal of Geophysical Research – Biogeosciences.
This work is funded by the Humber River Basin Project, a consortium of funding agencies in Canada including the Canadian Forest Service and Newfoundland and Labrador’s Forestry Office.
Nitrous oxide and climate change
Scientists do not understand how climate change will influence biosphere-atmosphere fluxes of N2O. Because N2O can be both produced and consumed in soil profiles, addressing this question is particularly challenging. Former Ph.D. student Dr. Lisa Tiemann and I are assessing how enzyme activities of soil microorganisms that cycle nitrogen can change under varying precipitation and temperature regimes, using carbon-rich grassland soils from the KU Field Station as a study system. We used qPCR to determine how relative abundances of genes linked to N2O production and consumption vary with short- and long-term temperature fluctuations. Part of this work is published in Biogeochemistry.
This work was funded by NSF grant DEB-0910343.
Modeling soil erosion as a carbon source or sink
Recent studies indicate that soil erosion may serve as a net sink for carbon, though this is an unresolved controversy in the literature. We have developed a user-friendly, spreadsheet-based model that permits investigators to examine an eroding soil profile of interest and determine the extent to which its erosion results in a net atmospheric carbon source or sink. Rather than asserting that erosion serves as either a net carbon source or sink, the model serves as a tool to assess which parameters are critical determinants of the net carbon flux into or out of an eroding profile. The model was developed in collaboration with Dr. Robert Buddemeier at the Kansas Geological Survey. Those interested in exploring the model can download it on the ‘Modeling erosion and CO2‘ tab, and read about it in our 2010 publication in Global Biogeochemical Cycles. Expanding on this work, we are collaborating with Dr. Rafael Bras‘ group at Georgia Tech to develop a spatially explicit understanding of erosion-CO2 dynamics.
Understanding and predicting tree responses to forest disturbance
In collaboration with Dr. Fred Stephen’s lab at the University of Arkansas, we are exploring forest responses to cyclical drought and a subsequent outbreak of an endemic insect, the red oak borer. This outbreak has resulted in high rates of northern red oak mortality over the past few years, but apparently healthy trees remain interspersed with their dying neighbors. Why did these forests suddenly become so susceptible to this disturbance, and why do some trees appear so unaffected when their neighbors are dying? Examining the carbon and oxygen isotopic composition of cellulose in tree rings from apparently healthy and dying trees, and untangling their different growth strategies, may help us understand different trees’ responses to climate-related disturbances. We also are collaborating with Dr. Sarah Glaser to incorporate the use of nonlinear forecasting models to better understand these forests’ responses to disturbance. Some of this work is in press at Ecosphere.
This work was funded by the U.S. Forest Service and NSF grant DGE-0801522.
Understanding tree carbon and water relations across a precipitation gradient
Encroachment of woody vegetation in grassland regions is a relatively recent phenomenon frequently linked to changing land use, but it is unclear how climate and land use will interact to shape vegetation patterns in the future. We are studying the stable isotopes of carbon and oxygen in tree rings across the Midwestern precipitation gradient to explore how tree carbon and water relations are influenced by climate in this region, and to make predictions about tree species’ abilities to adapt to altered climate in the future.
This work was funded in part by NSF grant DGE-0801522.