Current Research

Temperature sensitivity of soil organic matter respiration. Noah Fierer, Kendra McLauchlan and I are just beginning to test some of the controls on the temperature respiration of soil organic matter respiration by microbes. One prominent theory is that the temperature sensitivity of respiration is proportional to biochemical recalcitrance. High activation energy in an enzymatic reaction is associated with high temperature sensitivity. From this, we predict that more biochemically recalcitrant SOM should be more sensitivity to changes in temperature than less biochemically recalcitrant SOM. To test this, we will incubate soils from across North America at different temperatures to determine whether the Carbon-quality temperature sensitivity hypothesis explains the variation in temperature sensitivity observed among soils.

Konza Prairie. I am currently working at Kansas State's Konza Prairie on research associated with better understanding grassland dynamics. We just finished analyzing a long-term database of bison weights for both Konza and The Nature Conservancy's Tallgrass Prairie Preserve in Oklahoma. I am just now finishing up analyzing 25 years of flowering data for Konza to better understand the controls of climate over flowering. Ongoing research also includes characterizing the functional traits of Konza's grassland flora (>500 species), focusing on the importance of water and nutrients in shaping the plant community there.
Continental to global-scale patterns of nutrient cycling. From a synthesis I've led with over 10,000 data points, we've shown global relationships between foliar 15N and climate, the role of mycorrhizal fungi in determining values, and general relationships between N availability and foliar 15N. I've also been working with Texas A&M's GanLab to examine patterns of grass N concentrations in North America as assayed from over 20,000 cattle fecal samples collected over 15 years. Lastly, I've been running experiments that test the determinants of N and P limitation of North American grasslands. For example, by growing the same species of grass in the soils from over 100 grassland sites, we've seen that P limitation limits the response of plant growth to N addition.

 

Past research

Cedar Creek. For my dissertation, I worked at Cedar Creek Natural History Area in Minnesota with Dave Wedin and Terry Chapin. Most of the post-agricultural grasslands there are strongly N-limited and represent a great opportunity to undertand nutrient limitation. Ostensibly, my research began by measuring the longevity of prairie grasses. Unfortunately, most lived longer than my dissertation. Other research focused on the linkages among root traits, leaf traits, whole-plant traits, and nitrogen cycling among prairie species. We also began using soil CO2 flux as a index of belowground carbon allocation and worked to understand the determinants of soil CO2 flux across experimental and natural gradients.
New Zealand. The grasslands of the South Island of New Zealand represent fascinating contrasts. The alpine grasslands are strongly nutrient limited, have no evolutionary interaction with mammals, and the mild winters allow leaves to live for up to three years. There, I worked with Bill Lee of Landcare Research in comparing the roots and leaves of grasses that ranged from alpine tussocks to European grasses in improved pastures. There, we found congruent patterns between root and leaf traits that paralled those I saw at Cedar Creek. This research led to comparisons among New Zealand, Australian, South African, and North American grasslands. Of note were the strong separation between presumably N- and P-limited grasslands. Grasses from Australia and South Africa had much thicker roots (almost like chow mein noodles) when compared to their counterparts in New Zealand and North America, potentially associated with greater dependence on mycorrhizae in P-limited ecosystems. The research also showed that d15N can be used as an indicator of the openness of the N cycle.
Kruger National Park. Kruger National Park is located in the northeast corner of South Africa and is the country's premiere wildlife preserve. There, Willy Stock, Carl Morrow, and I are investigating basic questions regarding the role of N and P limitation in belowground processes and primary production. We have set up 6 sets of N and P addition plots that span a range of rainfall. Half of the plots are located on granite-derived soils and the other half on basalt-derived soils. Carl is investigating different aspects of the microbial assemblages at Kruger. We are also investigating the roles of nutrients in decomposition via a set of comparative experiments under controlled conditions focusing on the roles of microbial nitrogen mining in the decomposition of labile and recalcitrant C at different nitrogen availabilities.
Modeling competition for nutrients. One aspect of nutrient-limited ecosystems that is suprisingly (to me, at least) poorly understood is how plants compete for nutrients. Models of nutrient movement in soils have been in existence for almost 40 years and yet there is no generally agreed upon set of mechanisms by which plants compete for nutrients. In light of this, I've been working to put together models of the movement and uptake of nutrients to test basic ideas regarding competition. In terrestrial systems, the diffusion of nutrients is slow enough such that concentration gradients build up around roots. The modeling research has focused on the preemption of nutrient supplies in contrast to the reduction of nutrient concentrations in determining competitive outcomes.