The Global N Cycle

Human activities have driven profound changes to the global nitrogen (N) cycle, with estimates indicating that anthropogenic activities like food and energy production have roughly doubled the annual N added to terrestrial ecosystems via natural processes. The benefits of anthropogenic N are obvious; it fueled the Green Revolution, and food produced with N fertilizer feeds billions of people across the planet. However, anthropogenic N inputs also have a range of undesired ecological effects, and I am interested in understanding how increases in environmental N availability affect both humans and the ecosystems we depend on. I collaborate with several groups on a range of projects, including plot-level estimates of the effects of N deposition on soil and plant processes and diversity, analyses of the effects of salmon-derived nutrients on ecosystem processes, and global-scale estimates of the effects of N on human health, among others.

In addition, while it is unquestionable that humans have sharply increased the rate at which atmospheric N is fixed into reactive forms, comparisons between anthropogenic and natural inputs are only tenuous at best. Biological nitrogen fixation (BNF) provides the largest source of “new” N to natural ecosystems, and therefore estimates of the degree to which the N cycle has been perturbed require a thorough understanding of N inputs via BNF. However, for a number of reasons, both small- and large-scale rates of N fixation in natural ecosystems are very poorly constrained. Thus, I am interested in understanding the biotic and abiotic controls over free-living N fixation, and am using a suite of experimental manipulations to address this question. With scientists at the University of Colorado, I am investigating how nutrient availability regulates N fixation in both soils and in the canopies of tropical rain forests. N fixation rate estimates are now being augmented with detailed genetic (nifH) characterizations of the N-fixer community to determine what regulates the abundance and diversity of N fixing organisms, and how microbial community composition influences N fixation rates.

Finally, I am a member of the International Nitrogen Initiative (INI), an international scientific organization focused on understanding the global N cycle, how it is changing, and how our science may best be used to inform policy that could help mitigate the negative effects of N enrichment in the environment. Within the INI, I am leading an effort to improve estimates of N fixation in natural ecosystems using a “top-down” strategy that includes a modeling and an ecosystem N balance approach. This effort involves collaborators from the University of Colorado, the University of California Davis, Brazil and the UK.