Evidence, causes, and consequences of a global decline in ecosystem nitrogen availability

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Rachel Mason, SESYNC, University of Maryland, Annapolis, MD, Andrew J. Elmore, Appalachian Laboratory, University of Maryland Center for Environmental Science, Frostburg, MD, Joseph M. Craine, Jonah Ventures, Boulder, CO, Robinson W. Fulweiler, Dept. of Earth and Environment and Dept. of Biology, Boston University, Boston, MA, Peter Groffman, Environmental Science Initiative, Advanced Science Research Center at the Graduate Center, CUNY, New York, NY, Mathieu Jonard, UC Louvain, Nina K. Lany, USDA Forest Service, Scott V. Ollinger, Earth Systems Research Center, University of New Hampshire, Durham, NH and Quentin D. Read, SESYNC, Annapolis, MD

Background/Question/Methods

Emerging evidence suggests that ecosystems are experiencing widespread reductions in nitrogen (N) availability. Theoretical predictions and experimental results have suggested that rising atmospheric CO2 and other global changes can reduce the ability of terrestrial plants to acquire N. More recently, long-term observations of N in soil, stream water, wood, and leaves have demonstrated a decrease in N availability in several unmanipulated ecosystems. A reduction in the availability of this important nutrient would likely have profound impacts on the abundance and health of herbivores and the well-being of people who depend on a stable nitrogen cycle for food and livelihoods.

Our current understanding of trends in N availability rests largely on observations of several different N cycle variables from individual monitoring sites. To characterize trends on a large geographic scale using a set of closely-related indicators, we have assembled long-term isotopic datasets into a single time series of N availability. Similar to global temperature reconstructions based on long term measurements of paleoclimatological proxies and the more recent global temperature record, we model relationships between recent foliar d15N data and historical d15N time series from wood cores and lake sediments to derive a record of N availability over the North American continent dating back to the 18th century.
Results/Conclusions

We find that the large-scale N availability record exhibits stability through the early industrial period, followed by a sharp and sustained decline over the past century. While work is ongoing to identify the mechanisms underlying this decline, a combination of rising CO2, longer growing seasons, deacidification, and changing patterns of anthropogenic N deposition seems likely. In some situations, declining N may reverse some of the harmful effects of eutrophication, offering opportunities for ecosystem restoration. However, reductions in N availability may also manifest in lower foliar N concentrations, possibly contributing to ecological and economic issues such as declines in insect populations and slower growth of grazing livestock.
Most current research concentrates on the effects of excess N in the environment, which is a real and serious problem in many regions. However, we argue that a concurrent decline in available N over significant areas of the Earth is emerging as an under-appreciated phenomenon and potential threat. We encourage further research to better understand the extent, magnitude, and likely consequences of this issue, and more concerted monitoring of N availability to ecosystems worldwide.