COS 101-8 - Global nitrogen cycling changed during the Holocene as terrestrial ecosystems accumulated carbon

Wednesday, August 8, 2012: 4:00 PM
D135, Oregon Convention Center
Kendra K. McLauchlan1, Joseph J. Williams1, Joseph M. Craine2 and Elizabeth S. Jeffers3, (1)Geography, Kansas State University, Manhattan, KS, (2)Biology, Kansas State University, Manhattan, KS, (3)School of Geography & Environment, University of Oxford, Oxford, United Kingdom
Background/Question/Methods

Over the past century, humans have drastically altered the global nitrogen (N) cycle by increasing the supply of reactive N (Nr). Yet the net balance of N supply to vegetation and its demand for N is uncertain, so whether global terrestrial N availability has been increasing or decreasing during this time is unclear. Increases in atmospheric deposition of Nr have increased N availability in some ecosystems, but elevated carbon dioxide concentrations and increased carbon (C) storage have decreased N availability in others. Century- to millennial-scale reconstructions of ecosystem N cycling indicate historical patterns of N availability and provide insight into the controls on N cycling. To investigate potential global patterns of changes to N cycling, we assembled geochemical data, particularly stable N isotopes, from sediments of 86 lakes distributed on six continents. These sites ranged broadly in climate, vegetation cover, and size. We synthesized patterns of bulk sedimentary δ15N for the past 15,000 years. This time period encompasses changes during the Pleistocene-Holocene transition— a period of rapid warming, atmospheric CO2increases, and primary succession in newly-deglaciated landscapes— as well as unprecedented anthropogenic global changes during the past 500 years.

Results/Conclusions

Here we show that N availability, as indexed by stable N isotopes (δ15N) in lacustrine sediments from 86 lakes, declined from 15,000 yBP to 7056 yBP ± 597 y, a period of increasing atmospheric CO2 and terrestrial carbon accumulation. While there is no coherent global signal of changes in sedimentary δ15N during the past 500 years, both climatic and anthropogenic variables influenced the direction of change at individual sites. Sites with low mean annual temperature, high elevation, and a high human footprint index have been declining in sedimentary δ15N. As global N cycling appears to have been strongly linked to the C status of the terrestrial biosphere during the Pleistocene-Holocene transition, the lack of recent change in global terrestrial N availability might be caused by elevated atmospheric CO2 and net C sequestration in the biosphere offsetting increased supplies of Nr. Future consequences of anthropogenic global-scale manipulation of the N cycle depend on both N supply and N demand.