Global nitrogen cycling changed during the Holocene as terrestrial ecosystems accumulated carbon

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.