Paleobiogeochemical applications of nitrogen isotopes: terrestrial and lacustrine records from 27 to 15,000 years

Background/Question/Methods

Human activities have doubled the pre-Industrial supply of reactive nitrogen on Earth, and future rates of increase are expected to accelerate. Yet the net balance of nitrogen supply to vegetation and its demand for nitrogen is uncertain, so whether global terrestrial nitrogen availability has been increasing or decreasing due to human activities is unclear. Multi-decadal monitoring of terrestrial nitrogen availability indicates increased nitrogen availability in some ecosystems, but decreased nitrogen availability in others. Century- to millennial-scale reconstructions of ecosystem nitrogen cycling provide historical patterns of nitrogen availability and provide insight into the controls on nitrogen cycling. To investigate patterns of changes to nitrogen cycling on millennial timescales, we measured and synthesized geochemical data, particularly stable nitrogen isotopes, from archived plant foliar material, wood from living trees, and lacustrine sediments. These records range in duration from 27 years to 15,000 years, and they are located primarily in temperate biomes although there is some global representation. These paleorecords provide information about terrestrial nitrogen cycling, although the lacustrine sediments also contain an aquatic filter of the signal.

Results/Conclusions

Here, we present a variety of terrestrial and lacustrine paleorecords from temperate and high-latitude ecosystems that demonstrate declines in stable nitrogen isotopes (d¹⁵N) in the 20^th century toward present. Multiple working hypotheses for this decline include anthropogenic nitrogen deposition, land use change, climate change, changes in nitrogen source, increasing atmospheric carbon dioxide concentrations and terrestrial carbon accumulation. These hypotheses are evaluated by examining similarities and discrepancies between lacustrine and terrestrial records. Additionally, spatial differences among records indicate some regional-scale processes that may be influencing trajectories. Our results provide evidence that the carbon status of terrestrial ecosystems is a driver of nitrogen availability, independent of nitrogen supply rates. Future consequences of anthropogenic global-scale manipulation of the nitrogen cycle depend on both nitrogen supply and nitrogen demand.