Changes in global nitrogen cycling during the Holocene epoch.

Area of Science:

• Earth System Science
• Ecology
• Biogeochemistry

Background:

• Human activities have significantly increased reactive nitrogen levels globally, raising concerns about the biosphere's capacity to absorb this influx.
• Understanding past nitrogen cycling responses to environmental change is crucial for predicting future ecosystem behavior.

Purpose of the Study:

• To investigate how terrestrial ecosystems responded to nitrogen and carbon cycling changes during the Pleistocene-Holocene transition.
• To assess the impact of recent anthropogenic nitrogen increases and rising carbon dioxide on global nitrogen cycling.

Main Methods:

• Analysis of stable nitrogen isotope ratios (δ(15)N) in 86 lake sediment records spanning six continents.
• Comparison of nitrogen isotope data with paleoclimate records, atmospheric carbon dioxide concentrations, and terrestrial carbon accumulation estimates.

Main Results:

• Sedimentary δ(15)N values declined significantly from 15,000 to 7,056 years before present, indicating decreasing terrestrial nitrogen availability during the Pleistocene-Holocene transition.
• This decline in nitrogen availability correlated with increasing atmospheric carbon dioxide and terrestrial carbon storage.
• No consistent global trend in sedimentary δ(15)N was observed over the past 500 years, despite rising anthropogenic nitrogen inputs.

Conclusions:

• Terrestrial ecosystems experienced declining nitrogen availability during the last deglaciation, coinciding with rising carbon dioxide and carbon sequestration.
• Modern increases in atmospheric carbon dioxide and biosphere carbon uptake may be offsetting recent anthropogenic reactive nitrogen enrichment in some ecosystems.
• The biosphere's response to current nitrogen pollution is complex and potentially moderated by concurrent changes in carbon cycling.