N<sub>2</sub>O changes from the Last Glacial Maximum to the preindustrial – Part 2: terrestrial N<sub>2</sub>O emissions and carbon–nitrogen cycle interactions
oleh: F. Joos, R. Spahni, B. D. Stocker, B. D. Stocker, S. Lienert, J. Müller, H. Fischer, J. Schmitt, I. C. Prentice, I. C. Prentice, I. C. Prentice, B. Otto-Bliesner, Z. Liu
| Format: | Article |
|---|---|
| Diterbitkan: | Copernicus Publications 2020-07-01 |
Deskripsi
<p>Carbon–nitrogen (C–N) interactions regulate N availability for plant growth and for emissions of nitrous oxide (<span class="inline-formula">N<sub>2</sub>O</span>) and the uptake of carbon dioxide. Future projections of these terrestrial greenhouse gas fluxes are strikingly divergent, leading to major uncertainties in projected global warming. Here we analyse the large increase in terrestrial <span class="inline-formula">N<sub>2</sub>O</span> emissions over the past 21 000 years as reconstructed from ice-core isotopic data and presented in part 1 of this study. Remarkably, the increase occurred in two steps, each realized over decades and within a maximum of 2 centuries, at the onsets of the major deglacial Northern Hemisphere warming events. The data suggest a highly dynamic and responsive global N cycle. The increase may be explained by an increase in the flux of reactive N entering and leaving ecosystems or by an increase in <span class="inline-formula">N<sub>2</sub>O</span> yield per unit N converted. We applied the LPX-Bern dynamic global vegetation model in deglacial simulations forced with Earth system model climate data to investigate <span class="inline-formula">N<sub>2</sub>O</span> emission patterns, mechanisms, and C–N coupling. The <span class="inline-formula">N<sub>2</sub>O</span> emission changes are mainly attributed to changes in temperature and precipitation and the loss of land due to sea-level rise. LPX-Bern simulates a deglacial increase in <span class="inline-formula">N<sub>2</sub>O</span> emissions but underestimates the reconstructed increase by 47 %. Assuming time-independent N sources in the model to mimic progressive N limitation of plant growth results in a decrease in <span class="inline-formula">N<sub>2</sub>O</span> emissions in contrast to the reconstruction. Our results appear consistent with suggestions of (a) biological controls on ecosystem N acquisition and (b) flexibility in the coupling of the C and N cycles during periods of rapid environmental change. A dominant uncertainty in the explanation of the reconstructed <span class="inline-formula">N<sub>2</sub>O</span> emissions is the poorly known <span class="inline-formula">N<sub>2</sub>O</span> yield per N lost through gaseous pathways and its sensitivity to soil conditions. The deglacial <span class="inline-formula">N<sub>2</sub>O</span> record provides a constraint for future studies.</p>