N2O changes from the Last Glacial Maximum to the preindustrial – Part 2: terrestrial N2O emissions and carbon–nitrogen cycle interactions

• Published in: Biogeosciences
• Main Authors: Joos, Fortunat, Spahni, Renato, Stocker, Benjamin D., Lienert, Sebastian, Müller, Jurek, Fischer, Hubertus, Schmitt, Jochen, Prentice, I. Colin, Otto-Bliesner, Bette, Liu, Zhengyu
• Format: Text
• Language: English
• Published: 2020
• Subjects: ice core
• Online Access: https://doi.org/10.5194/bg-17-3511-2020; https://www.biogeosciences.net/17/3511/2020/

Carbon–nitrogen (C–N) interactions regulate N availability for plant growth and for emissions of nitrous oxide ( N 2 O ) 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 N 2 O 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 N 2 O 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 N 2 O emission patterns, mechanisms, and C–N coupling. The N 2 O 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 N 2 O 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 N 2 O 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 N 2 O emissions is the poorly known N 2 O yield per N lost through gaseous pathways and its sensitivity to soil conditions. The deglacial N 2 O record provides a constraint for future studies.