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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Apr 2019

Research article | 15 Apr 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

N2O changes from the Last Glacial Maximum to the preindustrial – Part II: Terrestrial N2O emissions constrain carbon-nitrogen interactions

Fortunat Joos1, Renato Spahni1, Benjamin D. Stocker2, Sebastian Lienert1, Jurek Müller1, Hubertus Fischer1, Jochen Schmitt1, I. Colin Prentice3, Bette Otto-Bliesner4, and Zhengyu Liu5 Fortunat Joos et al.
  • 1Climate and Environmental Physics, Physics Institute andOeschger Centre for Climate Change Research, University of Bern, Bern, CH-3012, Switzerland
  • 2CREAF, E08193 Bellaterra (Cerdanyola del Vallès), Spain
  • 3AXA Chair of Biosphere and Climate Impacts, Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot SL5 7PY, UK
  • 4Climate and Global Dynamics Division, National Center for Atmospheric Research, Boulder, CO 80307-3000, USA
  • 5Atmospheric Science Program, Department of Geography, Ohio State University, OH 43210, USA

Abstract. Land ecosystems currently take up a quarter of the human-caused carbon dioxide emissions. Future projections of this carbon sink are strikingly divergent, leading to major uncertainties in projected global warming. This situation partly reflects our insufficient understanding of carbon-nitrogen (C-N) interactions and particularly of the controls on biological N fixation (BNF). It is difficult to infer ecosystem responses for century time scales, relevant for global warming, from the comparatively short instrumental records and laboratory or field experiments. Here we analyse terrestrial emissions of nitrous oxide (N2O) over the past 21,000 years as reconstructed from ice-core isotopic data and presented in part I of this study. Changing N2O emissions are interpreted to reflect changes in ecosystem N loss, plant available N, and BNF. The ice-core data reveal a 40 % increase in N2O emissions over the deglaciation, suggestive of a highly dynamic global N cycle whereby sources of plant-available N adjust to meet plant N demand and loss fluxes. Remarkably, the increase occurred in two steps, each realized within maximum two centuries, at the onsets of the northern hemisphere warming events around 14,600 and 11,700 years ago. We applied the LPX-Bern dynamic global vegetation model in deglacial simulations forced with Earth System Model climate data to investigate N2O emission patterns, mechanisms, and C-N coupling. The reconstructed increase in terrestrial emissions is broadly reproduced by the model, given the assumption that BNF positively responds to increasing N demand by plants. In contrast, assuming time- and demand-independent levels of BNF in the model to mimic progressive N limitation of plant growth results in N2O emissions that are incompatible with the reconstruction. Our results suggest the existence of (a) strong biological controls on ecosystem N acquisition, and (b) flexibility in the coupling of the C and N cycles during periods of rapid environmental change.

Fortunat Joos et al.
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Fortunat Joos et al.
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Publications Copernicus
Short summary
Will nitrogen (N) limitation reduce the future land carbon (C) sink? We analyse global land N2O emissions reconstructed from ice core isotope data. These show a large emission increase during past abrupt warming events. Our new data and transient simulations of ecosystem N cycling and global N2O emissions over the past 21,000 years support the view of a dynamic, flexible C-N cycle. Biological N sources will likely adjust to support future land carbon uptake and alleviate N limitation.
Will nitrogen (N) limitation reduce the future land carbon (C) sink? We analyse global land N2O...