Abstract
Abstract. Carbon–nitrogen (C–N) interactions regulate N availability for plant growth and for emissions of nitrous oxide (N2O) 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 N2O 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 N2O 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 N2O emission patterns, mechanisms, and C–N coupling. The N2O 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 N2O 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 N2O 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 N2O emissions is the poorly known N2O yield per N lost through gaseous pathways and its sensitivity to soil conditions. The deglacial N2O record provides a constraint for future studies.
Highlights
Nitrous oxide (N2O) is a sensitive proxy of biogeochemical and ecosystem processes on land and in the ocean, and its past atmospheric variations are recorded in ice cores
The results indicate that N2O emissions from land and ocean increased over the deglaciation largely in parallel by 1.8±0.3 and 0.7± 0.3 Tg N yr−1, respectively, relative to the Last Glacial Maximum level
Vegetation is represented by plant functional types (PFTs) that are in competition for resources on each grid cell
Summary
Nitrous oxide (N2O) is a sensitive proxy of biogeochemical and ecosystem processes on land and in the ocean, and its past atmospheric variations are recorded in ice cores. N2O is produced through a variety of pathways both in the ocean and on land and N2O production is closely linked to the flows of C and N (Wrage et al, 2001; Chapuis-Lardy et al, 2006; Kato et al, 2013; Battaglia and Joos, 2018; Trimmer et al, 2016; Babbin et al, 2015; Gilly et al, 2013; Bange, 2008; Butterbach-Bahl et al, 2013; Firestone and Davidson, 1989). N2O is produced from nitrite through nitrification, often termed nitrifier denitrification, through anaerobic ammonium oxidation, chemautotrophic denitrification, abiotic processes, or photoautotrophic organisms in cryptogamic covers (Lenhart et al, 2015; Butterbach-Bahl et al, 2013)
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