Abstract
The magnitude of the nitrogen (N) limitation of terrestrial carbon (C) storage over the 21st century is highly uncertain because of the complex interactions between the terrestrial C and N cycles. We use an ensemble approach to quantify and attribute process-level uncertainty in C-cycle projections by analysing a 30-member ensemble representing published alternative representations of key N cycle processes (stoichiometry, biological nitrogen fixation (BNF) and ecosystem N losses) within the framework of one terrestrial biosphere model. Despite large differences in the simulated present-day N cycle, primarily affecting simulated productivity north of 40°N, ensemble members generally conform with global C-cycle benchmarks for present-day conditions. Ensemble projections for two representative concentration pathways (RCP 2.6 and RCP 8.5) show that the increase in land C storage due to CO2 fertilization is reduced by 24±15% due to N constraints, whereas terrestrial C losses associated with climate change are attenuated by 19±20%. As a result, N cycling reduces projected land C uptake for the years 2006-2099 by 19% (37% decrease to 3% increase) for RCP 2.6, and by 21% (40% decrease to 9% increase) for RCP 8.5. Most of the ensemble spread results from uncertainty in temperate and boreal forests, and is dominated by uncertainty in BNF (10% decrease to 50% increase for RCP 2.6, 5% decrease to 100% increase for RCP 8.5). However, choices about the flexibility of ecosystem C:N ratios and processes controlling ecosystem N losses regionally also play important roles. The findings of this study demonstrate clearly the need for an ensemble approach to quantify likely future terrestrial C-N cycle trajectories. Present-day C-cycle observations only weakly constrain the future ensemble spread, highlighting the need for better observational constraints on large-scale N cycling, and N cycle process responses to global change.
Highlights
Over the last decades, the terrestrial biosphere has sequestered roughly a quarter of the CO2 emitted by anthropogenic activities (Le Quéré et al, 2018)
We analysed future projections of the terrestrial C cycle using an ensemble of 30 C–N cycle models with alternative representations of C–N stoichiometry, biological nitrogen fixation (BNF) and N losses
We find that N dynamics reduce the global CO2 fertilization effect under the representative concentration pathway (RCP) 8.5 scenario by 24 ± 15% and reduce the C loss associated with global warming by 19 ± 20%
Summary
The terrestrial biosphere has sequestered roughly a quarter of the CO2 emitted by anthropogenic activities (Le Quéré et al, 2018). Terrestrial biosphere models (TBMs) that estimate the future C-uptake potential of land vegetation increasingly consider the dynamics of the global N cycle and its linkage to the C cycle (Arora et al, 2019; Zaehle & Dalmonech, 2011) These models generally suggest that N constraints attenuate the land C response to global change (Sokolov et al, 2008; Thornton et al, 2009; Wårlind, Smith, Hickler, & Arneth, 2014; Zaehle, Friedlingstein, & Friend, 2010; Zhang, Wang, Matear, Pitman, & Dai, 2014). We attribute model uncertainty to underlying process formulations and investigate to what extend a range of contemporary C-cycle benchmarks constrain the model spread Based on this analysis, we estimate the anthropogenic emissions compatible with each RCP scenario, given the RCP-specific atmospheric CO2 trajectory, simulated oceanic uptake (Dufresne et al, 2013), as well as the projected land C uptake considering N constraints
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