Representing the Dynamic Response of Vegetation to Nitrogen Limitation via Biological Nitrogen Fixation in the CLASSIC Land Model

2022

Preprint

The strength of CO2 fertilisation is a major uncertainty across terrestrial biosphere models (TBMs) and is suggested to be overestimated without a representation of nitrogen (N) limitation. Here, we compare TBM projections with and without coupled C and N cycling over alternative future scenarios (the Shared Socioeconomic Pathways) to examine how representing N cycling influences CO2 fertilisation as well as the effects of a comprehensive group of physical and socioeconomic global change drivers. Because elevated N deposition and N mineralisation (driven by elevated temperature) have stimulated terrestrial C sequestration over the historical period, a TBM without N cycling must exaggerate the strength of CO2 fertilisation to compensate for these unrepresented N processes and to reproduce the historical terrestrial C sink. As a result, it cannot reliably project the future terrestrial C sink, overestimating CO2 fertilisation as CO2 increases faster than N deposition and temperature in future scenarios.

Section: Classic Overviewmentioning

confidence: 99%

Section: Classic Overviewmentioning

confidence: 99%

Section: Simulationsmentioning

confidence: 99%

See 1 more Smart Citation

Compensatory Effects between CO2, Nitrogen Deposition, and Temperature in Terrestrial Biosphere Models without Nitrogen Compromise Projections of the Future Terrestrial Carbon Sink

2022

Preprint

“…Another input variable is lightning density, which is based on climatological monthly values. The main processes simulated by the biogeochemical component of CLASSIC include photosynthesis, canopy conductance, tissue turnover, allocation of carbon, and phenology (Arora & Boer, 2005b), dynamic root distribution (Arora & Boer, 2003), maintenance, growth and heterotrophic respiration (Melton et al, 2015), wildfires (Arora & Boer, 2005a;Arora & Melton, 2018), land use change (Arora & Boer, 2010), and nitrogen cycle (Asaadi & Arora, 2021;Kou-Giesbrecht & Arora, 2022b). The land carbon balance depends on how carbon fluxes respond to changes in environmental conditions and land use change.…”

Section: Canadian Land Surface Scheme Including Biogeochemical Cycles...mentioning

confidence: 99%

The Impact of Climate Forcing Biases and the Nitrogen Cycle on Land Carbon Balance Projections

Seiler

et al. 2023

Preprint

Earth System Models (ESMs) project that the terrestrial carbon sink will continue to grow as atmospheric CO$_2$ increases, but this projection is uncertain due to biases in the simulated climate and how ESMs represent ecosystem processes. In particular, the strength of the CO$_2$ fertilization effect, which is modulated by nutrient cycles, varies substantially across models. This study evaluates land carbon balance uncertainties for the Canadian Earth System Model (CanESM) by conducting simulations where the latest version of CanESM’s land surface component is driven offline with raw and bias-adjusted CanESM5 climate forcing data. To quantify the impact of nutrient limitation, we complete simulations where the nitrogen cycle is enabled or disabled. Results show that bias adjustment improves model performance across most ecosystem variables, primarily due to reduced biases in precipitation. Turning the nitrogen cycle on increases the global land carbon sink during the historical period (1995-2014) due to enhanced nitrogen deposition, placing it within the Global Carbon Budget uncertainty range. During the future period (2080-2099), the simulated land carbon sink increases in response to bias adjustment and decreases in response to the dynamic carbon-nitrogen interaction, leading to a net decrease when both factors are acting together. The dominating impact of the nitrogen cycle demonstrates the importance of representing nutrient limitation in ESMs. Such efforts may produce more robust carbon balance projections in support of global climate change mitigation policies such as the 2015 Paris Agreement.

“…By examining a single TBM with and without coupled C and N cycling, we can isolate the impact of explicitly representing coupled C and N cycling whereas intercomparisons across TBMs with and without N cycling are confounded by other structural and parametric differences between TBMs that may obscure the effects of N cycling. CLASSIC represents both flexible vegetation C:N stoichiometry and the upregulation of symbiotic biological N fixation under N limitation (described and evaluated in Asaadi and Arora (2021) and Kou‐Giesbrecht and Arora (2022)) thereby presenting an advanced representation of coupled C and N cycling in a TBM. We evaluate the role of N cycling under individual and combined contributions of a comprehensive group of physical and socioeconomic global change drivers: CO 2 , climate, N deposition, and land use (crop area and N fertilization).…”

Section: Introductionmentioning

confidence: 99%

Compensatory Effects Between CO₂, Nitrogen Deposition, and Nitrogen Fertilization in Terrestrial Biosphere Models Without Nitrogen Compromise Projections of the Future Terrestrial Carbon Sink

Geophysical Research Letters

2023

Self Cite

Although terrestrial biosphere models (TBMs) with and without nitrogen cycling successfully reproduce the historical terrestrial carbon sink, the influence of nitrogen cycling under interacting and intensifying global change drivers in the future is unclear. Here, we compare TBM projections with and without nitrogen cycling over alternative future scenarios (the Shared Socioeconomic Pathways) to examine how representing nitrogen cycling influences CO2 fertilization as well as the effects of a comprehensive group of physical and socioeconomic global change drivers. Because elevated nitrogen deposition and nitrogen fertilization have stimulated terrestrial carbon sequestration over the historical period, a model without nitrogen cycling must exaggerate the strength of CO2 fertilization to compensate for these unrepresented nitrogen processes and to reproduce the historical terrestrial carbon sink. As a result, it cannot realistically project the future terrestrial carbon sink, overestimating CO2 fertilization as the trajectories of CO2, nitrogen deposition and nitrogen fertilization diverge in future scenarios.