Can elevated CO2 experiments explain the magnitude of the land carbon sink?

Zhang, H., Prentice, I.C., Terrer, C., Keenan, T., & Franklin, O. ORCID: https://orcid.org/0000-0002-0376-4140 (2020). Can elevated CO2 experiments explain the magnitude of the land carbon sink? DOI:10.5194/egusphere-egu2020-11589. In: European Geosciences Union (EGU) General Assembly 2020, 4-8 May 2020, Vienna, Austria.

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Abstract

Based on Free Air Carbon Dioxide Enrichment (FACE) and other raised-CO2 experiments (eCO2), new hypotheses have been proposed to explain how the magnitude of the CO2 fertilization effect on biomass and biomass production depends primarily on soil nitrogen and phosphorus availability [1,2]. To test whether these hypotheses and measurements from eCO2 could explain the land carbon sink as independently determined from data and models, we combined a CO2 response curve for biomass production with a simple two-box model of biomass and soil to simulate the evolution of the land carbon sink during the past century. Results were compared to Dynamic Global Vegetation Model (DGVM) results, as reported by the Global Carbon Project, and to results from inversion studies based on atmospheric CO2 measurements. The interannual variability of the modelled land sink was realistic, dominated by the temperature dependence of heterotrophic respiration, and similar to DGVMs results. However, the magnitude of the derived land sink based on eCO2 results was smaller, and its geographical distribution was different to DGVMs average. Sensitivity tests showed that these findings were robust to reasonable variations of parameter values. The smaller sink is due to the smaller amount of vegetation biomass increment documented by eCO2 experiments in comparison with the mean predictions of DGVMs. A land sink closer to the observed one could be produced, however, when incorporating the hypothesis that nutrient-stressed plants export “excess” carbon (generated by increased photosynthesis, but unable to be used for growth) to the soil and that only a fraction of this excess carbon returns to the atmosphere. This hypothesis requires further exploration but hints at a reconciliation between DGVMs that explain the land carbon sink without nutrient limitations, with experimental findings of (sometimes severe) restrictions on CO2 fertilization due to nutrient stress.

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