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

Supervisor: Prof. Colin Prentice Cosupervisor: Dr César Terrer Dr Trevor Keenan Dr Oskar Franklin Huanyuan Zhang

2020 May EGU BG3.22

Huanyuan Zhang © . All rights reserved

Introduction

Based on Free Air Carbon Dioxide Enrichment (FACE) and other raised-CO

experiments, new theory (Terrer et al. 2019) has been proposed to explain how the magnitude of the CO

fertilization effect on biomass and biomass production depends on soil phosphorus (in ectomycorrhizal systems) and the soil carbon:nitrogen ratio (in arbuscular mycorrhizal systems). To test whether raised-CO

experiments and Terrer et al.’s theory could explain the known characteristic of land carbon sink, in this study, we

converted biomass increase into land carbon sink.

Fig. 1: Soil C:N and soil P are key plant resources driving the CO

fertilization effect on above-ground biomass. (Terrer et al. 2019)

Fig. 2: Potential above-ground biomass

enhancement in terrestrial ecosystems under

elevated CO2. (Terrer et al. 2019)

Method

So, for any grid of Fig.2 , we only know two points on the NPP enhancement vs CO

plot (Fig.3). We used a saturation curve to fit them, and computed NPP since 1901 (Fig 4) for any grid.

At a given location:

Fig. 2: Potential above-ground biomass enhancement in terrestrial ecosystems under elevated CO2. (Terrer et al. 2019)

Fig. 3: Saturation curve: the relationship between CO

and NPP enhancement (data shown are only for conceptual illustration).

Fig. 4: NPP calculated in this study, based on Terrer et al’s theory (about nutrient limitation) and experimental data from global eCO

experiments

We constructed a simple first order kinetic model to convert NPP into land carbon sink

Method

Heterotrophic respiration

Land Carbon Sink (net ecosystem productivity) = NPP - Heterotrophic respiration

NPP calculated with consideration of nutrient limitation (eCO2_NPP in Fig 5) is way smaller than other independent estimates, so is the derived land carbon sink.

Result

Fig. 5: NPP calculated in this study, based on Terrer et al’s theory (about nutrient limitation) and experimental data from global eCO

experiments, with comparison to Trendy (Sitch et al. 2008) and P_model (Stocker et al. 2019).

Fig. 6: Land carbon sink (NEP) calculated in this study and comparison with Trendy (Vege models) (Sitch et al.

2008) and the Global carbon project (residual land sink)

(Le Quéré et al. 2018).

Possible reason

A recent meta-analysis by showed that (Terrer et al. unpublished), at some elevated CO2 experiment sites, soil carbon pool is expanding while vegetation biomass remains unchanged. This was supported by several other studies (Bloom et al., 2010; Jiang et al., 2019; King et al., 2004).

These suggested that large amount of carbon might be exported to Csoil via Root exudates which were always neglected in field measurements.

Possible reason: The difference between

field measured “NPP” and actual NPP

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Question?

Photo Credits and Reference

Cover image: Aspen Face, provided by Photo: Rick Anderson/Skypixs Aerials, Lake Linden, Michigan, USA

https://news.wisc.edu/munching-bugs-thwart-eager-trees-reducing-the-carbon-sink/.

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Le Quéré, C., Andrew, R.M., Friedlingstein, P., Sitch, S., Hauck, J., Pongratz, J., et al. (2018). Global Carbon Budget 2018. Earth Syst. Sci. Data, 10, 2141–2194.

Stocker, B.D., Wang, H., Smith, N.G., Harrison, S.P., Keenan, T.F., Sandoval, D., et al. (2019). P-model v1.0: An optimality-based light use efficiency model for simulating ecosystem gross primary production. Geosci. Model Dev. Discuss., 2019, 1–59.

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Jiang, M., Medlyn, B. E., Drake, J. E., Duursma, R. A., Anderson, I. C., Barton, C. V. M., Boer, M. M., Carrillo, Y., Castañeda- Gómez, L., Collins, L., Crous, K. Y., De Kauwe, M. G., Emmerson, K. M., Facey, S. L., Gherlenda, A. N., Gimeno, T. E., Hasegawa, S., Johnson, S. N., Macdonald, C. A., … Ellsworth, D. S. (2019). The fate of carbon in a mature forest under carbon dioxide enrichment. BioRxiv, 696898. https://doi.org/10.1101/696898

King, J. S., Hanson, P. J., Bernhardt, E., Deangelis, P., Norby, R. J., & Pregitzer, K. S. (2004). A multiyear synthesis of soil respiration responses to elevated atmospheric CO2 from four forest FACE experiments. Global Change Biology.

https://doi.org/10.1111/j.1529-8817.2003.00789.x

Terrer, C., Jackson, R.B., Prentice, I.C., Keenan, T.F., Kaiser, C., Vicca, S., et al. (2019). Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass. Nat. Clim. Chang. Accessed by

https://www.researchgate.net/publication/335131249_Nitrogen_and_phosphorus_constrain_the_CO2_fertilization_of_globa l_plant_biomass

Acknowledgement

Thanks to Dr Benjamin David Stocker and Prof. Stephen Sitch for help in data processing.

David Sandoval and Wenjia Cai for help in running models.