A trade-off between plant and soil carbon storage under elevated CO₂

Area of Science:

• Environmental Science
• Ecology
• Climate Science

Background:

• Terrestrial ecosystems act as a crucial carbon sink, absorbing approximately 30% of annual anthropogenic carbon dioxide (CO2) emissions.
• The long-term stability of this carbon sink is uncertain due to varying responses of plant biomass and soil organic carbon (SOC) to elevated CO2 (eCO2).
• Previous experiments show inconsistent effects of eCO2 on SOC, leading to significant uncertainty in climate change models.

Purpose of the Study:

• To synthesize data from numerous eCO2 experiments to understand the factors influencing SOC stock responses.
• To identify the relationship between plant biomass stimulation and SOC storage under eCO2 conditions.
• To evaluate the performance of ecosystem models in reproducing observed SOC dynamics.

Main Methods:

• A meta-analysis was conducted, synthesizing data from 108 eCO2 experiments globally.
• The study analyzed the relationship between eCO2-induced changes in plant biomass and SOC stocks.
• Statistical analysis was used to identify key drivers of SOC responses and compare results between grasslands and forests.

Main Results:

• A significant trade-off was identified: strong stimulation of plant biomass by eCO2 led to a decline in SOC storage, while weak stimulation resulted in increased SOC.
• This trade-off is linked to plant nutrient acquisition strategies, where increased biomass may come at the expense of SOC.
• Overall, SOC stocks increased in grasslands (8% ± 2%) but showed no significant change in forests (0% ± 2%) under eCO2, despite greater biomass increases in forests.

Conclusions:

• The findings reveal a critical, previously underestimated, mechanism influencing terrestrial carbon storage under rising CO2 levels.
• Ecosystem models currently fail to capture this biomass-SOC trade-off, necessitating revisions for accurate climate projections.
• Understanding these dynamics is vital for predicting the future capacity of terrestrial ecosystems to mitigate climate change.