Quantification of the terrestrial CO2 fertilization effect on plant biomass

CO2 emissions from human activities play a double effect, on one hand CO2 causes global warming; on the other hand, CO2 can stimulate photosynthesis. The increase in photosynthesis can increase plant growth, sucking part of the CO2 in the atmosphere and slowing global warming. This is known as a feedback. However, scientists have feared that future plants may not take as much CO2 as CO2 levels continue rising, because plants need soil nutrients, and not just CO2 for growth. Understanding the limiting role of nutrients in the capacity of plants to help us slow climate change is a priority to predict future climate.

During the last three decades scientists have studied this question using experiments in which plants are grown with elevated concentrations of CO2, to recreate the levels of CO2 predicted for the end of this century. More than a hundred of these experiments have been constructed all over the world. In the picture below, FACE technology (Free-Air CO2 enrichment) is used to fumigate a forest with elevated levels of CO2. These experiments, however, haven’t provided a definitive answer: a few of them have shown that elevated CO2 boosts plant growth and carbon storage; other experiments have shown that soil nitrogen and phosphorus strongly limit the capacity of plants to store additional CO2.

“CO2 experiments give us snapshots of the future. We have put all these separate pieces of information together to predict the capacity of future plants to sequester CO2 from the atmosphere”.

EucFACE experiment in Western Sydney (Australia)

From CO2 experiments, we found that the range of biomass observations from CO2 fertilization experiments is best explained by:

N:Myc + P:Myc,

with N as nitrogen availability quantified as the soil C:N ratio, P as soil available phosphorus and Myc as the type of mycorrhizal association (ectomycorrhizae -ECM- or arbuscular mycorrhizae -AM-).

Model selection identified the most important drivers of the effect in the dataset of CO2 experiments (n = 138), indicating responses to CO2 were modulated by mycorrhizal type. a,b, Meta-analytic scatterplots showing the relationship between the CO2 effect and soil C:N (an indicator of nitrogen availability) in AM studies (n = 86) at 0–10 cm (a), and soil available phosphorus in ECM studies (n = 52) measured by the Bray method at 0–10 cm (b). The type of fumigation technology used (FACE, growth chamber and open top chamber) significantly influenced (P < 0.001) the magnitude of the CO2 effect. Regression lines represent the response found in FACE studies, based on a mixed-effects meta-regression model (pseudo-R2 = 0.94) and their 95% confidence intervals. Dot sizes are drawn proportional to the weights in the model and represent, on average, an increase in atmospheric CO2 of 250 ppm. G, growth chamber; OTC, open top chamber.

Using this statistical model identified through random-forest, we apply a meta-analysis regression to upscale the effect of elevated CO2 on plant biomass for CO2 levels we could unfortunately experience by the end of this century.

The results show that CO2 levels expected by the end of the century will stimulate plant biomass by 12%, storing about 60 additional petagrams of carbon, equivalent to 6 years of current CO2 emissions.

Elevated CO2 doesn’t boost biomass everywhere. Forests appear more capable of overcoming nutrient limitations, with higher potential to sequester additional carbon.

One note of caution. Even in the best-case scenario, plants can just take a fraction of CO2 emissions. Actually, the future CO2 effect we derive from experiments is ~3x lower than the past CO2 effect simulated by models.

Comparison of the magnitude of the effect of elevated CO2 on total biomass and the sensitivity of total biomass to the historical increase in atmospheric CO2 (β) in the period 1980–2010 as estimated by nine vegetation models. Results were standardized to 100 ppm ∆CO2. C, carbon-only models; CN, carbon models with coupled nitrogen cycle; CNP, carbon, nitrogen and phosphorus limitations

More details about this research: Terrer, C., Jackson, R.B., Prentice, I.C. et al. Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass. Nat. Clim. Chang. 9, 684–689 (2019). https://doi.org/10.1038/s41558-019-0545-2