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

[NNadir]

The paper I'll discuss in this post is this one: Terrer, C., Phillips, R.P., Hungate, B.A. et al. A trade-off between plant and soil carbon storage under elevated CO2. Nature 591, 599–603 (2021).

The abstract is open sourced, but it is useful to excerpt here to get some abbreviations understood:

Terrestrial ecosystems remove about 30 per cent of the carbon dioxide (CO2) emitted by human activities each year1, yet the persistence of this carbon sink depends partly on how plant biomass and soil organic carbon (SOC) stocks respond to future increases in atmospheric CO2 (refs. 2,3). Although plant biomass often increases in elevated CO2 (eCO2) experiments4,5,6, SOC has been observed to increase, remain unchanged or even decline7. The mechanisms that drive this variation across experiments remain poorly understood, creating uncertainty in climate projections8,9. Here we synthesized data from 108 eCO2 experiments and found that the effect of eCO2 on SOC stocks is best explained by a negative relationship with plant biomass: when plant biomass is strongly stimulated by eCO2, SOC storage declines; conversely, when biomass is weakly stimulated, SOC storage increases.

It currently seems likely to me that this Sunday, a new weekly average record for concentrations of the dangerous fossil fuel waste carbon dioxide will be set at the Mauna Loa CO2 observatory, probably near, at, or above 418 ppm. (The previous all time record was established in the week beginning February 28 of this year, 417.97 ppm.)

Last year, for the week beginning March 22, 2020, the weekly average reading at Mauna Loa was 415.52. The high for the year, which established a new record until this year, was 417.43, 1.89 ppm higher, observed for the week beginning May 24, 2020. All this suggests that we may see a weekly average reading of close to 420 ppm this year, possibly above it.

We are doing nothing to address climate change. Nothing beyond reciting the usual myths on our side of the political spectrum (so called "renewable energy" will save us) or simply engaging in lying and denial on the right ("climate change is not happening" or " it is not driven by human actions" or "it can be addressed by 'market' solutions.&quot

We may think if we plant a lot of trees, we can ameliorate the problem, but the fact is that we are destroying forests and other wildernesses at an alarming rate, and many pristine areas are now being rendered into industrial parks for land intensive and material intensive "renewable energy," which has not worked to address climate change, is not working to address climate change, and will not work to address climate change. But even if we were not engaged in these massive efforts to destroy forests, it appears, if this paper is correct, that improvements in the mass of biomass on the surface will be offset by decreased soil carbon storage.

From the introduction to the paper:

The future of the land sink, especially of SOC, is particularly uncertain9. Soils can become either sources or sinks of carbon with rising levels of atmospheric CO2, depending on the prevalence of gains via photosynthesis or losses via respiration9,10. This uncertainty in terrestrial ecosystem model projections reflects uncertainty in both the mechanisms and the parameter values controlling SOC cycling under eCO211.

Plant growth generally increases in response to eCO24,12, with soil nutrients identified as the dominant factor explaining variability across experiments12,13,14,15. The effect of eCO2 on SOC stocks (?soil) is more equivocal. Although the expectation is that SOC will accrue as eCO2 increases plant growth16, a few experiments show increases in ?soil, many show no change, and some even show losses7. The observed variation in ?soil across experiments is puzzling, and there is wide disagreement regarding the dominant mechanisms explaining this variation7,17,18.

A positive relationship between the effects of eCO2 on plant biomass and SOC pools is expected if increased plant production under eCO2 increases carbon inputs (litter) into the soil. Indeed, a positive relationship between inputs and SOC storage is formalized in first-order kinetics16 and is applied in most terrestrial ecosystem models19,20. Because the effect of eCO2 on plant aboveground biomass (?plant) is strongly correlated with the effect of eCO2 on litter production (Extended Data Fig. 1a, r = 0.81) and on root production21, a positive relationship between ?plant and ?soil can thus be expected from first-order kinetics. This hypothesis, however, ignores SOC losses associated with accelerated soil organic matter decomposition sometimes observed under eCO27,18. Plants acquire limiting resources from soils through carbon investment belowground in root growth, exudates and symbiotic bacteria and fungi. Accelerated decomposition of soil organic matter fuelled by plant carbon inputs can enable plant nutrient uptake (the “priming effect”22). The return on this belowground carbon investment is an increase in aboveground biomass production15. However, the priming effect can decrease SOC5. A negative relationship between ?plant and ?soil may thus emerge through the economics of plant resource acquisition.

Here, we evaluate the mechanisms of ?soil, including its relationship with ?plant, by synthesizing 268 observations of ?soil from 108 eCO2 experiments spanning the globe with coupled ?plant??soil data (Supplementary Table 1) using meta-analysis techniques...

Some graphics from the paper:

Fig. 1: Meta-analysis of the effect of eCO2 on percentage SOC across different factors.



The caption:

n = 108. Overall means and 95% confidence intervals are given; we interpret CO2 effects when the zero line is not crossed by the confidence intervals. Arrows represent 95% confidence intervals that extend beyond the limits of the plot. Soil carbon stocks represent values in ambient CO2 plots as a continuous variable, here expressed as intervals of equal sample size for illustration purposes. Values in parentheses are sample sizes. CO2 effects represent, on average, an increase in CO2 from 372 parts per million (ppm) to 616 ppm. FACE, Free Air CO2 Enrichment; OTC, Open Top Chamber; AM-ER, mix of AM and ericoid mycorrhizal; N-fixer, fixation of atmospheric nitrogen.

The authors state that overall, higher CO2 levels result, according to their meta analysis, in higher soil organic carbon (SOC).

However SOC is negatively correlated with surface biomass:

Fig. 2: Elevated CO2 experiments show an inverse relationship between the effects of eCO2 on plant biomass and SOC stocks due to plant nutrient-acquisition.



The caption:

This inverse relationship (a) can be explained by the different efficiencies in plant nutrient uptake (c) between AM and ECM nutrient-acquisition strategies driving opposite effects on plant biomass and SOC pools (b), including MAOM stocks (d). The regression line in a is based on a quadratic mixed-effects meta-regression model and 95% confidence interval (R2 = 0.67, P < 0.0001, n = 38). Dots in a represent the individual experiments in the meta-analysis, with dot sizes proportional to model weights. Dots in b?d represent overall effect sizes from a meta-analysis and 95% confidence intervals. Data shown here are for non-fertilized experiments (see Extended Data Fig. 3 for nutrient-fertilized experiments).

Fig. 3: Effect of eCO2 (about 240 ppm) on SOC stocks scaled up from 108 CO2 experiments.



The caption:

a, b, Relative effect of elevated CO2 on SOC scaled up on the basis of a meta-forest approach with data from CO2 experiments, with the spatial distribution shown on a map (a) and aggregated by ecosystem type (b). c shows the standard error in a, and d shows the standard error in b. Green dots in c represent the location of the CO2 experiments included in the analysis. e, f, Difference between expected CO2 effects on SOC stocks based on CMIP5 models and scaled up on the basis of experiments (shown in a) with the spatial distribution shown on a map (e) and aggregated by ecosystem type (f). Expected values result from the relationship between ?soil and ?plant coded in models. Positive values (reddish colours) indicate an overestimation by models; negative values (bluish colours) indicate an underestimation by models. Shaded areas between –15 to 15 and from 60° to 90° in latitude represent ecosystems not well sampled by experiments that we excluded from the analysis. Boxplots show the median, the first to third quartile, the 1.5× interquartile ranges, and outliers. On average, the difference between elevated CO2 and control plots in the experiments is 240 ppm.

Fig. 4: Comparison of modelled and measured relationships between aboveground biomass and SOC responses to CO2.




The caption:

a, Relationship observed (blue) and modelled (red) across six eCO2 experiments. Model results are based on 12 models applied to the same six experiments with a common forcing and initialization protocol. The experiments included are: Duke FACE (DUKE), Kennedy Space Center (KSCO), Nevada Desert FACE (NDFF), Oak Ridge FACE (ORNL), Prairie PHACE (PHAC), and Rhinelander (RHIN). The regression line across observations in a is based on a quadratic meta-regression model. Modelled simulations averaged in a for each experiment are from the FACE-MDS project phase 2 (ref. 34). b, c, Global-scale relationship simulated by ecosystem models from the TRENDY ensemble for the historical increase in CO2 since the year 1700 (b) and from the CMIP5 ensemble for an increase in CO2 from 372 ppm to 616 ppm as in eCO2 experiments (c). Dotted lines are the 1:1 line.

Some conclusions from the paper:

In summary, our synthesis of experiments shows that SOC stocks can increase by approximately 5% in response to a 65% step increase in CO2 concentrations, with a strong coupling between CO2-driven changes in plant aboveground biomass and SOC. However, the coupling between plant biomass and soils is an inverse relationship (Fig. 2a, Extended Data Fig. 1b), opposite to that simulated by many ecosystem models (Fig. 4). The effect of eCO2 on SOC storage is dependent on a fine balance between changes in inputs and changes in turnover18, where the latter is dependent on root?microbe?mineral interactions in the rhizosphere. Our results suggest that rhizosphere responses, and especially priming, explain much of the variation in ?soil across experiments (Fig. 2). Most models focus on carbon inputs and underestimate rhizosphere effects11,20,35, probably explaining the disagreement in ?soil between observations and models (Figs. 3, 4). We propose a framework to explain ?soil based on nutrient acquisition strategies15,36,37. On one end of the spectrum, substantial acquisition of soil N is possible via priming5 in ECM-associated plants, causing a stronger plant biomass sink at the expense of SOC accrual. On the other end, low nutrient availability strongly constrains the plant biomass sink38 in AM-associated plants.

This all suggests that forests have a limited capacity to address ever increasing carbon dioxide concentrations. The authors suggest that in terms of soil carbon, grasslands are more efficient than forests.

I note that it took many millions of years for the carbon we've released in the last century to be sequestered by photosynthesis and decay. This should be a sobering thought, given our propensity to believe sunlight alone can save the world. It won't be a sobering thought, but it should be.

Have a nice day tomorrow.

[Backseat Driver]

1. What's it all about, Alfie,...er NNadir??? I think you're telling me something important!!!

Here's another article from 2 years ago that should make our heads spin in terror? hope?: How to proceed...

https://www.theatlantic.com/magazine/archive/2018/03/charles-mann-can-planet-earth-feed-10-billion-people/550928/
(Yes, quite a graphic!)

Can Planet Earth Feed 10 Billion People?
Humanity has 30 years to find out.
Story by Charles C. Mann
MARCH 2018 ISSUE - THE ATLANTIC

(This article is long, as long as NNadir's offering, and just as puzzling to pick a good path through, but read it through for another prospective of likely effects on the limits of resources on our planet Earth in the time of climate change.)
==================================================================================
OK - go: All parents remember the moment when they first held their children—the tiny crumpled face, an entire new person, emerging from the hospital blanket. I extended my hands and took my daughter in my arms. I was so overwhelmed that I could hardly think.

Afterward I wandered outside so that mother and child could rest. It was three in the morning, late February in New England. There was ice on the sidewalk and a cold drizzle in the air. As I stepped from the curb, a thought popped into my head: When my daughter is my age, almost 10 billion people will be walking the Earth. I stopped midstride. I thought, How is that going to work?

[snip] all but the last paragraph...

My daughter is 19 now, a sophomore in college. In 2050, she will be middle-aged. It will be up to her generation to set up the institutions, laws, and customs that will provide for basic human needs in the world of 10 billion. Every generation decides the future, but the choices made by my children’s generation will resonate for as long as demographers can foresee. Wizard or Prophet? The choice will be less about what this generation thinks is feasible than what it thinks is good.