Plant Growth Takes A Steep Toll On The Soil – Supposedly

Guest post by Henry Gillard

[This originally arrived as a tip/suggestion for a post. The author was encouraged to turn it into a complete post. Be gentle~cr ]

I loathe reading scientific papers. Too often the grammar is appalling; the objectives of the work being described are unclear; the main body of the paper is a thicket of technical terminology; the summary of what has been found is unintelligible (and sometimes, the summary seems hardly related to the body of the paper).

Occasionally, just occasionally, I read something in a scientific paper that makes me smile. It has a ring of truth, expressed in clear language.  I came across just such a statement yesterday, and I feel the need to share it with a wider audience.

It started with a typically breathless, alarmist warning from Stanford University: 

“Stanford research finds that when elevated carbon dioxide levels drive increased plant growth, it takes a surprisingly steep toll on another big carbon sink: the soil.”

The claim is in a 29-03-2021 report;  https://news.stanford.edu/2021/03/24/one-earths-biggest-carbon-sinks-overestimated/?utm_source=Stanford+Report&utm_campaign=1a26a66d1f-EMAIL_CAMPAIGN_2021_03_26_03_17&utm_medium=email&utm_term=0_29ce9f751e-1a26a66d1f-53979409;

The report summarises a paper published earlier in the week (24-03-2021) in Nature, and adds comments from two of the co-authors.

The lead author lets us into one of his secrets:  “When plants increase biomass, usually there is a decrease in soil carbon storage”.

Wow! Who would have thought that?  When a plant grows, it extracts nutrients and carbon from the soil and converts that into its roots and stem and flowers and seeds. That sounds like something I learned at age 11 in Mr Collins’s Biology class, but Mr Collins was not a Fellow at Lawrence Livermore National Laboratory who worked on this research as a post-doctoral scholar at Stanford University, so perhaps I under-rated him all those years ago.

The senior study author – who is both the Michelle and Kevin Douglas Provostial (whatever that is) Professor at Stanford’s School of Earth, Energy & Environmental Sciences, and also a Senior Fellow at the Stanford Woods Institute for the Environment – confides: “It proved much harder than expected to increase both plant growth and soil carbon.”  At this point it would have been helpful if he had given us an insight into his theory of how the plant interacts with the air and the soil, and what he expected to happen.   But he didn’t.  See my initial grumpy remarks, above, about failure to state objectives.

Despairing of illumination from the Stanford report, I followed the link, https://www.nature.com/articles/s41586-021-03306-8 , to the original Nature article.  Let me quote the entire Abstract, verbatim other than for removal of the references.   Read it slowly and savour it.

Abstract

Terrestrial ecosystems remove about 30 per cent of the carbon dioxide (CO₂) emitted by human activities each year, 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 . Although plant biomass often increases in elevated CO₂ (eCO₂) experiments. SOC has been observed to increase, remain unchanged or even decline. The mechanisms that drive this variation across experiments remain poorly understood, creating uncertainty in climate projections. 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. This trade-off appears to be related to plant nutrient acquisition, in which plants increase their biomass by mining the soil for nutrients, which decreases SOC storage. We found that, overall, SOC stocks increase with eCO₂ in grasslands (8 ± 2 per cent) but not in forests (0 ± 2 per cent), even though plant biomass in grasslands increase less (9 ± 3 per cent) than in forests (23 ± 2 per cent). Ecosystem models do not reproduce this trade-off, which implies that projections of SOC may need to be revised.

I find it beautifully droll to read:  “(From) increases in elevated CO₂ (eCO₂) experiments, SOC (soil organic carbon) has been observed to increase, remain unchanged or even decline. The mechanisms that drive this variation across experiments remain poorly understood.”

Oops. They let the cat out of the bag, there!  I can almost hear them saying, “we’ve got these prestigious jobs, and we’re paid all this money, and we’ve done all this work, but we don’t understand what’s going on”.   Maybe I’m being cynical, but I really do wonder if that is going to be their justification for asking for more grants to do more work.  Either way, I got a smile out of that, and I hope you did too.

Seriously, what concerns me most is that the authors seem to be drawn to a crude causal relationship between the concentration of eCO2 and the change in SOC, in which the former affects the latter through the mechanism of plants “mining the soil for nutrients”.  It really is a shame that they have not made explicit their understanding of how eCO2, SOC and the plants interact, both over a single growing season and over an extended period.  The crucial failing in their model (if we may accord it such an honorific) is that it does not attempt to explain how the SOC can rise at all when plants are growing.   Mere correlation does not signify causation.  And “increase, remain unchanged or even decline” does not even signify correlation.

My interpretation of that interaction – based on the notes that I made in Arthur Collins’s Biology class 1965 – is:

• high eCO2 initially stimulates plant growth;
• plants take in CO2 from the air to build their (carbon-based) biomass;
• the healthy, growing plants take in SOC (soil organic carbon) for the same purpose;
• hence the SOC concentration falls;
• as the nutrients are depleted a plant’s growth slows, so there is less need for the plant to extract further SOC from the soil;
• when plants die, they decay and their constituents become part of the soil.

The corollary of falling SOC concentration – all other things being equal – is that there is more capacity in the soil to absorb more CO2 in the future, by the same mechanisms that have existed since time immemorial.

Looking at their figures:  With elevated eCO2:

• SOC stocks increase in grasslands                   by   8 ± 2 per cent
• SOC stocks increase in forests                         by   0 ± 2 per cent
• plant biomass increases in grasslands             by   9 ± 3 per cent
• plant biomass increases in forests                   by 23 ± 2 per cent

On my interpretation:

• Grasslands don’t build the biomass as efficiently as forests  
• Grassland biomass increases by 9%: the process stimulates initial SOC depletion and subsequent replenishment by nett 8%
• Forest biomass increases by 23%:     the process stimulates initial SOC depletion and subsequent replenishment by nett 0%

On the study’s figures, planting an acre of trees will absorb more CO2 than will be absorbed by planting an acre of corn. Crucially, over the timeframe of the study, the forests tend to leave the SOC concentration unchanged, while the grasslands increase it.  If we look at the sum of biomass increase + SOC increase for grasslands, compared to that for forests, they are not that different:  8+9 as against 23+0.  Is that surprising?   Not to me.  If there were a huge difference then I suspect that the one that performs better would have colonised the other one completely, over the course of the earth’s existence.

The Nature paper and the Stanford report on it do not concern themselves with what happens over decades.  Their key view seems short-term:  “when plant biomass is strongly stimulated by eCO₂, SOC storage declines; conversely, when biomass is weakly stimulated, SOC storage increases”  ie if there is lots of extra CO2 in the air, then the SOC will fall because the plants are growing strongly, and if there is just a little extra CO2 in the air the plants don’t grow as strongly so the SOC rises rather than falls.   

Moreover, the authors have an implicit belief that extracting SOC from the soil is self-evidently bad, because they fail to see the benefits in converting SOC into extra wheat, rice and other crops to feed a growing world population.  But let’s not get sucked off-topic.

So, does this mean that planting trees to reduce CO2 is a waste of time and money?

Well, if you think climate alarmism is baseless, then your answer is bound to be ‘yes’, so let me ask the question only to those who believe there is any merit in lowering atmospheric CO2.

Look at the paper; look at the numbers.  Make up your own mind. I’m not going to tell you what to think.

If you think the concerns in the paper are valid, then ask yourself what you are going to do.  Write to your local mayor and tell him to stop wasting public money on his virtue-signalling tree planting initiative?  (Every town seems to have such a scheme).  And are you going to risk incurring the wrath of the Greens and the hippies and the media?  They may not be amenable to persuasion by this paper.

Or if you think the report has questionable methodology, is poorly written, and has unconvincing conclusions, then are you doing to write to Stanford University and ask them to not waste their money giving another grant to these people?

Or will you do nothing?