A carbon sink that can’t be filled (misleading headline—“productive forests” may release more CO₂)

[OKIsItJustMe]

http://ose.utsc.utoronto.ca/ose/story.php?id=8109

[font face=Serif][font size=5]A carbon sink that can’t be filled[/font]

Jan 6, 2016 | Patchen Barss

[font size=3]Forests can store as much as 45 percent of the world’s terrestrial carbon, making them a critical part of the process of regulating climate change. As global temperatures rise, though, the organic matter in forests appears to break down more quickly, accelerating the release of carbon into the atmosphere.

This surprising conclusion comes out of a long-term study that was intended to find means to mitigate global warming, not exacerbate it.

…

Climate change could lead to “more productive” forests – bigger trees and more vegetation. This productivity would naturally increase the amount of litter, and therefore the amount of carbon sinking into the soil in the form of organic matter.

But in a paper published recently in the journal Biogeochemistry, Simpson and her co-authors describe how they simulated this change by doubling the amount of litter in sections of the forest in the hope that the soil could absorb more carbon. Instead, the increased litter stimulated bacterial and fungal activity. Organic matter broke down more quickly, eliminating any carbon storage benefit and releasing more CO₂ into the atmosphere.

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http://dx.doi.org/10.1007/s10533-015-0073-8

[jeff47]

1. Uhhhh.....

That litter was created from recently-fixed CO2. And since 100% of the litter can not decompose, it still will be a carbon sink.

This study, as reported in this article, doesn't make much sense. (Most likely the reporter didn't understand)

[OKIsItJustMe]

2. I believe you misunderstand

Normally, the litter accumulates and carbon is sequestered in the soil. So, the thinking is, if there was more litter, more carbon would be sequestered. (Seems reasonable. Right?) However, this study suggests otherwise…


As the researcher put it:

[font face=Serif][font size=3]…

“Altering the litter did more harm than good,” Simpson says. “Ours was a human manipulation, but it could as easily be altered through climate change.”

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http://dx.doi.org/10.1007/s10533-015-0073-8

[font face=Serif][font size=5]Soil warming and nitrogen deposition alter soil organic matter composition at the molecular-level[/font]

[font size=4]Abstract[/font]
[font size=3]Rising temperatures and nitrogen (N) deposition, both aspects of global environmental change, are proposed to alter soil organic matter (SOM) biogeochemistry. For example, increased plant productivity and enhanced microbial decomposition of litter and SOM may reduce soil carbon stocks and fertility. To better understand SOM biogeochemical shifts at the molecular-level, we employed an array of biomarker and nuclear magnetic resonance (NMR) techniques to investigate the composition and degradation of SOM components in the forest floor and mineral soil horizons of warmed (5 °C above average soil temperature) and N fertilized (5 g m[font size=1]⁻²[/font] year[font size=1]⁻¹[/font] N applied in the growing season) plots from the soil warming × nitrogen addition study at the Harvard Forest, MA, USA. Biomarker analyses indicated increased plant-derived inputs into the forest floor under N fertilization. Soil warming promoted the decomposition of plant-derived aliphatic and cyclic compounds in the forest floor. Cutin degradation was observed in the heated forest floor which also exhibited relatively higher microbial activity. Lignin oxidation was also observed but was most pronounced in the mineral horizon of the heated plots. These results suggest that continued soil warming may promote the degradation of lignin- and cuticle-derived SOM. N fertilization also enhanced lignin oxidation but to a lesser extent likely due to a decline in microbial activity. [font size=1]¹[/font]H NMR spectra of the mineral soils revealed enrichment of plant-derived alkyl structures and microbial-derived organic matter with both soil warming and N fertilization. Overall, this study shows that the decomposition and accumulation of molecularly distinct SOM components occurs with soil warming and N amendment and may subsequently alter soil biogeochemical cycling.[/font]

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[font size=4]Conclusions[/font]
[font size=3]To the best of our knowledge, this is the first study that combines two powerful molecular-level techniques (biomarkers and NMR methods) to determine the responses of SOM to rising temperature, N fertilization and both treatments. After 4 years of soil warming, N addition and soil warming + N addition, we observed distinct differences in the SOM composition. Consistent with our hypotheses, soil warming decreased labile SOM components and resulted in accelerated degradation of lignin and cutin. Soil warming likely stimulates the biodegradation of soil microbes which results in the enhanced degradation of more stable and recalcitrant forms of SOM. N fertilization did not markedly alter the SOM composition of the soil, likely through the suppression of microbial activity. However, we did observe some indication of slowed lignin degradation (oxidized lignin phenols along with intact macromolecular lignin by NMR) and the accumulation of microbial-derived cell wall components. When soil warming + N fertilization were combined, the results more closely mirrored those observed with N fertilization than soil warming alone. For instance, we observed the accumulation of labile SOM and more stable forms of SOM likely due to the decline in microbial activity. Interestingly, N amendment impacts on SOM superseded those observed with soil warming when the two treatments were performed simultaneously. These results suggest that future temperature increases and N deposition will control SOM decomposition patterns through changes in the soil microbial community and substrate utilization. Although we have identified molecular-level shifts in SOM composition with two important aspects of global environmental change, detailed comparisons with long-term studies combined with these molecular approaches will assist in the long-term elucidation of SOM biogeochemistry in forests.[/font]

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[jeff47]

3. Not quite.

Normally, the litter accumulates and carbon is sequestered in the soil. So, the thinking is, if there was more litter, more carbon would be sequestered. (Seems reasonable. Right?) However, this study suggests otherwise…

Not quite.

The article in the OP is written as if the extra litter means zero sequestration, possibly even a net CO2 release. This is impossible.

In order for a forest to not be a sink at all, all of the absorbed CO2 would have to be released. And that can't happen - no decomposition is 100% complete. And you can't release more CO2 than you took in (net CO2 release), since the carbon has to come from somewhere.

What this simulation shows is previous models may have overestimated how much of a sink a forest is, not that forests stop being a sink altogether.

[OKIsItJustMe]

4. No, I don’t believe that is what it says

i.e. it does not claim that the forest stops being a sink.

[kristopher]

5. "eliminating any carbon storage benefit"

I agree with you, but I'm guessing the origin of the impression might trace to a misreading of this line in the OP, "Organic matter broke down more quickly, eliminating any carbon storage benefit and releasing more CO₂ into the atmosphere."

It could be better phrased as "eliminating the expected improvement in carbon storage benefits".