Comment by mountaintimefrm | Hacker News Reader

Comment by mountaintimefrm | Hacker News Reader

mountaintimefrm

So having read the article summary, then the research paper, then reading the article summary again... it seems that the summary isn't at all an accurate summary of the research paper, but rather a narrow focus on the suboptimal cases where tree growing isn't super effective at sequestering carbon. It sort of completely ignores the cases where they observed enhanced carbon sequestration, increases in soil organic carbon, enhanced soil nitrogen availability, etc.

The emphatic message of the research paper is basically like, "tree growing to sequester carbon is very complicated, there's a lot we don't know, and there are a ton of different outcomes depending on how/where the tree growing is carried out."

One part of the paper I found most interesting was the section on nitrogen fixing microorganisms; they made it seem like the nitrogen fixation occurs via microbes pulling nitrogen from the soil and making it available to the plants. However my understanding is that those nitrogen fixing microbes pull N from the air, not the soil. Even good ol' wikipedia says "The bacteria are filamentous and convert atmospheric nitrogen into ammonia via the enzyme nitrogenase, a process known as nitrogen fixation." (https://en.wikipedia.org/wiki/Frankia) ... Undoubtedly there are microbes that can mine nitrogen from the soil, but why focus on those when the real bang-for-your-buck nitrogen fixation occurs when pulling nitrogen from the atmosphere.

Anyhow, great research paper, crappy summary.

"...tree growing to sequester carbon is very complicated, there's a lot we don't know, and there are a ton of different outcomes depending on how/where the tree growing is carried out.."

My guess is they say that because that's a much as they can say with full evidence backing. But suspect that most ecologists actually want to say "planting tree is a dumb solution for carbon sequestering or anything, please stop". That's what my ecologists say, certainly.

I mean, consider:

A) Trees are very good at spreading themselves. A tree adapted to it's environment will spread everywhere.

B) You can't get more carbon into an environment than ecosystem naturally sequesters - what it sequesters in long term, what's at the end of forest succession [1]. I'm in the California Sierras now and a lot of areas have a higher density of trees than the long term average and this along with global warming has contributed to the massive summer fires we've had. If anything, what this area's ecology needs is a thinning of the stick-like trees that have grown over the last 100 since all the existing trees were cut down during the Gold Rush. That can happen through fire or through human intervention but since human intervention is costly, fire is what it will be - fires made worse by fire suppression over many years. California's ecology is "fire based", etc.

https://en.wikipedia.org/wiki/Forest_succession

Regarding nitrogen fixing, my understanding is that 'green manure' & cover crops take from the air, leave in the soil - but microorganisms sounds like it's more in the context of composting, where whatever waste material is 'broken down'. So not 'soil' in the sense of 'it was already there anyway', but rather.. 'in the ground but needing to be made smaller and more available'.

Cover crops have a symbiotic relationship with nitrogen fixing bacteria that they maintain in their roots. When the crop is harvested (and the roots die), the nitrogen that was fixed by the bacteria in the roots remains in the soil.

Free living, nitrogen fixing bacteria are free living and have a protein that allows them to fix nitrogen to allow faster growth than the bacteria that need to get their nitrogen through other processes. They are often anaerobic (or functionally anaerobic) and so flourish in areas that are oxygen poor (like soil and decomposing organic matter) and by fixing the nitrogen present they enable other organisms to live there (their nitrogen fixing allows fungus to become established in the decomposing organic matter - the bacteria themselves aren't doing the decomposition). https://en.wikipedia.org/wiki/Paenibacillus_polymyxa is one such species of nitrogen fixing bacteria that forms a biofilm on plant roots, fixes nitrogen, and produces a substance that makes the plant roots more resistant to other pathogens.

Thanks for that explanation. I assume by "free living" you mean they can exist by themselves in soil apart from plant roots (or at least for part of their lifecycle)? I'm more familiar with the ones that are symbiotic with plants, especially the ones for woody plants (Frankia, etc), which I think are largely of a different type than the ones that live on cover crop plants like clover.

Soil and decomposing organic matter aren't necessarily oxygen poor environments, and in ideal conditions they aren't at all. Depending on factors like soil porosity, and rate/frequency of precipitation, there can actually be quite a lot of gas mechanically exchanged between the soil and the atmosphere (water fills up soil pore spaces and pushes out air, then water drains out of soil pore spaces, pulling air back in). Plant roots can respire atmospheric gases into soil as well. Part of the reason why compacted (ie minimal pore space) soil is harder to grow in than the same soil made friable, is because the lack of porosity causes the soil to go anaerobic, which is inhospitable to a lot of beneficial soil microorganisms.

Not all cover crops are nitrogen fixing.

hm not sure but I think at high growth rates the majority of soil N does come from decomposition even when nitrogen fixation is present.

Weird article. Almost all of Amazon soil is very poor.

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