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Soil respires CO2 (from microbial digestion, primarily), actually probably more actively than your compost pile does because it increases the surface area / mas

Soil respires CO2 (from microbial digestion, primarily), actually probably more actively than your compost pile does because it increases the surface area / mass ratio. Exactly how long carbon is sequestered in plant litter is an actively researched topic, and the general consensus seems to be that it depends a lot on temperature, soil composition, etc. But the overview is that worldwide, the feeling is that about 3x as much carbon is contained in soil as in living plants, almost all of that carbon being the result of plant litter. On a macroscopic level across all time-frames, clearly the carbon cycle is basically closed, so the amount respired out by the litter is the same amount as in. On a more local level, we may now have a lower-than-usual amount of carbon in the soil, as modern agricultural techniques and deforestation may have released more carbon from the soil than usual. Empirical results seem to confirm the intuition that woods and such sequester carbon longer than quicker-decaying substances, and indeed how could they not? So as a very, very rough rule of thumb, I'd say that you'd expect a fast-growing plant to sequester carbon for roughly 2-3x its lifespan (and of course some of that is carbon release happens quickly and some more slowly).

nkurz 12y ago
Looking into it a little more, I was definitely underestimating the amount of CO2 released from soil after the incorporation of compost. But I'm a little confused by your rule-of-thumb, or more particularly, the assumption that the soil carbon levels are constant. While the percentage may remain constant in the steady state, why doesn't the soil mass increase? For example, I presume the total mass of carbon in the soil is greater now than it was prior to the evolution of green plants. At what point did it transition from accumulation to steady-state? I'm not saying it didn't, just that I don't understand the logic that it must have. For example, I'd guess that there is more absolute organic matter in the soil in the northern US now than there was just after the retreat of the glaciers. Is this false? In the absence of erosion (and human agriculture) I'd presumed this process was still continuing.
- aetherson 12y ago
  On a medium-long timescale, it has to be basically even, right? If the amount of carbon captured in the soil were secularly increasing at 10% per century, then we wouldn't have any carbon anywhere besides the soil -- even if it was all released at the last ice age. If the amount of carbon in the soil is secularly increasing at 0.01% per century, well, that's maybe interesting geologically, but for the purposes of handling climate change in the next century or two, we can treat that as equivalent to "it's steady state."
  - lerouxb 12y ago
    It can't really be even, though. Otherwise how did all the fossil fuels form? Over geological time periods the co2 and o2 levels vary greatly. Maybe if not for mankind the atmosphere would actually have eventually ended up co2-poor over time ;) But I guess that timeframe isn't very relevant to any climate change we'll be experiencing in the near future.
- lerouxb 12y ago
  Also: where was all the carbon hundreds of millions of years ago before it was turned into fossil fuels? I guess "steady state" is a relative term when you get to long time periods. Your ever-growing compost pile might be analogous to peatlands, but on the other hand it will probably just be returned back to the atmosphere in the next 100 years. The future occupants of your property are unlikely to continue your experiment :) My guess is that anything we gain in that regard is more than offset by all the land that's covered in asphalt and concrete and also modern farming techniques and general deforestation. Very little carbon sequestration going on in the modern world..

nkurz 12y ago

Looking into it a little more, I was definitely underestimating the amount of CO2 released from soil after the incorporation of compost. But I'm a little confused by your rule-of-thumb, or more particularly, the assumption that the soil carbon levels are constant. While the percentage may remain constant in the steady state, why doesn't the soil mass increase? For example, I presume the total mass of carbon in the soil is greater now than it was prior to the evolution of green plants. At what point did it transition from accumulation to steady-state? I'm not saying it didn't, just that I don't understand the logic that it must have. For example, I'd guess that there is more absolute organic matter in the soil in the northern US now than there was just after the retreat of the glaciers. Is this false? In the absence of erosion (and human agriculture) I'd presumed this process was still continuing.

aetherson 12y ago
On a medium-long timescale, it has to be basically even, right? If the amount of carbon captured in the soil were secularly increasing at 10% per century, then we wouldn't have any carbon anywhere besides the soil -- even if it was all released at the last ice age. If the amount of carbon in the soil is secularly increasing at 0.01% per century, well, that's maybe interesting geologically, but for the purposes of handling climate change in the next century or two, we can treat that as equivalent to "it's steady state."
- lerouxb 12y ago
  It can't really be even, though. Otherwise how did all the fossil fuels form? Over geological time periods the co2 and o2 levels vary greatly. Maybe if not for mankind the atmosphere would actually have eventually ended up co2-poor over time ;) But I guess that timeframe isn't very relevant to any climate change we'll be experiencing in the near future.
lerouxb 12y ago
Also: where was all the carbon hundreds of millions of years ago before it was turned into fossil fuels? I guess "steady state" is a relative term when you get to long time periods. Your ever-growing compost pile might be analogous to peatlands, but on the other hand it will probably just be returned back to the atmosphere in the next 100 years. The future occupants of your property are unlikely to continue your experiment :) My guess is that anything we gain in that regard is more than offset by all the land that's covered in asphalt and concrete and also modern farming techniques and general deforestation. Very little carbon sequestration going on in the modern world..