We already have free self-replicating devices that do just that: plants. Just add water.
Not even close. The largest iron fertilization "experiment" to date was done by Russ George with just 100 tonnes off the Pacific coast of Canada in 2012.
It's now 2014 and the salmon population has quadrupled from the algal bloom. We don't know what sort of impact iron fertilization at scale will have on the ecosystem.
The real downside is that if did it at the scale necessary to have an impact, you're also going to be creating HABs (Harmful Algal Blooms) that could potentially change the Earth's climate themselves.
Most quick-growing plants have a short life-cycle after which they decay and put their carbon back into the atmosphere. The only plants that really lock up carbon for decades or centuries are trees, and growing new forests is slow and daunting. You can solve the fast-growing plant problem by continuously growing plants (so as the old ones die, new ones lock up the same amount of carbon), but that means that a given acre of land has to be devoted in perpetuity to locking up X amount of carbon. And fast-growing, large plants tend to degrade soil quality -- they need rich nutrients to support that fast growth. They also need water, which is not necessarily locally abundant.
Reforestation is still probably the best way to capture carbon from the atmosphere. Do something where you build a young-growth forest, cut down trees and do something with the wood other than allow it to decay or burn it, let new trees grow. But it's not just a matter of flinging some seeds on the ground and yelling "Done!"
You sound like you've looked into this, but at a glance this seems implausible.
I have a compost pile, and to it I add fast growing plants from my garden. I presume that the carbon mass of the plants has mostly been extracted from atmospheric CO2. Empirically, as time goes on the pile gets larger. I've always presumed that the a significant portion of the resulting pile is carbon. I'd be surprised if any significant fraction of my compost pile were to suddenly evaporate. If I plow the compost into the soil, I'd be even more surprised if it were to disappear.
So while there might be considerable loss of CO2 back to the atmosphere during during the composting process, and while (worse) there might be considerable methane released, isn't it safe to say that the increased carbon mass of the pile represents CO2 that has been removed from the atmosphere? And that if this compost is buried and the soil mass increases, the difference can be counted as sequestered CO2? Where's the flaw in my logic?
To quote the EPA "Composting also results in biogenic CO2 emissions associated with decomposition, both during the composting process and after the compost is added to the soil." - http://www.epa.gov/climatechange/wycd/waste/downloads/compos...
"Overall, EPA estimates that centralized composting of organics results in net carbon storage of 0.20 MTCO2E per wet short ton of organic inputs composted and applied to agricultural soil."
"Based on the expert judgment of Dr. Michael Cole from the University of Illinois, EPA found that between 4 to 20 percent of the carbon in compost degrades very quickly, and the remainder can be considered either slow or passive. Dr. Cole found 400 years to be the average of the reported sequestration times of carbon in the soil."
Elsewhere they mention that the carbon content of wet compost is about 20%. Again, so while the conversion is not perfect, is there dispute that composting green materials and adding the compost to the soil results in net CO2 sequestration?
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).
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.
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."
But I guess that timeframe isn't very relevant to any climate change we'll be experiencing in the near future.
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..