Render all the carbon dioxide right out of the air into some solid form and dump it on the ground evenly and it would make a layer about about an eighth of an inch high... e.g. a thin layer of mulch.

I think the problem here is that you keep thinking that something that emits it's carbon over a century, instead of months or years, is some sort of 'stopgap', it's not, you need your annual absorption rate to equal your annual increase rate to be carbon neutral, that's the key, and you're thinking of carbon like it was a leaking pipe connected to an effectively infinite reservoir where we can store water in buckets but eventually run out of place to put the buckets and drowned... but it's also not. There is X amount of Fossil Fuels, if we burn them all we must sequester an equal quantity of carbon to keep the atmospheric CO2 levels even, ignoring for the moment that soil and ocean both sequester and release CO2 naturally and that you're standing on more carbon in the first foot of dirt beneath you than the entire atmosphere above you and all that's been in the air before at some point Assuming that air to dirt interchange is zero and there's some fixed supply of air and fossil fuel and biomass carbon, A, F, and B.... and I'm sorry to keep banging this drum but your last set of comments makes it more clear you're missing this so we'll do equations.

So C = A + B + F; where C is a fixed value,

Pre-industrial: C = A + B and after all fossil fuels are used up this is also true.

Now, we want A to remain constant, therefore B(t) = B-initial + F and to keep a constant B'(t) = F'(t), that is, the rate of biomass increase would have to equal the rate of fossil fuel decrease, or use rate. Obviously this is not the case, were our equation correct, as CO2 ppm has risen.

To sequester this carbon, one must merely make sure at B-final = B initial + F, you are making the incorrect assumption that B'(t) = biomass creation = B(t), effectively the same as assuming miles and miles per hour were the same thing. Two men jogging to New York at the same speed only get to NYC at the same time if they began equidistant, not if one starts in Jersey and the other LA, see? Ultimately B'(t) must equal zero once F is depleted anyway, all that matters is that B-initial + F = B-final so that A final = A initial.

Now, just to repeat, while some things like Hemp happening to suck down carbon very quickly and cheaply, which is good, a field of Hemp might be an inferior sequestering source compared to pine, if its overall turnover rate meant that at any given time the Hemp acre had 50 tons of Carbon locked up and the Pine Acre had 100 tons, even if the pine grows slower. A Strawberry field has a fairly decent absorption rate for instance, but virtually no turnover time and might stores only a few tons per acre, making it very bad for this purpose, however that same field, if it were, say, mulched with tree bark that had to be replenished at a rate of 10% a year, might have a couple pounds of mulch per foot on it at all times, or about 50 tons an acre, suddenly making it a fairly decent sequestering point and cheaper to irrigate and potentially making a faster growing mulch source that natively stored less carbon than, say, redwoods, an appealing sequester method. The rate of absorption, while an important factor, is essentially secondary, it is the tonnage per acre that interests us. To place this in perspective, this planet has some 32 billion acres of land, to sequester 320 GT of CO2 on that would require 10 tons of additional material per acre, but would not require those acres grow faster, merely that the material on them be more massive. If, to pull some number out of my rear, an acre of pine, suddenly ripped out root and stump, weighed 50 tons of carbon, and was replaced with an acre of oak, fully grown at 60 tons, this would result in a net permanent reduction of 10 tons for that acre, even if the oak took twice as long to grow, similarly knocking down that pine and replacing it with potatoes, at say 40 tons of stored carbon on average per acre, would result in an atmosphere increase of 10 tons of carbon, but is basically yanking 40 tons of carbon out of the air each year compared to much lower amounts from those tree type [these are all made up numbers] making it an inferior crop for sequestering... however if it were used in tandem with an acre of the faster growing pine, where old pines were mulched down to make room for new pines at a rate of say 31 tons of pine mulch a year, suddenly the pine and potato acres would be at 121 tons of sequestering per two acres, or 60.5 tons, exceeding the Oak acre, and serving as a superior sequestering depot... because at any given time they'd have more carbon stored. There release rate doesn't matter any more than a chunk of limestone's does, the stuff isn't in the air.