I think the problem with soft wood and carbon sequestration are possibly the same underlying problem.
If they are still gathering sunlight and carbon the same way but gaining size quicker then there is just less carbon per cubic cm. There'll be some advantage in being able to get at more light and carbon sooner, but as carbon stored per square meter of land it's probably less.
That is not to say that you could not in principle make trees better, but I think the changes would have to be something fundamental to the cell physiology. A C4 carbon fixation pine tree might be better but it might be easier to turn bamboo into a pine tree than it would be to make a pine tree work with C4. Evolution is really good at balance, tweaking a bit can knock a lot off balance. It's not an impossible feat to make something better but it isn't easy.
They're more water, less carbon. So, they are less efficient at carbon sequestration in terms of density. And, they are probably easier for microbes to break back down so they hold the carbon for less time.
Poplars are considered to be among the best trees in terms of carbon sequestration according to studies done thus far. In addition, if they're converted to paper and other wood products, they continue to sequester the carbon for the life of those products.
I would regard it as prima facia deeply unlikely that processing wood into paper is carbon-negative, regardless of how long you then store the paper. It takes power to turn trees into paper.
It's easy enough to calculate: Do you need more than an equivalent amount of paper burned into energy, to make that paper.
The answer is no, paper mills power themself from trimmings from paper making. They use much less than the mass of paper they produce.
Making paper does not take very much energy. The energy used in cutting and preparing wood is not that much when compared to the amount of wood produced.
So yes, turning wood into paper is carbon negative (in a way, technically it's the growing of the tree).
