Will the Soil Save Us?

The model projected changes (above) in
global soil carbon as a result of root-soil interactions,
with blue indicating a greater loss of soil carbon to the atmosphere.
Courtesy: Benjamin Sulman, Princeton Environmental Institute

Conventional projections of global carbon dioxide emissions suggest that enhanced plant growth may help to sequester more soil organic carbon (SOC)--particularly in our soils. Under this scenario, Canada comes out a winner in Global Climate Change (GCC), with the prairie provinces expected to grow a wider range of crops with a lengthened growing season.
But, as with every aspect of GCC, this may not be the case. The paper Microbe-driven turnover offsets mineral-mediated storage of soil carbon under elevated CO2 in Nature Climate Change suggests that this is only partly true. Morgan Kelly's piece for Reporting Climate Science says:

Researchers based at Princeton University report...that the carbon in soil — which contains twice the amount of carbon in all plants and Earth’s atmosphere combined — could become increasingly volatile as people add more carbon dioxide to the atmosphere, largely because of increased plant growth. The researchers developed the first computer model to show at a global scale the complex interaction between carbon, plants and soil, which includes numerous bacteria, fungi, minerals and carbon compounds that respond in complex ways to temperature, moisture and the carbon that plants contribute to soil.

The study in NCC suggests that SOC sequestration increases under rising atmospheric carbon--but only in stabilization of ‘new’ carbon in protected SOC pools which may equal or exceed microbial priming of ‘old’ SOC in ecosystems with readily decomposable litter and high clay content (emphasis mine). However, carbon losses induced through accelerated decomposition dominate the net SOC response in ecosystems with more resistant litters and lower clay content.
So, little from column A, little from column B. An undisturbed deciduous  forest with readily composting leaves and grasses might sequester more SOC than, say, a coniferous forest. And there's no guarantee that farmland will sequester more SOC.