There is a cool new paper in Nature showing that semiarid ecosystems can have big effects on overall carbon concentrations in the global atmosphere, primarily through enhanced productivity of vegetation in wet years. The authors write:
As the dynamics of dryland systems, which cover 45% of the Earth’s land surface, increase in global importance, more research is needed to identify whether enhanced carbon sequestration in wet years is particularly vulnerable to rapid decomposition or loss through fire in subsequent years, and is thus largely transitory. Such behaviour may already be reflected by the larger-than-average atmospheric growth rate in 2012 (ref. 30) that was associated with a return to near-normal terrestrial land sink conditions.
In other words, the next question is how long can these systems lock up this carbon in the transient vegetation of deserts? I would guess that, as the authors suggest, a string of dry years would cause these gains in carbon storage to be lost, but it might take a couple of decades to work the carbon out of the soil, and increased fertility in the short term can lead to soil stabilization, thus increasing overall ecosystem fertility, particularly with good land management practices. So this could be a good thing for the climate, and an essential role played by dryland ecosystems.