As the planet warms, many of its dryer regions (~ 40% of the land surface), would be expected to become more arid. This is because heat drives evapotranspiration, and evapotranspiration strongly influences dry land soil moisture (even more than the amount of precipitation that falls). Based on what we know of more arid climates, we might expect that increased aridity will result in less plant productivity. But what will happen to soil microbes? Plants and soil microbes are the twin pillars of ecosystem function. If they decline, we might also see a decline in the functions they contribute to such as the creation and maintenance of soil fertility. That would be further bad news.
So, how would somebody study the effect of increasing aridity on soil microbes at the global scale. Experimentally, you'd have to decrease rainfall input into soils or warm soils in several locations, then measure the diversity and abundance of the microbes. The problem is this gets expensive, and its hard to find research funding to spend in different areas of the world. Also, we'd have to pick one or two experimental treatments to focus on (for example, a 50% reduction of precipitation or a 2 degree warming of soils). We don't know for sure how climate change will play out in local areas, so we could be picking the wrong treatments if we went that route.
But, what if you wanted to do this for much less money, and at the same time increase the number of sites you sampled. You could just sample aridity gradients in many places around the world, and find the general relationship between aridity and microbial abundance and diversity. From that you could make predictions about what warming and increasing aridity would do to soil microbes without having to know how much aridity might increase.
We've collectively published a lot of cool studies from this sampling effort, and I'm happy to say the latest is out in PNAS. So, what about the microbes? It looks like both the abundance and diversity of both fungi and bacteria are likely to decline if aridity increases in a given dryland. Of a handful of possible specific mechanisms, this seems most closely linked to less soil carbon in more arid regions, although the variation in temperature is also influential. Finally, on the bacterial side, there are also shifts in which phyla dominate the community as one goes from less to more arid sites.
Press release from NAU news