5 Things to Know about Drought and Your Soil Microbiome
One of the most valuable resources to growers is the soil in which they grow their crop. Measuring the health of their soil is complicated since it isn’t well-defined; there isn’t one soil test (or even an agreed-upon set of tests) that can be called a universal soil health measurement. Despite this, we do know that the soil microbiome—all the microscopic organisms that call soil home—is an essential part of soil health.
As agronomists and scientists have learned more about soil biology, growers are investing in regenerative agriculture practices like cover cropping, no-till, and diversifying crop rotations. There are also incentives to improve carbon sequestration in agricultural soils, a process that is heavily dependent upon soil microbes. With the substantial investment of time, money, and energy in soil health, growers may be wondering—what is drought doing to my soil microbiome?
1. Your soil microbiome is resistant and resilient to change, even during drought.
In ecological terms, resistance refers to the ability of a community to stay the same during a stress, and resilience is the ability of a community to return to its starting composition before the stress occurred. Research has shown the soil fungal community is more resistant to change than bacteria (1, 2), and changes in the bacterial community generally only last as long as the drought (1, 3, 4).
2. Biodiversity in the microbiome stays the same.
While there may be changes in the microbial community during drought, the overall biodiversity usually stays the same (5, 6, 7). There are a few different ways to measure biodiversity, but most of them incorporate richness (the number of species present) and evenness (the relative abundance of the different species). During drought, the population numbers for soil bacteria shift to favor species that have thicker cell walls and are able to form spores, both beneficial to survival during dry periods (1, 8). Despite the population shift, the list of species present, and thus the biodiversity, usually stays the same.
3. Microbial biomass is stable during drought.
You may be beginning to think you could’ve stopped reading this list after the first point, but stick with me here. Research on soil microbial biomass is more variable than biodiversity, but overall, biomass often remains stable during drought (5, 6). Microbial biomass is used as a measurement of the size of microbial communities; higher biomass is associated with higher rates of soil organic matter decomposition and nutrient cycling.
4. Different soil microbes are recruited into the root zone during drought.
The soil surrounding crop roots is called the rhizosphere—the microbes living in this area are heavily impacted by the crop (9, 10). Although these microbes come from the surrounding soil microbiome, the rhizosphere community is distinct. The reason for this difference is because plants release a wide array of chemical signals into the soil through their roots called exudates, and these vary based on plant genetics, age, and growth conditions (4, 11). Drought causes a change in root exudate, resulting in an increased amount of groups like Actinobacteria living inside the root and rhizosphere (4, 12, 13).
5. Soil microbes release a burst of CO2 when drought ends.
When a rain event occurs after a period of drought, soil microbes release a burst of CO2 known as the Birch effect (14). Research has shown that the CO2 released mostly comes from microbial biomass (not long-term stored carbon), but depends upon soil texture and chemistry (6, 7). However, the amount of CO2 expelled increases with the amount of carbon already in the soil, so carbon-rich soils may lose more during drought (15).
So…why does this matter?
If you are incorporating soil biology testing into your management plan, it is helpful to understand how different variables may affect your results. In the case of drought, measurements of biodiversity and microbial biomass are likely to be similar regardless of drought conditions. If you are interested in specific bacteria and took your soil sample during a drought period, it may be helpful to know whether their abundances may be higher or lower as a result.
In general, gram negative bacteria have thinner cell walls and are unable to form spores, so they decrease in abundance during drought. These include members of the genus Pseudomonas, and the nitrogen-fixing legume symbiont, Bradyrhizobium japonicum. Conversely, gram positive bacteria increase in abundance during drought; these include members of the genera Bacillus and Streptomyces. Additionally, since we know that the soil microbiome is both resistant and resilient to drought stress, growers can be confident that any investment into regenerative agriculture practices to improve soil health will not be nullified by future droughts.
Note from the author: Most PhD scientists who transition away from academia (and research altogether) may expect to never use the specifics of their graduate research ever again. Imagine my delight when I was asked to write this post, just a few years after completing my PhD on how drought affects the sorghum root microbiome (16). I am grateful to Trace for valuing my expertise as a scientist and for putting in the hard work to bring innovations in microbiome science to folks with “boots on the ground.”
Pictures from author’s PhD work: holding a young sorghum plant in the field (left) and isolating microorganisms from the root microbiome (right).
About the author: Dr. Tuesday Simmons is the Science Writer at Trace Genomics. She earned her Ph.D. in Microbiology from the University of California, Berkeley, studying the root microbiome of cereal crops.
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