The secret’s in the soil: Can soil organic matter protect crops from drought?
Reference: Renwick, L. L., Deen, W., Silva, L., Gilbert, M. E., Maxwell, T., Bowles, T. M., & Gaudin, A. C. (2021). Long-term crop rotation diversification enhances maize drought resistance through soil organic matter. Environmental Research Letters, 16(8), 084067.
It’s getting dry out there
Farmers across the world are facing increasingly difficult growing conditions as a result of global climate change. In many agricultural areas, more frequent and severe droughts pose a major threat to crop yields. This has researchers and farmers alike seeking out strategies to bolster the resilience of agricultural systems to such extreme weather patterns in order to maintain crop production and feed a growing population. In a new study, Dr. Leah Renwick and her colleagues reveal how the key to weathering these challenges may lie beneath our feet.
In recent years, researchers have started noticing that certain types of farm management strategies are associated with greater agricultural resilience than others. A key theme is crop diversity. Farms with higher levels of crop diversity (i.e., a greater number of different crop species) over time are often less vulnerable to extreme weather, like droughts, and show more stable crop yields from year to year. On the other hand, farms that contain only a couple of crop species, like corn and soy, are often left with larger yield losses when weather conditions are poor. The latter situation is characteristic of most farmland these days, leaving our food system at risk for major yield losses as climate change progresses.
Fortunately, there are multiple ways to increase crop diversity on farm fields. For example, farmers can add different types of food crops to their rotations; instead of just alternating corn and soybean every other year, they could also include other important food crops, like wheat and other cereals every few years. Another great way to add diversity is by using cover crops, which are non-harvested crops grown when farmers would otherwise leave their fields bare during the off-season. Cover crops can provide an array of important environmental benefits. Although numerous experiments and anecdotal evidence from farmers indicate that these and other crop diversity practices are associated with greater drought resilience, we still don’t know exactly how crop diversity protects against yield losses under drought.
Healthy soils, healthy crops
Many researchers suspect that crop diversity improves agricultural drought resilience through its effects on soil health and fertility. Soil plays a critical role in sustainable and resilient agricultural systems, because it is the medium through which plants acquire the water and nutrients they need to grow. One particularly important aspect of soil health and fertility is soil organic matter, or SOM. When plant tissues are left in the soil, like the roots of food crops or the entire plant in the case of cover crops, they decompose and become part of SOM. The amount of organic matter in soil has a large impact on water availability to plants, and thus might be an important resource to manage when adapting farming systems to more frequent and severe droughts. Different crops have different characteristics that contribute differently to SOM, though, such that growing a greater diversity of crops seems to increase SOM and its associated impacts on soil health and fertility more than when growing only a few crop types. However, until recently, no one had experimentally demonstrated these relationships.
Dr. Renwick and her team set out to determine whether or not crop diversity improves agricultural drought resistance through its effects on SOM in a field experiment at the University of Guelph Research Station. The experiment was performed in several different farm management systems – some with a history of low crop diversity and others with a history of high crop diversity. Within each management system, the research team also created two precipitation conditions – drought (using shelters that excluded rain from the experimental plots), and regular rainfall. For each management system and precipitation condition, they measured corn yields and soil properties. In this way, they were able to compare whether the management systems with higher crop diversity had lower yield losses under drought than the low diversity systems, and test whether this was due to soil properties.
Soil organic matter matters!
As expected, they found that the high crop diversity systems were more resilient to drought, and that the greater drought resilience could, in turn, be attributed to differences in SOM between the high and low diversity systems. But, what they didn’t expect to find was that SOM did not improve drought resilience in the high diversity system through its ability to increase soil water availability; instead, there was something else about the characteristics of SOM in the high diversity systems that allowed the corn crop to thrive despite water-limited conditions, and the researchers aren’t quite sure what that is yet. Until they can conduct another experiment to dig deeper into the mysteries of SOM, they still have an important take-home message for farmers: crop diversity and its impacts on soil properties can act as a major defense against droughts.