Picture an open field of soil sitting in wait until a single grass seed finds its way to the surface and receives just the right amount of water to germinate. Soon more and more seeds of a variety of species make it into the community, with some never living to see the light of day and others taking off like a shot. As the community develops, different microbes become attracted to the roots of specific plants. At the same time decaying plants can change soil conditions just enough to shift the types of fungi and bacteria that can live underground.
Some plants even purposefully release compounds from their roots that microbes find to be “tasty” so that the microbes will hang out in the root zone and provide benefits such as nutrient digestion or defense against pathogens. Now it’s a whole new world underground and a different set of plants are able to germinate and out-compete some of the resident species. On and on these changes cycle and shift the plant community through generation after generation, ultimately creating a legacy in the soil. Ecologists want to know more about how this process takes place in order to understand more about community assembly after severe disturbances as well as to encourage successful restoration of degraded lands.
There are several ways that soil can be altered to create a legacy. Beyond the possibility of shifting the microbial community, natural or human-caused events can change soil structure, moisture content, and nutrient concentrations in drastic ways that change which plants are suited to a particular area. New research is finding that wildfires, drought, and other extreme weather events caused by global change also leave soil legacies which shape future generations of plants. Though our knowledge surrounding these long lasting effects on soil-plant interactions is growing by the day, there is still little known about the ways that each change in the soil will shift plant communities in the long and short term.
Robin Heinen and colleagues set out to get a better understanding of the legacies that plants leave in the soil through a grassland community experiment. Prior work on soil legacies has rarely looked at whole communities in this way and has largely focused on analyzing changes in soil caused by individual potted plants. [In this study, by manipulating the ratio of grasses and flowers planted in the community, the researchers were able to “condition” the soil to create a particular legacy and see the resulting effects. After one or two years of growth, the whole top layer of plants and roots was cut away and then reseeded with a new community to see how legacy conditions impacted the next generation of plants to establish.
The researchers collected soil samples both after the initial conditioning and after the second community was fully established to see what changes occurred. They measured nutrient concentrations and pH and recorded the diversity of microbes in the soil via DNA analysis. The study found that fungi were the main reason for changes in the soil during the conditioning phase and that the composition of the communities which developed after the conditioning were highly dependent on the composition of the predecessor community. A very strong trend seen was that communities which were grass-dominated prevented following communities from having a high grass content. The same was seen in respect to flowers, showing that opposites attract in the sense that the second community tended to develop with the opposite dominant plant type as the first community. According to DNA analysis of the soil, this occurred largely because of changes to the composition or abundance of fungal pathogens which were specialized to grasses. Studies like this one show just how powerful soil legacies can be in shaping the development of post-disturbance communities and perhaps will someday lead to a unique way to more precisely engineer plant communities during restoration.
What Legacy are We Leaving?
Plants aren’t the only cause of soil legacies. Land uses such as agriculture can drastically change the microbial community and structure of soil in ways that make it very difficult for native vegetation to recolonize. Tilling can make soils too “even” as unique, diversity-harboring soil pockets are churned up and lost. Some of these changes even provide invasive colonizing species with a greater than normal advantage, further exacerbating the challenge of restoring native species in old fields. Luckily, as our understanding of these legacies progresses through scientific study we too have the opportunity to progress our views of soil as a major tool for ecosystem recovery and to alter our practices to prevent negative soil legacies in the future, such as the adoption of no-till agriculture. As we learn more about the connection between land use and plant-soil interactions it is up to us to decide what legacy we want to leave on the land.