Legacies of human impacts linger in longleaf pine

Crunch. Crunch. Crunch. The sound of my hiking boots in the dry soil marks the miles as the trail meanders through the shrublands near my home. Each plant has a different shade of green, flowers are visited by dozens of insect species, and the trail is marked by footprints of mountain lions and deer that came before me. But most of the biodiversity that goes uncounted lives beneath the sole of my boot. 

A single gram of soil, about the size of a fingernail, contains over 1 million microbial cells. Some of these microbes are “bad,” meaning they cause disease in humans (such as Giardia) but more often than not, we rely on them to keep our soil healthy and maintain biodiversity. Soil microbes are key members in global nutrient cycles. They take in nitrogen from the atmosphere and turn it into a form plants can use which helps plants grow and increases diversity. Fungi form relationships with plant roots and help them move nutrients from the soil to the plants. A high diversity of soil microbes help create the unique ecological communities we see above the ground.

A depiction of the variety of textures found in soil. Note, the diversity of small and large chunks, all potential habitat for soil microbes. “Soil Health_NR_06” by NRCS Montana is licensed under CC PDM 1.0

However, our actions can change what microbes are present and have cascading effects throughout the ecosystem. Imagine the crunch of a hiking boot. This probably disrupts some microbes but may not have ecosystem level effects. Now imagine the crunch of industrial agricultural equipment. How would this large scale disturbance change the diversity of microbial species living in the soil?

In a new paper, Turley et al. (2020) from Michigan State University describe the long term effects of agricultural tilling and vegetation restoration on the soil microbial community in longleaf pine forests in the Southeastern United States.

Longleaf pine communities are dominated by pine tree species and a dense grassy understory. Many restoration efforts target removing pine trees to promote understory growth and return the ecosystem to a savanna with widely-spaced trees and a diverse plant community, Additionally, many current longleaf pine ecosystems have a history of agricultural tillage before the 1950s when many agricultural fields were converted back into longleaf pine forests. The researchers wanted to find out if tillage and tree thinning changed the soil microbial community. To do this they had four treatments: tilled and thinned, not tilled and thinned, tilled and not thinned, and neither tilled nor thinned. 

Upland longleaf pine savannah at the Lake Thoreau Environmental Center of the University of Southern Mississippi. By Flacourtophile – Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=66224461

Flacourtophile, CC BY 4.0 https://creativecommons.org/licenses/by/4.0, via Wikimedia Commons

They found that sites with a history of agriculture had a greater diversity of bacterial species compared to sites that were not tilled, but there was a decrease in fungal diversity. Restoration, or thinning, increased both the bacterial and fungal diversity. Interestingly, there were no interactions between thinning and tilling, meaning the effects of the restoration did not depend on the history of the site. The researchers found evidence that environmental variables strongly affect the soil microbial communities. For example, plots with more bare ground, more understory plants, and more plant species supported greater microbial biodiversity.  

A recently tilled field, showing how tilling can change the soil structure. Art Anderson / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0)

Each of the four treatments resulted in four distinct microbial communities showing how land use can drastically change the species present. Although the tilled sites have not been tilled in over 60 years, the microbial community has not recovered. Historically, restoration ecology has emphasized the plants and all the cool things we can see above the ground. But focusing on the vegetation does not move the microbial communities towards the pre-agriculture state indicating more active restoration is needed specifically addressing the microbes that are so important for the functioning and longevity of ecosystems. 

As the field of soil microbial ecology treks forward and makes progress towards restoring degraded soil communities, pay attention to the soil crunching beneath your boots. A billion microbes are working hard fueling the above-ground biodiversity that we all admire. Next time you stumble upon some soil, give it a gentle high-five and thank the microbes for all their hard work. They deserve to be appreciated.

Source: Turley, NE, Bell‐Dereske, L, Evans, SE, Brudvig, LA. Agricultural land‐use history and restoration impact soil microbial biodiversity. J Appl Ecol. 2020; 57: 852– 863. https://doi.org/10.1111/1365-2664.13591

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Brianne Palmer

I am a PhD candidate at San Diego State University and the University of California, Davis studying how biological soil crusts respond and recover from fire. Most of my research is in coastal grasslands and sage scrub. We use DNA and field measurements to understand how cyanobacteria within biological soil crusts help ecosystems recover after low severity fires. I am also involved with local K-12 outreach within the Greater San Diego Metro Area.

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