Crawford, K. M., Busch, M. H., Locke, H. and Luecke, N. C. (2019), Native soil microbial amendments generate tradeoffs in plant productivity, diversity, and soil stability in coastal dune restorations. Restoration Ecology. Accepted Author Manuscript. doi:10.1111/rec.13073
In those warmer days of summer, everyone at the beach faces their towels to the beautiful, breezy ocean. But something equally expansive and interesting is to our backs: the dunes. Without plants to stabilize them, however, these dunes can disappear.
Climate change threatens to bring storms of greater strength to our shores, as well as raise the level of the seas– putting coasts at risk (see coastal damage to Puerto Rico after Hurricane Maria). In some ways, everyone inland relies on these barrier beaches for protection. The dune ecosystems provide buffers from storms, stabilize inland soils, limiting further erosion or inland flooding. When you slip along the sandbanks on the way to the ocean, you’re crossing a great protector.
But not all dunes are fulfilling their duties; some are degraded ecosystems, impacted by human disturbance, with eroding soils. Levees can be built in their place to mitigate storm damage, but what if those dunes could be restored to a state of better health?
The key to keeping the dunes stable is to have plants with roots that bind the soil in place. Restoration projects have already been in the works all over coastal areas, but the reality is that dune restoration can be difficult. Think about life on a sand dune: you’re sprayed by the ocean, battered by wind, rooted in loose soil that often lacks the nutrients you need, and if you’re a plant, you can’t move. These plants, the real workers of restoration, need all the help they can get.
Kerri Crawford and her team of researchers from the University of Houston had an idea that microbes could lend a helping hand. All plants have complex relationships with below-ground microscopic organisms, even in the soils of sand dunes. These bacteria and fungi that live in the soil can form relationships with plants. Some species harm plants by causing disease, while others help the plants by taking up nutrients, or generally benefit the ecosystem by increasing plant diversity and producing compounds that stabilize the soil.
If you take a plant from a nursery and plop it in the sandy soil, you may need more ingredients than just water, nutrients, and sunlight to make the plant successful. If the soil doesn’t have the key essential, unseen, living components of the microbiome, as is often the case if the dune is made from dredged sand, the plants might not establish well. While microbes can arrive at the site with time, restoration practitioners can give the plants a head start by introducing microbes like mycorrhizal fungi– their needed companions.
By giving plants a starting dose of microorganisms, also known as inoculating the soil, the researchers hoped to see that the microbes would jumpstart the plants growth, help maintain a diverse assemblage of grasses, and even work to increase the stabilization of the soil.
Microbes Remain Mysteries
Rather than do the work in the dunes themselves, the researchers worked in a more controlled setting, planting into sand-filled pots, some with the microorganisms from a natural dune community, and some without. The stars of the show were two species of grasses which are naturally found in dune ecosystems: coastal panic grass (Panicum amarum) and sea oats (Uniola paniculata). The researchers used these grasses to see how the microbe addition would affect them independently, and also together in the same pot.
Even with this wonderful setup, the researchers did not get the results they expected. When the panic grass
and sea oats competed with each other, the panic grass usually won. But then when the native microbes were added, the sea oats had more growth than before. So while the microbes did aid in leveling the playing field of grass diversity, instead of increasing the grass’s growth, the addition of the microbes actually decreased their growth. And while the mycorrhizal fungi did make associations with the plants, they didn’t actually work to aggregate and stabilize the soil– though the potential was there.
Why the unexpected? The researchers believe the answers lie in the complex relationships that microbes can have with each other, and how this can impact other plants. For example, the microbes that were added were antagonistic to the panic grass instead of beneficial. In short, the impact of microbes can be context-dependent. This all goes to show how much there is left to learn about the interactions between plants and microbes.
While the addition of microbes didn’t have all the ecosystem benefits the researchers hoped for, this is far from the end of the road. When it comes to soil organisms and ecosystem restoration, the soil microbiome is like a black box. Researchers know there’s interesting stuff going on in there, and can see the effects of it through the plants growth, but it’s much more difficult to see the details: which organism is doing what, how are the microbes interacting, and how, ultimately this can optimize the restoration of ecosystems. But research like this, is one sandy step in the right direction.
Feature Image: Sea Oats. Paul Brennan. From https://www.needpix.com/photo/1552152/beach-sand-seaoats-seascape-landscape-nature-outdoors-vacation-tourism