How will soil invertebrates respond to climate change? Ask the plants!

Climate has been changing on this planet, and will continue to change. We hear a lot about warmer temperatures, but that's just one piece of the story. Another important part of climate is also changing, and that's precipitation.

Here in the Sonoran Desert, we get rain during two different seasons: during winter and during our summer monsoons. With climate change, we are seeing our precipitation change. One prediction for the future of the southwestern U.S. is that we will get rain less frequently, but when it does rain, the storms will be larger. How will less frequent precipitation influence the ecosystem? Even though the rain that comes may deliver more rain at once, a lot of that extra rain can't be absorbed into the soil. So that might mean less water available overall for the organisms.

Certainly the plants will respond to that change. If they can't rely on receiving water as frequently as they're used to, it's harder for some plants to survive. A lot of studies have looked into plant responses to changing precipitation patterns. In our research group, we like to learn about the soil community that support the plants: the microbes and invertebrates that recycle nutrients so that the rest of the ecosystem can thrive! How will they respond to changes in precipitation? Much less is known about that!

Soil invertebrates include organisms like this velvet mite.

So we did an experiment to find out! In the Sonoran Desert, we spent one monsoon season changing the timing and size of rainfall and measured how the soil community responded. We used watering cans to make "fake" rainfall events. 

We watered the soil using either the normal pattern for precipitation during monsoon season (as our control treatment) or with changes to the frequency and size of the storm. Some of our plots received rain at the same size of storm but less frequently. Other plots received rain at the same frequency but with larger amounts of rain. Then other plots received rain in what is predicted for the future: less frequently but with larger amounts.

We then took soil samples and extracted the soil invertebrates so that we could see how they changed in response to the experimental rain. We predicted that the communities would become less abundant and diverse if they received rain less frequently. But we had another question: Do plants help protect the invertebrate community from changes in rainfall? Plants provide shelter for the soil beneath them, providing shade, pulling water towards the surface of the soil, and creating food for the soil organisms. If plants are present, does that mean the invertebrates don't feel the impact of precipitation as much? 

So we did our experimental rainfall experiment in soils directly beneath plants and compared that to the rainfall experiment on bare soil, Here, Kelly is "making it rain" beneath a creosote bush. 

We also wanted to know whether type of plant matters. Creosote bush is a larger shrub that has deep roots and keeps its leaves all year round. We also did the experiment under a different type of shrub. Bursage is a smaller shrub that goes dormant during periods of drought. They drop their leaves, stop photosynthesizing (so stop bringing up water). That means that a bursage in spring might look like this:

But during periods of drought (like a hot, dry summer!) might look like this:

That creates a very different habitat for soil organisms than a big, evergreen creosote bush! So we "made it rain" with either less frequent or larger rainfall events (or both!) under creosotes, bursage, and bare soil over the course of one monsoon season.

We collected soil at the start of the experiment to look at initial differences between creosotes, bursage, and bare soil. Then we collected soil again at the end of the experiment to see how soil communities responded to the change in precipitation. We predicted that we would see more change in the bare soils compared to soils beneath plants, because we expected the plants to help protect the invertebrates from the lack of water we created.

In the lab, we extracted the soil invertebrates from the soils. Then, we used microscopes to look at the invertebrates, so that we could identify and count them. A LOT of students helped us with the microscope work!

From the results, we learned that plants have a big influence on the soil invertebrates. The community of invertebrates living beneath creosote and bursage plants is much bigger and more diverse. The invertebrates must really like having the shade and resources from a plant above them! Both creosotes and bursage housed larger communities, but creosotes had the strongest effect. The invertebrates really like the shade and resources from deeper rooted plants that are active during monsoon season!

Changing precipitation didn't have a big influence over how many invertebrates lived in the soil, but it did change the diversity of the invertebrates. There are many different ways to count "diversity", and two of these measures were influenced by changing rainfall (Shannon diversity index and evenness). Making rain fall less frequently reduced the diversity of invertebrates in the soil. However, as we predicted, the communities beneath plants were less effected than in the bare soil. Soil invertebrates living away from plants are exposed to the changes in precipitation, but they are more protected when they have the help of a plant. That means that if the abundance of plants like creosote and bursage change in response to the climate, the soil invertebrate community will change. And those invertebrates do a lot of important processes for nutrient recycling and decomposition in soil, so there can be major consequences for the overall ecosystem!

The results of this study are published in: Ball, B.A., K. Bergin, A. Morrison. 2023. Vegetation influences desert soil arthropods and their response to altered precipitation. Journal of Arid Environments 208: 104873. DOI:10.1016/j.jaridenv.2022.104873

Urban forestry... in the desert?

Cities, like Phoenix, burn a lot of fossil fuels and release a lot of CO₂ into the atmosphere. Think of all of the cars on the road, houses using electricity, and businesses and industries that use fossil fuels, too! The CO₂ is a greenhouse gas that influences the climate of the entire planet. How can cities reduce how much CO₂ is released into the atmosphere? One way is to burn less fossil fuels. We try this through efforts like increasing cars' fuel efficiency and switching to solar power. Another way is to try to re-absorb the CO₂ that was put into the atmosphere. So, even though the fossil fuels are being burned and CO₂ is going into the atmosphere, we can try to pull some of it back into the city so that it doesn't stay in the atmosphere. This lets cities "mitigate" the amount of CO₂ they create.

There are a lot of scientists who are trying to engineer fancy new technology to absorb CO₂ from the atmosphere. But there is also a simple way that just uses nature: trees! Trees pull a lot of CO₂ from the atmosphere when they photosynthesize, and they hold onto that carbon for a long time. Trees live for decades or more, which means the CO₂ stays in their biomass for decades. ("Biomass" is the scientific term for the entire mass of the plant, and about half of that biomass is made up of carbon!) Every year they shed parts of their biomass, like leaves and twigs, onto the ground, which gets decomposed and turned into "soil organic matter". That soil organic matter can become very stable in the soil, meaning it only continues decomposing VERY slowly. The soil organic matter builds up over time in the soil as trees shed their leaves, bark, and twigs, locking it into the soil instead of the atmosphere. So, not only do trees hold a lot of carbon, they also build up carbon in the soil where it can be stored for hundreds of years.

The carbon cycle in an urban forest ecosystem: The trees take up CO₂ through photosynthesis, and that carbon is stored in the tree for decades or more. When the tree sheds its dead parts (called "litterfall"), it decomposes in the soil, creating a form of soil organic matter called "humus". The humus can slowly decompose, which will release some of the carbon back to the atmosphere through respiration, but it is VERY SLOW. More litterfall every year creates humus faster than it is decomposed, which means there's a buildup of humus over the years, storing carbon in the soil for centuries!

Growing trees is easy in a lot of cities and has other benefits, like providing shade, cooling the surrounding area thanks to transpiration, making a home for birds and other animals, and generally looking pretty! "Urban forestry" is becoming popular in some cities. But... we live in a desert! Can Phoenix use urban forestry as a way to mitigate our CO₂ emissions? Many trees that have been planted in yards and along streets aren't native to the desert, so they require a lot of extra water to survive. That creates its own problems! Is there a way that Phoenix can grow native trees without having to give them a lot of irrigation? Can urban forestry be a sustainable way to capture CO₂ in a desert city?

That's what Arizona State University is trying to find out. We have planted an "urban forest" on campus, using only native trees that are good at surviving in a desert climate (without needing to be watered every day). The forest was planted in an unused area of the West Campus where some mesquite trees had "volunteered" to grow. But, most of the area was bare. One thousand mesquite and palo verde saplings were planted into those bare areas!

This is the area of campus, just before the forest of saplings was planted.

1,000 mesquite and palo verde saplings were planted into the bare areas to grow into a forest.

Right now, the trees are little saplings. As they grow, they will capture CO₂ during photosynthesis and store it in the tree biomass, and as they shed their leaves and wood, they will start to build up the carbon stored in the soil, too. However, they grow slowly, especially since we're only giving them water early in their life until they establish. After that, they are on their own to find water! 

So, how much carbon can a slow-growing desert forest absorb? We will be measuring that every year. We are measuring the growth of the trees, to estimate how much carbon is being stored in the tree biomass. We are also measuring how much carbon is being stored in the soil. The trees and soil are the two main places we expect to see the carbon stored.

One of the saplings whose biomass we measure every year. Hopefully we will see its biomass increase year-after-year, meaning it has removed CO₂ from the atmosphere and stored that carbon in the plant.

We are also making other measurements, to better understand the carbon cycling in the forest. We are measuring processes like photosynthesis and respiration rates (where CO₂ is moved between the forest and the atmosphere), the amount of "litterfall" every year, and the biomass of microbes living in the soil who do the respiration.

Kevin measures soil respiration using an infrared gas analyzer.

David measuring microbial biomass living in the soil using a process called "chloroform fumigation extraction".

It will take a long time before we have results from our experiment, because the trees grow slowly. If it is successful, it will grow into a beautiful mesquite bosque that not only stores carbon, but also provides shade, a habitat for birds, and a food source for pollinators. It will take decades to reach maturity! Until then, we will keep making our measurements every year, as we (hopefully) watch the trees grow.