Decomposition: the Sonoran Desert's nutritious recycling program

When plants die in nature, they get recycled through the process of decomposition. When a plant dies and falls to the ground, we scientists start to refer to it as "plant litter". Microscopic bacteria and fungi living in soil can eat away at the dead plant litter to return the plant's nutrients to the soil. That's the only way new plants can grow in natural ecosystems! Without the recycled nutrients, new plants couldn't get the nutrition they need to grow.

In a desert ecosystem, though, the bacteria and fungi that decompose plants might have a hard time doing their job. When it is hot and dry, microbes might not be active enough to decompose the plant litter. Then how do plants get recycled in the desert when it's too hot and dry for the microbes?

Another way plant litter can be broken down is by the sun. UV radiation from the sun can break apart the molecules inside the plant. (Anyone who lives here in the Sonoran Desert knows how fast our sun's rays can break down anything we leave out in the yard!) This process is called "photodegradation". Photodegradation of plant litter can happen at the same time as soil microbes are breaking down plant litter, as long as there's sunlight. In a desert, though, sometimes photodegradation can be more important than the biological decomposition by microbes, just given how tough it is for the microbes to survive and how much sunlight we have!

Many scientists have studied the recycling that happens during decomposition by microbes. We know a lot about how nutrients get released into the soil when plant decomposes, especially in places that are cooler and wetter than the Sonoran Desert. We don't know nearly as much about nutrients getting recycled during photodegradation. The biological processes by microbes work differently than the sun to degrade plant litter. We wanted to know if that meant that nutrients are recycled differently when plant litter is being broken down by microbes or the sun.

How did we answer that question? Well, we took plant litter from one species of plant native to the Sonoran Desert. We chose triangle leaf bursage (Ambrosia deltoidea).

Max collecting litter from bursage plants

We put dried bursage leaves into clear pouches that were made out of plastic that either allowed UV radiation to penetrate (and therefore allows photodegradation to happen) or blocked UV radiation (and therefore prevented photodegradation).

Small holes in the plastic allowed soil microbes to invade and biologically decompose the litter when it was able to be active. We put the pouches on the ground to decompose for almost an entire year, and every few months we collected some of the bags to measure the decomposition happening inside the bags.

Bursage litter in their UV pouches in the Sonoran Desert

On all of the pouches we brought back, we measured how fast the plant litter was disappearing. That tells us how fast the litter is decomposing. We also measured how much of the original nutrients are still in the litter. Whatever nutrients are no longer in the litter must have been returned to the soil. The difference between the pouches that allow UV and the pouches that block UV is the result of photodegradation, and tell us about how sunlight changes the way nutrients are recycled compared to the microbes doing it alone.

Measuring mass loss and nutrient chemistry on plant litter samples in the lab

The reason we wanted to know about nutrient recycling during photodegradation is because air pollution in Phoenix can add extra nitrogen to the soil. That means we're fertilizing the plants inside the city with extra nutrients, which can change the starting chemistry of the plants. Does that mean city plants recycle nutrients differently from outside plants? We already know that the soil microbes decompose litter faster when there's more nutrients in the litter, because the microbes need their nutrition just like humans! But UV rays shouldn't care about the amount of nutrients in the litter, so will fertilized plants recycle nutrients differently?

To answer that second question, we added another experimental treatment. In the pouches that allowed UV radiation, half of the plant litter was collected from plants that were fertilized while they were growing. We also filled half of the pouches that blocked UV radiation with fertilized litter. The remaining half was filled with natural litter that wasn't fertilized. We also did the experiment in two different places at the same time: Inside the city where air pollution is happening, and outside the city where there's less air pollution.

Bursage litter in their UV pouches in the city of Phoenix.

So what did we learn? Like other studies, we saw that UV radiation sped up the loss of the plant litter from the bags. We also noticed that nitrogen and phosphorus recycling was changed a bit by the UV radiation. UV radiation tended to increase the recycling of nitrogen and phosphorus from the plant litter. That means UV radiation can speed up nutrient recycling when soil microbes aren't being as active as they would be in other cooler, wetter ecosystems. The one exception to this pattern was fertilized litter inside the city... the microbes decomposing the litter in this high-nitrogen setting (of being fertilized AND receiving the city's air pollution) didn't much care for the UV radiation!

We also learned that plant litter grown and decomposed inside the city (where there is more nitrogen pollution in the soil) recycles nitrogen and phosphorus more quickly than litter outside the city.

Why does this matter? It tells us that one of the consequences of the air pollution in Phoenix, which is a rapidly urbanizing area of the Sonoran Desert, is that the way nutrients are recycled during decomposition can change, and that the UV radiation that is so abundant in the desert will play a big part in how it changes. That is important if you are a new plant trying to survive in the Sonoran Desert, because you rely on those recycled nutrients!


The results of this study are published in: Ball, B.A., M. Christman, S.J. Hall. 2019. Nutrient dynamics during photodegradation of plant litter in the Sonoran Desert. Journal of Arid Environments 160: 1-10. DOI:10.1016/j.jaridenv.2018.09.004