Nitrogen pollution in the urban Sonoran Desert

 Phoenix is one of the fastest growing cities in the United States. That means a LOT of people live in the city, which means a lot of fossil fuels are consumed. When we think about the environmental impacts of fossil fuels, we usually hear about carbon dioxide (which is a greenhouse gas and contributes to global warming and climate change). But that's not the only down-side to fossil fuels. Another form of pollution caused by burning fossil fuels is "nitrogen deposition". 

When fossil fuels are combusted in, for example, a car engine, nitrogen compounds are released into the air. While these compounds are in the air, they go through chemical reactions and eventually fall back to the ground. This is one of the components of "acid rain", which you've probably heard of before. But here in the Sonoran Desert, it's a bit different. We don't have a lot of rain. So sometimes the nitrogen comes down in the rain, but sometimes it comes down in dry particles, kind of like dust. That's why we call it "nitrogen deposition". Deposition just means everything coming back down to the ground - it can be either wet (like rain or snow) or dry (like dust).

We know that the Sonoran Desert is receiving extra nitrogen deposition around Phoenix. How do we know? We measure it! Our research group (CAP-LTER) has many research sites inside and outside of Phoenix where we measure how much nitrogen is coming down to the ground. We use what we call "deposition collectors", which are a group of funnels that catch the dust and rain. Filters connected to the bottom of the funnels collect all of the nitrogen compounds that came as part of the dust and rain. The filters get collected on a regular basis for measurement.

You can watch this video where our Site Manager, Quincy, shows you the "deposition collectors" and how he gathers the filters for measurement:

From these "deposition collectors", we know that the desert areas inside the Phoenix urban area are receiving more nitrogen than outside the city. The large population of Phoenix is having an impact on how much nitrogen is in the Sonoran Desert! Not only do we measure how much nitrogen is coming down, but we also measure how that nitrogen influences in the Sonoran Desert plants and soil.

Nitrogen is a nutrient that all organisms need, but it becomes a problem when there's too much of it. (You CAN have too much of a good thing!) Nitrogen can become toxic in high concentrations and kill the organisms living in the soil. It can even make humans sick if it gets into the groundwater! Plus, adding a lot of nitrogen can upset the balance for the ecosystem. (For example, you can read here how excess nitrogen causes problems at some national parks.)

Our research group (the CAP-LTER) has been measuring the impacts of that nitrogen pollution since 2006. The nickname we give that experiment is "DesFert", which is short for "Desert Fertilization Experiment". CAP-LTER has been following the excess nitrogen in the plants and soils to see how the Sonoran Desert changes with these extra nutrients. We also experimentally add extra fertilizer to our research plots to predict how the Sonoran Desert will continue to change if the pollution continues.

If you want to see what the research plots look like, and learn a little bit about the actual measurements we make, you can watch this video:

BioArt in the Sonoran Desert

BioArt: Sonoran and Arctic Environments is an interdisciplinary course at Arizona State University’s West Campus that pairs science and art majors to conduct independent scientific research and science communication through art. While the disciplines of science and art are usually considered to be very different, they actually require a similar set of skills: observation, interpretation, creativity, and communication. Students in this course hone these skills by studying both art and science in two ecosystems that are also considered to be very different, yet in fact similar in many ways: the Sonoran Desert and the Arctic. The goal is to train a broader group of students in both disciplines and engage them in science communication.

The product of these scientist-artist teams gets displayed in an exhibition, including the traditional scientific presentation of their research, as well as the creative work that conveys the research through a different medium. The most recent set of projects was exhibited in August & September 2019 at the Fletcher Library at ASU. This spring, the work was set up for display at the South Mountain Environmental Education Center, to be enjoyed by visitors to South Mountain Park. That way, visitors to this Sonoran Desert reserve could see science and art based on that ecosystem! Unfortunately, that display was cut very short. It was up for only one weekend before the visitor center was closed due to the COVID-19 pandemic! Since it can no longer be enjoyed by in person, I thought I could perhaps publish it here, for people to enjoy remotely all over the world.

So, without further ado, I present to you the BioArt projects from the Sonoran Desert!

Lourdes, Mohammad, and Rebecca explored how different amounts of precipitation can influence the abundance and diversity of wildflowers in the Sonoran Desert. They learned that different locations have different wildflower communities, though humans may have a role in creating those communities, not just precipitation alone. Learn more about it in their research poster.

This trio of canvases represents how rainfall can influence heterogeneity of wildflower communities around Phoenix. The blue paint at the bottom of the canvas represents how many centimeters of rain fell from December 31st, 2018 through March 1st, 2019 at three different locations in the Phoenix area. Our goal is to show which species of wildflowers were most common at each site that was surveyed. The most common species of wildflowers were centered around the least common wildflower species.

Paul & Shauny's experiment investigated the amount of excess nutrients can be found in lakes next to recreation areas that use different amounts of fertilizers, and whether that makes algal blooms likely in those areas. They found more algae in lakes near fertilized areas, with the amount of nutrients depending upon the management practices used.  Read more in their research poster.

Essential nutrients include nitrogen (N), phosphorus (P), and potassium (K) to nourish and sustain life in both terrestrial and aquatic environments when present in properly balanced quantities. An imbalance in nutrient levels, such as that caused by the addition of fertilizer to a terrestrial ecosystem which runs off into a nearby waterway, can cause a rapid spike in primary production, depleting resources and ultimately leading to the death of organisms within the system. In their artwork, an unsuspecting abundance of ciliates, flagellates, and multiple algae species swim on shimmering silver stream currents as they feast on an unexpected influx of NPK, the result of fertilizer run-off from a nearby field. Just like phytoplankton, this painting requires light to be vibrant. (Unfortunately, I took this photo at night, so you can't see the light reflection that makes it shimmer!)

Brittany and Kamber investigated how soil fertility changes as you increase in elevation up mountains in Phoenix's park reserves. They found that some nutrients increase with elevation, while others decrease. Read more in their research poster.

Their short film is a visual and auditory representation of the Sonoran Desert and some of the many species of organisms that inhabit it. All photography and videography is original work inspired by our driving question of: “Does human interaction in the Sonoran Desert affect overall soil fertility?” Although experimentation heavily relied on test tubes and various lab work, immersing ourselves in the environment of interest and sharing these experiences through photographs and video has helped support our findings. Yes, human interaction in the Sonoran Desert affects overall soil fertility. We are striving to help others understand that even something as basic as “dirt” is actually nutritionally complex soil in need of our protection. Through both art and science, we can make the world a better place. You can watch their video here:

(Like BioArt and want to see a few examples from the Arctic, as well? See our sister Polar Soils Blog at this post, as well as this post.)

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