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

Rodents of the Sonoran Desert

The city of Phoenix, AZ is one of the largest and fastest growing cities in the U.S. My lab studies how this urbanization changes soil ecology, but one thing we didn't know much about was how it will change herbivores in the Sonoran Desert. We have a lot of small herbivores who rely on the plants growing in the soil. We know a bit about how plants and soils change... but what about the animals that eat them?

One of my students, Jessica, decided to find out the answer to that question for a particular group of herbivores: small rodents. There are many species of small rodents in the Sonoran Desert that eat either plants or their seeds. These include cute critters like kangaroo rats, pocket mice, and ground squirrels!

For two years, she conducted population surveys of small rodent populations at four sites inside urban Phoenix and four sites outside in the rural, non-urbanized areas. She wanted to find out whether abundances and diversity of small rodents are different when you compare the urban and rural areas. That matters, because small rodents are common vertebrate herbivores who can impact the plants in the desert. Jessica hypothesized that rodents will be more abundant inside the city, because there would be more food for them and fewer predators than out in the rural desert, but that there would be more biodiversity of rodents out in the non-urbanized rural desert, because some species wouldn't have the necessary habitat to survive inside the city.

Jessica used a capture & release method, where she caught mice in humane live-traps, identified their species, and then let them go. In order to do this, we had to have a lot of permits to verify that we were not causing any harm to the rodents. She carefully avoided bad weather and protected them from predators. She became an expert rodent handler and identifier! The surveys were conducted at desert sites inside and outside the city.

We learned from these surveys that abundance is actually the same inside and outside the city. We expected more rodents inside the city, but in fact they are the same inside and out! We did notice, though, that the communities inside the city were very different from outside the city. The rodents inside the city were mostly from just a couple of groups of pocket mice and deer mice, with only a few rodents from other groups present. Outside the city, though, there were pocket mice, kangaroo rats, woodrats, grasshopper mice, and many other types!

We think that this difference is probably related to the food available. The species we found outside the city have particular plants or habitat types that they need to survive which might not be available inside the city. However, we didn't specifically measure their food sources yet, so that is work for another future study!

The results of this study are published in: Alvarez Guevara & Ball 2018. Urbanization alters small rodent community composition but not abundance.. PeerJ 6:e4885. DOI 10.7717/peerj.4885

But what about the microbes?

Hello! My name is Nikita Kowal and as Dr. Becky stated in her last blog post, I will be sharing my research experience on here throughout this summer. I am part of the Ecological Society of America SEEDS Fellowship, and am conducting an REU through the Central Arizona Project Long-Term Ecological Research (CAP LTER for short). This summer I am working for Dr. Becky Ball and Dr. Pam Marshall. Under Dr. Ball, we will be doing the chemistry behind the soil composition and Dr. Marshall is the expert in microbial communities. But enough about myself; let's get to the real fun - science! 

Usery Mountain Preserve

This project aims to find patterns in the different kinds of microbial communities who live in various levels of nitrogen-enriched environments. Nitrogen deposition is most common in inner-city ecosystems due to the nitrogen emissions in cars. Nitrogen deposition is when nitrogen from the atmosphere falls (or deposits) into the biosphere. Because there is more in the atmosphere in the middle of a huge metropolitan area, such as Phoenix, previous studies from CAP LTER have proven that preserves inside the city will have more nitrogen in the soil than farther out of the city. We will/have been sampling from different sites in the city as well as the outer edges of the city. Our inner-city sites include Piestewa Peak area and South Mountain and our outer-city sites include the White Tanks, Lost Dutchman Trail (in the Superstition Mountains), Salt River Reserve, the Usery Mountain region and the Estrella Mountains. So far, we have hit Piestewa Peak, Salt River Reserve, Usery Mountain, White Tanks, and one of the South Mountain sites. At each site, there is a control plot and there is a nitrogen plot, where nitrogen has been added to the soil. 

Carbon Utilization Plate

The field work is an enjoyment, because we get to enjoy nature while collecting our samples. The way samples are collected is by taking a soil core, which is shaped as a cylinder, and pound it into the dense desert soil, then scoop it up into a whirlpool bag. The cylindrical shape helps to ensure multiple layers of the soil is collected, and not just the surface. The samples are then taken back to the lab and prepared for the next steps. To test what kind of microbes are in the soil, we use carbon utilization plates, which have different types of carbon in each well. By putting our soil samples into these, the various microbes in the samples eat the different kinds of carbon and respire chemicals that turn the wells purple over time. The intensity of purple is tested every 24 hours for the next 4 days.

Additionally, the chemical composition of the soil will also be tested, including the phosphorus and nitrogen levels in the soil, the water content, and the texture of the soil. 

Lab member, Paul, and I collecting samples

A busy summer of Sonoran Desert research

We have begun what will be a very busy summer! There are several students and researchers working in the lab this summer on several different projects. Here is a brief overview:

Soil restoration project at the McDowell Sonoran Preserve

We are processing soil samples from an experiment testing methods to restore old mountain bike trails. Mountain biking might be fun, but it can have a hard impact on the soil. The soil along mountain bike trails becomes very compacted, making it difficult for plants to establish, and the constant action of the tires causes erosion. The McDowell Sonoran Field Institute is testing the best way to help restore old trails to be native desert. Coby is assisting with the soil sampling and analyses.

Elena and Matt are studying how different decomposer organisms influence plant leaf decomposition during composting. We made litterbags full of sycamore leaves, and each of the bags holding the leaves have different hole sizes. Some of the bags only let in microbial decomposers (like bacteria and fungi). Some of the bags let in very small invertebrates, like mites and nematodes. And some of the bags let in bigger invertebrates, like worms, beetle larvae, and termites. In this picture, Nikita and Paul are helping Matt bury the litterbags in the compost pile at Phoenix College's garden in downtown Phoenix. It's pretty rare for composters to think about invertebrates, but boy there's a lot of interesting critters living in that compost pile! At the end of the project, we will be able to show how important they are in the breakdown of the leaves as they turn into compost.

Sampling soil at the Salt River Rec Area in Tonto National Forest

We are also continuing our work to investigate the impacts of nitrogen deposition from air pollution on soil ecosystems here in Phoenix. Our main project this summer is to see how that extra nitrogen influences soil microbial communities. We are sampling soil from parks inside and outside of Phoenix, and we will profile the microbial community living in the soil, as well as characterize the chemical and physical properties of the soil that provides the habitat for these microbes. In this photo, three students are helping with the soil sampling: Nikita and Miranda in front and Paul behind. Nikita is the student responsible for the work. (Paul and Miranda are helping out, but they actually have their own, separate project to work on!) Nikita will be posting to the blog throughout the summer about what she's doing and learning:

Meet Nikita! Here she's collecting a soil sample.

Can moss help remove nitrogen pollution from the soil?

A while ago, I told you about soil biological crusts, and how they are an important part of nutrient cycling in deserts. One of the main parts of crusts that I'm interested in is moss.

Soil biotic crust dominated by moss (the green fuzzy stuff)

Moss is really neat, because it can tolerate the very harsh conditions that are common in the desert. They can survive being desiccated (almost completely dried out), extreme heat, and freezing. As soon as conditions improve, they perk right up! That means that when it rains, moss can quickly "wake up" and start exchanging nutrients with the soil and water. (You can see how quickly dry moss "wakes up" after I add water in the video below.) Other, larger plants are not able to respond to rain quite so quickly.

Here in Arizona's Sonoran Desert around Phoenix, we receive a fair amount of nitrogen from air pollution. That means that soil inside the city of Phoenix has more nitrogen than soil outside the city. A lot of our research tries to understand what happens to that nitrogen that's deposited by air pollution. Nitrogen can fertilize plants (which is why you might put it on your yard or garden), but too much of it can cause a problem, both for human health and for the ecosystem's health!

I was curious whether moss might be able to take up some of that extra nitrogen from pollution. Because it can "wake up" so quickly after rain, we wanted to know whether moss can take up some of that nitrogen, even if larger plants can't!

To answer our question, we sampled moss, and the soil beneath it, from different places in and around Phoenix. We sampled from both inside the city (where nitrogen pollution is a bigger problem) and rural areas to the east and west of Phoenix (where nitrogen pollution is less of a problem).

Undergraduate research student, Jessica, sampling moss.

We measured the amount of nitrogen in the moss and soil, so that we could learn whether moss takes up the extra nitrogen when it's in the soil. If moss nitrogen is higher where soil nitrogen is higher inside the city from pollution, it would suggest that moss is taking up the extra nitrogen in the soil. If moss nitrogen is the same across all of the sites, regardless of the amount of soil nitrogen, it would suggest that moss is not able to take up the extra nitrogen.


What did we learn? In our samples, there was more nitrogen in the soil inside the city of Phoenix (as expected), and the moss growing on that soil was also higher in nitrogen. That suggests that moss can take up nitrogen from pollution.

Moss nitrogen increases as soil nitrogen increases. The extra nitrogen inside the city of Phoenix ("core") compared to rural areas to the east and west is probably from nitrogen pollution.

However, moss is much less abundant inside the city where N pollution is a problem. Moss are sensitive to pollution and human disturbance, so they have a more difficult time surviving inside the city.

Moss is less abundant inside the city of Phoenix ("core") compared to the rural areas to the west and east.

So what does all of this mean? Moss can take up extra nitrogen from pollution, but because there is less moss where the pollution occurs, they aren't going to solve the problem. We need to help preserve the fragile moss crusts in order for them to help deal with the nitrogen pollution!

The results of this experiment are published in the article: Ball, B. and Alvarez Guevara, J. The nutrient plasticity of moss-dominated crust in the urbanized Sonoran Desert. Plant and Soil.