8.20.22

Nitrate: Good for plants, bad for drinking water

Evelyn is a scientist at the University of Minnesota. She studies nitrate pollution and how growing perennial crops may prevent it from entering our drinking water.

The activities are as follows:

Nitrogen is the most abundant element in our atmosphere. All living things need nitrogen to live and grow, but plants and animals can’t use the atmospheric form. Instead, many plants extract nitrogen from the soil and in the case of crops, we supply nitrogen through fertilizer, in a form called nitrate.

Nitrate dissolves well in water. This helps make it easy for plants to use, but it can also end up in rivers and groundwater. Groundwater with just 10 milligrams of nitrate per liter is not safe to drink because it can lead to a higher risk of cancer and birth defects. It is really expensive to remove nitrate from drinking water. Towns whose groundwater is contaminated must either pay to remove it or find a new drinking water source. Virtually all nitrate pollution comes from fertilizers used on crops, so one way to address this problem is to change the way we farm.

Annual plants live for just one season and typically have smaller shallower root systems than perennial plants, which live for multiple seasons. Most farmland grows annuals like corn and soybeans, but we get some of our food from perennials like apples, hazelnuts, and raspberries. Perennials stay in the ground all year and start growing right away in the spring before annual crops are even planted. Perennial grasses are particularly good at growing deep roots and taking up lots of nitrate from the soil. If we could produce more food from perennial plants instead of annual plants, crops may absorb enough nitrate to prevent it from getting into our drinking water.

For twenty years, researchers at The Land Institute in Kansas and at the University of Minnesota have been working on a new perennial grain crop called Kernza®, the seeds from a plant called intermediate wheatgrass. Kernza® can be used like wheat or rye, but it has a larger, deeper root system than regular annual wheat. Perennial plants’ deep roots are really good at absorbing dissolved nitrate in soil, so scientists wanted to study Kernza® in the field to see if it would prevent nitrate getting into groundwater.

Evelyn is one of these researchers. She grew up in Minneapolis, Minnesota and as a high school student, she was surprised to learn that agriculture has a huge impact on soil and water quality, wildlife habitat, and biodiversity. She wanted to help protect the environment, so she studied Food Systems at the University of Minnesota. A few years later, she joined a project that involved planting Kernza® in rural areas to prevent and reduce nitrate contamination of drinking water. Farmers, city officials, water managers, and scientists worked together to find solutions. This project inspired Evelyn to study Kernza® and nitrate for her master’s degree.

In her experiment, Evelyn planted plots of Kernza® (foreground) and plots with a corn-soybean rotation (background). This photo was taken in a corn year. Lysimeters are used to collect groundwater samples. The white posts are holding up the lysimeter sampling tubes.

To see if Kernza® helped absorb more nitrate from soil than annual crops, Evelyn and her colleagues ran an experiment. They planted plots of Kernza® and other plots that rotated between corn and soybean every year. Plots with Kernza® and corn were fertilized with nitrogen. Soybean plots were not fertilized.

In the plots, they installed lysimeters: long tubes that go down several feet to collect soil water from below where most plant roots can reach it. Soil water is the water that sits between soil particles. It can be taken up by plant roots or trickle down into the groundwater that is used for drinking wells. Once it moves deeper than a plant’s roots, it can’t be taken up and is very likely to reach the groundwater. Evelyn took water samples from the lysimeters every ten days and analyzed them for nitrate concentration.  If more nitrate is found in soil water under corn and soybean plots than Kernza®, this would be good evidence that Kernza® takes up more nitrate and helps protect groundwater.

Featured scientist: Evelyn Reilly (she/her) from University of Minnesota

Flesch–Kincaid Reading Grade Level = 10.9

12.10.20

Mangroves on the move

mangrove in marsh
A black mangrove growing in the saltmarshes of northern Florida.

The activities are as follows:

All plants need nutrients to grow. Sometimes one nutrient is less abundant than others in a particular environment. This is called a limiting nutrient. If the limiting nutrient is given to the plant, the plant will grow in response. For example, if there is plenty of phosphorus, but very little nitrogen, then adding more phosphorus won’t help plants grow, but adding more nitrogen will. 

Saltmarshes are a common habitat along marine coastlines in North America. Saltmarsh plants get nutrients from both the soil and the seawater that comes in with the tides. In these areas, fertilizers from farms and lawns often end up in the water, adding lots of nutrients that become available to coastal plants. These fertilizers may contain the limiting nutrients that plants need, helping them grow faster and more densely.

One day while Candy, a scientist, was out in a saltmarsh in northern Florida, she noticed something that shouldn’t be there. There was a plant out of place. Normally, saltmarshes in that area are full of grasses and other small plants—there are no trees or woody shrubs. But the plant that Candy noticed was a mangrove. Mangroves are woody plants that can live in saltwater, but are usually only found in tropical places that are very warm. Candy thought the closest mangrove was miles away in the warmer southern parts of Florida. What was this little shrub doing so far from home? The more that Candy and her colleague Emily looked, the more mangroves they found in places they had not been before.

Candy and Emily wondered why mangroves were starting to pop up in northern Florida. Previous research has shown nitrogen and phosphorus are often the limiting nutrients in saltmarshes. They thought that fertilizers being washed into the ocean have made nitrogen or phosphorus available for mangroves, allowing them to grow in that area for the first time. So, Candy and Emily designed an experiment to figure out which nutrient was limiting for saltmarsh plants. 

mangrove saltmarsh researchers
Candy (right) and Emily (left) measure the height of a black mangrove growing in the saltmarsh.

For their study, Candy and Emily chose to focus on black mangroves and saltwort plants. These two species are often found growing together, and mangroves have to compete with saltwort. Candy and Emily found a saltmarsh near St. Augustine, Florida, in which they could set up an experiment. They set up 12 plots that contained both black mangrove and saltwort. Each plot had one mangrove plant and multiple smaller saltwort plants. That way, when they added nutrients to the plots they could compare the responses of mangroves with the responses of saltwort. 

To each of the 12 plots they applied one of three conditions: control (no extra nutrients), nitrogen added, and phosphorus added. They dug two holes in each plot and added the nutrients using fertilizers, which slowly released into the nearby soil. In the case of control plots, they dug the holes but put the soil back without adding fertilizer.

Candy and Emily repeated this process every winter for four years. At the end of four years, they measured plant height and percent cover for the two species. Percent (%) cover is a way of measuring how densely a plant grows, and is the percentage of a given area that a plant takes up when viewed from above. Candy and Emily measured percent cover in 1×1 meter plots. The cover for each species could vary from 0 to 100%.

Featured scientists: Candy Feller from the Smithsonian Environmental Research Center and Emily Dangremond from Roosevelt University

Flesch–Kincaid Reading Grade Level = 8.3

7.24.20

Limit by limit: Nutrients control algal growth in Arctic streams

The Arctic Stream Team. Frances, Breck, Abby, Alex, Jay, and Arial at Toolik Field Station in 2019. 

The activities are as follows:

You rely on the nutrients from the foods you eat to grow and thrive. Other organisms, like microbes, do as well! Aquatic algae, a type of microbe that live in the water, need to take in nutrients from their surroundings for growth. Two important nutrients for algal growth are nitrogen (N) and phosphorous (P).

Sometimes the environment does not have all the nutrients that aquatic algae need to grow. When one nutrient is less available compared to others, algae can become nutrient limited. Research on nutrient limitation started with Justus Liebig, a 19th century scientist who proposed the “law of the minimum.” The law states that the nutrient available in the lowest amount relative to demand will limit overall growth and production. This means that growth is not controlled by all the nutrients, but by the scarcest one (the “limiting factor”). When more than one nutrient limits growth, algae are considered co-limited. This just means that a combination of two nutrients are needed for algae to grow. Knowing what nutrients are limiting growth helps scientists understand how an ecosystem is working.  

From other research we know that many ecosystems, including those in the Alaskan Arctic, are phosphorus-limited. Scientists figured this out because they found if they added phosphorus, then algae growth increased. However, climate change could change this. As the Arctic warms, ecosystems on land might start to release nutrients in higher amounts or new proportions into the water. These extra nutrients will likely cause increases in algae growth in streams and ponds, which in turn could change food webs and nutrient cycling. It is therefore important to understand which nutrients are currently limiting algae growth before climate change changes things even more. This starts with tests to see how Arctic algae grow in response to changes in N, P, and N and P in the water.  

A team of scientists got to work on this question! Arial, Jay, Frances, Alex, Breck, and Abby are all interested in understanding how climate change may alter nutrient limitations in Arctic streams. Each team member has a unique role in the larger research project. For example, undergraduate researcher Abby spent her 2019 summer at Toolik Field Station in Northern Alaska as part of a research opportunity. She explored nutrient limitation in one particular lake, called Lake I8. 

Abby used small cups that placed into the streams that fed into Lake I8. These cups were filled with agar gel, a material used in labs to grow microbes. Each cup contained different nutrient treatments. Abby used four different treatments in her cups: (1) a control (agar only), (2) agar + nitrogen, (3) agar + phosphorus, and (4) agar + nitrogen + phosphorus. On the top of each cup, she placed a glass disk to provide a surface for the algae to grow.

A. Cups before going into the stream. B. Abby putting out her cup treatments into an Arctic stream. C. Cups incubating under water in an Arctic stream. D. Analyzing Chlorophyll a extracted from the cups. 

Abby put 5 replicate cups for each treatment at both the Inlet and Outlet streams on the I8 Lake. She left them underwater for 4 weeks. She brought the cups back to the lab to measure the algae that grew on each glass disk. Abby measured how much algae grew on each disk by measuring the amount of Chlorophyll a, the green pigment that helps plants photosynthesize. The more pigment, the more the algae is growing. Abby compared the data from the control to each of the other treatments. When there is more growth in a treatment compared to the control, that means a particular nutrient was limiting at that location. Abby expected that the streams would be limited by the amount of phosphorus, but not the amount of nitrogen. She predicted algae would grow more when they are given additional phosphorus compared to the control treatment.

Featured scientists: Abigail Rec from Gettysburg College; Frances Iannucci, Alex Medvedeff, and Breck Bowden from University of Vermont; Arial Shogren and Jay Zarnetske from Michigan State University

Flesch–Kincaid Reading Grade Level = 8.6

11.4.19

Streams as sensors: Arctic watersheds as indicators of change

Jay taking field notes next to a rocky Tundra stream.

The activities are as follows:

The Arctic, Earth’s region above 66° 33’N latitude, is home to a unique biome, known as tundra. A defining trait of tundra ecosystems is the frozen land. Permafrost is the underground layer of organic matter, soil, rock, and ice that has been frozen for at least 2 full years. Plant material decays slowly in the Arctic because of the cold temperatures. Building up over thousands of years, the plants become frozen into the permafrost. Permafrost represents a very large “sink” of dead plant material, nutrients, and soil that is locked away in a deep freeze. 

Though the Alaskan Arctic may seem far away from where you live, tundra permafrost is important for the entire globe. During the past few thousand years, Earth’s climate has naturally changed a little over time, but because humans are adding greenhouse gases to the atmosphere, the average global temperature may increase by as much as 2 to 4oC over the next century. As a result of global climate change, permafrost has become less stable. By causing warmer and wetter conditions in the Arctic, thawing permafrost soils release ancient material that was previously frozen and locked away. Two important materials are dissolved nitrogen (N), which is a nutrient critical for plant growth, and carbon (C), which is stored in plant matter during photosynthesis. These released materials can be used again by plants, but some is carried away as melted water flows from the land into rivers and streams. You can imagine N and C in permafrost like a bank account where the landscape is the savings account. The land slowly deposits or withdraws N and C from the savings account, while the water receives any excess N and C that the land does not save.

Arial downloads data from a water quality monitoring station at the Kuparuk River. The station is connected a sensor that stays in the river and takes a reading for both carbon and nitrogen concentrations every 15 minutes.

The water that melts as permafrost thaws flows into a stream, ultimately ending up in an ocean. Watersheds are the network of streams and rivers that flow to a single point as they empty out into the ocean. The water at the end of the watershed therefore reflects all the changes that happened across a large area. Scientists use Arctic watersheds as large “sensors” to understand how and when landscapes may be releasing material from thawing permafrost. 

Because the Alaskan Arctic is a vast, sparsely populated area, scientists often rely on established field stations to conduct experiments, collect observational data, and develop new understanding of Arctic ecosystems. One of these field sites is Toolik Field Station. Scientists working at Toolik have been monitoring terrestrial and aquatic Arctic ecosystems since the late 1970s. 

Arial and Jay are aquatic scientists who work at Toolik. They are interested in how Arctic watersheds respond to climate change. Together, Arial and Jay act like ecosystem accountants: they use the chemistry within the water to monitor changes in ecosystem budgets of C and N. Arial and Jay used both historic data and water quality sensors deployed in 2017 and 2018 to monitor the N and C budget in the Kuparuk River. They use this data to calculate how much N and C the river is spending. They measure this as the total export in units of mass per year. This mass per year is determined by multiplying concentration (mass/volume) by flow (volume/day) and adding these values across the whole season (mass/year). These budgets at the watershed outlet help reveal signals of how this tundra landscape may be changing. In this way, they can assess if the landscape savings account for N and C is being depleted due to climate change. 

Featured scientists: Arial Shogren and Jay Zarnetske from Michigan State University

Flesch–Kincaid Reading Grade Level = 10.8

8.24.15

Urbanization and estuary eutrophication

Charles Hopkinson out taking dissolved O2 measurements.

Charles Hopkinson out taking dissolved O2 measurements.

The activities are as follows:

An estuary is a habitat formed where a freshwater river or stream meets a saltwater ocean. Many estuaries can be found along the Atlantic coast of North America. Reeds and grasses are the dominant type of plant in estuaries because they are able to tolerate and grow in the salty water. Where these reeds and grasses grow they form a special habitat called a salt marsh. Salt marshes are important because they filter polluted water and buffer the land from storms. Salt marshes are the habitat for many different kinds of plants, fish, shellfish, and birds.

Hap Garritt removing an oxygen logger from Middle Road Bridge in winter.

Hap Garritt removing an oxygen logger from Middle Road Bridge in winter.

Scientists are worried because some salt marshes are in trouble! Runoff from rain washes nutrients, usually from lawn fertilizers and agriculture, from land and carries them to estuaries. When excess nutrients, such as nitrogen or phosphorus, enter an ecosystem the natural balance is disrupted. The ecosystem becomes more productive, called eutrophication. Eutrophication can cause major problems for estuaries and other habitats.

With more nutrients in the ecosystem, the growth of plants and algae explodes. During the day, algae photosynthesize and release O2 as a byproduct. However, excess nutrients cause these same algae grow densely near the surface of the water, decreasing the light available to plants growing below the water on the soil surface. Without light, the plants die and are broken down by decomposers. Decomposers, such as bacteria, use a lot of O2 because they respire as they break down plant material. Because there is so much dead plant material for decomposers, they use up most of the O2 dissolved in the water. Eventually there is not enough O2 for aquatic animals, such as fish and shellfish, and they begin to die-off as well.

Two features can be used to identify whether eutrophication is occurring. The first feature is low levels of dissolved O2 in the water. The second feature is when there are large changes in the amount of dissolved O2 from dawn to dusk. Remember, during the day when it’s sunny, photosynthesis converts CO2, water, and light into glucose and O2. Decomposition reverses the process, using glucose and O2 and producing CO2 and water. This means that when the sun is down at night, O2 is not being added to the water from photosynthesis. However, O2 is still being used for decomposition and respiration by animals and plants at night.

The scientists focused on two locations in the Plum Island Estuary and measured dissolved O2 levels, or the amount of O2 in the water. They looked at how the levels of O2 changed throughout the day and night. They predicted that the upper part of the estuary would show the two features of eutrophication because it is located near an urban area. They also predicted the lower part of the estuary would not be affected by eutrophication because it was farther from urban areas.

A view of the Plum Island estuary

A view of the Plum Island estuary

Featured scientists: Charles Hopkinson from University of Georgia and Hap Garritt from the Marine Biological Laboratory Ecosystems Center

Flesch–Kincaid Reading Grade Level = 9.6

1.10.14

The ground has gas!

Measuring nitrogen (N2O) gas escaping from the soil in summer.

Measuring nitrogen (N2O) gas escaping from the soil in summer. Photo credit: Julie Doll, Michigan State University

The activities are as follows:

If you dig through soil, you’ll notice that soil is not hard like a rock, but contains many air pockets between soil grains. These spaces in the soil contain gases, which together are called the soil atmosphere. The soil atmosphere contains the same gases as the atmosphere that surrounds us above ground, but in different concentrations. It has the same amount of nitrogen, slightly less oxygen (O2), 3-100 times more carbon dioxide (CO2), and 5-30 times more nitrous oxide (N2O, which is laughing gas!).

Measuring nitrogen (N2O) gas escaping from the soil in winter.

Measuring nitrogen (N2O) gas escaping from the soil in winter. Photo credit: Julie Doll Michigan State University.

Nitrous oxide and carbon dioxide are two greenhouse gasses responsible for much of the warming of global average temperatures. Sometimes soils give off, or emit, these greenhouse gases into the earth’s atmosphere, adding to climate change. Currently scientists are working to figure out why soils emit different amounts of these greenhouse gasses.

During the summer of 2010, Iurii and his fellow researchers at Michigan State University studied nitrous oxide (N2O) emissions from farm soils. They measured three things: (1) the concentration of nitrous oxide 25 centimeters below the soil’s surface (2) the amount of nitrous oxide leaving the soil (3) and the average temperature on the days that nitrous oxide was measured. The scientists reasoned that the amount of nitrous oxide entering the atmosphere is positively associated with how much nitrous oxide is in the soil and on the soil temperature.

Featured scientist: Iurii Shcherbak from Michigan State University

Flesch–Kincaid Reading Grade Level = 9.2

More information on the research associated with this Data Nugget can be found here. 

Data associated with this Data Nugget can be found on the MSU LTER website data tables under GLBRC Biofuel Cropping System Experiment. Bioenergy research classroom materials can be found here. More images can be found on the LTER website.

logo

SaveSave

1.9.14

Fertilizing biofuels may cause release of greenhouse gasses

An aerial view of the experiment at MSU where biofuels are grown

An aerial view of the experiment at MSU where biofuels are grown. Photo credit: K. Stepnitz, MSU

The activities are as follows:

Greenhouse gases in our atmosphere, like carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), trap heat from the sun and warm the earth. We need some greenhouse gases to keep the planet warm enough for life. But today, the majority (97%) of scientists agree that the levels of greenhouse gases are getting dangerously high and are causing changes in our climate that may be hard for us to adjust to.

Scientist Leilei collecting samples of gasses released by the growing of biofuels

Scientist Leilei collecting samples of gasses released by the growing of biofuels. Photo credit: K. Stepnitz, MSU

When we burn fuels to heat and cool our homes or power our cars we release greenhouse gasses. Most of the energy used today comes from fossil fuels. These energy sources are called “fossil” fuels because they come from plants, algae, and animals that lived hundreds of millions of years ago! After they died, their tissues were buried and slowly turned into coal, oil, and natural gas. An important fact about fossil fuels is that when we use them, they release CO2 into our atmosphere that was stored millions of years ago. The release of this stored carbon is adding more and more greenhouse gases to our atmosphere, and much faster than today’s plants and algae can remove during photosynthesis. In order to reduce the effects of climate change, we need to change the way we use energy and think of new ways to power our world.

One potential solution could be to grow our fuel instead of drilling for it. Biofuels are a potential substitute for fossil fuels. Biofuels, like fossil fuels, are made from the tissues of plants. The big difference is that they are made from plants that are alive and growing today. Unlike fossil fuels that emit CO2, biofuel crops first remove CO2 from the atmosphere as the plants grow and photosynthesize. When biofuels are burned for fuel, the CO2 is emitted back into the atmosphere, balancing the total amount that was removed and released.

Scientists are interested in figuring out if biofuels make a good replacement for fossil fuels. It is still not clear if the plants that are used to produce biofuels are able to absorb enough CO2 to offset all of the greenhouse gases that are emitted when biofuels are produced. Additional greenhouse gases are emitted when producing biofuels because it takes energy to plant, water, and harvest the crops, as well as to convert them into fuel. In order to maximize plant growth, many biofuel crops are fertilized by adding nitrogen (N) fertilizer to the soil. However, if there is too much nitrogen in the soil for the crops to take up, it may instead be released into the atmosphere as the gas nitrous oxide (N2O). N2O is a greenhouse gas with a global warming potential nearly 300 times higher than CO2! Global warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere.

Leilei is a scientist who researches whether biofuels make a good alternative to fossil fuels. He wondered what steps farmers could take to reduce the amount of N2O released when growing biofuel crops. Leilei designed an experiment to determine how much N2O is emitted when different amounts of nitrogen fertilizer are added to the soil. In other words, he wanted to know whether the amount of N2O that is emitted into the atmosphere is associated with how much fertilizer is added to the field. To test this idea, he looked at fields of switchgrass, a perennial grass native to North America. Switchgrass is one of the most promising biofuel crops. The fields of switchgrass were first planted in 2008 as a part of a very large long-term study at the Kellogg Biological Station in southwest Michigan. The researchers set up eight fertilization treatments (0, 28, 56, 84, 112, 140, 168, and 196 kg N ha−1) in four replicate fields of switchgrass, for a total of 32 research plots. Leilei measured how much N2O was released by the soil in the 32 research plots for many years. Here we have two years of Leilei’s data.

Featured scientist: Leilei Ruan from Michigan State University

Flesch–Kincaid Reading Grade Level = 10.1

Additional teacher resources related to this Data Nugget:

logo

SaveSave

SaveSave

1.8.14

Fair traders or freeloaders?

Measuring chlorophyll content in the greenhouse

Measuring chlorophyll content in the greenhouse

The activities are as follows:

When two species do better when they cooperate than they would on their own, the relationship is called a mutualism. One example of a mutualism is the relationship between a type of bacteria, rhizobia, and legume plants. Legumes include plants like peas, beans, soybeans, and clover. Rhizobia live in bumps on the legume roots, where they trade their nitrogen for sugar from the plants. Rhizobia fix nitrogen from the air into a form that plants can use. This means that legumes that have rhizobia living in their roots can get more nitrogen than those that don’t.

Under some conditions, this mutualism can break down. For example, if one of the traded resources is very abundant in the environment. When the plant doesn’t need the nitrogen traded by rhizobia, it doesn’t trade as many sugars to the rhizobia. This could cause the rhizobia to evolve to be less cooperative as well. Less-cooperative rhizobia may be found where the soil already has lots of nitrogen. These less-cooperative bacteria are freeloaders: they fix less nitrogen, but still get sugars from the plant and other benefits of living in nodules on their roots.

Photo by Tomomi Suwa, 2013

Rhizobia nodules on plant roots. In exchange for carbon and protection in the nodules from plants, rhizobia provide fixed nitrogen for plants.

One very important legume crop species is the soybean. Soybeans are used to produce vegetable oil, tofu, soymilk, and many other food products. Soybeans trade with rhizobia for nitrogen, but often farmers add more nitrogen into the field as fertilizer. Since farms use a lot of nitrogen fertilizer, researchers at KBS were interested in how different types of farming affected the plant-rhizobia mutualism.

They grew soybean plants in a greenhouse and added rhizobia from three different farms: a high N farm, low N farm, and organic farm that used no N fertilizer. After four weeks, the researchers measured chlorophyll content of the soybean plants. Healthy plants that have lots of nitrogen will have high chlorophyll content, and plants with not enough nitrogen will have low chlorophyll content. Because high nitrogen could lead to the evolution of less-cooperative rhizobia, they expected that rhizobia from organic plots would be most cooperative. They predicted rhizobia from high N plots would be the least cooperative, and rhizobia from low N plots would fall somewhere in the middle. More-cooperative rhizobia provide more nitrogen, so the researchers expected plants grown with cooperative rhizobia to have higher chlorophyll content than plants receiving less-cooperative rhizobia.

Featured scientist: REU Jennifer Schmidt from the Kellogg Biological Station

Flesch–Kincaid Reading Grade Level = 10.1

For more information on the evolution of cheating rhizobia, check out these popular science articles:

If you are interested in performing your own classroom experiment using the plant-rhizobium mutualism, check out this paper published in the American Biology Teacher describing methods and a proposed experimental design: Suwa and Williamson 2014

SaveSave