Q: The Wetlands Initiative is helping farmers install wetlands to naturally reduce nutrient runoff. How exactly do wetlands remove nutrients?

One of the valuable services provided by natural, restored, or constructed wetlands is that they protect downstream waterways from the impact of nutrient pollution.

Our past modeling work in the Big Bureau Creek Watershed, an agricultural watershed in north-central Illinois, has shown that small, precisely placed wetlands can cost-effectively reduce the excess nutrients coming off farm fields. By sharing these findings with farmers through targeted outreach, we now have seven candidates lined up to install a “demonstration wetland” on their properties to show their peers how they work to improve water quality.

Wetlands are often described as “filtering out” pollutants from water, acting as “nature’s kidneys.” While this is a useful basic way to conceptualize it, there’s actually a lot more going on in a wetland than that. I usually explain to landowners that wetlands are very complex systems, and they don’t take nutrients out of incoming waters in just one way. Wetlands are able to remove nitrogen and phosphorus through a combination of physical, chemical, and biological processes. These naturally occurring processes adsorb/absorb, transform, sequester, and remove the nutrients and other chemicals as water slowly flows through the wetland.

The main physical processes of nutrient removal are particle settling (sedimentation), volatilization (releasing as a gas into the atmosphere), and sorption. Sorption includes a nutrient adhering to a solid (adsorption) or diffusing into another liquid or solid (absorption). Chemical processes include transformations of nutrient forms and chemical precipitation, in which a solid compound is formed out of a liquid through a chemical reaction. The main biological processes are uptake (or assimilation) by plants, algae, and bacteria and transformation processes conducted by microbes. All of these processes occur throughout the different wetland compartments, which include water; biota (plants, algae, and bacteria); litter; and soil.

A simplified illustration of the nitrogen and phosphorus cycles in a wetland (modified from Kadlec and Knight (1996), “Treatment Wetlands”; images from IAN, University of Maryland).

Both nitrogen and phosphorus can be present in many forms (particulate, dissolved, organic, inorganic, etc.), and these forms are acted upon differently by the various processes within the wetland compartments. For example, some forms are volatile and released into the atmosphere, others fall to the bottom of the wetland, and other forms are used by plants and microorganisms. These wetland processes are affected by the presence or absence of oxygen, season, temperature, water inflow rate, nutrient loading rate, and retention or holding time of the water within the wetland. So while a wetland is always working to remove nutrients, the rate of this removal depends on a great variety of factors.

While the dominant removal processes for nitrogen and phosphorus are different, both nutrients are utilized by wetland biota. Wetland plants uptake inorganic nitrogen and phosphorus forms (i.e., nitrate, ammonia, and soluble reactive phosphate) through their roots and/or foliage during the spring and summer and convert them into organic compounds for growth. However, this only provides temporary storage of the nutrients. The majority of these assimilated nutrients are released back into the water and soils when plants grow old and decompose during the fall and winter. A small amount of the nutrients (10–20%) does remain stored in hard-to-decompose plant litter and becomes incorporated in wetland soils, but this is relatively minor compared to other removal processes.

Nitrogen removal involves a large suite of bacteria (or microbes) that mediate or conduct numerous chemical reactions. These microbes are found on solid surfaces within the wetland, such as soil, litter, and submerged plant stems and leaves. The main transformation processes are ammonification (organic nitrogen to ammonia), nitrification (ammonia to nitrate or nitrite), and denitrification, where nitrate (NO3) is converted to harmless nitrogen gas (N2), which composes 85% of our atmosphere.

Denitrification is the dominant, sustainable removal process in wetlands that receive high nitrate loadings from agricultural runoff or wastewater treatment plant discharge. Denitrification is primarily performed by bacteria that are heterotrophic, meaning they require a carbon source for growth and energy. Wetland plants are a key source of this carbon. Since denitrification is facilitated by microbes, the process is temperature-dependent. Higher rates of denitrification occur during higher temperatures when the bacteria are more active. Therefore, wetlands designed for nutrient removal like the ones that TWI is promoting work hardest at removing nitrogen during the summer months (when runoff is also highest!), and it’s important that native plants are installed in them to help fuel the process.

Phosphorus, on the other hand, is removed primarily through physical and chemical processes. Phosphorus typically enters wetlands attached to suspended material like small soil particles (particulate form) or as PO4 (dissolved form). Particulate phosphorus is deposited in wetlands (the process of sedimentation). The leaves and stems of emergent and submerged vegetation help to settle out particles by slowing the water down and allowing the particles to fall. The dissolved form of phosphorus (phosphate) accumulates quickly in sediments by sorption (to aluminum and iron oxides and hydroxides) and precipitation (to form aluminum, iron, and calcium phosphates).

However, wetland soils have a limited amount of phosphorus they can hold. In order to continually remove phosphorus, new soils need to be “built” within the wetland from remnant plant stems, leaves, root debris, and undecomposable parts of dead algae, bacteria, fungi, and invertebrates. The growth, or accretion, of new material in the wetland is the only sustainable removal and storage process for phosphorus. The farm-based wetlands TWI is designing will primarily remove nitrogen, but they will accomplish some phosphorus removal as well.

As you can see, wetlands don’t just filter: They also transmogrify, release into the atmosphere, and consume nutrients. And we haven’t even touched on the carbon cycle in wetlands!

Considering all these complex processes, TWI is working to learn more about how to optimize farm-based wetlands’ nutrient removal. In 2015, we’re beginning a partnership with a professor of environmental engineering at the University of Illinois at Chicago, Dr. Karl Rockne, to conduct water quality monitoring of the demonstration wetlands. Dr. Rockne will place automated sampling equipment in the wetland to gather data on various forms of nitrogen and phosphorus and will deploy “tracer particles” to study the movement of particles within the wetland. With this more detailed understanding, we can then enhance the design of farm-based wetlands to achieve maximum nutrient removal—helping them work even better to clean water!

 ~Jill Kostel, Ph.D., senior environmental engineer, the Wetlands Initiative