Water Quality: Nitrogen

In this month’s exploration of water quality, we’re going to focus on nitrogen. Nitrogen is an important element on the periodic table and it comes in many forms. To better understand the role of nitrogen in our environment, we first need to talk about the nitrogen cycle.

In environmental science, we think about the cyclical nature of many different compounds, including water, carbon dioxide, nitrogen, and phosphorus. These materials pass through different forms and cycle through living organisms and the environment, hence why we talk about cycles.

For nitrogen, we are going to start the cycle in atmospheric nitrogen, which takes the form of N2. N2 is Nitrogen gas and it makes up 78% of the air we breathe. However, nitrogen in this form is typically considered inert. This means that the bond between the two nitrogen molecules is very strong so it’s difficult to break and it doesn’t react with other molecules at normal temperatures and pressures. However, some bacteria are able to use N2 gas and convert it into nitrogen that is usable by other organisms. In particular, these bacteria include symbiotic species that live in the root systems of certain plants like legumes (peas) and grasses. These bacteria are able to take N2 and convert it into ammonia (NH3) and ammonium (NH4+).

At high concentrations, ammonia and ammonium are toxic to living organisms. Thankfully, other kinds of bacteria take this form of nitrogen and convert it to nitrite (NO2-) and then other bacteria convert that form to nitrate (NO3-). Nitrate is the form of nitrogen that is most usable to plants, which can be nitrogen limited. 

The Nitrogen Cycle

This is why we apply fertilizer to plants that we want to increase the growth of. Those plants take that nitrate and use it to build new plant tissue (ie. to grow bigger or produce more fruit). From there animals eat those plants and take up that nitrogen to incorporate into their own tissues. Animals, including humans, then pass waste nitrogen in the form of urea (CO(NH2)2). More bacteria then break urea down into ammonia to re-enter the cycle. When organisms die, bacteria also break down the nitrogen stored in those once living tissues, back into ammonia which then re-enters the cycle. Finally, there are still more bacteria that take nitrate (NO3-) and turn it back into nitrogen gas (N2), which is released back into the atmosphere. There are a few other components that are part of the nitrogen cycle, such as lightning and the burning of fossil fuels, but those are beyond the scope of what we are talking about today.

The main takeaway from all of this is that bacteria are an important part of ensuring that all living organisms have nitrogen available to them. The other takeaway is that nitrate (NO3-) makes plants grow.

This means that in aquatic systems, an excess of nitrogen can cause algal blooms. This happens when phytoplankton in the water take advantage of the surplus of nitrogen and start growing like crazy. This makes water clarity go down and, when the algae eventually run out of nitrogen, they begin dying. The process by which bacteria break down these dead plants uses up oxygen, which causes the water to become hypoxic (low oxygen) and even anoxic (no oxygen). This rapid decrease in available oxygen can then kill other animals, like fish and crabs.

So, we want some nitrogen, in the form of nitrate, in the environment, including the Chesapeake Bay. However, when we over fertilize land or have extreme levels of animal or human waste, all of this nitrogen ends up in the water after a rain event.

When it comes to measuring nitrogen in the Chesapeake Bay, there are several different ways we can get this data. However, it’s important to know what form of nitrogen you are looking for as you can test for individual forms such as just nitrate or just ammonia, or you can test for total nitrogen, but you won’t get a breakdown of the different forms making up the total.

The easiest measurement is to use test strips that produce a color change based on the amount of nitrogen in a water sample. These test strips are cheap, easy to use, and give results relatively quickly. These results are also relatively accurate, though not as accurate as laboratory tests that use spectrophotometry. At Phillips Wharf, we monitor nitrogen in the forms of ammonia and nitrate in our aquariums to ensure that bacteria are properly completing the breakdown of waste nitrogen from our animals to nitrate. We use liquid chemical based tests that require you to take a sample of water, add several drops of 1 or 2 indicator chemicals, mix, then let sit for 1 to 5 minutes depending on the test. Again, a color change will indicate the level of what is being tested and you compare the color of your sample to a standard to get your result. The results from both test strips and liquid tests provide results that are accurate enough to make decisions about an aquarium or the water in your home (where nitrate should be 0), but not necessarily accurate enough for scientists recording and tracking nitrogen for scientific studies. In those cases, a water sample will be collected and processed depending on what form of nitrogen you want to measure. For example, to measure Total Kjeldahl Nitrogen (TKN), the water sample will be treated with sulfuric acid to convert all nitrogen to ammonia, which is then quantified using colorimetry (the same method as the chemical based tests we use at Phillips Wharf but with a machine to register tiny changes in color and intensity) or titration.

Nitrate Test Strips
Nitrate Liquid Chemical Test

Nitrogen waste is a larger problem than just one person but there are things we can do to help reduce our nitrogen footprint. Food waste and fertilizer are major contributors to nitrogen waste. Creating a meal plan, composting food waste, and knowing how to properly store produce and leftovers can help reduce food waste in your home. It’s also important to understand that much food waste happens before the food even gets to us, the consumer. A lot of produce isn’t considered “pretty” enough for the consumer, so purchasing from companies that help use this food is a good idea. This might include purchasing from local farms that donate leftover food (this happens at the end of the St. Michaels Farmers Market), or finding companies that take advantage of this “waste” and make sure it still enters the food market, such as Hungry Harvest, Misfit Market, or Wtrmln Wtr, which turned watermelons that were deemed to ugly to sell, into cold pressed juice. You can read more about companies turning food waste into usable products by clicking here.

Other ways to reduce nitrogen running off into the Chesapeake Bay include reducing and ceasing the use of fertilizer on your land, such as in your home landscape. If you do use fertilizer, making sure that there are buffer zones between your land and stormwater drains or nearby waterways can help make sure that plants pull out any excess nitrogen before it reaches the Bay.

Finally, if you’re interested in learning about your nitrogen footprint, you can check out this calculator from the University of Virginia.

[1] https://www.usgs.gov/centers/chesapeake-bay-activities/science/greatest-opportunities-future-nitrogen-reductions

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