Using Denitrifying Plants in the Landscape

Turns out we have an addiction in this country. “Call it the nitrogen fix. It is like a drug mainlined into the planet’s ecosystems, suffusing every cell, every pore — including our own bodies.”1 In response, some jurisdictions (apparently finding dealing with addiction at the source – i.e. agriculture, wastewater, feed lots, poor landscape practices, etc. too difficult) are requiring the end user of non-potable reclaimed effluent to utilize only denitrifying plants in the landscape. Their theory being that if the ornamental plants can take up the nitrogen, then problem solved.

This discussion of overabundance of nitrogen in our water and soil could get really scientific, really fast (and unfortunately make for very dry reading) so instead I’m going to keep things pretty simple and straightforward and provide some great resources for seeking additional information.

The nitrogen problem. “Over the last century, the intensive use of chemical fertilizers has saturated the Earth’s soils and waters with nitrogen. Now scientists are warning that we must move quickly to revolutionize agricultural systems and greatly reduce the amount of nitrogen we put into the planet’s ecosystems.”1 Excessive nitrogen leaches into waterways, feeds algal blooms and contributes to eutrophication of water systems….. effectively starving plant and aquatic life of oxygen. It can even starve our children of oxygen, known as Blue Baby Syndrome, as a result of nitrogen contaminated drinking water.

The good news. Nitrogen is an inorganic compound which, unlike other macronutrients, can be turned to gas and released into the atmosphere. More good news. This means that the use of denitrifying plants can be addressed via phytometabolism in a relatively short period of time and presents good opportunities for field application.2 “Since all plants use nitrogen and support denitrifying bacteria, any kind of plant can provide some form of nitrogen remediation from soils and water. However, the method that provides the quickest remediation tends to be a system that includes plants with very high growth rates and evapotranspiration rates. Nitrogen is used up quickly, or the plant acts like a large reactor, priming the soil bacteria for speedy conversion of the nitrogen into a gas. Plants species that produce a lot of biomass have been those most successfully used in studies to remove high levels of nitrogen in soils and groundwater.”3 Some would then argue for the use of bluegrass and other fast growing turf, but studies have shown that a mixed species landscape will produce a more diverse microbial soil, and therefore denitrify faster via plants AND bacteria.

Finding the right plants. While in no way an exhaustive list, the book Phyto: Principles and resources for site remediation and landscape design, suggests a brief list of high biomass plants such as Bambuseae, Brassica juncea, Brassica napas, Cannabis sativa, Linum usitatissimum, Panicum virgatum, Populus, Salix and Sorghum. Additionally, some high evapotranspiration-rate plants include Alnus, Betula, Eucalyptus, Fraxinus, Populus, Salix, Sarcobatus vermiculatus and Taxodium distichum. Again, while not an exhaustive list one thing to note is that these tend to be high water use species. In a large scale remediation setting this is desirable (because you are purposefully applying large amounts of contaminated water), however, it tends to fly in the face of end user goals in the ornamental landscape that are working hard to reduce overall water consumption.

Unfortunately, it would seem that municipalities, especially in drought plagued areas trying to encourage water reuse, may end up further discouraging effluent reuse with these unnecessary regulations and may find themselves dealing with unintended consequences, such as forcing the use of plants that require even more water to both thrive and denitrify the soil. It is clearly a discussion worth continuing.

 

I’d like to thank Kate Kennen for the amazing information gathered from her book for this article. Kate is always an inspiration and this book is an INCREDIBLE resource for all practical applications, large or small, for phytoremediation and productive landscapes. It is a must read!

Find Kate here….  http://offshootsinc.com/

Find her book here…. https://www.amazon.com/Phyto-Principles-Resources-Remediation-Landscape/dp/0415814154

  1. Fred Pearce. Copyright 2009. http://e360.yale.edu/mobile/feature.msp?id=2207
  2. Kate Kennen and Niall Kirkwood, Phyto: Principles and resources for site remediation and landscape design. Routledge, 2015. Figure 3.1, Page 63
  3. Page 128

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