Today’s guest post is from Nathalie Shanstrom, a sustainable landscape architect with the Kestrel Design Group. It’s a little science-y, but still very readable. If you’re not a stormwater geek, don’t be deterred – there’s a lot of great information in here. -LM

Parts 1 and 2 of this blog series looked at how plants improve performance of bioretention systems for retention of dissolved nitrogen and phosphorus, respectively.

Today’s final post will get a little more practical, with a discussion of what factors affect efficiency of plants in retaining dissolved nutrients.

Read et al (2008) studied how plants vary in their effectiveness for bioretention and found that effluent concentrations varied to more than 20-fold for nitrogen (NOx and NH₄^+,) and 2- to 4-fold for total phosphorus. Some of this variation was attributable to plant size. Other factors that they and others found to influence N retention efficiency include:

• Root size, including length of longest root, rooting depth, total root length, and root mass appeared to be the most common and strong contributors to N and P removal in Read et al 2010.
• Plant maturity: N retention improved with maturity (Lucas and Greenway 2011).
• Plant vigor (Henderson 2009): some species can accumulate biomass much more rapidly than others, and this in turn affects their potential to act as a nutrient sink within the bioretention mecocosms.
• Growth rate: plants that contributed most to pollution removal also had high growth rates (Read et al 2010).
• Time of year: seasonal differences in N retention correlate with plant uptake (Lucas and Greenway 2011).
• Soil type: growing medium significantly affected the mass of nutrients retained by plants in Henderson’s 2009 thesis study as well as in Lucas and Greenway 2008, with loam providing better nutrient retention than sand or gravel in both studies. Lucas and Greenway (2008), for example, found 92% total phosphorous removal in vegetated loam, 67% in vegetated sand, and 44% in vegetated gravel after 50 weeks of stormwater loading. According to Henderson, “the greater water holding capacity of the finer grained media such as loamy sand and sand gave the vegetation in these systems a longer period of access to moisture and nutrients and thus facilitated greater nutrient removal” (Henderson 2009).
• Nutrient loading rate: proportion of nutrients removed that can be attributed to plant uptake is dependent on the nutrient loading rate. Leaf tissue nitrogen increased in response to nutrient availability and decreased in response to nutrient scarcity (Henderson, 2009).
• Increased frequency of harvesting increases nitrogen uptake (Tuncsiper et al 2006 in Lucas and Greenway 2011).
• Rates of litterfall: litterfall can contribute to nutrients being immobilized as they build up as organic matter builds up in the media (Henderson, 2009).

In short, larger, healthier, more vigorous plants and roots lead to better retention of dissolved nutrients, and better soils lead to more vigorous plants.

Part I: How plants improve dissolved nitrogen retention.

Part II: How plants improve dissolved phosphorous retention.

References

Denman, L.; May, P.; and P. Breen. 2006. An investigation of the potential to use street trees and their root zone soils to remove nitrogen from urban storm water. Australian Journal of Water Resources: 1 (3): 303-311.

Henderson, C.F.K. (2009) The Chemical and Biological Mechanisms of Nutrient Removal from Stormwater in Bioretention Systems. Thesis. Griffith School of Engineering, Griffith University.

Lucas, W. C.; Greenway, M. (2008) Nutrient Retention in Vegetated and Non-vegetated Bioretention Mesocosms. J. Irrig. Drain. E-ASCE, 134 (5): 613-623.

Lucas, W. C.; Greenway, M. (2011). Hydraulic response and nitrogen retention in bioretention mesocosms with regulated outlets: part II–nitrogen retention. Water Environ Res. 2011 Aug;83(8):703-13.

Read, J.; Fletcher, T. D.; Wevill, T.; Deletic, A. (2010) Plant Traits that Enhance Pollutant Removal from Stormwater in Biofiltration Systems. Int. J. Phytoremediation, 12, 34–53.

Read, J.; Wevill, T.; Fletcher, T. D.; Deletic, A. (2008) Variation Among Plant Species in Pollutant Removal from Stormwater in Biofiltration Systems. Water Res., 42, 893–902.

Tuncsiper, B.; Ayaz, S.C; Akca, L. (2006). Modeling and Evaluation of Nitrogen Removal Performance on Subsurface Flow and Free Water Surface Constructed Wetlands. Water Sci. Technol., 53 (12), 111-120.

Image: milan.ilnyckyj