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
While bioretention has been shown to be very effective for removing total suspended solids (TSS), heavy metals, particulate nutrients, and hydrocarbons from water, dissolved nutrient removal performance has been much more variable. Several studies have even shown increased nutrient concentrations in stormwater runoff from bioretention systems.
However, research indicates that bioretention can also be designed to consistently provide effective reduction of dissolved nutrients. I’ll be writing a series of blog posts that examine how plants contribute to nutrient reduction in bioretention systems. Today’s addresses nitrogen removal.
Nitrogen Cycle In Bioretention Soils
Nitrogen (N) occurs in several different forms in typical bioretention soils: as particulate organic N, dissolved organic nitrogen, ammonium (NH4+), and nitrate (NO3-). Nitrate is the most common dissolved form of N. Because nitrate has a negative charge, and typical bioretention soil is also negatively charged, nitrate is not adsorbed to soil and often leaches out of soil. N is primarily exported from bioretention systems as nitrate. Improving N performance of bioretention systems, therefore, targets minimizing loss of nitrate. Nitrate retention in bioretention systems is dependent on (1) adequate time for biological processes to occur, and (2) presence of plants. The role of plants in nitrogen removal of bioretention systems is described below.
How Plants Affect Nitrogen Efficiency In Bioretention
Plants affect nitrogen retention of bioretention system on several timescales:
(1) on the most rapid timescale, soils with plants have higher microbial populations then barren soils, and therefore higher rapid N uptake by microbes;
(2) on a longer timescale, plants then take up nitrogen taken up by microbes;
(3) and on an even longer timescale, N taken up by plants is converted to soil organic matter.
Increased Microbial N uptake with Presence of Plants
Because microbial immobilization (uptake) of nutrients occurs much faster (30-100 times faster!) than uptake by plants, it is the initial pathway in which nitrate is taken up from stormwater in bioretention systems. In an experiment by Henderson (2009), up to 100% of nutrients were removed this way. However, the presence of plants is crucial to capitalizing on this microbial nitrate uptake in two ways:
(1) Plant roots release carbon exudates into the surrounding soil, and microbes use this carbon as an energy source. As a result, soils with plants have been found to have much higher microbial populations than those without plants (e.g. Henderson 2009). Bacteria and fungi are 20-50 times more abundant in the rhizosphere of plants than in the bulk soil (Newman 1978, Atlas and Bartha 1998 in Henderson 2009).
(2) Microbial lifespans are short and nutrients are not retained for very long in microbes. If plants are present, plants can take up the nitrogen immobilized by microbes. Without plants, much of the N taken up by the microbes is eventually flushed out of the soil (Henderson 2009). An experiment by Henderson found that “Vegetated bioretention systems retain the nutrients that were removed from stormwater, whereas in unvegetated systems large quantities of soluble nutrients are flushed from the media after an inter-event dry period. Mineralized nutrients were taken up by the vegetation and soil microbes before they could be flushed from the media. The majority of the nitrogen irrigated onto the bioretention systems was recovered in the vegetation” (2009).
Uptake by Plants
Several studies have shown that with adequate retention time, plants can take up most, if not all, nitrogen from stormwater in bioretention systems (e.g. Lucas and Greenway 2011, Henderson 2009). Late on I’ll discuss some factors that affect exactly how much nitrogen is taken up by bioretention plants.
Research by Henderson (2009) found that nutrients taken up by plants appear to be transformed into recalcitrant (stable) soil organic matter, and “thence remained trapped in the media.” In comparing vegetated bioretention mecocosms to unvegetated ones, he found that more organic matter accumulated in the vegetated ones than in the unvegetated ones. He also cites that Tanner (2001) found that “accumulated organic matter in the media…contained twice the mass of N that was held by dead and living standing biomass” and “even suggests that peat or recalcitrant organic matter accumulation is the most important nutrient sink. He states that plants play an important role in acquiring labile nutrients and converting them into less biodegradable forms, but the final sink for those nutrients is the deposition and accumulation of peat on the bottom of the wetland or within the filter media.”
In conclusion, studies show that the presence of vegetation is crucial to the N retention performance of bioretention systems. But do all plants perform equally? My next blog will look at how plants improve dissolved phosphorous retention.
Atlas and Bartha (1998) Microbial Ecology. Fundamentals and Applications. Menlo Park, CA. Benjamin/Cummins Science Publishing.
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.
Newman, E.J. (1978). “Root Microorganisms: Their significance in the ecosystem.” Biological Reviews 53: 511-554.
Tanner, C.C. (2001). “Plants as ecosystem engineers in subsurface-flow treatment wetlands.” Water Science and Technology 44(11): 9-17.