Possible fates for nitrate in grassy vs. xeriscaped stormwater retention basins

Stormwater retention basins are common flood control structures found in many urban environments. Due to the concentration and delivery of local runoff replete with nitrate, these basins may be locations of vertical transport of nitrate into deeper soils via infiltration, or they may hot spots for denitrification. In the Phoenix, AZ metropolitan area, there are two basin types: grassy and xeriscaped (typically a gravel cover with drought-resistant shrubs and trees).This research explores the influence of landscaping design on basin soil characteristics and processes. Soil organic matter, moisture, extractable nitrogen (N) pools, and potential denitrification were measured from 32 retention basins (16 of each type). Organic matter, water content, and potential denitrification were much higher in grassy basins than in xeric ones, but soil nitrate did not differ between the two types. Investigation of vertical soil profiles at a subset of these sites (5 of each type) demonstrated that grassy basins quickly reduce soil nitrate concentrations within the first few centimeters whereas xeric basins are more variable and tend to have some storage of nitrate at lower depths. These patterns indicate that the predominant fate (denitrification or storage) of stormwater nitrate entering retention basins depends on the landscaping design. Certain fates may be more desirable with respect to the ecosystem services provided by stormwater retention basins; future research will examine gaseous products of denitrification and the implications of alternate fates for ecosystem service valuation.