Vegetation type alters water and nitrogen budgets in a controlled, replicated experiment on residential-sized rain gardens planted with prairie, shrub, and turfgrass

Vegetation type alters surface hydrology and nitrogen (N) cycling by partitioning evapotranspiration (ET) and drainage, but has not been fully utilized for this purpose in the design of urban rain gardens. Replicated treatments of three vegetation types (prairie, shrub, and turfgrass) commonly planted in rain gardens, as well as bare soil (control), were evaluated 1 year after establishment in 12, free-drainage bioretention cells designed to specifications for residential rain gardens in the Midwestern United States. Water and N budgets were calculated to assess differences in ET, drainage, soil moisture, and N transport following three stormwater applications in July, August, and October of 2006. Evaporative demand was also estimated as potential or reference ET during the study period using Hamon, Priestley-Taylor, and FAO Penman-Monteith models. Results show that after 1 year of establishment, vegetation type changed the water budgets of rain gardens thus altering their provisioning of ecosystem services via observed tradeoffs between daytime evaporative cooling (ET), stormwater storage, N-load reduction, soil and plant N retention, and groundwater recharge (drainage). Vegetation effects on hydrology and N transport were most significant when evaporative demand was high during the middle of the growing season. Observed changes in ET, drainage, soil moisture, and N transport support the incorporation of different vegetation types into conceptual and numerical models of rain gardens to assess ecological outcomes and tradeoffs at a variety of spatiotemporal scales.