A Comparison of Subcanopy Versus Overhead Application of Constructed Wetland Treated Nursery Runoff on Short and Long Rotation Nursery Crops

Abstract

Abstract The nursery/greenhouse industry is the fastest growing segment of U.S. agriculture. Consumer demand for excellent product quality requires luxury applications of water and agricultural chemicals. These cultural practices tend to yield significant volumes of runoff rich in nutrients and pesticides. A capture and recycle system at the Nursery/Floral Crops Research and Education Center at Texas A&M University was fitted with 12 gravel filtration sub-surface flow (SSF) and 12 bog-like free-surface flow (FSF) wetland cells. Three cells of each type were planted with Canna x generalis Bailey ‘Cherry Red’, Iris L. x ‘Clyde Redmond’, both species, or no wetland plants. Runoff was continually collected from the nursery and recycled through wetland cells prior to application via overhead impact sprinklers or subcanopy microsprinklers. Short-term (10 wk) differential effects between overhead and subcanopy irrigation during production of Fraxinus pennsylvanica, Pistacia chinensis von Bunge, Quercus virginiana P. Miller, and Taxodium distichum (L.) Richard in 9.4 liter (#3) containers or Ilex vomitoria W. Aiton ‘Nana’ and Catharanthus roseus G. Don in 5.8 liter (#2) containers were limited in magnitude. However, overhead irrigation reduced height and caliper growth or injured the foliage compared to plants irrigated with subcanopy microsprinklers during longer-term (14 months) production in large 87.9 liter (#27) containers. The extent of reduction was species dependent with Pinus elliottii Englemann being minimally impacted, Pyrus calleryana Descaisne ‘Bradford’ intermediate, and Lagerstroemia L. x ‘Basham's Party Pink’ (purportedly a Lagerstroemia indica L. x Lagerstroemia fauriei B. Koehne hybrid) and T. distichum exhibiting more pronounced effects. Damage appeared to be largely a result of high dissolved salt concentrations in irrigation water contacting the foliage. Recycling of runoff through the FSF cells concentrated soluble salts more so than passing the water through the SSF cells. Efficacy of nitrate nitrogen removal varied with species, season, loading rate, and wetland type. However, the constructed wetlands were generally effective under our test conditions at maintaining effluent nitrate levels at ≤10 mg/liter (10 ppm) when loading rates were ≤50 mg/liter (50 ppm). Presence of emergent wetland plants (those with roots imbedded in the substrate and shoots extending above the water surface, rather than floating or submerged plants) in the system was more important for effectively reducing nitrate levels in effluent from SSF than from FSF cells.