RESEARCH UPDATE: N LOSSES TO URBAN RUNOFF AND LEACHING

Turf is increasingly blamed as a source of non-point source (NPS) pollution, including nutrients such as nitrogen. Some turf critics suggest using permanent plantings of ornamentals rather than turf because these plantings require less-intensive management. Therefore, the argument goes, less nutrient-loaded runoff and leaching will occur.

However, there's been little research to back this up. By contrast, several studies have shown that healthy, properly managed turf typically loses very little N to the environment.

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To test the hypothesis that mixed-species plantings might result in less nitrogen loss, researchers at the University of Florida set up two sets of research plots — one set consisting of St. Augustinegrass, and the other of mixed-species ornamental plantings — for a comparison of runoff and leaching losses of applied nutrients. The mixed-species planting was designed by a member of the Florida Yards and Neighborhoods Program, a group that promotes landscaping principles to reduce environmental impacts of landscapes.

Each set of plots was fertilized in 4-month cycles. The mixed-species plantings were fertilized once each cycle, the St. Augustinegrass turf twice each cycle. Thus, the total N applied to the turf was twice that of the mixed species planting (approximately 6 lbs/1,000 sf/year for the turf; half that for the mixed species). According to the researchers, these rates are considered appropriate for these types of plantings.

The researchers found that surface runoff was insignificant for both types of plantings. In fact, during the entire study it occurred only one time (after an especially intense rainfall) and, even then, only in small quantities.

However, leaching occurred in both plots and correlated closely with precipitation received. Despite the fact that the quantity of water leaching through the landscapes was similar in both types of plots, the mixed-species-planting leachate consistently contained higher levels of N. While less than 2 percent of the applied N leached from the turf, more than 30 percent of the applied N leached from the mixed species planting. This was exhibited throughout the study, in varying environmental conditions.

The researchers think that the relatively dense cover and rooting of the St. Augustinegrass was primarily responsible for the greater N immobilization in this study. This suggests that the mixed-species plantings might eventually be able to trap more of the applied nutrients as they mature and grow more extensive root systems. The researchers note that ongoing monitoring of these landscapes should answer questions about the long-term characteristics of the mixed-species planting. Regardless, the turf exhibited qualities confirmed by other studies — it was an excellent trap for applied nutrients.

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