Linda Benedict, Strahan, Ronald E., Beasley, Jeffrey S. | 11/23/2011 9:53:50 PM
Jeffrey S. Beasley, Robert Burwell Jr., Ron E. Strahan and Gregg C. Munshaw
Flooding from Hurricane Katrina has resulted in federal and state mandates to raise levees in New Orleans and surrounding parishes. Levees are constructed from heavy soils compacted into embankments to reduce water infiltration and prevent flooding. As a result, levees are susceptible to surface runoff from intense rainfalls during and after construction. In the case of levees, these earthen structures border water bodies with little to no buffer zones to trap off-site nutrient and sediment movement.
To reduce erosion and maintain structural integrity, grass establishment on levees, referred to as soft armoring, is standard. However, during establishment, levees are more susceptible to nutrient and sediment losses via surface runoff into water bodies compared to mature grass swards. Nitrogen and phosphorus contamination of surface waters via runoff has been shown to significantly contribute to eutrophication, a process involving algal blooms and water-oxygen depletion and subsequent deterioration of surface water quality.
A common practice by levee construction contractors is to accelerate grass establishment using water-soluble nitrogen at seeding. However, water-soluble nitrogen is extremely susceptible to off-site movement. One factor that should affect nitrogen losses to surface runoff is the type of nitrogen fertilizer applied. Application of slow-release nitrogen fertilizers could reduce nitrogen losses during surface runoff while providing sufficient nitrogen to accelerate grass growth to limit erosion during establishment.
To test the effect of nitrogen fertilizers on grass establishment and surface runoff losses, runoff trays (see photos) were installed on the Bonnet Carré Spillway levee near LaPlace, La., in 2008 and 2009. Within each runoff tray common bermudagrass was seeded and fertilized with 50 pounds of nitrogen per acre from a water-soluble nitrogen or slow-release nitrogen with unfertilized trays serving as controls. In 2008, urea was the water-soluble nitrogen fertilizer and sulfur-coated urea was the slow-release nitrogen applied. The second year, urea and sulfur-coated urea were replaced by ammonium nitrate as the water-soluble nitrogen fertilizer and urea-formaldehyde as the slow-release nitrogen applied. All fertilizers were applied at seeding with the exception of ammonium nitrate, which was applied as a split-application at seeding and 35 days after seeding, once grass coverage exceeded 50 percent. This was done to assess the effect of nitrogen application timing on nitrogen losses.
During bermudagrass establishment natural rainfall was collected, and rainfall simulations were performed. Rainfall simulations were conducted three times in 2008 and four times in 2009. For each runoff event, measurements included the time until runoff occurred, total runoff volume and bermudagrass coverage. Water samples were collected for nitrogen and sediment analyses for 70 days in 2008 and 56 days in 2009. All water samples were analyzed for ammonium and nitrate concentrations and sediment. To simplify the presentation of the results, nitrogen data for ammonium and nitrate were combined and presented as total nitrogen.
Nitrogen-fertilized bermudagrass established the fastest at over 90 percent for both urea and sulfur-coated urea, compared to 75 percent coverage for unfertilized bermudagrass 70 days after seeding. In 2009, nitrogen fertilizer had a similar effect on bermudagrass growth with 89 percent coverage for unfertilized and 95 percent coverage for fertilized bermudagrass at 56 days after seeding. Nitrogen has been shown to accelerate grass growth and improve characteristics such as density, shoot growth and root growth, factors that can retard runoff. However, bermudagrass establishment was comparable for slow-release nitrogen fertilizers and water-soluble nitrogen fertilizers. Similarities in bermudagrass growth between fertilizers may be due to southern Louisiana’s subtropical climate, which allowed faster nitrogen release of slow-release fertilizers for greater grass uptake and growth.
Increases in bermudagrass cover led to greater runoff resistance. For example, the onset of runoff was delayed up to 538 seconds with over 90 percent bermudagrass cover compared to 10 percent bermudagrass cover. Stoloniferous grasses such as bermudagrass develop dense canopies that slow water flow and increase infiltration to delay runoff from occurring and reduce runoff volumes. Runoff volumes decreased from 12 gallons at less than 20 percent bermudagrass cover to 2 gallons at 90 percent bermudagrass cover in 2008. In 2009, runoff volumes followed a similar pattern with a four-fold reduction in runoff volume 56 days after seeding compared to 14 days after seeding. Decreasing runoff volumes from increasing bermudagrass cover suggests bermudagrass affected the soil’s infiltration capacity. Scientists have long attributed reductions in runoff severity from grassed areas to a combination of dense vegetation and increased rooting.
Although runoff is not directly detrimental to water quality, surface runoff has been identified as a major mode of transport for sediment and dissolved nutrients. Surface runoff is not only an environmental concern but also a threat to levee structural integrity. Therefore, practices to accelerate grass establishment post-construction to attain higher grass coverage for greater runoff resistance must be evaluated to assess environmental costs versus benefits. Among them are the benefits of nitrogen fertilization on grass growth versus potential nitrogen losses during establishment.
Given the positive effect of nitrogen fertilization on bermudagrass establishment, nitrogen fertilization should not only lead to faster bermudagrass establishment but also to decreased erosion. Although sediment losses were much higher in 2008 than 2009, due to a slope difference of 33 percent to 23 percent and slightly faster bermudagrass establishment in 2009, the pattern of higher bermudagrass coverage decreasing sediment loading was observed in both years. Once 95 percent bermudagrass coverage was present, sediment losses were reduced to 95 percent in 2008 and 85 percent in 2009. However, total sediment losses for the establishment periods did not differ across nitrogen fertilizers.
Even though nitrogen fertilizers accelerated bermudagrass establishment, if no benefit occurs concerning erosion, the costs of potential nitrogen losses from surface runoff must be accounted. Nitrogen losses were highest for all fertilizers following initial surface runoff events. At 14 days after seeding in 2008, two natural rainfall events resulted in nitrogen losses from sulfur-coated urea and urea-fertilized bermudagrass accounting for 82 percent and 75 percent of the total nitrogen lost during the 70-day establishment period. Again in 2009, nitrogen losses were highest during initial rainfall with 60 percent and 75 percent of total nitrogen lost from urea-formaldehyde and ammonium nitrate, respectively, for the 56-day establishment period.
In the second year of the study, water-soluble nitrogen was applied as a split application to test whether nitrogen losses would be reduced with greater bermudagrass nitrogen uptake. However, nitrogen losses were similar to losses observed when fertilizers were applied at 14 days after seeding. Regardless of fertilizer type, nitrogen losses were highest with the initial runoff after application and declined for subsequent rainfalls.
Although delaying the second application of ammonium nitrate did not reduce total nitrogen losses, it did affect total nitrogen losses. Bermudagrass fertilized with water-soluble fertilizers, urea and ammonium nitrate, resulted in 72 percent greater nitrogen losses in 2008 and 86 percent greater nitrogen losses in 2009, compared to unfertilized bermudagrass. However, the effect of slow-release nitrogen fertilizers on nitrogen losses from runoff varied, depending on the type of slow-release nitrogen applied. Sulfur-coated urea resulted in the highest total nitrogen losses compared to the control, whereas total nitrogen losses from urea did not differ from the control. Only urea-formaldehyde accelerated grass establishment without significantly increasing total nitrogen losses from unfertilized grass during the establishment period.
In this study, all nitrogen concentrations in surface runoff exceeded the concentration reported to accelerate eutrophication and favor detrimental algal growth. The majority of nitrogen lost from runoff occurred from rainfalls immediately post-nitrogen applications for all fertilizers. Large initial runoff losses from fertilized bermudagrass emphasize the inability of plant densities less than 50 percent to shorten the period of greater runoff susceptibility by faster growth from nitrogen fertilizers to create a more resistant bermudagrass cover.
Under the conditions tested, there was no benefit from nitrogen fertilization at seeding on levee embankments to increase erosion resistance. However, levees typically receive fertilization only during establishment. Therefore, benefits of increased bermudagrass growth would not be realized. Of course, if less fertile clay were used for levee construction, nitrogen applications could have a more significant impact on grass establishment. Alternate solutions such as mulches or fertilizer application timing could reduce nutrient and sediment loading.
Jeffrey S. Beasley, Assistant Professor; Robert Burwell Jr., Graduate Student; and Ron E. Strahan, Associate Professor, School of Plant, Soil & Environmental Sciences, LSU AgCenter, Baton Rouge, La.; and Gregg C. Munshaw, Associate Professor, Department of Plant and Soil Sciences, University of Kentucky, Lexington, Ky.
(This article was published in the fall 2011 issue of Louisiana Agriculture magazine.)