NITROGEN FERTIGATION OF SUBSURFACE DRIP IRRIGATED BERMUDAGRASS M.A. Maurer* 1, J.A. Moken 2 and J.L. Young 1 1 Department of Agriculture, Stephen F. Austin State University, P.O. Box 13000, SFA Station, Nacogdoches, TX Department of Environmental Science, Stephen F. Austin State University, P.O. Box SFA Station, Nacogdoches, TX ABSTRACT The effects of five nitrogen fertigation rates (0, 12.2, 24.4, 48.8, 97.7 kg N·ha -1 ·month -1 ) and three fertigation frequencies (weekly, bi-monthly, monthly) on 'Tifway 419' hybrid bermudagrass (Cynodon dactylon x C. transvaalensis) were studied, using a combination of subsurface drip irrigation (SDI) and fertigation with urea ammonium nitrate (UAN, ) as the nitrogen source. Soil analysis showed soil nitrate levels to exceed 10 mg·L -1 in the 48.8 and 97.7 kg N·ha -1 ·month -1 rates. This led to excessive leaching in nitrate and nitrite over EPA limits at these rates as well as the 24.4 kg N·ha -1 ·month- 1 rate. Leaf tissue analysis showed the fertigation rates of 12.2 kg N·ha -1 ·month -1 and up had nitrogen levels between the 3-5% sufficiency range. This research indicates that a fertigation rate of 12.2 kg N·ha -1 ·month -1 produced sufficient quality turfgrass and maintained leachate levels below EPA limits. Even though frequency did not play a statistically significant role in this study, it is recommended that a frequency of 7 to 14 days be used when sandy or sandy-loam soil is being used. INTRODUCTION Irrigation and fertilization are two essential processes for obtaining optimum quality turfgrass. However, resulting environmental impact and increasing droughts call for more efficient and less polluting means of fertilization and irrigation. Subsurface drip irrigation in combination with fertigation may be the answer. When compared to conventional irrigation methods, SDI has been shown to reduce water usage by 33-55%. Fertigation can reduce nitrogen leaching through smaller and exact applications and when applied through SDI it can reduce fertilizer loss through surface runoff. This combination places both water and nutrients directly at the root zone of the turfgrass increasing efficiency and maximizing uptake. This study was designed to look at a wide range of fertigation rate and frequency combinations to minimize nitrate leaching levels below EPA limits while still producing quality turfgrass. MATERIALS & METHODS The study was conducted at the Stephen F. Austin State University (SFASU) Mast Arboretum with the experimental set up and sodding of the ‘Tifway 419’ hybrid bermudagrass in the Fall of 2007 and data collection being performed during the 2008 growing season. Treatments were arranged as 3 fertigation frequencies (monthly, bi-monthly, weekly) X 5 nitrogen fertigation rates (0, 12.2, 24.4, 48.8, 97.7 kg N·ha -1 ·month -1 ) factorial design with 4 replicates per treatment to form a completely randomized block design experiment. Nitrogen applications began on March 28, 2008 with all treatments receiving their appropriate UAN dosage. To see the variation in nitrate levels due to the UAN application, an intense four-week soil sampling was performed during the 2008 growing season (July 23rd through August 16th 2008). Two leachate collections were made prior to this intense soil sampling (April 1st, and May 7th) and one during (August 12th) to see the amounts of nitrate and nitrite being leached and compare them to the EPA limits of 10 mg·L -1 and 1 mg·L -1 respectively. A biomass collection and several visual quality ratings were performed throughout the 2008 growing season to analyze the health of the turfgrass. Two and three way analysis of variance (ANOVA) were performed on the collected data using the Statistical Analysis Software (SAS ® ) general linear model (GLM) procedure at the 0.05 probability level with the Tukey multiple-comparison test used to determine differences in means between the treatments. RESULTS & DISCUSSION Soil analysis showed large amounts of nitrate-nitrogen being retained in the soil and not being utilized by the turfgrass at rates of 48.8 and 97.7 kg N·ha -1 ·month -1 (Table 1). During a natural rain event (August 18, 2008) resulting in 8.28 cm of precipitation, this large buildup of nitrate-nitrogen in the soil was leached causing excessive nitrate-nitrogen leachate over the EPA limits (Table 2). The first two water samples were taken early in the growing season when the applied nitrate-nitrogen was not being effectively used by the turfgrass. This led to higher nitrate-nitrogen leachate values in the lower 12.2 and 24.4 kg N·ha -1 ·month -1 rates which lowered significantly once the turfgrass was fully established. Leaf tissue analysis showed only the control as having a deficiency in nitrogen. The other four rates had leaf tissue nitrogen percentages between the 3-5% sufficiency range for a fairway turfgrass. Visual quality ratings showed the 24.4 and 48.8 kg N·ha -1 ·month -1 rates having the highest turfgrass color and thickness quality. The 12.2 kg N·ha - 1 ·month -1 showed visual deficiency symptoms and the 97.7 kg N·ha - 1 ·month -1 rate had some leaf firing. Both these rates however did still produce an acceptable quality turfgrass. CONCLUSIONS Results from the study indicate that a fertigation rate of 12.2 kg N·ha - 1 ·month -1 produced a quality turfgrass while minimizing nitrate- nitrogen leaching to a level at or below EPA limits in sandy-loam soil. Optimum quality was obtained at the 24.4 and 48.8 kg N·ha - 1 ·month -1 rates. This range is currently recommended for optimum bermudagrass quality and growth. However, in this study, the 24.4 and 48.8 kg N·ha -1 ·month -1 rates produced nitrate leachate levels that were well above EPA limits. These rates may have produced such high nitrate leaching values due to the limited root zone and soil profile of only 30 cm. If a deeper soil profile of had been used, leachate values may not have been as high. Even though fertigation frequency was not statistically significant in this study, a frequency of 7 to 14 days should be used in order to minimize the amount of nitrate and nitrite that can be leached at any one time. A monthly frequency allows for larger amounts of nitrogen leaching if a rainfall or large irrigation event were to occur right after application. Also, the monthly frequency had excessive nitrate leaching above EPA limits at all rates except the control. Table 1. Weekly soil nitrate-nitrogen levels (mg·L-1) at 0 to 15 and 15 to 30 cm soil depths over a one month fertigation cycle (28 days). Table 2. Levels of nitrate-nitrogen (kg N·ha -1 ·month -1 ) following rainfall events taking place 4, 16, and 26 days after initiation of three separate fertigation cycles (28 days). Blue coloring shows values below EPA limit of 10 mg·L -1 orange exceeds this limit. May 12, 2008