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Distribution of Applied Nitrogen in Soil—Pensacola Bahiagrass Components as Indicated by Nitrogen-15¹

ABSTRACT

The fate of fertilizer N applied to coarse-textured soil-plant systems is not well understood. To determine the effects of N rates and seasonal N applications on plant response and recovery of applied N from these soils, three field experiments with ¹⁵N-enriched fertilizer were superimposed in 1976 on nontracer N experiments on Myakka fine sand (sandy, siliceous, hyperthermic Aeric Haplaquods), Ona fine sand (sandy, siliceous, hyperthermic Typic Haplaquods), and Astatula sand (hyperthermic, uncoated Typic Quartzipsamments) planted to Pensacola bahiagrass (Paspalum notatum Flugge). At each site, a split-plot experimental design was used with two N rates (100 and 200 kg ha^–1) as main plots and three seasonal N applications as subplots. Forages, stolon-root masses, and soils were analyzed for total N and ¹⁵N concentrations. In 1976, forage yields and ¹⁵N-labeled fertilizer N contents were larger at the higher N rate, but were not affected by seasonal N applications. Recovery of fertilizer N in forage was 36, 31, and 29% for the Myakka, Ona, and Astatula soils, respectively. An average of 48% of the total N in forage was from the fertilizer. Mineralization of immobilized N from previous fertilizations probably did not account for the large amount of nonfertilizer N in the forage. Recoveries of fertilizer N in stolon-root masses were 11, 11, and 16%; recoveries in soils were 28, 13, and 11%; and recoveries in soil-plant systems were 74, 54, and 54% from the Myakka, Ona, and Astatula soils, respectively. Denitrification was suspected as an important N loss mechanism from the poorly drained Myakka and Ona soils, and leaching was likely dominant from the well-drained Astatula soil; however, soil-solution samples collected from depths of 60 and 120 cm in these soils showed no evidence of N leaching. Not more than 5% of the ¹⁵N-labeled fertilizer applied in 1976 was recovered in forage produced in 1977. At the end of 1977, total fertilizer N accounted for was essentially the same as at the end of 1976. Two important results were the relatively large amounts of nonfertilizer N in forage, especially in 1976, the year of ¹⁵N-labeled fertilizer application, and the relatively high percentage of fertilizer N retained in these three sandy soils. In 1983, 7 yr after fertilization with labeled N, there was little change in labeled N in the soil compared to 1977.

¹ Florida Agric. Exp. Stn. Journal Series no. 1848.

² Formerly graduate student, Soil Science Dep., Univ. of Florida, presently Dep. of Soil Science, Kasetsart Univ., Bangkok, Thailand; Professor (Soil Chemistry and Fertility); and Assoc. Professor (Environmental Quality), Soil Science Dep., Florida Agric. Exp. Stn., Gainesville, FL 32611.

Received for publication January 3, 1984. Accepted for publication June 12, 1984.