DISTRIBUTIONS OF LABELED NITROGEN IN THE PROFILE OF A FERTILIZED SANDY SOIL1

Abstract

Isotopically labeled (/sup 15/N-depleted) ammonium sulfate (115 kg N ha/sup -1/) was applied to a Spodosol in a citrus grove, to determine the fate and subsequent distributions of NO/sub 3/-N and NH/sub 4/-N in the soil profile. The soil was tile-drained, and citrus trees were located on soil beds. The authors examined three soil management treatments: the original A1 horizon (ST): deep mixing of A1 and A2 horizons with the underlying Spodic horizon (DT); and incorporation of 56 Mg ha/sup -1/ of dolomitic limestone, along with deep mixing (DTL) of the profile. Soil samples were taken to the 70-cm depth in ST and to the 95-cm depth in DT and DTL soils and were extracted with 1 M KCl. Extracts were chemically analyzed for NH/sub 4/-N and NO/sub 3/-N concentrations and isotopic analysis was performed by mass spectroscopy. At 12 d after fertilization, both NH/sub 4/-N and NO/sub 3/-N values in the ST soil profile exceeded corresponding values for DT and DTL soils. This N was derived primarily from the fertilizer. With time, concentrations of both NH/sub 4/-N and NO/sub 3/-N decreased in profiles for all three soils; however, N concentrations in the ST soil consistently exceeded corresponding values formore » DT and DTL profiles.« less