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

Abstract

We applied isotopically labeled (15N-depleted) ammonium sulfate (115 kg N ha−1) to a Spodosol in a citrus grove, to determine the fate and subsequent distributions of NO3-N and NH4-N in the soil profile. The soil was tile-drained, and citrus trees were located on soil beds. We examined three soil management treatments: (1) the original A1 horizon (ST): (2) deep mixing of A1 and A2 horizons with the underlying Spodic horizon (DT); and (3) incorporation of 56 Mg ha−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 NH4-N and NO3-N concentrations, and isotopic analysis was performed by mass spectroscopy. At 12 d after fertilization, both NH4-N and NO3-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 NH4-N and NO3-N decreased in profiles for all three soils; however, N concentrations in the ST soil consistently exceeded corresponding values for DT and DTL profiles. The shallow root system in the ST soil was observed to be less efficient for uptake of available N than was the deeper root system in DT or DTL soils. Labeled, as well as unlabeled, forms of plant-available N decreased rapidly during the first few weeks after application of fertilizer to these soils. Rapid uptake of fertilizer N by plant roots resulted in low levels of available soil N throughout the profile and small quantities of fertilizer discharged in drainage effluent.