Recovery of residual fertilizer-N and cotton residue-N by Acala and Pima cotton

Little is known about the availability of residual fertilizer-N and cotton (Gossypium spp.) residue-N to subsequent crops. This study determined the effects of two soil types and N fertilization rates on the recovery of residual [sup.15]N-fertilizer and [sup.15]N from labeled cotton residue in the soil and plants of subsequent cotton crops. Microplots in the San Joaquin Valley, California, on a Panoche clay loam [fine-loamy, mixed (calcareous), thermic Typic Terriorthents] and a Wasco sandy loam (coarse-loamy, mixed, nonacid, thermic Typic Torriorthents) were used to trace the fate of [sup.15]N from labeled abovcground cotton residue separately from the [sup.15]N in roots and soil. Total [sup.15]N -fertilizer recovery in the second year after application averaged 5.8% for Acala (G. hirsutum L.) and 2.9% for Pima (G. barbadense L.) cotton. In the third year after application, total [sup.15]N-fertilizer recovered by Acala averaged 2.0% on the clay loam soil and 3.3% on the sandy loam. Most of this recovered [sup.15]N-fertilizer was cycled through soil pools and roots and only small amounts originated from labeled aboveground residue. Virtually all of the [sup.15]N applied in the form of labeled aboveground residue and recovered in the soil was found in the top 0.3 m. The [sup.15]N-fertilizer which cycled through belowground pools was found mainly in the top 0.6 m of the soil. Recovery of residual [sup.15]N-fertilizer appears to contribute little to total cotton N uptake in the second and third crop after application.