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 15 N‐fertilizer and 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 15 N from labeled aboveground cotton residue separately from the 15 N in roots and soil. Total 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 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 15 N‐fertilizer was cycled through soil pools and roots and only small amounts originated from labeled aboveground residue. Virtually all of the 15 N applied in the form of labeled aboveground residue and recovered in the soil was found in the top 0.3 m. The 15 N‐fertilizer which cycled through belowground pools was found mainly in the top 0.6 m of the soil. Recovery of residual 15 N‐fertilizer appears to contribute little to total cotton N uptake in the second and third crop after application.