Fixation and release of n‐15‐labeled ammonium during soil drying

Abstract

Abstract Nitrogen applied to soils as ammonium fertilizers often is found as nonexchangeable, or fixed, ammonium soon after application. The importance of ammonium fixation under field conditions has not been well established, and studies to assess the importance of ammonium fixation are hampered by lack of information about the effects of air drying of soil samples on ammonium fixation. The work reported here examines the effects of air drying on fixation and release of 15N‐labeled ammonium in soil samples collected at 10, 45, and 90 days after labeled anhydrous ammonia was applied to two soils in an Iowa cornfield. Comparisons of field‐moist and air‐dried samples revealed that air drying induced fixation of ammonium in samples collected soon after fertilization and induced release of fixed ammonium in samples collected after exchangeable ammonium concentrations had been depleted by nitrification or other processes. The magnitude of the effects were proportional to the amount of fertilizer N present as f...