Soil Water and Ammonia Volatilization Relationships with Surface‐Applied Nitrogen Fertilizer Solutions

Abstract

Abstract Soil water content is an important factor influencing NH 3 volatilization from N fertilizers. Information on the relationship between water evaporation and NH 3 volatilization from urea (U) or urea‐ammonium nitrate (UAN) solutions, and on the kinetics of both processes is lacking. We examined NH 3 volatilization from surface‐applied solutions of U and UAN in samples of St. Bernard (loamy, mixed, nonacid, frigid Typic Hapludoll) and Ste. Sophie (sandy, mixed, nonacid, frigid Typic Haplorthod) soils exposed to water potentials ranging from < −1.5 to −0.01 MPa. An air‐train system was used. Ammonia volatilization increased as soil water content increased. Volatilization differences between moist (> −0.038 MPa) and air‐dry (< −1.5 MPa) samples were reduced as the clay content of the soil increased. However, the effect of clay content on NH 3 volatilization became more pronounced as soil approached air dryness. Water evaporation was consistent with first‐order kinetics, whereas NH 3 volatilzation followed two first‐order kinetics. The initial rate constants ( k 1 ) for NH 3 volatilization (0.338–0.348 d −1 ) from U solution were consistently higher than those for water evaporation (0.099–0.104 d −1 ) at −0.01 MPa water potential and 70% relative humidity. Differences could be attributed to different energy requirements. Ammonia volatilization vs. water evaporation followed a logarithmic relationship.