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

Abstract

AbstractSoil water content is an important factor influencing NH3 volatilization from N fertilizers. Information on the relationship between water evaporation and NH3 volatilization from urea (U) or urea‐ammonium nitrate (UAN) solutions, and on the kinetics of both processes is lacking. We examined NH3 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 NH3 volatilization became more pronounced as soil approached air dryness. Water evaporation was consistent with first‐order kinetics, whereas NH3 volatilzation followed two first‐order kinetics. The initial rate constants (k1) for NH3 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.