Nitrogen losses from urea by ammonia volatilization are higher from no-tillage than from conventional tillage. The objective of this study was to evaluate the magnitude of this process under cool and wet spring conditions in the South-Central region of the State of Parana and to evaluate the influence of two winter cover crops (black oat and common vetch) on ammonia volatilization in no-tillage. The tillage systems were compared in a long-term tillage experiment (28 years) and the cover crops tested separately in a long-term (>15 yr) no-tillage area. Maize was grown in both experiments. Urea was applied at rates of 0, 80 and 160 kg ha -1 N in a single application in the tillage experiment and at rates of 0, 100 and 200 kg ha -1 N, split in two applications, in the cover crop experiment. Volatilization of NH 3 was measured for 20 days after urea application in a semi-open static system. Urease activity was evaluated in both experiments. The NH 3 loss rates were highest 5 days after urea application. Cumulative ammonia losses reached 18% of the applied N in no-tillage and 3% in conventional tillage. The higher losses from no-tillage may be partially related to the greatest urease activity in the soil surface layer. Ammonia volatilization was not affected by cover crops. As an isolated practice, split surface N fertilization does not ensure a decrease of NH 3 losses, which are primarily related to rain events immediately after urea application.
