Volatiliza？？o de am？nia da ureia alterada por sistemas de preparo de solo e plantas de cobertura invernais no Centro-Sul do Paraná

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 paraná 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 nh3 was measured for 20 days after urea application in a semi-open static system. urease activity was evaluated in both experiments. the nh3 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 nh3 losses, which are primarily related to rain events immediately after urea application.