Ammonia volatilisation following urea fertilisation in an irrigated sorghum crop in Italy

Ammonia (NH3) fluxes were estimated by three inverse modelling methods over a sorghum field following the application of 240 kg N ha(-1) of urea pills under a semi-arid Mediterranean climate. Ammonia volatilisation started following irrigation, which coincided with the third urea application. The maximum volatilisation rate was reached 7 days after irrigation. A clear dependence of the NH3 volatilisation on irrigation and rainfall events was observed. The NH3 fluxes ranged from 2.5 to 45 mu g NH3 m(-2) s(-1). The canopy compensation point jumped from 9 mu g NH3 m(-3) before urea hydrolysis to 131 mu g NH3 m-3 afterwards, while the soil compensation point varied in the meantime from 24 to 800 mu g NH3 m-3 on average. The soil-dominated observed NH3 emissions were reasonably well reproduced by a two-layer resistance model. Overall, between 10% and 14% of the total nitrogen applied was volatilised. (C)