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

• Ammonia fluxes were measured with three inverse modelling approaches.
• The three approaches agreed within 20%.
• Ammonia emission increased sharply after each water supply by irrigation or rain.
• Ammonia emission mainly originated from soil.
• Ammonia losses reach 10% (14%) of the total applied nitrogen in 8 days (20 days).

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 μg NH3 m−2 s−1. The canopy compensation point jumped from 9 μg NH3 m−3 before urea hydrolysis to 131 μg NH3 m−3 afterwards, while the soil compensation point varied in the meantime from 24 to 800 μg 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.

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