Estimates of ammonia (NH3) volatilisation in fertilised and flooded rice systems range from 2 % to 60 % of total N applied. A reasonable estimation of NH3 volatilisation is important in order to quantify the N loss to the environment and to pre-determine how much N is available for uptake by rice crop. Hence, one of our objectives is to determine an appropriate model structure for estimating NH3 volatilisation in fertilised and flooded rice systems, given limited availability of observational data sets. Previously, a process-based model for estimating NH3 volatilisation in fertilised and flooded rice systems and is of complexity appropriate for small data sets was developed. The model was calibrated with data sets from the Philippines.
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The proposed model was co-validated with modelling concepts developed by Chowdary et al. (2004) with respect to their common outputs: concentrations of urea-N in the floodwater, concentrations of total ammoniacal-N in the floodwater, and cumulative NH3 volatilisation. The key question then is: Can the proposed model produce comparable or better estimations than Chowdary’s model?
Unlike the proposed model, the model of Chowdary et al. (2004) could not properly predict the trend of total ammoniacal-N in floodwater observed (Fig. 1). The result supports the concept of two-step urea hydrolysis for rice systems with low urease activity. Nevertheless, the cumulative NH3 volatilisations estimated by both models are comparable. The calibrated value of constant rate coefficient of NH3 volatilisation of each model differed due to the underlying concepts.
Chowdary, V.M., Rao, N.H., & Sarma, P.B.S. (2004). A coupled soil water and nitrogen balance model for flooded rice fields in India. Agric Ecosyst Environ, 103, 425-441
Fillery, I.R.P., Simpson, J.R., & De Datta, S.K. (1984). Influence of field environment and fertilizer management on ammonia loss from flooded rice. Soil Sci Soc Am J, 48, 914-920.