Coupling atmospheric ammonia exchange process over a rice paddy field with a multi-layer atmosphere–soil–vegetation model

To understand the process of atmospheric ammonia (NH3) exchange over a paddy field, an existing multi-layer atmosphere–SOiL–VEGetation model (SOLVEG) was modified. Heat transfer at the paddy water layer and dry deposition of water-soluble gases such as NH3 and sulfur dioxide (SO2) onto the wet canopy, as well as the emission potentials of NH3 from the rice foliage and floodwater or soil surface, were newly modeled. The performance of the modified model was tested using flux data derived using the eddy covariance and gradient methods used for single rice crops in central Japan. The modified model reproduced the observed fluxes of momentum, heat, and CO2, as well as the observed net radiation, Bowen ratio, paddy water temperature, and soil temperature and moisture during both the fallow (bare soil incorporating rice residues) and cropping (flooded) seasons. By adjusting the NH3 emission potentials of the sub-stomatal cavity, the observed upward and near-zero downward fluxes of NH3 were simulated. The calculated deposition velocity of NH3 was 0.4–0.8 and 0.2–1.0 cm s−1 in the fallow and cropping seasons, respectively. Numerical experiments were conducted using the modified model to investigate the effects of canopy structure on stomatal uptake or emissions of NH3 for various rice growth stages. The NH3 exchange (uptake and emissions) rate within a canopy decreased with an increase in the leaf area index (LAI) and became constant at LAI > 1 because of decoupling between in-canopy flow and above-canopy turbulence. Since much of the volatilized NH3 is absorbed within a dense canopy with no stomatal emission potentials, the recapture process is important in reducing NH3 vaporization loss of fertilizer broadcast to the growing rice.