Gas exchange between plant canopies and the atmosphere: Case-studies for ammonia

We first present the elements of an inverse Lagrangian model of gas transport in plant canopies. The model allows the inference of sites of gas exchange in the canopy and their source and sink strengths from measured profiles of mean gas concentration and statistics of the canopy turbulence. The practical application of the model is demonstrated through a case study of the fate of ammonia volatilized from fertilizer applied to the floor of a sugarcane crop. Some of the lost ammonia was absorbed by the foliage of the crop; the rest was lost to the atmosphere above. While there was excellent agreement between model predictions of the net flux from the canopy and independent micrometeorological measurements of ammonia flux in the air-layer above it, verification of flux predictions within the canopy was much more difficult. Appeal was made to a process-based model of canopy gas exchange that describes gas transport to and from foliage surfaces in terms of diffusion across aerodynamic, boundary-layer and stomatal resistances in response to a difference in ammonia concentration between the air and leaf sub-stomatal cavities. There was acceptable agreement between the two models in their predictions of foliage ammonia uptake. Next, we apply the process model to a study of the recapture of volatilized ammonia by sugarcane crops with different leaf area indices (LAI). The study indicated recoveries increasing almost linearly with LAI and suggested probable recoveries in excess of 20% for canopies with LAIs of 2 or more. These and other published studies of ammonia exchange between canopy and atmosphere that employed both the inverse Lagrangian and process models suggest that their coupling provides a powerful tool for studying canopy gas exchange.