Distribution of Carbon Dioxide Released in a Field Crop

Abstract

A model to deal with the two-dimensional diffusion of carbon dioxide, released at ground level, within and above a crop canopy was developed and tested experimentally. The CO2 was released from a uniform area source at ground level and at a constant rate. The crop canopy was assumed to be horizontally uniform and infinite. The steady-state CO2 distribution was obtained as a solution of the two-dimensional diffusion equation by the forward finite-difference approximation, using a digital computer. Theoretical wind and eddy diffusivity profiles for above and within the canopy were incorporated. Crop assimilation was neglected.The crop canopy was characterized by its aerodynamic parameters: crop height, zero-plane displacement, roughness length and exponential coefficient of the within-canopy wind profile. The model calculated the CO2 profile as a function of down-wind distance, wind speed in the crop surface boundary layer, CO2 release rate, and CO2 concentration at the upwind edge of the source. The influence of crop canopy structure on the CO2 profile was found by changing the aerodynamic canopy parameters.A series of CO2 diffusion field experiments was conducted in a cotton canopy. All input parameters in the model, except the exponential coefficient of the within-canopy wind profile, were measured. The CO2 profiles at a fixed down-wind distance were determined and compared with those predicted by the model.The model overestimated the CO2 concentration in the lower layer of the canopy by about 50% when the leaf area index was 2.33 and the canopy was open. The difference between predicted and measured values was 20 to 30% when the leaf area index was 2.74 and the canopy was denser due to branching in the upper layers. It was concluded that the model can give a useful approximation of the distribution of CO2 in a dense and uniform canopy, when wind fluctuations are small, and under thermally neutral conditions.