Numerical experiments on the influence of CO 2 release at ground level on crop assimilation and water use

Abstract

A model to simulate the effect of increased carbon dioxide levels on the gas exchange by a crop canopy was developed. The distribution of carbon dioxide in the canopy, released from a ground level area source, was calculated by a method reported elsewhere. In this submodel, the wind speed in the crop surface boundary layer, the CO 2 concentration at the upwind edge of the release area, the CO 2 release rate, and the aerodynamic canopy parameters (crop height, zero-plane displacement and roughness length) were the inputs. The radiant energy distribution inside the canopy was computed by a modified Duncan-Stewart submodel, using leaf architecture and optics as inputs. Using the outputs of the submodels and the air temperature, dewpoint and effective soil water potential as inputs, a leaf gas and energy exchange submodel calculated CO 2 and water vapor fluxes for each leaf layer, each characterized by physiological parameters that were assumed constant with height. The gas exchange of two sorghum canopies was calculated by the proposed model for a crop grown in central Texas. The efficiency of CO 2 enrichment (the increase in the CO 2 assimilation rate divided by the CO 2 release rate) was significant only at a high irradiance level and a low wind speed. An efficiency of 13% was predicted for a sparse and rough canopy at a global irradiance of about 1,000 W m −2 and at a wind speed of 0.5 m sec −1. It was 16% for a dense and smooth canopy. The CO 2 concentration halfway into the denser canopy was 3.6 g m −3 (1,997 ppm) higher than the ambient level, at a wind speed of 0.5 m sec −1 at 2 m height, for a release rate of 0.01 g m −1 sec −1 (360 kg ha −1 h −1). When CO 2 was continuously released in a sparse and rough canopy during a calm (average wind speed of 1.2 m sec −1) and clear day at a rate of 36 g m −2 h −1, the CO 2 concentration at the crop height was maintained at around 390 ppm. Daytime CO 2 assimilation was increased from 64 g m −1 without release to 84 g m −2 with release. The efficiency in this case was 4%, and would have been 5% for a dense and smooth canopy. The daily water use efficiency (crop assimilation divided by the crop water use) was enhanced from 0.7% to 1.1%. The results indicate that the proposed model is useful to evaluate the possibilities of CO 2 enrichment in field crops in terms of crop CO 2 assimilation and water use under steady state conditions.