Partitioning of ozone deposition over a developed maize crop between stomatal and non-stomatal uptakes, using eddy-covariance flux measurements and modelling

Ozone (O3) flux measurements, obtained by eddy-covariance technique, over a developed maize crop were used to partition the overall O3 deposition between stomatal and non-stomatal uptakes. Data were analysed using a big-leaf model, which was developed from current knowledge on O3 deposition. The classical parameters used in dry deposition models (i.e. the in-canopy aerodynamic resistance Rac, the intrinsic ground resistance Rig and the cuticular resistance Rcut) were determined for the maize crop from the relationship between the experimental non-stomatal conductance (gns) and the friction velocity (u*) in dry conditions (relative humidity (RH) < 60%). gns was determined as the difference between the O3 canopy conductance (gc) and the O3 stomatal conductance (gs), where gs was estimated by a method which combines the Penman–Monteith approach and the use of the CO2 assimilation flux.Data analysis revealed that chemical reactions between O3 and nitrogen monoxide (NO) between the canopy top and the O3 flux measurement level (zm) could induce high values of the observed O3 conductance, not representative of ozone deposition to the canopy. The actual O3 canopy conductance was derived from the observed O3 conductance by including a correction term function of zm and the NO concentration at this height, based on the previous studies on O3 destruction above canopies.The estimations of Rac, Rig and Rcut given by the non-linear regression of gnsvsu* are in agreement with previously published results. Our analysis also confirms previous studies which have shown that the cuticular conductance (gcut) increases exponentially with RH, and we propose a new parameterization of gcut as a function of RH, based on experimental evidence.Using our model to partition the total O3 deposition to the canopy, we showed that the relative contributions of stomatal and non-stomatal uptakes varied strongly with the physiological activity of the maize and the meteorological conditions. This point is of major importance for studies dedicated to the impact of ozone on plant physiology, since it emphasizes the necessity to determine accurately the amount of O3 actually absorbed by the plants via their stomatal activity.