Partitioning of ozone deposition over a developed maize crop between stomatal and non-stomatal uptakes, using eddy-covariance flux measurements and modelling
Résumé
Ozone (O-3) flux measurements, obtained by eddy-covariance technique, over a developed maize crop were used to partition the overall O-3 deposition between stomatal and non-stomatal uptakes. Data were analysed using a big-leaf model, which was developed from current knowledge on O-3 deposition. The classical parameters used in dry deposition models (i.e. the in-canopy aerodynamic resistance R-ac, the intrinsic ground resistance R-ig and the cuticular resistance R-cut) were determined for the maize crop from the relationship between the experimental non-stomatal conductance (g(ns)) and the friction velocity (u(*)) in dry conditions (relative humidity (RH) < 60%). g. was determined as the difference between the O-3 canopy conductance (g(c)) and the O-3 stomatal conductance (g(s)), where g(s) 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 O-3 and nitrogen monoxide (NO) between the canopy top and the O-3 flux measurement level (z(m)) could induce high values of the observed O-3 conductance, not representative of ozone deposition to the canopy. The actual O-3 canopy conductance was derived from the observed O-3 conductance by including a correction term function of z(m) and the NO concentration at this height, based on the previous studies on O-3 destruction above canopies. The estimations of R-ac, R-ig and R-cut given by the non-linear regression of g(ns) vs u(*) are in agreement with previously published results. Our analysis also confirms previous studies which have shown that the cuticular conductance (g(cut)) increases exponentially with RH, and we propose a new parameterization of g(cut) as a function of RH, based on experimental evidence. Using our model to partition the total O-3 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 O-3 actually absorbed by the plants via their stomatal activity. (C) 2009 Elsevier B.V. All rights reserved.