A COMBINED USE OF EDDY-COVARIANCE AND OPEN-TOP CHAMBERS FOR THE IDENTIFICATION OF DOSE-EFFECT RELATIONSHIPS FOR OZONE ON CROPS. A CASE STUDY AND PERSPECTIVES

In the latter years a flux-based approach for ozone risk assessment on crops was strongly supported by the scientific community. Nevertheless, the amount of ozone absorbed by plants through stomata is hardly measurable and very few dose-response relationships are available in the literature. Field experiments are usually performed in OTC (or Free Air Ozone Enrichment Systems, where available) and ozone dose is calculated (not measured) by stomatal conductance models fed with climatic parameters measured in the OTCs. At the moment OTCs are the only affordable system able to remove ozone molecules in field conditions and able to provide a “control” treatment for the validation of the ozone effects. At the same time the only way to measure ozone fluxes on vegetation is the Eddy Covariance (EC), a micrometeorological technique that, unfortunately, cannot be applied inside OTCs. These two techniques were combined and simultaneously used to study the effects of the ozone uptake by alfalfa fields in a two years pilot experiment in Italy. An EC tower mounted in the middle of the field and three OTC were placed randomly on a side of the field, two of them flushed with charcoal-filtered air (ozone-free) and one with ambient air only. The plant productivity within the OTCs and in the open field, as well as the forage quality, were compared at the end of each growing cycle. With the total ozone fluxes and water and heat ones obtained from the EC measurements, a resistive analysis was conducted in order to get the stomatal resistance of plants in the open field and thus their ozone uptake. Then, the ozone uptake by the plants inside the OTCs was calculated by assuming the same stomatal resistance obtained in the open field but applying an atmospheric Ra and sub-laminar Rb resistances fit on the OTC geometry and fan system flow. The results show that a considerably amount of the ozone deposited on the field was absorbed by plants through the stomata, with values between 66% and 94% in the different growing cycles. The plants exposed to ozone in the open field and in the ambient air flushed OTC showed on average a dry biomass production of 20% less than in the charcoal filtered chambers, but the forage quality resulted a little bit increased since the raw fibres content, as well as the ratio between acid and neutral deterged fibres fractions and the protein contents increased. Finally the results of five harvests were combined and related to both the ozone exposure (calculated as AOT40) and the cumulated ozone dose (flux) received by crop. The cumulated stomatal flux resulted a more reliable predictor (R2=0.67) of the biomass productivity than AOT40 (R2=0.03), and an average linear dose-biomass relationship of 0.0225 Kg of dry weight reduction per mmol m-2 of O3 taken up by crop was found. The same behaviour was confirmed more or less for all the parameters taken into account. This pilot study showed that the combined use of Eddy Covariance and OTC techniques could be promising for studies aimed to quantify dose-effect relationships.