Nitrogen modulates the ozone response of Mediterranean wheat: Considerations for ozone risk assessment

The experiment was conducted in an Open Top Chamber facility located in the Mediterranean basin to investigate how nitrogen (N) fertilization affects the response of wheat to ozone (O₃) exposure. The study considered the response of Artur Nick, a modern wheat cultivar commonly used in the area, to three O₃ exposure levels (ambient and elevated ambient, +20 and +40 nL L^-1 O₃), and two N fertilization doses (100 and 200 kg ha^-1). Measurements included leaf gas exchange, leaf chlorophyll content, leaf and grain N content, plant growth and yield parameters. Ozone × N interactive effects were studied and quantified based on accumulated O₃ concentrations above a 40 nL L^-1 threshold (AOT40) and phytotoxic O₃ dose (POD) indices, which are used in O₃-risk assessments, from which critical levels (CL) for a 5 % effect were derived. Results revealed that O₃ impacts on growth and yield parameters were stronger under the highest N fertilization dose. In consequence, O₃ Critical Levels (CL) were as much as 3-4 times lower for grain yield in the high-N compared to the low-N treatment. Interestingly, O₃ limited the fertilizer stimulus, strongly reducing the N use efficiency for grain yield and the agronomic efficiency of N for protein yield. Another important aspect was that 71 % of the POD was accumulated before anthesis, stressing the potential importance of O₃ exposure during the vegetative phase of wheat under Mediterranean conditions, which is usually considered less important than post-anthesis exposure. In conclusion, this study suggests the need to consider crop N management in the derivation of O₃ CLs, due to its effect on dose-response relationships used for CL derivation, including the potential O₃ effects in N use efficiency. Therefore, N modulation could be considered in the O₃-risk assessment methodology to be applied in risk exercises when negotiating air pollution abatement policies.