Heterogeneous responses to ozone and nitrogen alter the species composition of Mediterranean annual pastures

Air pollution represents a threat to biodiversity throughout the world and particularly in the Mediterranean area, where high tropospheric ozone (O₃) concentrations and atmospheric nitrogen (N) deposition are frequently recorded. Mediterranean annual pastures are among the most important ecosystems in southern Europe due to their high biodiversity and extension. Aiming to study the responses of these communities to the main atmospheric pollutants in the Mediterranean region, an experimental study was performed in an open-top chamber (OTC) facility. A mixture of six species representative of annual pastures was grown under field conditions inside the OTC. Plants were exposed for 39 days to four O₃ treatments and three doses of N. The species responded heterogeneously to both factors. Legumes did not react to N but were very sensitive to O₃: Trifolium species responded negatively, while Ornithopus responded positively, taking advantage of the greater sensitivity of clovers to O₃. The grasses and the herb were more tolerant of O₃ and grasses were the most responsive to N. Significant interactions between factors indicated a loss of effectiveness of N in O₃-polluted atmospheres and an ability of O₃ to counterbalance the damage induced by N input, but both effects were dependent on O₃ and N levels. The inclusion of plant competition in the experimental design was necessary to reveal results that would otherwise be missed, such as the positive growth responses under elevated O₃ levels. Surprisingly, competition within the legume family played the most important role in the overall response of the annual community to O₃. Both tropospheric O₃ and N deposition should be considered important drivers of the structure and biodiversity of Mediterranean annual pastures.