Current ozone levels threaten gross primary production and yield of Mediterranean annual pastures and nitrogen modulates the response

Pastures are among the most important ecosystems in Europe considering their biodiversity and distribution area. However, their response to increasing tropospheric ozone (O₃) and nitrogen (N) deposition, two of the main drivers of global change, is still uncertain. A new Open-Top Chamber (OTC) experiment was performed in central Spain, aiming to study annual pasture response to O₃ and N in close to natural growing conditions. A mixture of six species of three representative families was sowed in the field. Plants were exposed for 40 days to four O₃ treatments: filtered air, non-filtered air (NFA) reproducing ambient levels and NFA supplemented with 20 and 40 nl l−1 O₃. Three N treatments were considered to reach the N integrated doses of “background”, +20 or +40 kg N ha⁻¹. Ozone significantly reduced green and total aboveground biomass (maximum reduction 25%) and increased the senescent biomass (maximum increase 40%). Accordingly, O₃ decreased community Gross Primary Production due to both a global reduction of ecosystem CO₂ exchange and an increase of ecosystem respiration. Nitrogen could partially counterbalance O₃ effects on aboveground biomass when the levels of O₃ were moderate, but at the same time O₃ exposure reduced the fertilization effect of higher N availability. Therefore, O₃ must be considered as a stress factor for annual pastures in the Mediterranean areas.

Calvete-Sogo, H., S. Elvira, J. Sanz, I. González-Fernández, H. García-Gómez, L. Sánchez-Martín, R. Alonso, & V. Bermejo-Bermejo (2014) Current ozone levels threaten gross primary production and yield of Mediterranean annual pastures and nitrogen modulates the response, Atmospheric Environment 95:197-206.