• Mediterranean grasslands;
  • herbage quality;
  • ozone damage;
  • global change;
  • ozone fluxes;
  • dehesa


A study was conducted on the effect of tropospheric ozone (O3) on soft brome (Bromus hordeaceus) and the modulation of its response by nitrogen (N). Two assays were conducted using open-top chambers (OTCs). Three O3 treatments were considered: filtered air, with concentrations below background levels (charcoal-filtered air), non-filtered air (NFA) that simulates ambient O3 concentrations, and unfiltered air to which 40 nL L−1 O3 above-ambient concentrations was added (NFA+), simulating elevated values recorded in natural areas of annual pastures in the Iberian Peninsula. Three N rates were used, simulating the increase in soil N through atmospheric deposition and excreta from livestock grazing. Ozone caused an augmentation in foliar senescence, whereas green biomass was not altered; consequently, an increased senescent/green biomass ratio was produced. A stronger O3 effect was detected in the second assay compared with the first. This was related to the estimated absorbed O3 fluxes, which were double the value calculated in the former. Increasing N input enhanced biomass production, but its effectiveness was greater in the first assay, under less-favourable weather conditions and lower plant growth. In the first assay, the O3 response was modulated by N availability, which mitigated the effects of O3 to medium concentration values. In the first assay, O3 reduced the aerial/subterranean biomass ratio, caused by a positive-trend effect on roots. Foliar concentration of lignin was increased by O3, and in vitro digestibility of aerial biomass and the plant cell wall fraction tended to decrease with increasing O3.