Ozone sensitivity of the Mediterranean terophyte Trifolium striatum is modulated by soil nitrogen content

The influence of nitrogen (N) fertilization on the sensitivity to ozone (O3) has been studied using the O3 sensitive species Trifolium striatum, an annual species frequently found in therophitic grasslands of the dehesas in the central Iberian Peninsula. The experiment was carried out in open-top chambers using three different O3 exposure treatments: charcoal-filtered air (CFA), non-filtered air (NFA) and non-filtered air plus 40 nl l−1 added over ambient concentrations (NFA+). After 30 days of exposure to the different treatments, plants were placed under ambient conditions until seed ripeness was completed. Three N fertilization treatments were established receiving final doses of 10 (low), 30 (medium) or 60 (high) kg N ha−1 throughout the growing period divided into four proportional doses at every 15 days. Ozone affected phenology and reduced total aerial and flower biomass and seed production in NFA and NFA+ treatments. Carry-over effects of O3 on the reproductive ability of this species were found resulting in reductions in flower biomass and seed production detected 1 month after O3 exposure. By contrast, N fertilization increased aerial and flower biomass. Significant O3×N interactive effects were detected since N fertilization counterbalanced O3-induced effects only when plants were exposed to moderate O3 levels (NFA) but not under high O3 concentrations (NFA+). The observed O3 effects on reproductive ability of T. striatum might determine changes in species viability and future diversity of the ecosystem.