Increased nitrogen wet deposition triggers negative effects of ozone on the biomass production of Carpinus betulus L. young trees

During a two-year Open-Top Chamber experiment performed in 2012 and 2013, young trees of hornbeam (Carpinus betulus L.) were exposed to four levels of ozone (O-3) ( - 40% of ambient O-3 in charcoal-filtered OTCs, - 5% in non-filtered OTCs, + 30% and +75% in O-3 -enriched OTCs) and two levels of Nitrogen (N) wet deposition (CoN = tap water and +N = tap water + 70 kg N ha(-1) yr(-1)). Stomatal conductance at leaf level and growth parameters (stem diameter and height) were measured during the two years of experimentation, while at the end of each growing season plants dry biomass partition between the roots and the stem was assessed. Hornbeam plants resulted highly tolerant to O-3, which did not cause any significant negative effects on the biomass yield of plants grown in CoN conditions. Furthermore, the dose-response relationships based on the Phytotoxic Ozone Dose (POD S ) indicated that the stem biomass increased under high levels of O-3 ( +1.48% per 1 mmol O-3 m(-2) absorbed by CoN plant), while root biomass was not affected. Nitrogen alone caused a general increase of total biomass in both years of the experiment ( + 14.8% and + 38.1% on average in 2012 and 2013, respectively). However, in 2013 plants subject to the highest level of O-3 showed an opposite behavior, with a -11% decrease of total biomass in +N conditions. Ozone and Nitrogen in combination caused a statistically significant negative effect on the stem biomass and stem diameter of plants. Dose-response relationships based on POD 1 for plants in +N conditions revealed significant linear regressions for stem, roots and total biomass, which decreased by -1.04%, - 0.80% and - 0.92% for each mmol m(-2) of O-3 absorbed, respectively. These results show that on a long-term scale, an increase of N wet deposition could significantly modify C. betulus response to O-3 stress, playing a potential role in the risk assessment of O-3 impact on the growth of this species.