Influence of Nitrogenous Air Pollutants on Carbon Dioxide and Ozone Effects on Vegetation

Abstract

Nitrogenous air pollution can have a major impact on ecosystems. Ambient concentrations of NO_x and NH₃ are increasing in many regions due to the increasing intensity of traffic and livestock farming, respectively. On a more universal scale, concentrations of CO₂ and O₃ are also increasing. This chapter discusses whether nitrogenous air pollutants must be taken into account when evaluating the effects of elevated CO₂ and O₃ on plants. Results reported here indicate that combination effects are generally additive. An exception was the combination of CO₂ and NH₃; plant growth was stimulated by both compounds when applied singly, but not in combination. The growth-stimulating effect of CO₂ was also counteracted by NO_x and O₃. The effect-threshold for O₃ was higher in the presence of elevated CO₂ and NH₃. In more general terms, it appears that the stimulatory effect of elevated CO₂ concentration is reduced by ‘conventional’ air pollution. This could mean that reductions of NO_x and NH₃ emissions may enhance the effects of CO₂ and O₃, and that in polluted regions the beneficial effect of elevated CO₂ on biomass production may be less than previously assumed. It is suggested that nitrogenous air pollutants must be taken into account when evaluating the true impact of elevated CO₂ and O₃ concentrations.