Ozone (O3) and nitrogen oxides (NOx: nitrogen monoxide NO and nitrogen dioxide NO2) are reactive gases with an important role in atmospheric chemistry. Terpenes are a reactive subgroup of BVOCs (biogenic volatile organic compounds) emitted by plants. Needle or leaf surfaces are the first point of contact between the atmosphere and a plant. Boreal forests represent a significant portion of the global land area available for atmosphere-biosphere interactions.
The aim was to develop methods for observing the exchange of NOx in field conditions and to explore the roles of terpenes on needle surfaces and nitrate fertilization on the fate of O3 and NOx in plant-soil-atmosphere interfaces. The methods included whole-canopy measurements, shoot-scale chamber measurements, needle sampling and laboratory analyses, utilizing both continuous observations and experimental setups.
In the studied low- NOx environment, the shoot-level NOx fluxes were too small to be monitored accurately in field conditions with an automated dynamic chamber. In addition to interference, the signal to noise ratio was low, and a significant proportion of the observed fluxes were to/from chamber walls. No clear NOx fluxes from Scots pine foliage were detected, and there was no effect of nitrogen fertilization on the observed fluxes. It seems unlikely that a fertilization treatment could cause significant NOx emission from boreal pine forests. The fluxes reported in our earlier studies included compounds other than NOx.
Shoot terpene emissions and needle wax extracts were both dominated by monoterpenes. There was variation in the terpene spectra of both emissions and wax extracts. The proportion of sesquiterpenes was higher in the epicuticular waxes than emissions, and the observed sesquiterpene compounds were for the most part different in the emissions and wax extracts. The role of direct transport through the cuticle from sites of terpene synthesis may be more important than has been assumed.