Resumen de Ecologia global de les emissions de vocs per les plantes
Biogenic volatile organic compounds (BVOCs) play important roles in ecology and atmospheric chemistry. Their emissions from terrestrial ecosystems are driven mainly by plants, and are greatly influenced by the variations in environmental variables and therefore are altered by increasing global environmental change (GEC). However, these emissions and their influences are not well known in natural Mediterranean ecosystems.
While soil BVOCs apparently play a minor role in atmospheric chemistry, owing to their usually low fraction with respect to the total ecosystem emissions in Mediterranean-type ecosystems, they play important ecological roles in soil processes. Much previous studies on soil VOCs have looked at fluxes, while the actual VOC concentrations in soils from Mediterranean ecosystems have never been considered.
In this work, I carry out the monitoring of environmental manipulation experiments in typical Mediterranean ecosystems to study the exchanges of VOCs, including emissions from plants, and exchanges and concentrations of soils, and their possible drivers.
The shrub Erica multiflora L. abundant species in Mediterranean shrublands and the tree Quercus ilex L. dominant species in many Mediterranean forests were the species studied, the former in the Garraf Natural Park (shrubland) and the latter in Les Muntanyes de Prades (forest), two typically Mediterranean ecosystems. Both species emitted terpenes, but a significant emission of isoprene was only detected from E. multiflora. Isoprenoid emissions increased with air temperature and generally decreased as the amount of soil moisture increased. Terpene emissions increased synergistically due to heat stress and drought in summer. Nitrogenous fertilizer did not significantly affect the emission of isoprene, but it significantly increased the total emissions of terpenes and decreased their diversity. The results suggest that higher isoprenoid emissions can be expected as the Mediterranean region becomes warmer and drier over the next few decades and that N deposition could further stimulate these emissions.
For the soils of the same shrubland and holm oak forest ecosystems, those of shrubland showed higher total VOC concentrations than forest. The concentrations of methanol, acetic acid, formaldehyde, ethanol, and acetaldehyde were the dominant compounds in both ecosystems. Soil temperature and water content, CO2 efflux, and enzyme activity were the best explanatory variables for variation in soil VOC concentrations in the two ecosystems: there was a stronger association between concentration of dominant compounds, except formaldehyde, with soil temperature and/or CO2 efflux than with soil water content. Activity of C- and N-degrading enzymes was positively associated with the concentration of VOCs, depending on ecosystem, and consistently correlated with high soil water content. In the holm oak forest soils, net photosynthetic rate (A) was positively correlated with soil concentration of monoterpenes. These results show that soil VOC concentrations in these Mediterranean ecosystems are driven by soil temperature and water content, and microbial activity, in combination with ecosystem plant activity.
I also analyzed a Mediterranean turf with Pinus pinea L. trees. I quantified the exchange of isoprenoids between soil (with litter) and atmosphere along a horizontal gradient from the trunks of Pinus pinea. Isoprenoid emissions were greatest and most diverse, and also can be roughly estimated by litter dry weight near the trunk, where the needle litter was denser. Irregular emission and adsorption of isoprenoids with low exchange rates were recorded, and exchange rates were correlated positively with soil temperature and negatively with soil moisture in open turf.
In conclusion, higher BVOC emissions can be expected owing to the increases in foliar emissions, and exchanges and concentrations of soils, and N deposition will also further stimulate these emission trends by increasing foliar isoprenoid emissions in the warmer and drier conditions predicted for the coming decades in the Mediterranean region.
