Environmental controls of methanol emissions from a grazed grassland in Dorinne, Belgium

Despite the growing interest for oxygenated volatile organic compounds (OVOC) over the last 15 years due to their role in the atmospheric chemistry, current knowledge about OVOC exchanges by grassland and the environmental factors driving them remains lacunar. However, those ecosystems represent an important part of the total earth surface (13.37%). This study conducted on a grazed grassland therefore aims to quantify OVOC exchanges over full grazing seasons in order to understand the mechanisms behind these exchanges. It took place within the activities of the CROSTVOC project (CROp Stress VOC) and therefore gives an important attention to the stress induced fluxes. BVOC flux measurements were performed with a PTR-MS for the measurements of OVOC mixing ratios on two different scales: the eddy Covariance method was used during two whole grazing seasons (2014 and 2015) and in the 2016 summer, measurements were also performed on a smaller scale by using all-teflon automated dynamic chambers. The chambers allowed measuring accurately the impact of grazing by following simultaneously undisturbed and grazed grassland patches.

This study pointed out that several OVOC were exchanged in variable quantities, with methanol being by far the most important. Methanol fluxes exhibited a clear diurnal cycle with close-to-zero fluxes at night and maximum fluxes at midday. The flux was also much larger in the summer than during autumn or spring. For the eddy Covariance data, the average methanol flux in the summer (0.033 μg.m-2s-1) was in the same range of other studies, being lower than the average found by Bamberger et al. in 2010 (0.080 μg.m-2s-1) and by Ammann (0.077 μg.m-2s-1). Fluxes from the chambers were slightly lower with an average value of 0.27 μg.m-2s-1 but they were measured in the late summer. Driver analysis is still ongoing but first results showed that the flux was strongly correlated at short time scale (half hourly for the EC method and one and a half hourly for the chambers) to the photosynthetic photon flux density and the latent heat flux. We believe that their influence comes mainly from the control practiced by stomatal conductance in non-steady state conditions and expansion of cell walls in steady state conditions. The cuvette data showed that grazing resulted in significantly enhanced emissions of methanol that lasted for several days. On the contrary, the eddy Covariance method cannot distinguish between grazed and non-grazed grass and only a small fraction of the method footprint had been grazed the previous days on average. Our eddy-covariance data suggested that an increase in the stocking density did not contribute to a significant rise of methanol emission. The limited variability in stocking density during the two growing seasons and the more powerful effect of other drivers likely induced that the flux variability due to the SD was too low to be measured by the eddy Covariance method. In depth comparisons between the eddy-covariance data and the dynamic chambers data will be carried out.