Collecting grassland volatile organic compounds in open air through passive and active sampling strategies

Volatile organic compounds (VOCs) act as mediators in plant communication, pollination, and plant defense against herbivory. The set of VOCs in an environment constitutes the volatilome, and it may contain valuable information for understanding ecosystem functioning. Here we set out to obtain a representative inventory of VOCs such as those that a bee might encounter while foraging in grazed permanent grasslands. We chose the passive diffusion trapping (PDT) method, because this method, designed for this type of measurement in large air volumes, is the easiest to implement. We thus exposed Tenax TA sorbent tubes to air over the daytime on nine different plots at five periods during the vegetation cycle. GC-MS analysis after thermal desorption detected more than 100 VOCs encompassing a broad diversity of chemical families, with oxygenated compounds (alcohols, ethers, carbonyls, esters) and alkanes dominating in terms of number of compounds as well as total relative area, benzenoids and terpenes being also abundant. For subsets of observations, we checked if solid-phase microextraction using polydimethylsiloxane-carboxen-divinylbenzene fiber coatings, or active-mode trapping (AcT) using a pump that enabled a highly controlled volumes of air to cross the Tenax tubes should be preferred to PDT. Compared to the two passive sampling methods for the same observations, the AcT method, which collected the greatest diversity of compounds and the highest number of known semiochemicals, seemed best suited to monitoring the grassland chemical landscape.