On-line screening of soil VOCs exchange responses to moisture, temperature and root presence

The exchanges of volatile organic compounds (VOCs) between soils and the atmosphere are poorly known. We investigated VOC exchange rates and how they were influenced by soil moisture, temperature and the presence of plant roots in a Mediterranean forest soil. We measured VOC exchange rates along a soil moisture gradient (5%-12.5%-20%-27.5% v/v) and a temperature gradient (10 degrees C-15 degrees C-25 degrees C-35 degrees C) using PTR-MS. Monoterpenes were identified with GC-MS. Soils were a sink rather than a source of VOCs in both soil moisture and temperature treatments (-2.16 +/- 0.35 nmol m(-2) s(-1) and -4.90 +/- 1.24 nmol m(-2) s(-1) respectively). Most compounds observed were oxygenated VOCs like alcohols, aldehydes and ketones and aromatic hydrocarbons. Other volatiles such as acetic acid and ethyl acetate were also observed. All those compounds had very low exchange rates (maximum uptake rates from -0.8 nmol m(-2) s(-1) to -0.6 nmol m(-2) s(-1) for methanol and acetic acid). Monoterpene exchange ranged only from -0.004 nmol m(-2) s(-1) to 0.004 nmol m(-2) s(-1) and limonene and alpha-pinene were the most abundant compounds. Increasing soil moisture resulted in higher soil sink activity possibly due to increases in microbial VOCs uptake activity. No general pattern of response was found in the temperature gradient for total VOCs. Roots decreased the emission of many compounds under increasing soil moisture and under increasing soil temperature. While our results showed that emission of some soil VOCs might be enhanced by the increases in soil temperature and that the uptake of most soil VOCs uptake might be reduced by the decreases of soil water availability, the low exchange rates measured indicated that soil-atmosphere VOC exchange in this system are unlikely to play an important role in atmospheric chemistry.