Soil uptake of VOCs exceeds production when ambient VOCs are readily available

Volatile organic compounds (VOCs) is a group of highly reactive gaseous species in the atmosphere with significant environmental implications, such as influencing air quality and Earth’s radiation balance. Natural ecosystems constitutes a large part of VOCs inventory with vegetation as well-known sources and soils as potential unidirectional interface yet relatively less studied. Here, we collected soil samples from two representative temperate ecosystems: beech forest and heather heath, and incubated them under manipulated conditions, such as at different temperatures,  and/or exposed to different ambient VOC levels, using a dynamic flow-through system coupled with a PTR-ToF-MS, from which production and/or uptake rates of some selected VOCs were measured and calculated. Results showed that these soils were natural sources of a variety of VOCs, and their emission strength and profile were influenced by soil biogeochemical properties (e.g., soil organic matter, moisture) and temperature. These soils were switched to natural sinks of most VOCs when supplying VOC substrates to the headspace of the enclosed soils at parts per billions level, and the sink size positively responded to the amount of VOCs available in the ambient air. Further analysis indicated that the observed VOC uptake by soils were likely driven by microbial metabolism plus a minor contribution from physical adsorption to soil particles. Overall, our study suggests that soil uptake of VOCs may conceal the simultaneous production and turn it into VOC sinks when ambient VOCs become readily available, such as significant VOC sources existing near surface, thereby regulating the net performance of ecosystem exchange of these environmentally important trace gases.