Strong toluene and p-cymene emission from waterlogged hotspots of a temperate mixed-forest soil

Abstract. Ecosystem processes taking place at hotspots, i.e., at areas of small spatial extent, but at high intensity, might potentially affect the whole system. Due to their great importance for atmospheric chemistry, we studied the exchange of volatile organic compounds (VOCs) from the forest floor of the ECOSENSE forest, a temperate mixed forest, at selected waterlogged sites. These sites were mainly located along skid trails, hence, characterized by soil compaction and strongly reduced drainage of rainwater. In addition, we monitored VOC exchange from undisturbed, well-drained forest floor during summer and the transition from summer to autumn/winter when strong changes in soil moisture occurred. We complemented these field measurements by experiments under controlled conditions in the laboratory, in which soil cores collected from well-drained and waterlogged sites in the forest were exposed to anoxia. Anoxic conditions enormously stimulated emissions of aromatic compounds, particularly of toluene and the aromatic monoterpene p-cymene 50-fold and 20-fold, respectively. Moreover, anoxia also strongly enhanced the emission of the monoterpenes camphene, limonene, δ-3-carene and α-pinene. In the field, the forest floor changed from a sink of toluene and p-cymene to a strong source of these compounds in response to waterlogging. We observed enormously increased toluene and p-cymene emissions from the waterlogged forest floor along skid trails, amounting to over 5000 ng m^-2 h^-1 and 3000 ng m^-2 h^-1, respectively. Considering that the share of skid trails of managed forests can be considerable, we further investigated a potential impact of these hotspots of toluene emission at the ecosystem scale. Indeed, ambient atmospheric mixing ratios of toluene in the ECOSENSE forest peaked in autumn when soil moisture levels were increased. The work therefore suggests that the soil of temperate forests might be a yet underestimated source of toluene and p-cymene for the atmosphere. Still, elevated ambient toluene levels co-occurred with litterfall and senescence of European beech leaves which might be an additional source of atmospheric toluene. At periods with high ambient toluene mixing ratios, the well-drained forest floor acted as a significant sink of toluene. Our results underline the importance of the forest floor’s spatial heterogeneity to act as a concurrent sink (main area) and a source (from waterlogged hotspots) of aromatic VOCs.