Chiral Volatile Organic Compound Fluxes from Soil in the Amazon Rainforest across seasons

Abstract. The rainforest floor is an underexplored source and sink of biogenic volatile organic compounds (BVOC), and its contribution to the ecosystem BVOC budget remains poorly understood. We performed multi-seasonal measurements in the Amazon rainforest on the soil-atmosphere exchange of enantiomer-resolved monoterpenes (C₁₀H₁₆) and sesquiterpenes (C₁₅H₂₄), isoprene, and two isoprene oxidation products: methacrolein and methyl vinyl ketone. Soil uptake of isoprene and isoprene oxidation products was stronger during dry seasons than wet seasons and peaked in the afternoon hours. Sesquiterpene emission was highest during the El Niño- influenced dry season. Monoterpene fluxes showed changes in speciation across seasons. The presence or removal of the litter layer strongly altered the speciation of the monoterpene and sesquiterpene fluxes, partly shifting from emission with the litter layer to uptake without it. At the same time, the litter had no significant effect on isoprene. Enantiomeric ratios of α-pinene, limonene, β-pinene, and camphene differed between soil emissions and ambient air and shifted seasonally, suggesting distinct soil sources and processes. For each sesquiterpene only one enantiomer was detected. Although soil BVOC fluxes contribute little to the overall atmospheric budget in rainforests dominated by the plant canopy, they may affect near-surface chemistry and play important roles in soil ecology.