Seasonal dynamics of Volatile Organic Compound Fluxes from Soil in the Amazon rainforest

Volatile organic compounds (VOCs) are important to atmospheric chemistry as they readily react with ambient oxidants, such as ozone (O₃) and hydroxy radicals (OH) to produce particles, thereby influencing air quality, trace gas lifetimes and climate. The Amazon rainforest is the largest natural source of VOCs in the atmosphere, with net emissions resulting from a complex balance between sources and sinks across the different ecosystem compartments, such as the canopy and soil. The aim of this study was to characterize the seasonal dynamics of VOC fluxes from Amazon rainforest soil. The experiments were conducted at the Amazon Tall Tower Observatory research facility during four seasons: the dry-to-wet transition and dry season of 2023 ( with the latter being influenced by El Niño) and the wet and dry seasons of 2024.

Soil VOC samples were collected on sorption tubes from three manual steady-state soil chambers. The samples were analysed using thermal desorption-gas chromatography - time of flight mass spectrometry with a chiral column to further separate chiral terpenoids into their separate mirror image forms known as enantiomers. We focused on terpenoid species including isoprene, monoterpenes and sesquiterpenes.

Results showed that in all seasons the Amazon rainforest soil was a net sink for both isoprene (average flux of -12 nmol m^-2 h^-1) and its oxidation products methacrolein (average fluxes of -4 nmol m^-2 h^-1) and methyl vinyl ketone (-5 nmol m^-2 h^-1). In contrast, the Amazon rainforest soil was a source of sesquiterpenes with an average flux of 2 nmol m^-2 h^-1. The most abundant sesquiterpenes were β-Caryophyllene and α-Copaene. Monoterpene behaviour varied among species, time of day, season and rainfall events.

The highest VOC emissions from soil were observed during the El Niño-influenced dry season of 2023, likely driven by intense heat and drought stress, which significantly reduced soil microbial VOC uptake and increased VOC emissions from abiotic degradation processes. During the wet season and dry-to-wet transition season, the magnitude of the observed soil fluxes was smaller, indicating a more balanced state between uptake and emission processes, likely attributed to the restoration of the microbial uptake in the more humid soil.