Forest diversity and environmental factors shape contrasting soil-litter fluxes of biogenic volatile organic compounds and methane in three central Amazonian ecosystems

Abstract. Biogenic volatile organic compounds (BVOCs) play a crucial role in biosphere-atmosphere interactions and the global carbon cycle. While vegetation is recognized as the primary source of BVOC fluxes in forest ecosystems, recent studies suggest that the carbon-rich soil-litter compartment plays a significant role in gas fluxes. However, the drivers, variability, and magnitude of these fluxes across different forest types remain poorly understood. This is particularly notable in the Amazon rainforest, the world's largest source of BVOCs, where measurements remain scarce. In this study, we investigated the net soil-litter gas exchange of BVOCs and methane, along with their potential drivers – soil and litter nutrient content, soil and litter microbial biomass, soil temperature, and soil moisture – across three forest types in central Amazonia: White Sand Forest (WS), Upland Forest (Up), and Ancient River Terrace Forest (AR). Our results showed distinct soil-litter gas exchange patterns across the forest types. WS exhibited both high emissions and consumption of gases, notably high acetaldehyde and methane emissions, along with an uptake of monoterpenes. Up showed lower overall fluxes, with moderate emissions and consumption of dimethyl sulfide (DMS), isoprene, and acetaldehyde. In contrast, AR presented no significant fluxes. Among the variables tested, models indicated that soil moisture and temperature were the strongest drivers of fluxes in WS, whereas microbial biomass was the main driver in Up. Our measurements suggest that, despite covering a relatively small area in the Amazon basin, WS can be a significant ecosystem for BVOC and methane fluxes, where these fluxes are influenced by soil moisture and temperature. Our findings underscore the need to account for forest-type-specific fluxes when modeling BVOC and methane emissions in the Amazon, particularly under changing climate conditions.