Volatile Organic Compound (VOC) Exchange in Tropical Leaf Litter in Response to Wetting: An Automated Scheme to Classify Flux Pulse Dynamics

Leaf litter emits volatile organic compounds (VOCs) that can impact atmospheric and soil processes, particularly in ecosystems with episodic litterfall and decomposition such as dry-wet transitions in tropical forests. Litter VOCs may originate from both plant and microbial sources that are challenging to disentangle but may be reflected in the temporal patterns of litter VOC fluxes to wetting. Here, we collected Clitoria fairchildiana litter after an ecosystem-scale experimental drought in the Biosphere 2 Tropical Rainforest and measured litter VOC fluxes over a 10-day incubation to: (a) identify and quantify litter VOC fluxes; (b) examine the impacts of moisture; and (c) distinguish plant from microbial VOCs. In total, we observed 121 masses exhibiting either significant emission (88%) or uptake (12%) fluxes. Emissions of methanol, acetaldehyde, and acetone were the dominant fluxes. Wetting dry litter altered the flux of 47% of VOCs: 66 decreased to pre-wetting levels within 24 hr although 25 sustained higher emission rates. We categorized VOCs during wetting as plant derived (55%), microbial-derived production (21%), microbial uptake (12%), and unknown (13%) by visual inspection of the flux time series. Automated classification of the wetting pulses with fitted model parameters was consistent with the visual categorization approximately 80% of the time. Our results provide measurements of litter VOC fluxes for a widespread tropical plant. Moreover, we illustrate an automated data-model approach to efficiently characterize and categorize trace gas pulses for litter VOC fluxes that is translatable to other types of trace gases, forcings, and ecosystem components including soil.