The Composition and Diversity of Volatile Organic Compounds (VOCs) From Leaf Litter in the Biosphere 2 Tropical Rainforest

Background: Volatile Organic Compounds (VOCs) are organic compounds with high vapor pressure at room temperature released by plants, bacteria, archaea, fungi, and protists. Plants and microbes can produce VOCs as a means of communication (i.e., signaling and species interactions) or as a mechanism to ameliorate abiotic stress (e.g., isoprene with high temperatures). The majority of microbial VOC (mVOC) studies have focused on volatiles produced from soils, but recent evidence suggests that leaf litter can have greater VOC production, microbial biomass, and respiration rates adjacent soil. However, it is difficult to differentiate plant VOCs from mVOCs and identify the different mechanisms driving their release into the atmosphere. Thus, as part of an ecosystem-scale project at the Biosphere 2 Tropical Rainforest (B2 TRF) that addressed the impact of drought on VOCs from soil and living leaves (i.e., Biosphere 2 Water, Atmosphere, and Life Dynamics: B2 WALD), I performed a 10-day VOC experiment of leaf litter to: (i) quantify and identify VOCs produced by Clitoria leaf litter in B2TRF; (ii) examine the impact of moisture on litter VOC flux; and (iii) determine whether flux patterns can be used to distinguish plant VOCs and mVOCs. Methods: Leaf litter was collected from five individuals of Clitoria fairchildiana distributed across the B2 TRF. VOCs were continuously measured over a 10-day period from four replicate chambers and a control chamber using proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS). To examine the impact of moisture on VOC fluxes, leaf litter was wet after seven days to simulate a rainfall event. VOCs were identified by comparing each mass to a reference database and flux calculations were performed to examine change in VOC abundance over time, after accounting for the control. Results: In total, 304 VOCs were identified across all four replicate chambers. Wetting altered the flux of 35% of litter VOCs. Among VOCs emitted after wetting, 72 decreased to pre-wetting levels within 24 hours, while 25 sustained higher production with increased moisture. Conclusions: Leaf litter represents a significant source of VOCs yet even with high resolution real-time data it is difficult to differentiate plant-derived VOCs from mVOCs due to shared metabolic pathways, as well as limited information on mVOCs. In addition, although I hypothesized that wetting would stimulate mVOC production, strong fluxes of VOCs after wetting were likely plant-derived VOCs whose release from the leaf surface was amplified by Henry’s law. Future work is needed to identify mVOCs from microbial cultures and link to leaf-level measurements