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

Schüttler, Johanna M.; Pugliese, Giovanni; Byron, Joseph; Quaresma Dias-Júnior, Cléo; de A. Monteiro, Carolina; Harder, Hartwig; Lelieveld, Jos; Williams, Jonathan

doi:10.5194/egusphere-2025-5530

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https://doi.org/10.5194/egusphere-2025-5530

Preprints

21 Nov 2025

| 21 Nov 2025

Status: this preprint is open for discussion and under review for Biogeosciences (BG).

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

Johanna M. Schüttler, Giovanni Pugliese, Joseph Byron, Cléo Quaresma Dias-Júnior, Carolina de A. Monteiro, Hartwig Harder, Jos Lelieveld, and Jonathan Williams

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.

Received: 07 Nov 2025 – Discussion started: 21 Nov 2025

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Johanna M. Schüttler, Giovanni Pugliese, Joseph Byron, Cléo Quaresma Dias-Júnior, Carolina de A. Monteiro, Hartwig Harder, Jos Lelieveld, and Jonathan Williams

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RC1:
'Comment on egusphere-2025-5530', Anonymous Referee #1, 26 Nov 2025 reply
The paper by Schüttler et al. offer some interesting data on BVOC exchanges between the soil and atmosphere and extends our understanding of these fluxes in tropical systems. As a Biologist, not a Chemist, I found the methods descriptions commendably clear, logical, and easy to follow. I do NOT know enough to comment critically on these methods, but the descriptions made perfect sense to me.
The importance of BVOC fluxes in these systems has been known for over thirty years, but we still have very few data sets of soil fluxes, per se; this study, even though it is based on few chambers in one site, offers some tantalizing insights. The authors need to bear in mind in their statistical models when their samples are truly independent, when they are time series, and when they are pseudo-replicated. I suspect they will want to either re-structure their models or, at the very least, acknowledge when their data violate assumptions of independence. I do not see this as a major issue because the results are so striking.

A few results stood to me, with respect to the underlying Biology. I hope the authors find these useful as they revise.
the lack of isoprene uptake in the litter layer suggests that litter microbes have not adapted to this carbon source. Given the earlier described role of litter microbes in methanol and terpene consumption, this result is worth highlighting.

the difference in enantiomeric variability between the monoterpenes and the sesquiterpenes is congruent with earlier ideas that the sesquis come from the MVA pathway while the monoterps are made by the DOX-P pathway.

the idea of terpene emission from leaf litter, per se, rather than microbial activity during decomposition, could be developed a bit. The reason for pursuing this distinction is that a fair bit is known about the distribution of different terpenes in different plant taxa, so the direct leaf-contribution to BVOC fluxes should be predictable if the plant species composition is known.

For over 30 years, thanks to Carleton White, we’ve been aware of the importance of soil microbes as consumers of BVOCs. When the flux is net upward, as Schüttler et al. found here for the 10 and 15 C terpenes, then the consumption processes are smaller than the production ones. The results of this manuscript suggest that it is time for some more process-based studies of soil BVOC fluxes so that we can begin to estimate gross fluxes also.

I applaud the authors for their wet & dry season measurements, but I think the link to El Nino is rather tenuous, and, imo, they should de-emphasize this point. But I leave this to the editor’s discretion.

the authors make the important point that soil fluxes could result from microbes and/or roots. There have been similar observations on volatile sulfur fluxes in tropical systems. Distinguishing plant from microbial sources of production and consumption is crucial for developing predictive modeling. If the authors choose to discuss this, they should bear in mind McDonald and Falls’s older work showing microbial consumption on the leaf surface, which could look like leaf consumption.

In short, this is a fine contribution and one that I look forward to citing when it appears in the literature.

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Citation: https://doi.org/10.5194/egusphere-2025-5530-RC1

Johanna M. Schüttler, Giovanni Pugliese, Joseph Byron, Cléo Quaresma Dias-Júnior, Carolina de A. Monteiro, Hartwig Harder, Jos Lelieveld, and Jonathan Williams

Data sets

Soil fluxes and volume mixing ratios (VMR) of volatile organic compounds (VOCs) measured at the ATTO Site in 2023 and 2024 J. M. Schüttler et al. https://doi.org/10.17871/ATTO.612.7.2472

Johanna M. Schüttler, Giovanni Pugliese, Joseph Byron, Cléo Quaresma Dias-Júnior, Carolina de A. Monteiro, Hartwig Harder, Jos Lelieveld, and Jonathan Williams

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Short summary

We measured how Amazon rainforest soils release and uptake compounds commonly produced by plants across seasons, including the severe 2023 El Niño drought. Soils took up isoprene most in the afternoon of dry seasons, while emissions of very reactive sesquiterpenes peaked during the drought. Removing the leaf litter changed which compounds transferred to and from the soil. These soil exchanges, though small compared to the canopy, can shape air chemistry near the ground and influence soil biota.


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