Root exudation in coastal wetlands: measuring an overlooked but significant carbon flux in mangroves

Hanemann, Lea; Dang, Huyen Thuong; Huot, Hermine; Rumpel, Cornelia; Maillard, Lucie; Gunnell, Yanni; Dahdouh-Guebas, Farid; Le, Thi-Minh-Tam; Bui, Nguyen The Kiet; Arnaud, Marie

doi:10.5194/egusphere-2026-2081

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https://doi.org/10.5194/egusphere-2026-2081

Preprints

28 Apr 2026

| 28 Apr 2026

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

Root exudation in coastal wetlands: measuring an overlooked but significant carbon flux in mangroves

Lea Hanemann, Huyen Thuong Dang, Hermine Huot, Cornelia Rumpel, Lucie Maillard, Yanni Gunnell, Farid Dahdouh-Guebas, Thi-Minh-Tam Le, Nguyen The Kiet Bui, and Marie Arnaud

Abstract. Coastal wetlands are among the most carbon-dense ecosystems on Earth, yet belowground carbon cycling remains poorly understood compared to aboveground processes. Root exudation, the release of labile organic compounds from live roots, represents a critical pathway for transferring plant-derived carbon to soils. This flux is poorly quantified because existing measurement techniques fail under flooded and tidal conditions, leaving a knowledge gap in carbon budgets. Here, we adapted and applied a sealed-cuvette system to quantify root exudation in situ across the two most common and widespread mangrove genera worldwide (Rhizophora and Avicennia) and across contrasting wet and dry seasons in a deltaic mangrove (Cần Giờ, Vietnam). The sealed cuvette method was successful in measuring root exudation, improving current methods for coastal wetlands. Mean root exudation rates were 126 ± 172 μg C g⁻¹ h⁻¹ for Avicennia and 68.5 ± 96.1 μg C g⁻¹ h⁻¹ for Rhizophora, with seasonal rates of 52.4 ± 67.2 μg C g⁻¹ h⁻¹ for the wet season and 135 ± 168 μg C g⁻¹ h⁻¹ for the dry season. Root exudation did not differ significantly across genera and seasons and was similar in magnitude to those of terrestrial forests. When upscaled at the ecosystem level, the root exudation is estimated to represent a considerable portion of the mangrove GPP (6.1–11.9 %). We conclude that root exudation is a non-negligible and previously unaccounted-for component of mangrove carbon budgets and highlight the need for quantification of this ‘missing carbon flux’.

Received: 15 Apr 2026 – Discussion started: 28 Apr 2026

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Lea Hanemann, Huyen Thuong Dang, Hermine Huot, Cornelia Rumpel, Lucie Maillard, Yanni Gunnell, Farid Dahdouh-Guebas, Thi-Minh-Tam Le, Nguyen The Kiet Bui, and Marie Arnaud

Status: open (until 09 Jun 2026)

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RC1:
'Comment on egusphere-2026-2081', Anonymous Referee #1, 14 May 2026 reply
General
Methodological paper that also show range and magnitude of tropical root exudation over seasons. The paper provide novel advances in belowground biogeochemistry methodology for coastal wetlands. It is well written, and the main comments are around clarification for some method aspects, and accuracy/usefulness of the scaled-up calculations.

Abstract
Make sure genera are italicised throughout paper

Line 27-30- because the rate errors (range) are so large it’s causing the non-significant, although the means are are 2-2.5-fold different depending on the comparison. Suggest making that clearer in abstract

Clarify if the GPP value just for the ecosystems in Vietnam?

Intro
L50: It’s not clear what ‘other carbon fluxes’ refers to

L66: not clear how the ad hoc adapted method is insufficient and misses carbon production

Methods
Two different sized cuvette beads between the seasonal measures adding to the variability. How will this change the interpretation between the seasons or what approach readers should use when replicating the method?

Typically DOC samples are preserved with zinc chloride to stop any turnover/shift in DOC. Understanding this may be complex to do in the field, but 7 d storage at 4C may cause unintended changes in DOC. Have the authors tested how the 0.22um filtration would select for some types/sizes of DOC while also removing living organisms? Please state if this may impact the results for both exudate and sediment control DOC including in discussion as required.

4.1. Where the two sizes of beads used or the 2 mm kind?

Can the author clarify why DBD was taken for only the top 5 cm while most of the other measures were to 30 cm? Will this bias the ecosystem scale calculations?

Results
There seem to be exudation spikes, ie outliers in figure 6. What may be happening here that is causing such broad ranges of exudate production? Was there any trend or consistent factor related to those samples in particular?

For ecosystem-scale calculations, the exudation values are so wide (SD is much higher than the mean), that reporting means here may not be the best approach for scaling to annual flux. It may be more transparent to report the ranges of the scaled-up values.

Discussion
L423: since senescence in this case is an experimental artifact, rather than natural, it would be worth restricting exudate measure in the future to the 24-48 hour period. How would the shock and senescence also impact the scaled up results?

Given the methodological development focus of the paper, low replication and high variability in exudation, I suggest the scaling-up exercise be presented as a back-of-the-envelope calculation in the discussion with a subset of data that takes into account the potential bias/limitation of the data. Or else the scaling-up results should be remove in this instance, and revisited in future studies.

Could plant productivity measures or indicators be useful in explaining root exudation?

Is there value of including more than 1 root in the cuvette if the data are normalised to mass or volume of the roots. Would that simulate a more natural rhizosphere condition?

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Citation: https://doi.org/10.5194/egusphere-2026-2081-RC1
RC2: 'Comment on egusphere-2026-2081', Anonymous Referee #2, 26 May 2026 reply

The manuscript addresses a genuinely important and understudied topic – root exudation in mangroves – and presents a creative adaptation of the cuvette method for waterlogged, tidal conditions. The dataset, while limited in sample size and spatial scope, provides some of the first in situ exudation rates for Avicennia and Rhizophora. The upscaling attempt is useful for highlighting the potential significance of this flux. However, several methodological weaknesses, statistical limitations, and overstatements in the interpretation require major revision before the work meets the standards for a high-quality journal like Biogeosciences.
The sample sizes are highly uneven across species and seasons (e.g., Avicennia dry season: n=5 vs. wet season: n=13). How do the authors account for potential statistical bias from unequal replication, and could they justify the minimum sample size for robust comparisons?
Section2.5.1 Exudation rates are calculated using the dry mass of roots after they have been incubated for 48 h and then oven-dried. This assumes that root mass did not change during incubation. However, roots may lose mass through respiration, exudation itself, or tissue death. Conversely, they might gain mass if any microbial biofilm develops. This is a critical source of error. Did you measure root fresh mass or estimate initial dry mass from a parallel set of roots?
Section3.2 The incubation duration test was based on only three roots. No statistical pairwise differences were significant, yet the authors use this to justify the 48 h protocol. This is insufficient. Please either provide a larger time-series experiment or present the existing data more cautiously, acknowledging that the choice of 48 h remains provisional.
Section 4.3 The Spearman correlation matrix shows essentially no significant correlations between exudation rate and any measured variable (temperature, rainfall, humidity, soil EC, pH, bulk density, root traits). The authors suggest this is because the sampled conditions were not sufficiently contrasted. If the study cannot identify any driver, what is the mechanistic contribution of the work? Please discuss more honestly whether the method may be too noisy to detect environmental signals, or whether sample size is simply too small.
Section 4.4 The manuscript compares exudation fluxes to a “global mean mangrove GPP of 16–20 Mg C ha⁻¹ yr⁻¹” from Adame et al. (2024). However, your site-specific GPP may differ. If available, please report local GPP or at least discuss the uncertainty in applying a global average to a specific Vietnamese mangrove.
Dry-season exudation was higher but non-significant. Could the authors link this to seasonal changes in salinity, temperature, or plant water stress, and provide mechanistic explanations?
Porewater salinity, redox potential (ORP), and nutrient concentrations (NH₄⁺, NO₃⁻) were not measured. These variables strongly regulate root exudation—could the authors supplement these data or discuss how their omission limits mechanistic interpretation?
The study focuses on one mangrove site. Could the authors contextualize results with data from other tropical/subtropical mangroves to assess generalizability?
The sealed cuvette contains microbes that may re-uptake DOC. Could the authors discuss how incubation time and sterile controls mitigate this bias?

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Citation: https://doi.org/10.5194/egusphere-2026-2081-RC2

Lea Hanemann, Huyen Thuong Dang, Hermine Huot, Cornelia Rumpel, Lucie Maillard, Yanni Gunnell, Farid Dahdouh-Guebas, Thi-Minh-Tam Le, Nguyen The Kiet Bui, and Marie Arnaud

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Short summary

Mangroves store large amounts of carbon below ground, offsetting global carbon emissions. But root exudation — the release of carbon compounds from living roots into soil — is a key carbon flux that remains poorly constrained, as wetland conditions make standard techniques unworkable. We adapted an established method to measure this flux in two widespread mangrove genera, finding it equals 6–12 % of tree carbon uptake; a proportion that must be accounted for in mangrove carbon budgets.


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