the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Mar 2026
Divergent mercury sequestration dynamics in tropical dry and moist broadleaf forests
Abstract. Forests are major sinks for atmospheric mercury (Hg) due to the efficiency of stomatal uptake and litter deposition. Tropical forests, highly productive ecosystems, remain understudied despite their pronounced climatic and phenological variability. We investigated whether seasonal rainfall regimes and associated tree adaptations regulate the Hg dynamics – its uptake by leaves, deposition through litter, and storage in soils– in secondary tropical moist broadleaf (TMBF) and tropical dry broadleaf (TDBF) forests from Costa Rica. Seasonality strongly controlled Hg sequestration in TDBF. Deciduous trees showed 4.7 times higher foliar Hg concentration in the wet season, when leaves were mature, compared to the dry season, when newly flushed leaves emerged after leaf shedding. Evergreen trees in TDBF demonstrated 2.3 times lower foliar Hg concentrations in the dry season than in the wet season (23 vs. 53 µg kg-1), likely due to various associated physiological processes (e.g., leaf flushing). However, the primary mechanism remains unclear given the complex and unexplored Hg dynamics in TDBF. TMBF showed no clear seasonal variation in foliar Hg in either deciduous (dry: 48; wet: 54 µg kg-1) or evergreen trees (dry: 57; wet: 53 µg kg-1), likely due to longer leaf lifespan sustaining year-round transpiration and stomatal Hg uptake under high humidity. Atmospheric Hg concentrations in TDBF were two times higher than in TMBF (1.2 vs. 0.6 ng m-3) across both seasons, likely reflecting greater Hg capture per unit area in TMBF due to denser vegetation and enhanced wet deposition via rainfall. Foliar Hg was not correlated with stomatal density or specific leaf area in either forest type. Soil Hg concentrations, however, were correlated with litter-derived inputs, supporting litter as the dominant Hg transfer pathway. Higher seasonally averaged Hg inputs via litter (34 μg m-2) in TMBF than in the TDBF (19 μg m-2) resulted in 3.4 times higher soil Hg concentrations (0–30 cm) in TMBF (115 µg kg-1) than in TDBF (34 µg kg-1). Soil Hg stocks were 2.6 times lower at a previously deforested TMBF site, indicating persistent disturbance effects despite almost three decades of reforestation. Overall, seasonality regulated Hg sequestration in TDBF, yet both TDBF and TMBF serve as important global Hg sinks with contrasting dynamics that are potentially sensitive to climate change.
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Status: open (until 23 Jul 2026)
- RC1: 'Comment on egusphere-2026-1354', Anonymous Referee #1, 22 May 2026 reply
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RC2: 'Comment on egusphere-2026-1354', Anonymous Referee #2, 01 Jul 2026
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Reviewer Comments
This study examines seasonal changes in atmospheric, foliar, forest-floor litter, and soil Hg in tropical dry and moist broadleaf forests in Costa Rica. Seasonal foliar Hg data from tropical dry forests remain limited, and the observed differences between deciduous and evergreen trees across seasons provide valuable field evidence for understanding how leaf phenology may influence Hg accumulation in tropical forests. However, the study directly measured Hg concentrations in leaves, litter, and soils, together with the standing Hg pools of forest-floor litter. These measurements represent Hg concentrations and a static litter Hg pool at the time of sampling, rather than Hg uptake rates, litterfall Hg fluxes, ecosystem Hg inputs, or net sequestration. The manuscript therefore overinterprets these metrics when describing forest Hg uptake, storage, and sequestration capacity. The authors should clearly distinguish concentrations and standing pools from fluxes and net ecosystem sequestration, and revise the mechanistic and global-scale conclusions accordingly.
Major comments
1. Lines 165–170: The authors collected forest-floor litter once using approximately 400 cm² quadrats and calculated litter mass and Hg burden per unit area. This metric represents the amount of litter present on the forest floor at the time of sampling, i.e., the forest-floor litter standing stock, rather than a litter Hg input flux over a defined period. Therefore, the terms “litter Hg input,” “litter Hg flux,” “litter-derived Hg deposition,” and “Hg transfer to soil” are not appropriate. These metrics should instead be described as “forest-floor litter mass” or “standing litter Hg burden,” and they should not be directly compared with annual Hg input fluxes measured using litterfall collectors.
2. Section 2.2: The study mainly measured Hg concentrations in leaves, litter, and soils, together with the standing mass and Hg burden of forest-floor litter. These metrics do not directly represent ecosystem-scale Hg uptake rates, input fluxes, storage, or net sequestration. However, the terms “Hg sequestration,” “uptake,” “storage,” and “global Hg sink” are repeatedly used in the title, abstract, and conclusions, which may give readers the impression that these processes were directly quantified. The authors should clearly distinguish among Hg concentration, standing burden, flux, stock, and net sequestration, and restrict their interpretations to what is directly supported by the data.
3. Lines 170–175: The authors measured soil Hg concentrations at different depths in units of μg kg⁻¹, but did not report soil bulk density, coarse-fragment content, or other parameters required to calculate areal soil Hg stocks. Therefore, statements such as “soil Hg stocks were 2.6 times lower” in the abstract and conclusions are not supported by the available data. Unless the necessary parameters can be provided and soil Hg stocks recalculated, “soil Hg stocks” should be replaced throughout with “soil Hg concentrations.”
4. In the TDBF, leaves collected during the dry season were mainly newly flushed after rainfall, whereas those collected during the wet season were mature leaves. The higher Hg concentrations in wet-season mature leaves may therefore primarily reflect greater leaf age and longer exposure to atmospheric Hg, rather than higher water availability, greater stomatal opening, or stronger transpiration during the wet season. Because leaf age, season, and water availability are confounded in the current sampling design, the present results do not demonstrate that water availability is the main driver of foliar Hg variation.
5. Lines 640–650: The authors compare plots G1–G2 with G3–G6 in the TMBF and attribute the lower soil Hg concentrations in G1–G2 to historical grazing, cacao cultivation, and lower tree-species diversity. They further suggest that recovery of soil Hg stocks requires a long time. However, this comparison lacks an undisturbed primary-forest reference and pre-disturbance soil Hg data. In addition, the study measured soil Hg concentrations rather than Hg stocks. Therefore, the current evidence is insufficient to demonstrate that anthropogenic disturbance caused a 2.6-fold loss of soil Hg stocks or to determine the time required for recovery. The conclusion that maintaining species diversity enhances Hg sequestration also extends beyond what the data can support.
6. Lines 680–685: Based on mature-leaf Hg concentrations at the two study sites being higher than the global average, the authors conclude that TDBF and TMBF are important global Hg sinks and recommend including TDBF in global Hg budgets. However, high foliar Hg concentrations do not necessarily indicate a strong ecosystem-scale Hg sink. Forest Hg sequestration also depends on foliar biomass, leaf age, leaf turnover, leaf area index, and Hg re-emission, among other factors. The current results are therefore insufficient to demonstrate that these forest types function as major global Hg sinks, and the relevant statements should be substantially weakened.
Minor comments
1. The Methods state that four soil cores were collected from each plot. With 12 plots, the total should therefore be 48 cores, yet the manuscript reports n=24n=24n=24 cores. In addition, 192 samples across four depth intervals correspond to 48 cores. Please verify and correct the number of soil cores and soil samples throughout the manuscript.
2.The Methods report a total of 391 leaf samples, whereas the sample sizes shown in Fig. 3 sum to 355. Please verify the sample numbers and explain the discrepancy.
3.The manuscript treats water availability and seasonality as key mechanisms explaining foliar Hg variation, but does not report actual precipitation, air temperature, relative humidity, or soil moisture during the three sampling campaigns or the two MerPAS deployment periods. Please provide site-specific measurements or data from nearby meteorological stations to better support these interpretations.
4.Section 2.6 is numbered twice. The Statistical Analysis section should be renumbered as Section 2.7.
5.The phrase “A significance level of significance” in line 214 is redundant and should be revised.
6.“Specific leaf area index” in line 402 should be corrected to “specific leaf area.”
7.The unit “20–40 μg μg kg⁻¹” in line 418 contains a duplication and should be corrected.Citation: https://doi.org/10.5194/egusphere-2026-1354-RC2
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All my comments are in the attached PDF file below.