the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Dec 2024
Quantifying the soil sink of atmospheric Hydrogen: a full year of field measurements from grassland and forest soils in the UK
Abstract. Emissions of hydrogen (H2) gas from human activities are associated with indirect climate warming effects. As the hydrogen economy expands globally (e.g. the use of H2 gas as an energy source), the anthropogenic release of H2 into the atmosphere is expected to rise rapidly as a result of increased leakage. The dominant H2 removal process is uptake into soils; however, removal mechanisms are poorly understood and the fate and impact of increased H2 emissions remains highly uncertain. Fluxes of H2 with soils are rarely measured, and data to inform global models is based on few studies. This study presents soil H2 fluxes from two field sites in central Scotland, a managed grassland and a planted deciduous woodland, with flux measurements of H2 covering full seasonal cycles. A bespoke flux chamber measurement protocol was developed to deal with the fast decline in headspace concentrations associated with rapid H2 fluxes, in which non-linear regression models could be fitted to concentration data over a 7-minute enclosure time. We estimate annual H2 uptake of -3.1 ± 0.1 and -12.0 ± 0.4 kg H2 ha-1 yr-1 and mean deposition velocities of 0.012 ± 0.002 and 0.088 ± 0.005 cm s-1 for the grassland and woodland sites, respectively. Soil moisture was found to be the primary driver of H2 uptake at the grassland site, where the high clay content of the soil resulted in anaerobic conditions (near zero H2 flux) during wet periods of the year. Uptake of H2 at the forest site was highly variable and did not correlate well with any localised soil properties (soil moisture, temperature, total carbon and nitrogen content). It is likely that the high clay content of the grassland site (55 % clay) decreased aeration when soils were wet, resulting in poor aeration and low H2 uptake. The well-drained forest site (25 % clay) was not as restricted by exchange of H2 between the atmosphere and the soil, showing instead a large variability in H2 flux that is more likely to be related to heterogeneous factors in the soil that control microbial activity (e.g. labile carbon and microbial densities). The results of this study highlight that there is still much that we do not understand regarding the drivers of H2 uptake in soils and that further field measurements are required to improve global models.
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RC1: 'Comment on egusphere-2024-3654', Anonymous Referee #1, 09 Jan 2025
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The paper presents a time-resolved dataset of H2 flux measurements in two different ecosystems in central Scotland—a managed grassland and a deciduous woodland. It addresses a critical data gap in H2 biogeochemistry that constrains accurate projection of atmospheric H2 levels in a changing world. Deposition velocities reported for both sites indicate net uptake, although with greater spatial variability in the woodland site. While uptake was primarily controlled by soil moisture at the grassland site, no significant driver was identified at the woodland site, underscoring critical knowledge gaps in H2 cycling across soil and ecosystem types. The paper also presents a customized flux chamber protocol and regression analyses for measuring rapid H2 fluxes. The flux data as well as the measurement/analyses protocols are valuable contributions to the field of H2 biogeochemistry.
Specific comments:
Lines 55-56: “The soil H2 sink is caused by microbial activity, both under aerobic and anaerobic condition” clarify that the soil sink for atmospheric H2 (which is typically what term “soil sink” refers to) occurs under aerobic conditions.
Lines 60-61: The referenced paper does not discuss “the high H2 demand of microbes”, rather I believe the implication of their findings is that the large capacity of soils for H2 uptake is primarily constrained by diffusive limitation.
Lines 61-62: Clarify that such measurements exist, but are sparse/few in number (e.g., Conrad and Seiler, 1985; Yonemura et al., 1999, 2000a, 2000b; Smith-Downey et al., 2006, Smith-Downey 2008, Lallo et al., 2009, Khdhiri et al., 2015, etc.)
Lines 68-71: The second part of this sentence does not seem tied in with the idea of temperature controlling enzyme activity.
Lines 70-72: This appears to be an inaccurate/incomplete discussion of the interaction between the soil sinks of CH4 and H2. The authors seem to be referencing H2 dynamics in methanogenic environments, as suggested by the cited reference. If such environments, H2 is not competing with CH4 as the energy source for microbes but is instead being consumed by methanogens to produce CH4 (i.e., CH4 is the product of metabolism, not a direct competitor for H2). This is likely not directly relevant to the atmospheric H2 sink, except insofar as H2 emissions from such environments could contribute to atmospheric H2 levels. In terms of H2 vs. CH2 specifically as energy sources with implications for the soil sinks, a more relevant discussion would be the interaction between high-affinity H2 oxidizers and methanotrophs in aerated soils.
Line 73: Specifically, spatial variation in microbial diversity.
Line 255-260: Looks like total carbon varies between 2.2-4% at the woodland site (Fig. S4)—does this really indicate high spatial variability? Which figure or table compares the variability in C and N at the chamber-scale vs at the plot scale?
Line 295-296: Suggestion to clarify that the “increase” is referring to the difference between measured and predicted uptake rates. Also reference Table 4 (this should be called Table 3?).
Line 303: Table 3 or 4?
Lines 347-350: Perhaps, it is worth emphasizing that while both soils have similar porosity values (based on Table 1), the difference in texture implies variations in pore size distribution and connectivity, leading to different sensitivities to moisture changes.
Lines 350-351: Minor suggestion, but it may be helpful to rephrase this sentence to clarify that it’s the “well-drained” property of the soil that’s providing “ideal conditions” for H2 uptake, and not the litter lying on top. (The litter layer being thin is possibly contributing to this effect).
Figure S1: This figure is supposed to show the deposition velocity, but it looks identical to Fig. 2. The y-axis label also says “H2 flux” not “deposition velocity”
Citation: https://doi.org/10.5194/egusphere-2024-3654-RC1 -
RC2: 'Comment on egusphere-2024-3654', Anonymous Referee #2, 20 Jan 2025
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Cowan and coauthors present valuable field measurements of H₂ soil uptake from two UK ecosystems: grassland and forest. The study combines gas chamber flux measurements with soil temperature and moisture data, two key abiotic drivers that serve as proxies for biotic H₂ uptake. These field measurements are urgently needed to constrain the global H₂ budget, particularly the soil sink, the most uncertain term in the global budget. The work hence holds significant implications for global H2 modeling and net-zero scenarios.
Overall, I am highly enthusiastic about the authors’ field campaign and the potential impact of their dataset. However, while I understand the authors’ decision not to include a model-data comparison —which I believe could provide additional value— I have concerns regarding some aspects of the data analysis. I outline these concerns below, along with suggestions for improving the presentation of the results.
Main issue: decoupling of temperature and moisture.
The authors analyze temperature and moisture effects on H₂ uptake as independent variables, as evidenced by Figure 3, which separately plots uptake against temperature (panel a) and moisture (panel b). This approach obscures the joint effects of these variables, as it is unclear what range of soil moisture corresponds to each temperature data point in panel b (likely contributing to the large scatter).
Decades of experimental and modeling studies (since Conrad’s work in the 1980s) demonstrate that temperature and moisture act jointly on H₂ uptake. At least a contour plot showing uptake as a function of both variables (e.g., Smith-Downey et al., GRL, Figure 4) would clarify the interplay between them.
This issue also extends to the polynomial fit presented in Figure 4, where the results imply additive effects of temperature and moisture (uptake = f(moisture) + g(temperature)). However, these variables typically interact multiplicatively (uptake = f(moisture) × g(temperature)). Incorporating this distinction is critical. While I acknowledge the authors’ decision not to use uptake models for their analysis, I recommend reviewing relevant studies (e.g., Smith-Downey 2006, Ehhalt et al. 2011, 2013, Tellus B; Bertagni et al. 2021, GBC) to inform the interpretation of their results.
Definitions and variables.
There is some confusion in the definition of equations and variables.
Concentration Decay. The authors model the decay with an exponential law, a solution of a linear rate closure. It is known that H2 decays with this behavior, which allows us to define the deposition velocity. Because there is an equilibrium concentration at the end of the experiment (Ceq), the differential equation is simply:
dC/dt = - k (C-Ceq)
to be solved with the initial condition C=C0 at t=0. Notice that dC/dt cannot be defined as a constant as it varies in time (linearly, not nonlinearly). What is nonlinear is the concentration C(t). The initial rate used in the deposition velocity could be indicated as (dC/dt)t=0. The references to Hutchinson and Mosier (1981) and HM/HMR models (acronyms not defined) seem unnecessary; this is a simple first-order closure for the rate. Cmax is an unfortunate definition if it is the (minimum) value at equilibrium (proposing Ceq?).
Soil moisture. The authors claim to report results as water-filled pore space (WFPS), defined as the volume of water per volume of pore space. However, the reported values (always below 0.5) suggest they may have used volumetric soil water content (volume of water per volume of soil). I support the Authors’ choice of using the WFPS, but I think those values should be divided by the porosity.
Additionally, it is unclear the range of soil moisture values encountered by the authors on the different days of measurements. How much of the spread of H2 uptake measurements in a single day of measurements is due to soil/soil moisture heterogeneity? I believe this critical aspect should be addressed, maybe with an additional graphical element.
Additional comments.
The measurements of CH4 and N2O seem somewhat tangential to the story. In principle it is interesting to have coupled measurements of direct and indirect greenhouse gas. In practice, the reader learns that there are these measurements on page 5, and the connection with H2 fluxes struggles to appear in the next pages.
I would suggest defining the classes of the soils: in USDA, the two soils are clay (grassland) and sandy clay loam (forest). Soil texture is fundamental in the biogeophysical processes of H2 uptake, including moisture dynamics and soil H2 diffusion (e.g., Bertagni et al., 2021).
Minors
In the keywords, I would suggest having hydrogen rather than methane and carbon
Line 11. Although H2 might be present geologically, it is usually referred to as an energy carrier rather than a source.
Line 14. Within soils?
Line 18. Nonlinear fit i.e., exponential?
Line 44. Add stratospheric to water vapor.
Line 54. Although both H2 sinks are uncertain, the soil sink is much more uncertain.
Line 55. As I understood, most atmospheric H2 uptake would happen in aerobic conditions.
Line 62-68. The authors are right that soil porosity and structure (together) greatly influence H2 uptake, especially regarding H2 diffusivity in the soil (Bertagni et al., 2021).
Lines 70-72. I am no microbiologist, so take this as a discussion comment, but I am unsure that H2 and CH4 are competing energy sources for soil microbes.
Paragraph lines 76. I would add that the only extensive H2 uptake dataset with coupled measurements of soil moisture and temperature is Meredith et al., 2016. Although at a lower temporal resolution, your measurements are a critical addition to what we have in terms of complete data (very little!).
Section 2.1 It would be very helpful to have a map, or at least indications, about the spatial extension of the campaigns. How far were the point measurements taken? This is somewhat related to the comment on moisture variability.
Lines 208-210. Clarification is needed: is the deposition velocity negative for H2 emissions?
Lines 208–210: Does the deposition velocity become negative for H₂ emissions?
Figure 2: Since these data are not equally distributed in time, you may consider using an x-axis that reflects the actual temporal spacing of measurements.
Figure 3: This is, in my view, the most important figure in the paper, but it could benefit from significant improvement. Refer to my earlier comments on representing temperature and moisture interactions more effectively.
Lines 310–311: The phrase “While dealing with the exponential non-linearity of the rate of change” is wrong. While the concentration follows an exponential (and nonlinear) decay, the rate of change itself is linear, as discussed earlier.
Lines 320–321: Consider adding “unknown microbial and geological processes” to account for the possibility of abiotic H₂ production pathways.
Lines 342–350: I agree that soil types (clay and sandy clay loam) are critical factors. Coupling these measurements with existing models could help determine if soil type alone explains the observed variability. However, it’s also possible that differences in biotic constants may need to be accounted for.
Lines 366-367. Is this due to H2 leakage? This a very interesting aspect that you may want to highlight, given the interest in H2 containment/leakages.
Line 408: Besides soil aeration, I suggest discussing H₂ diffusion, as it plays a crucial role in the uptake process.
Citation: https://doi.org/10.5194/egusphere-2024-3654-RC2
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