The process of water vapor adsorption (WVA) by soil (i.e. water vapor movement from atmosphere to soil, forming liquid water on soil particles) is likely a substantial contributor to the water cycle in drylands. However, several gaps remain in our knowledge of WVA: (1) continuous in situ estimates of WVA are still very scarce; (2) the underlying mechanisms involved in its temporal patterns are still not well constrained, and (3) the understanding of its coupling with the carbon cycle and ecosystem processes remains at an incipient stage.

Here, we aimed to (1) identify periods of WVA and improve the understanding of the underlying mechanisms involved in its temporal patterns by using the gradient method (GM); (2) characterize a potential coupling between water vapor and CO₂ fluxes, especially expected in drylands due to the water-limitation of ecosystem processes. In particular, we assumed that the nocturnal soil CO₂ uptake increasingly reported in those environments (including at our study site) could come from WVA enhancing reactions with CaCO₃; (3) explore the effect of soil properties and biocrusts ecological succession on fluxes.

To this end, in the Tabernas Desert (Almería, Spain), we measured continuously during ca. 2 years the relative humidity and CO₂ molar fraction in soil and atmosphere, in association with below- and aboveground variables, in microsites representative of the biocrusts ecological succession. We estimated water vapor and CO₂ fluxes with the GM, and cumulative fluxes over the study. Then, we used linear and non-linear statistical modelling to explain relationships between variables.

Our main findings are (1) WVA during hot and dry periods, and a new insight into the micrometeorological conditions triggering those fluxes; (2) a diel coupling between water vapor and CO₂ fluxes (including the uptake of both gases by soil at night) and between cumulative fluxes, well predicted by our models; and (3) cumulative CO₂ influxes increasing with specific surface area in early succession stages, thus mitigating CO₂ emissions. We suggest that the GM is a suitable approach to monitor WVA in-situ since it offers several advantages such as providing direct low-cost measurements of water vapor fluxes with good spatio-temporal resolution and low soil disturbance. Over a year, the WVA represented between ca. 0.2% and 2.8% of the precipitation amount, depending on the microsite and the diffusion model that was used to estimate the fluxes.

Therefore, WVA constituted a non-negligible input of liquid water in this dryland. In particular, during summer drought, as WVA was the main water source, it probably maintained ecosystem processes such as microbial activity and mineral reactions. We propose that the nocturnal CO₂ uptake reported in this dryland may arise from (i) WVA enhancing geochemical reactions involving CaCO₃ and/or biological dark CO₂ fixation; (ii) the co-adsorption of CO₂. Further research is now needed to (1) disentangle those processes; (2) monitor soil water vapor and CO₂ uptake by soils as those sinks could grow with climate change; (3) improve the accuracy of the water vapor fluxes estimated with the GM, for example by calibrating the GM with lysimeters.

How to cite: Lopez-Canfin, C., Lázaro, R., and Sánchez-Cañete, E. P.: The water vapor adsorption by dry soils potentially links the water and carbon cycles: insight from a semiarid crusted ecosystem , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2027, https://doi.org/10.5194/egusphere-egu22-2027, 2022.

Gas transport in the soil is dominated by molecular diffusion in the air-filled pore network. A study in the 1970s could show that Radon emissions from soil increased during the passage of a low-pressure system which temporarily enhanced soil gas transport rates (Clements & Wilkening, 1974). Enhanced wind speed near the soil surface was also found to speed up gas transport rates in the soil (Kimball & Lemon, 1971). Further studies followed confirming the observations that wind and substantial atmospheric pressure changes have the potential to affect soil gas transport, including studies conducted in snow and firn, deserts, forest soil, arid systems, and soils near water saturation. Especially during recent years, wind and air- pressure-related effects on soil gas transport received increasing attention, with diverse concepts and methodologies, and also a wider ecological relevance.

While the slow (hours) and relatively large atmospheric pressure changes (up to 50 hPa) reported in Clements & Wilkening (1974) cause a kind of steady piston flow in the soil, the effect in Kimball & Lemon, (1971) was explained as the result of dynamic wind-induced pressure fluctuations, which are much smaller in amplitude (2-20 Pa) and occur at higher frequencies (0.1-1.0 Hz). Although the effect of wind-induced pressure fluctuations on gas transport in the soil has been confirmed by a few studies, there is still only little knowledge about the underlying processes. Additional effects between the pure “static piston flow “and the dynamic pressure fluctuations certainly occur. Different approaches and methodologies were used to derive estimates for the impact (if quantified) of air pressure fluctuations on soil gas transport, which makes inter-study comparisons complicated and limits further progress.

We overview relevant studies, their methods, concepts and explanations to identify research gaps and develop a plan for further research concepts.

Clements, W. E., & Wilkening, M. H. (1974). Atmospheric pressure effects on 222 Rn transport across the Earth-air interface. Journal of Geophysical Research, 79(33), 5025–5029. https://doi.org/10.1029/jc079i033p05025

Kimball, B., & Lemon, E. (1971). Air Turbulence Effects Upon Soil Gas Exchange. Soil Science Societyof America Journal 35(1), 16–21. https://doi.org/10.2136/sssaj1971.03615995003500010013x

How to cite: Maier, M., Osterholt, L., and Schindler, D.: Blowing in the wind: a review of wind and air- pressure-related effects on soil gas transport , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8713, https://doi.org/10.5194/egusphere-egu22-8713, 2022.

The reactive transport of gas components in the subsurface significantly influences key biogeochemical processes. For instance, reactive transport of oxygen in soil influences mineral dissolution/precipitation and control pore water chemistry. The dynamics of such processes is affected by land-atmosphere interactions and controlled by the exchange processes occurring at the soil/atmosphere interface. One notable example is soil water evaporation that is driven by the exchange of water vapor and energy across the soil/atmosphere interface. This process creates a two-phase system in soil pores and induces a non-linear and complex distribution of the fluid phases (i.e., liquid and gaseous phase) and gas components in the individual phases. The spatiotemporal evolution of the fluid phases and the transport of gas components with and across the phases, in turn, exert important controls on key subsurface biogeochemical processes.

In this study, we explore the impact of evaporation on reactive transport of oxygen in soil using well-controlled laboratory experiments and numerical simulations. We performed a set of evaporation experiments in which an initially water saturated, anoxic soil column containing a layer of pyrite is exposed to a low-humidity atmospheric condition. This resulted in the formation of a partially saturated zone, the invasion of a drying front, and the penetration of oxygen into the porous medium, leading to oxidative dissolution of pyrite. In parallel, we also performed similar experiments under fully water-saturated conditions in order to compare the extent of mineral dissolution with and without evaporation. The spatiotemporal distribution of oxygen was measured using a non-invasive optode technique during the experiments and the concentration of dissolved reaction products (i.e., sulfate, iron and pH) was quantified at the end of the experiments. We developed a non-isothermal multiphase and multicomponent reactive transport model and applied the model to quantitatively interpret the experimental datasets and to understand the coupling between fluid displacement, component transport and geochemical processes.

How to cite: Ahmadi, N., Muniruzzaman, M., Battistel, M., and Rolle, M.: Multicomponent transport and geochemical reactions under evaporative conditions at the soil/atmosphere interface, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3975, https://doi.org/10.5194/egusphere-egu22-3975, 2022.

The flux-gradient method (FGM) is a versatile approach for modelling soil gas fluxes from concentration profiles. It is especially useful for continuous and long-term estimations of gas fluxes based on concentrations from permanently installed probes or sensors, focussing on relative changes and trends in time. However, there are inherent uncertainties in the parametrisation, e.g. diffusivity estimates or installation depths of probes. This can make it challenging to estimate absolute fluxes, as small differences in some parameters can lead to disproportionately high changes in the model output. The relative uncertainty of the input parameters can be assessed by multiple replicate measurements. However, further analysis often requires the use of a single value, where usually the mean or median value is used. Yet, the “effective” parameter value that best describes real-world conditions can deviate from a mathematically precise mean value, so that rather than one-size-fits-all, a range of values (e.g. mean ± standard deviation) should be considered. This can be solved by calibration of FGM models on the basis of reference measurements.

The FGM requires estimation of both, gas concentration gradients and diffusivity in the soil. Gas concentration can be measured relatively easily and consistently, whereas diffusivity is often harder to estimate reliably. One possibility is in-situ measurement using a tracer gas. However, due to relatively high cost and work requirement, diffusivity is often modelled from air-filled pore space (AFPS) instead, using soil-specific transfer functions (TF´s). Modelling soil gas diffusivity in turn requires several input parameters, including porosity, soil water content, temperature and barometric pressure. While modelling diffusivity can have satisfactory results when analysed in the laboratory on soil cores, there are far more challenges in the field, which eventually result in a mismatch between the concentration profiles, diffusivity, and modelled efflux. As a result, FGM-modelled efflux may have an offset compared to more reliable chamber measurements.

Hence, rather than following a one-size-fits-all approach, the inherent uncertainties of diffusivity modelling should be accepted and compensated for by finding effective values of input parameters that close the gap between concentration and diffusivity measurements. Here, we introduce a procedure to run a sensitivity analysis on FGM models to identify the most influential input parameters, as well as find a suitable model parametrisation of effective values. Input parameters of FGM models are varied within a range around the original value and several quality parameters are calculated from the comparison of the model output to reference flux measurements and to the original gas concentration profile. The parametrisation with the “best” quality parameters are then used as “effective” values for the enhanced final model. The process was developed on a dataset of continuous gas concentration measurements in forest soils and is now being applied to long-term datasets as well. This may enhance the quality of FGM models and in turn help to balance gas fluxes in soils.

How to cite: Gartiser, V., Lang, V., Osterholt, L., Jochheim, H., and Maier, M.: Beyond one-size-fits-all: Estimating effective soil physical parameters for gas flux modelling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10393, https://doi.org/10.5194/egusphere-egu22-10393, 2022.

Gas transport in soils is generally dominated by molecular diffusion. Yet, several studies showed that other factors such as wind-induced pressure-pumping can substantially enhance soil gas transport for a certain time. The underlying processes behind wind-induced enhancement of soil gas transport are very complex and there is an ongoing discussion about it. It has been observed that turbulence associated with high above-canopy wind speed generates pressure fluctuations that propagate into the air filled soil pore network. The resulting 2D pressure field travels in wind direction over the ground and generates lateral pressure gradients in the soil (Laemmel et al., 2019). We hypothesize that the 2D oscillation of the pressure gradient in the soil significantly contributes to the pressure-pumping effect (PPE) compared to a purely 1D pressure oscillation.

Previous studies relied on a monitoring of gas transport rates in the soil, which needed to cover calm and windy periods. In order to quantify PPE at different soils and to investigate the influence of 2D versus 1D pressure fields we develop a large mobile chamber system (approx. 2 x 4 m) with separated compartments to simulate dynamic 2D fields of pressure fluctuations in the field. By alternately pumping air in and out of the chamber sinusoidal pressure fluctuations can be generated. Pressure fluctuations in the different compartments can be set with a time-lag to create a lateral gradient between the compartments and thereby simulate 2D pressure fields.

Combined with automated chamber measurements and soil gas profile measurements inside the chamber system the influence of pressure-pumping on soil gas efflux can be investigated while the influence of other environmental drivers can be excluded. In the natural environment windy periods often coincide with other parameters like precipitation or temperature which also influence gas transport in soil. Excluding these factors could allow a clearer quantification of PPE. With this chamber system also the influence of wind speed directly above the ground in comparison to the influence of pure pressure-pumping could be investigated. Artificially simulating pressure-pumping has the advantage over the monitoring of natural pressure-pumping events that different scenarios can be run under controlled conditions and with replications. Additionally, artificially simulating pressure-pumping saves a lot of time since there is no need to wait for the right wind conditions. We believe that this set up will help to gain a better understanding of wind-induced pressure-pumping on a process level.

Literature:

Laemmel, T., Mohr, M., Schack-Kirchner, H., Schindler, D., & Maier, M. (2019). 1D Air Pressure Fluctuations Cannot Fully Explain the Natural Pressure-Pumping Effect on Soil Gas Transport. Soil Science Society of America Journal, 83(4), 1044-1053.

How to cite: Osterholt, L. and Maier, M.: Towards a better understanding of wind-induced pressure-pumping - a chamber system to simulate dynamic fields of pressure fluctuations  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9503, https://doi.org/10.5194/egusphere-egu22-9503, 2022.

Nitrous oxide (N₂O) emissions are notoriously variable at different spatial and temporal scales. As recognized in the literature, peaks in emissions of N₂O occur after fertilization, precipitation and freeze-thaw events. Although the individual microbial processes have been extensively studied, the understanding of the underlying mechanisms behind the pulse emissions is still subject to many uncertainties. The N₂O produced in connection with a rain event can either be entrapped in the soil matrix and be subject to N₂O reduction or be released later when soil diffusivity increases as water infiltrate into the soil or evaporate.

To understand the mechanisms behind the observed flux emissions related to precipitation events, we are conducting a laboratory experiment to quantify the N₂O movement in the soil. In 50 cm tall soil columns exposed to a simulated rain event, gas samples are extracted from the soil matrix at three depths via reinforced silicone tubes. At the surface, gas is sampled for flux estimates.

A common trigger of pulse emissions is a lowered soil oxygen content. Continuous monitoring of the soil oxygen with sensors at three depths provides measurements of O₂ dynamics in the soil simultaneously with the N₂O content. This can add to the understanding of how O₂ relates to N₂O production, reduction and movement. Tensiometers will additionally provide data on the soil water status during simulated precipitation events.

The experimental set-up can furthermore be used for studying the effects of other factors affecting N₂O movement and emission in soil e.g., soil types, type of fertilizers, soil temperature etc. 

How to cite: Hansen, L. V., Brændholt, A., Tariq, A., Jensen, L. S., and Bruun, S.: Nitrous oxide emission peaks and distribution of nitrous oxide in the soil profile during rain events: A soil column experiment, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4585, https://doi.org/10.5194/egusphere-egu22-4585, 2022.

Soils play an important role in the Earth's greenhouse gas cycle. The gas dynamics in soils are tightly coupled to gas dynamics in plants, trees, and surface waters. Riparian soils receive and process solutes leaching from upland areas and act as crucial buffers of land-use effects on various ecological and biogeochemical properties of surface waters. However, their role in greenhouse gas cycling is poorly understood. Forest clear-cutting often increases the leaching of organic carbon, nutrients and greenhouse gases in groundwater. Unfortunately, the fate of these substances on their way from upland clear-cut areas through riparian forest buffer zones left along streams after clear-cutting is unknown, but highly relevant for watershed-scale greenhouse gas budgets. Here, we performed a watershed-scale experiment to investigate the effect of clear-cutting on greenhouse gas dynamics in riparian forest buffer zones in a Swedish boreal headwater catchment. The experiment included weekly to monthly sampling during April-October before (2020) and after (2021) forest clear-cutting performed in February 2021, and included a treatment watershed and an untreated reference watershed. We measured concentrations of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in soils using gas probes installed at various depths within the zone of groundwater level fluctuations along four transects from the clear-cut area through riparian forest buffer zones to near-stream sites. We also measured fluxes of these gases between the atmosphere and the forest floor, as well as tree stems, using flux chamber techniques. Initial results suggest that the clear-cutting increased CO₂ and CH₄ concentrations in clear-cut soils and the center of riparian buffer zones, but not in near-stream sites. In contrast, the concentrations of N₂O in soils were not affected by forest clear-cutting across the full transects. In terms of greenhouse gas exchange with the atmosphere, the clear-cutting did not affect CO₂, CH₄ and N₂O fluxes at the forest floor. Tree stems were consistent emitters of CO₂ and CH₄ in 2021, but the clear-cut effect remains unclear due to missing reference data before the clear-cut. Together, these results suggest that the clear-cut induced excess of CO₂ and CH₄ in upland groundwater was likely consumed in riparian soils or emitted through tree stems, assuming that upland and riparian soils were hydrologically connected. Our results stress the potential importance of riparian buffer zones in mediating clear-cut effects on catchment-scale greenhouse gas budgets.

How to cite: Klaus, M., Machacova, K., Falk, A., Wallin, M., Soosaar, K., and Öquist, M.: Forest clear-cutting effects on greenhouse gas dynamics in riparian buffer zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4511, https://doi.org/10.5194/egusphere-egu22-4511, 2022.

As a reactive gaseous hydrocarbon, the phytohormone ethylene (Ethene, C₂H₄) influences root growth, senescence, and fruit ripening. While plants produce ethylene, microorganisms and fungi are also capable of degrading it. Ethylene therefore acts as an indicator for soil biological processes, but due to its reactivity it is hardly detectable in the atmosphere and soil air. In the 1970s to 1990s, studies were able to demonstrate that up to several ppm of C₂H₄ occur in soil under certain conditions. However, these studies were limited to laboratory experiments and have a limited transferabilty to undisturbed forest soils.

We investigated the occurrence of ethylene as well as the influencing environmental parameters in forest soils in southwestern Germany using long-term measurement series from the Forest Environmental Monitoring (ICP Forests), as well as from project studies over the past 30 years. In total, soil gas data were available from 24 sites covering a period from 1994 to 2021. Data from gas samplers were used which were installed at various soil depths, at which the soil gas concentration was determined at regular intervals.

The data analysis showed that ethylene in the forest soil very rarely reached the detection limit of our highly sensitive gas chromatography system and that the occurrence is not subject to a regular temporal pattern, but rather cluster in hotspots and hot moments. Ethylene is measured far more frequently under spruce than under deciduous trees. The observed tree species effect indicates a correlation between rooting intensity and ethylene occurrence, as revealed by the evaluation of the root profiles. Artificial soil compaction also leads to increased ethylene concentrations, whereas no effect of liming could be observed.

Thus, the extensive field measurements confirm the patterns known from laboratory studies and show that ethylene, despite its rare occurrence in forest soils, is potentially found at all sites. The accumulation of ethylene in soil air could be observed significantly more frequently in compacted soils than in well-aerated forest soils, where the faster exchange with ethylene free atmospheric air makes accumulation and thus detection difficult.

How to cite: Lang, V., Schneider, V., Schengel, A., Schäffer, J., Schack-Kirchner, H., and Maier, M.: Spotting C2H4 in forest soils- what influences the occurrence of the phytohormone? , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9183, https://doi.org/10.5194/egusphere-egu22-9183, 2022.

Greenhouse gas (GHG: CO₂, CH₄ and N₂O) concentrations continue to increase in the earth’s atmosphere and they are fully implicated in current global warming. There is a critical need to understand of the cause–effect relationships of GHG emissions and quantify their sources/sinks in the natural systems, as well as its main reservoirs and quantity. In particular, there is a need to understand and quantify GHGs within the vadose zone (as an unknown reservoir), because depending on its porosity it can store different amounts of these gases. The vadose zone, the space between the surface and the groundwater, has an important contribution to the global GHG due to both its high concentrations and the enormous capacity to store gases in its pore space.

At present, the measurements of these three GHGs have been widely studied mainly in the first few meters of the soil, not taking into account the transport and storage processes in deep areas. However, the study of the whole column of the vadose zone should not be neglected since it can make an important contribution to the global GHG balance.  

This study analyses GHG concentrations in the vadose zones of several aquifers of the Andalusian Mediterranean basins. For this purpose, air samples were taken from more than one hundred wells in a total of 22 aquifers with water table depths between 7-240 meters; samples were collected at different depths: 12.5, 25, 50, 100 and 200 meters and one sample was collected at the groundwater boundary; for these reasons, the number of samples per well varied depending on the depth to the water table. These samples and analyses provide profiles of GHG concentrations: with values for CO₂ between 103-75030 ppm, for CH₄ between 0.02-755 ppm and for N₂O between 0.31-1504 ppm. The ultimate objective of the project is to know the GHG  profile, the porosity, depth to the water table, groundwater chemistry and aquifer extension, to estimate underground GHG storage.

How to cite: Echeverría Martín, E., Kowalski, A. S., Serrano-Ortiz, P., and Pérez Sánchez-Cañete, E.: Analyzing CO2, CH4 and N2O Concentrations in the Vadose Zone of Several Aquifers of the South of Spain, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9993, https://doi.org/10.5194/egusphere-egu22-9993, 2022.

Agricultural managed aquifer recharge (Ag-MAR) is an emerging method for groundwater replenishment, in which farmland is flooded during the winter using excess surface water to recharge the underlying aquifer. Successful implementation of Ag-MAR projects requires careful estimation of the soil aeration status, as prolonged saturated (waterlogged) conditions in the rhizosphere can damage crops due to O₂ deficiency. We studied the soil aeration status under almond trees and cover crops during Ag-MAR at three sites differing in drainage properties. We used O₂ and redox potential as soil aeration quantifiers to test the impact of forced aeration compared with natural soil aeration. Forced aeration treatments included air-injection through subsurface drip irrigation, or dissolution of calcium peroxide powder (scattered on the soil surface before flooding). Our results suggest that forced soil aeration methods have an average increase of up to 2% O₂ compared to natural soil aeration. Additionally, only a minor impact on crop yield was observed between treatments for one growing season. Results further suggest that natural soil aeration can support crop O₂ demand during Ag-MAR if flooding duration is controlled according to O₂ depletion rates. According to this concept, we developed a simple model based only on soil texture and crop type, for estimating Ag-MAR flood duration with minimal crop damage.

How to cite: Ganot, Y. and Dahlke, H.: Natural and forced soil aeration during agricultural managed aquifer recharge (Ag-MAR), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10407, https://doi.org/10.5194/egusphere-egu22-10407, 2022.