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Background: Gas exchange between soil and atmosphere is of great importance for greenhouse gas cycles. As gas transport in soil is generally dominated by diffusion, the soil gas diffusion coefficient (D S ) is crucial to understand fluxes between soil and atmosphere.Estimating D S is still a great source of uncertainty when calculating soil gas fluxes such as soil respiration from soil gas profiles. In situ measurement of the effective exchange coefficient (D eff ) not only reduces this uncertainty, but also allows to quantify non-diffusive transport processes in addition to the purely diffusive exchange (D S ), which cannot be investigated by laboratory measurements or the application of soil gas diffusivity models.Even though several methods for in situ D eff measurement exist, they often lack in the temporal resolution to identify short-term effects on D eff or require laborious set-ups, which makes them unsuitable for a fast and mobile application.Aims: Our objective was to test an improved profile probe for model-based soil gas flux analyses that allows in situ monitoring of (1) soil CO 2 profiles with high temporal resolution, (2) soil gas transport coefficients, including non-diffusive transport processes, (3) soil-atmosphere CO 2 flux, and (4) soil respiration profiles. Methods:We developed a CO 2 profile probe with build-in sensors that can easily be installed in soil to gain continuous CO 2 concentration profiles. The probe includes the option to inject CO 2 as a tracer gas to estimate D eff . To account for changes in natural CO 2 concentrations in the soil, we tested two approaches: firstly, a differential approach using two probes, an injection probe and a reference probe, and secondly, a stand-alone approach in which changes in natural CO 2 concentrations are estimated by a statistical model using its main environmental drivers. The resulting tracer gas profiles were used to fit a finite element gas diffusion model to derive D eff . Using the derived D eff values and the CO 2 profiles allowed calculating CO 2 fluxes. The approach was tested with controlled laboratory experiments using different mineral substrates to compare the diffusivity estimates of the in situ method with laboratory measurements on soil cores. Additional laboratory experiments included artificial CO 2 sources to simulate soil respiration in order to evaluate the gradient-based estimation of soil respiration profiles. In a second step, both approaches were tested under natural conditions in the field.

This paper reports statistical relationships between measured airflow, air pressure fluctuations, and the wind-induced motion of planted Scots pine trees (Pinus sylvestris L.). The results presented illustrate the potential of low-cost, ground-based air pressure measurements for monitoring wind-induced tree response. It is suggested that air pressure fluctuations can be used as surrogate information for above-canopy airflow, often used to estimate wind loads on forest trees. We demonstrate that air pressure fluctuations can be measured representatively at the forest floor and correlate very well with wind speed and direction at mean canopy-top (18 m a.g.l.) and above the 18 m high, 56-year-old forest. Their strong correlation (coefficient of determination R2 > 0.77) allows a good approximation of airflow conditions above the canopy, and, with some limitations, in the below-canopy space. Air pressure fluctuations also correlate very well with wind-induced tree motion with a similar correlation to that between wind speed and tree motion. Furthermore, the main directions of wind-induced tree motion agree very well with the propagation direction of air pressure waves. Above-canopy airflow measurements in forests with a large vertical extent are rare, and often require tall wind measurement towers. Therefore, we consider the estimation of airflow conditions over forests using ground-based air pressure measurements a promising option for monitoring the airflow conditions of relevance for predicting wind-induced tree response over large areas using a minimum of measurement infrastructure.

<p>Gas fluxes between soil and atmosphere play an important role for the global greenhouse gas budgets. Several methods are available to determine soil gas fluxes. Besides the commonly used chamber methods the gradient method becomes more and more important. Chamber methods have the disadvantage that the microclimate can be influenced by the chamber which can affect gas fluxes. This problem does not occur with the gradient method. Furthermore the gradient method has the advantage that it can provide information about the depth profile of gas production and consumption in the soil.</p><p>The concept of the gradient method is to calculate gas fluxes by the vertical concentration gradient of a gas in the soil. For the calculation of the flux the effective diffusivity coefficient of the soil is needed. This can be approximated by models or by lab measurements. However, both of these approaches often fail in explaining site specific characteristics and spatial variability. Another way to determine soil gas diffusivity is to apply the gradient method using a tracer gas. By the injection of a tracer gas with known flux soil gas diffusivity can be measured in-situ.</p><p>We developed an innovative sampling set-up to apply an improved gradient method including the possibility to determine soil gas diffusivity in situ. We designed a sampler with build-in CO<sub>2</sub> sensors in multiple depths that can easily be installed into the soil. With this sampler CO<sub>2</sub> concentrations can be measured continuously in several depths. This enables the identification of short-time effects such as the influence of wind-induced pressure pumping on gas transport. The sampler allows tracer gas injection into the soil for in-situ diffusivity measurement. We decided for CO<sub>2 </sub>as a tracer gas because it can be measured with small sensors which keep the set-up simple. To account for the natural CO<sub>2</sub> production in the soil we developed a differential gas profile approach. Using an additional reference sampler allows measuring the natural CO<sub>2</sub> gradient without the tracer signal, and thus subtracting the tracer CO<sub>2</sub> signal from the natural CO<sub>2</sub> signal.</p><p>The sampler consists of one 3D print segment per depth each containing one CO<sub>2</sub> sensor. These parts can be combined to a sampler with flexible amount of measurement depths. The construction with individual segments allows a better maintenance in case of sensor defects. For the installation of the sampler a hole has to be drilled, into which the sampler is inserted. To prevent gas bypassing along the wall of the drill hole we equipped each segment with an inflatable gasket between the measurement locations.</p><p>In a next step we will evaluate the sampler and test it in the lab and under different environmental conditions. We expect that with this sampler we will be able to run gas transport experiments in the field with a high temporal resolution and relatively low effort.</p><p><em>Acknowledgements</em></p><p><em>We thank Alfred Baer and Sven Kolbe for the technical support.</em></p>

<p>Soils are important terrestrial biological reactors and play a central role in the global carbon (C) and nitrogen (N ) cycle. Soils can store large amounts of C and N, but they also can be a major source (or sink) of greenhouse gases. The highest C and N concentrations are usually found in the topsoil, which is also the biologically most active soil layer, and the origin of most soil respiration. Subsoil (>0.5m depth) usually has lower C and N contents, and the contribution to the soil surface gas fluxes, e.g. soil respiration is low. Nevertheless, the total amount of C and N stored in the subsoil (e.g. 0.5-3m) can be large. Slow changes due to global climate change (e.g. in subsoil moisture or temperature) might affect subsoil respiration, i.e. subsoil C mineralization, and thus, might have a substantial long-term effect on subsoil C and N storage.</p> <p>While gas fluxes from soil surfaces are usually measured by chamber methods or the Eddy-covariance method, these methods are not suitable to assess subsoil gas fluxes. The gradient method allows calculation of gas fluxes in a soil profile, that means also in the subsoil, based on a measured soil gas profile and a known soil gas diffusivity (Maier & Schack-Kirchner, 2014). Estimating the latter is a major challenge, especially in subsoils, and the (unreflective) application of a general soil gas diffusivity model without prior knowledge of the soil physical characteristics of the subsoil can result in large uncertainties.</p> <p>We present soil CO<sub>2</sub> data from a deep soil profile (1m) of a forest site (Jochheim et al., 2022) from which we chose special and typical situations of daily CO<sub>2</sub> cycles at different soil depths. We used time-dependent Finite Element Modelling (COMSOL) to run different scenarios to investigate the phase shift and damping of diurnal CO<sub>2 </sub>cycles in the atmosphere/topsoil and subsoil, which allows to derive soil gas diffusivity of the subsoil. We tested the susceptibility of the approach to misinterpretation due to possible inaccurate assumptions by further scenarios. To evaluate the effect on the derived subsoil gas flux, we will use diffusivity values from this new in situ approach and known general soil gas diffusion models as well.</p>