A differential CO₂ profile probe approach for field measurements of soil gas transport and soil respiration^#

Abstract

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.

Small lateral air pressure gradients generated by a large chamber system have a strong effect onCO2transport in soil

“…The gas exchange between soil and atmosphere inside the chamber has been monitored by two CO 2 profile probes with built‐in sensors, as described by Osterholt et al (2022). The probes measure CO 2 concentrations continuously in seven depths (3.5, 7, 10.5, 14, 17.5, 21 and 24.5 cm) underneath the chamber.…”

The Spatiotemporal Dynamic Impact Mechanism of Soil Greenhouse Gases under Precipitation Based on Environmental Law

“…The adjacency matrix is attributed to the set of edge lengths. To reduce the number of parameters and increase the speed of the model calculation, the adjacency matrix will be shared in the network [25][26][27]. Equation (3) yields the sensor node connectivity definition.…”

Dynamics of Soil CO2 Efflux and Vertical CO2 Production in a European Beech and a Scots Pine Forest

“…Soil temperature and soil moisture are the main factors that influence soil CO 2 efflux, which is usually interpreted as instantaneous soil CO 2 production, which is not always true (Maier et al, 2011). Nevertheless, the effect of these factors on the distribution of CO 2 production in a soil profile has been little studied since it requires partitioning vertical CO 2 production, e.g., by applying the flux-gradient method (FGM) (De Jong and Schappert, 1972;Tang et al, 2003;Maier and Schack-Kirchner, 2014;Osterholt et al, 2022).…”