Quantifying and understanding fluxes of methane (CH4) and carbon dioxide (CO2) in natural soil–plant–atmosphere systems are crucial to predict global climate change. Wetland herbaceous species or tree species at waterlogged sites are known to emit large amounts of CH4. Upland forest soils are regarded as CH4 sinks and tree species like upland beech are not known to significantly emit CH4. Yet, data are scarce and this assumption needs to be tested.

We combined measurements of soil–atmosphere and stem–atmosphere fluxes of CO2 and CH4, and soil gas profiles to assess the contribution of the different ecosystem compartments at two upland beech forest sites in Central Europe in a case study. Soil was a net CH4 sink at both sites, though emissions were detected consistently from beech stems at one site. Although stem emissions from beech stems were high compared to known fluxes from other upland tree species, they were substantially lower compared to the strong CH4 sink of the soil. Yet, we observed extraordinarily large CH4 emissions from one beech tree that was 140% of the CH4 sink of the soil. The soil gas profile at this tree indicated CH4 production at a soil depth > 0.3 m, despite the net uptake of CH4 consistently observed at the soil surface. Field soil assessment showed strong redoximorphic color patterns in the adjacent soil and supports this evaluation. We hypothesize that there is a transport link between the soil and stem via the root system representing a preferential transport mechanism for CH4 despite the fact that beech roots usually do not bear aerenchyma. The high mobility of gases requires a holistic view on the soil–plant–atmosphere system. Therefore, we recommend including field soil assessment and soil gas profiles measurements when investigating soil–atmosphere and stem–atmosphere fluxes to better understand the sources of gases and their transport mechanisms.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jpln.201600405

Combining soil and tree‐stem flux measurements and soil gas profiles to understand CH4 pathways in Fagus sylvatica forests

A dual-gas sensor based on the combination of a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and an electronic hygrometer was realized for the simultaneous detection of methane (CH4) and water vapor (H2O) in air. The QEPAS sensor employed an interband cascade laser operating at 3.34 μm capable of targeting a CH4 absorption line at 2988.8 cm−1 and a water line at 2988.6 cm−1. Water vapor was measured with both the electronic hygrometer and the QEPAS sensor for comparison. The measurement accuracy provided by the hygrometer enabled the adjustment of methane QEPAS signal with respect to the water vapor concentration to retrieve the actual CH4 concentration. The sensor was tested by performing prolonged measurements of CH4 and H2O over 60 h to demonstrate the effectiveness of this approach for environmental monitoring applications.

https://www.mdpi.com/1424-8220/20/10/2935/pdf

Environmental Monitoring of Methane with Quartz-Enhanced Photoacoustic Spectroscopy Exploiting an Electronic Hygrometer to Compensate the H2O Influence on the Sensor Signal.

Abstract. Uptake (or negative flux) of nitrous oxide (N2O) in agricultural soils is a controversial issue which has proved difficult to investigate in the past due to constraints such as instrumental precision and methodological uncertainties. Using a recently developed high-precision quantum cascade laser gas analyser combined with a closed dynamic chamber, a well-defined detection limit of 4 μg N2O-N m−2 h−1 could be achieved for individual soil flux measurements. 1220 measurements of N2O flux were made from a variety of UK soils using this method, of which 115 indicated uptake by the soil (i.e. a negative flux in the micrometeorological sign convention). Only four of these apparently negative fluxes were greater than the detection limit of the method, which suggests that the vast majority of reported negative fluxes from such measurements are actually due to instrument noise. As such, we suggest that the bulk of negative N2O fluxes reported for agricultural fields are most likely due to limits in detection of a particular flux measurement methodology and not a result of microbiological activity consuming atmospheric N2O.

Investigating uptake of N2O in agricultural soils using a high-precision dynamic chamber method

Trees are known to emit methane (CH4 ) and nitrous oxide (N2 O), with tropical wetland trees being considerable CH4 sources. Little is known about CH4 and especially N2 O exchange of trees growing in tropical rain forests under nonflooded conditions. We determined CH4 and N2 O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400-yr-old tropical lowland rain forest situated on a basaltic lava flow (Reunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows. The stems studied were net sinks for atmospheric CH4 and N2 O, as were cryptogams, which seemed to be co-responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH4 . Their N2 O fluxes were negligible. Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH4 and N2 O. The volcanic tropical lowland rain forest appears to be an important CH4 sink, as well as a possible N2 O sink.

https://hal.univ-reunion.fr/hal-03047906/document

Trees as net sinks for methane (CH 4 ) and nitrous oxide (N 2 O) in the lowland tropical rain forest on volcanic Réunion Island.

High-precision differential air pressure measurements were conducted in the below-canopy space of a Scots pine forest and in the forest soil to investigate small air pressure fluctuations and their effect on soil gas flux. In addition to air pressure measurements, tracer gas concentration in the soil and airflow characteristics above and below the canopy were measured. Results suggest that air pressure fluctuations in the frequency range of 0.01 Hz–0.1 Hz are strongly dependent on above-canopy wind speed. While amplitudes of the observed air pressure fluctuations (<10 Pa) increase significantly with increasing above-canopy wind speed, the periods decrease significantly with increasing above-canopy wind speed. These air pressure fluctuations are associated with the pressure-pumping effect in the soil. A pressure-pumping coefficient was defined, which describes the strength of the pressure-pumping effect. During the measurement period, pressure-pumping coefficients up to 0.44 Pa·s−1 were found. The dependence of the pressure-pumping coefficient on mean above-canopy wind speed can be described well with a polynomial fit of second degree. The knowledge of this relation simplifies the quantification of the pressure-pumping effect in a Scots pine forest considerably, since only the mean above-canopy wind speed has to be measured. In addition, empirical modeling revealed that the pressure-pumping coefficient explains the largest fraction of the variance of tracer gas concentration in the topsoil.

https://www.mdpi.com/2073-4433/7/10/125/pdf

Analysis of Air Pressure Fluctuations and Topsoil Gas Concentrations within a Scots Pine Forest

2D profiles of CO2, CH4, N2O and gas diffusivity in a well aerated soil: measurement and Finite Element Modeling

From above the forest into the soil – How wind affects soil gas transport through air pressure fluctuations

Direct Observation of Wind-Induced Pressure-Pumping on Gas Transport in Soil

Summary
Soil aeration is central to the biogeochemistry of soil. Gas transport in soil is governed mainly by molecular diffusion and depends on the soil gas diffusion coefficient (DS). Several methods exist to determine this property based on field and laboratory measurements. These methods, however, are unsuitable for continuous monitoring of DS over time. Moreover, non-diffusive processes can affect gas transport in soil during certain situations that last several hours only and discontinuous measurements might fail to identify such processes. We developed a novel in situ method for the real-time monitoring of gas transport in soil. Helium (He) was injected continuously into the soil and the resulting steady-state gas profile was monitored. We used a gas sampling probe with very permeable membranes, which enables passive sampling of the soil gas at different depths. The DS profile was modelled by inverse finite-element modelling (FEM) of an exact geometrical model of the sampler and soil. Molecular diffusion was assumed to be the only process of gas transport. The method was tested in the laboratory with different granular materials and in two field studies. The DS values obtained with this new method agreed well with reference measurements from soil cores and diffusion models. Soil gas diffusivity was monitored over a few days that included a major rain event. During this period the effect of increasing soil moisture on gas transport in soil could be observed in real time. Our novel method is suitable for monitoring gas transport in soil over several days. This enables the monitoring and investigation of situations including non-diffusive transport processes that result from barometric pressure changes or strong wind events.

Highlights
We present a novel method for in situ monitoring of gas transport in soil.
Helium was continuously injected into the soil and its distribution in the soil modelled.
Soil gas diffusivity profiles determined in situ agreed well with laboratory measurements of DS.
Our novel method is ideal for the investigation of specific situations in gas transport in soil.

Thomas Laemmel

Papers

Analysis of Air Pressure Fluctuations and Topsoil Gas Concentrations within a Scots Pine Forest

2D profiles of CO2, CH4, N2O and gas diffusivity in a well aerated soil: measurement and Finite Element Modeling

From above the forest into the soil – How wind affects soil gas transport through air pressure fluctuations

Direct Observation of Wind-Induced Pressure-Pumping on Gas Transport in Soil

An in situ method for real‐time measurement of gas transport in soil