Analysing the relationship of CO2 and O2 concentrations and flux patterns in forest soils

Soils act as bioreactors for the production and consumption of different gases. CO₂ is usually produced in soils due to the oxidation of organic material. Under aerobic conditions, this production is coupled to a consumption of O₂ resulting in concentration profiles that increase with depth for CO₂ and decrease for O₂. Depending on the organic material present, the exchange of O₂ and CO₂ is approximately equimolar in well aerated soils. This can be deduced from vertical gradients of both gases which should reflect the ratio of their diffusion coefficient (Massmann 1998). The ratio between the CO₂ and O₂ flux is often called the respiratory coefficient. However, certain soil types or conditions may invoke anaerobe processes that may lead to a decoupling of CO₂ production and O₂ consumption. Such a decoupling can also result from oxidation of minerals or dissolution and relocation of carbonates.

Here we present long-term data of soil CO₂ and O₂ concentrations from forest sites in South West Germany. Gas samples were collected passively starting 1998 until now using permanently installed gas wells at different depths. The samples were then analysed using gas chromatography for CO₂ and O₂ (and additionally N₂, Ar, N₂O, CH₄, and C₂H₄).

CO₂ and O₂ fluxes were calculated using the gradient approach (Maier et al 2020). At sites with well aerated soils, the observed CO₂ and O₂ fluxes followed a clear linear relationship, with high effluxes of CO₂ corresponding to high influxes of O₂. The exchange was furthermore approximately equimolar with the calculated fluxes following a -1:1 trend.

We will compare these data from well aerated soils to concentration data of CO₂ and O₂ from less well-aerated soils with temporally suboxic conditions to further analyse the respiratory coefficient under oxygen limited conditions. Furthermore, diffusion-coefficient-normalised gradients are calculated to obtain information about the stoichiometry of the production and consumption patterns involved.

 

Literature:

Maier M, Gartiser V, Schengel A, Lang V. Long Term Soil Gas Monitoring as Tool to Understand Soil Processes. Applied Sciences. 2020; 10(23):8653.

Massman, W J. A review of the molecular diffusivities of H₂O, CO₂, CH₄, CO, O₃, SO₂, NH₃, N₂O, NO, and NO₂ in air, O₂ and N₂ near STP. Atmospheric Environment 1998; 32(6), 1111–1127