Groundwater changes affect soil CO2 dynamics

Soils play a central role in the global carbon (C) cycle and can be a major source or sink of greenhouse gases. The highest concentrations and turnover of soil C is typically found in the top soil (0-0.5m depth). Yet, the total amount stored of soil organic carbon (SOC) in the subsoil (e.g. 0.5-3m) can be large and could be mobilized when soil environmental conditions change. These could be slow changes due to global climate change e.g. in subsoil moisture or temperature affecting subsoil respiration, but also more abrupt changes e.g. subsoil SOC mineralization after a decline in the groundwater level when prior submerged SOC gets exposed to higher O₂ levels.

Our objective was (1) to study the effect of a changing groundwater level on soil CO₂ concentration dynamics and (2) to test if the gradient method is a suitable tool to identify subsoil respiration effects.

For this pilot study we used a multilevel soil CO₂ profile probe that allowed simultaneous online monitoring of soil CO₂ concentration at different depth (Osterholt et al 2022), and calculation of the depth distribution of the soil respiration profile based on the gradient method (Maier & Schack-Kirchner, 2014). The CO₂ probe and additional sensors were installed in a large (3 m diameter) lysimeter field station with sandy soil where the groundwater table was kept constant for >10 years at 0.8m depth. During the study period the ground water table was changed several times between 0.3-1.0m depth directly inducing effects on the CO₂ concentration itself (piston flow) and also on the soil respiration profile (enhanced mineralization after groundwater level drops), which would not have been observed by pure chamber measurements at the surface.

 

Osterholt, L.; Kolbe, S.; Maier, M. (2022): A differential CO₂ profile probe approach for field measurements of soil gas transport and soil respiration #. In J. Plant Nutr. Soil Sci. 185 (2), pp. 282–296. DOI: 10.1002/jpln.202100155.

Maier, M., & Schack-Kirchner, H. (2014). Using the gradient method to determine soil gas flux: A review. Agricultural and Forest Meteorology, 192–193, 78–95. https://doi.org/10.1016/j.agrformet.2014.03.006