Online Determination of 18O Fractionation Between CO2 and Soil-Water during Soil Dessication by a Novel Mid-Infrared CO2 Isotope Analyzer Coupled to an Dynamic Chamber Incubation System

Abstract:

The stable oxygen isotope composition of CO₂ is an important tracer for quantifying gas interactions between soils and atmosphere. Soils impact atmospheric ¹⁸O-CO₂ signatures by CO₂-H₂O equilibration during diffusion of CO₂ through the soil column. However, recent research has revealed that also catalytic reactions by carbonic anhydrase, an enzyme used by microorganisms for triggering the conversion of CO₂ and water to bicarbonate and protons, is an important factor influencing the oxygen isotopic signature of CO₂.

In order to study the importance of biotic and abiotic factors for ¹⁸O-CO₂, we used a novel mid infrared ¹⁸O/¹³C-CO₂ analyser coupled to a dynamic chamber system, which allowed us to measure online ¹⁸O and ¹³C of a continuous CO₂ stream percolating through soil samples while drying out from fully water saturated to air dry.

Our results indicate that changes in CO₂- ¹⁸O signatures peak at certain soil moistures levels, which is most probably catalysed by the activity of certain microbial groups under optimum growth conditions.

More analyses with different soil types and depths, combined with molecular analyses are planned in order to understand the importance of microbial processes and dynamics for influencing soil-CO₂ interactions.