Influence of soil pore structure on the rate of microbial oxygen consumption

O₂ availability is one of the main factors influencing microbial processing of soil carbon and nitrogen and their cycling, and soil pore structure is what drives micro-scale patterns of O₂ availability. The diffusivity of O₂ is known to be a function of soil porosity and moisture content. However, the actual distribution of O₂ in the soil is a product of dynamic interactions between physical (O₂ diffusion) and microbial (O₂ consumption) processes and is influenced by the soil pore structure. Measurements of gas diffusivity can be achieved via several laboratory techniques, while the determination of O₂ consumption by microorganisms is challenging. The objectives of this study are, first, to propose a method for measurement of microbial O₂ consumption under steady-state conditions in saturated soil and near saturated soil, and, second, to quantify the rate of O₂ consumption in soil materials with contrasting pore structures but similar microbial compositions. The proposed method is based on Fick’s second law of diffusion, given as , where R(z) is an O₂ consumption term, C is the concentration of O₂, and D_s is the effective molecular diffusion coefficient of O₂. The equation was solved for R(z) under steady-state conditions (near saturated soil) where the flux (J)=0. Two soil materials with contrasting pore structures, namely dominated by > 30 μm Ø pores (i.e., large-pore soil) and by < 10 μm Ø pores (i.e., small-pore soil), were prepared. The O₂ profile was measured to the depth of 1 cm in the two materials under saturated and near-saturated conditions using O₂ microsensor (Unisense, Aarhus, Denmark). As expected, the O₂ diffusion was higher in large-pore soil as compared to the small-pore soil, however, the estimated rate of volumetric O₂ consumption was also higher in the large-pore soil as compared to the small-pore soil. This finding supports the notion that large pores provide a better micro-environment for soil microorganisms stimulating their activity with subsequent increases in O₂ consumption. Our ongoing work builds on these findings and explores the rate and spatial distribution patterns of O₂ diffusion and microbial O₂ consumption in soils with contrasting pore structures in the presence of plant residues.