Layered structure significantly inhibits CO2 transfer through the depositional profile: as simulated by well-mixed vs. interlaid soil columns

Abstract

Depositional profiles often feature sorted layers with stratified porosity and water retention, but the vertical partitioning of CO₂ production and transfer remain unclear. In this study, fine soil and coarse sand were refilled to form three layering patterns: Layer-Mix (soil and sand well-mixed), Layer-Thin (8 thin layers interlaid), and Layer-Thick (4 thick layers interlaid). Three doses of ¹³C-labelled glucose were respectively added to the top, middle, and bottom. The results show that the Layer-Thin and Layer-Thick cumulatively released 62% and 67% less CO₂ than the Layer-Mix. The ¹³C-CO₂ contributed 14.1~60.3% to the total CO₂ released from the Layer-Mix, but was only responsible for 7.3~48.8% of that from the Layer-Thin and 7.0~37.0% of that from the Layer-Thick. The peaks of δ¹³C-CO₂ of the two interlaid columns were lowered and lagged by 1~2 days, but the δ¹³C residue remaining in the soil were on average 3~6‰ more negative than that of the Layer-Mix. The ¹³C-CO₂ contributed more to the total CO₂ when the glucose was added at the top, but the δ¹³C-soil was 3‰ more negative when added at the bottom. Overall, the lagged outgassing and lower share of ¹³C-CO₂ from the two interlaid columns did not match with the more negative ¹³C residue remaining in the soil. Such inconsistency collectively highlights that the interlaid layers did not inhibit the decomposition of ¹³C-labelled glucose (i.e., potentially abundant CO₂ produced), but the low diffusivity of the fine layers significantly impeded CO₂ transfer through the heterogeneously structured soil profile.