O₂:CO₂ exchange ratio and isotopic composition of atmospheric O₂ measured using soil chambers, and their application to evaluating enhanced rock weathering and the Dole–Morita effect

Abstract. Soil chamber measurements of δ(O₂/N₂), CO₂ amount fractions, and the δ(¹⁸O) of O₂ in air (δ_atm(¹⁸O)) were conducted at a forest site in Takayama (TKY), Japan, and at agricultural fields in Tsukuba (TKB) and Miyakojima (MYK), Japan. The latter fields included plots with and without crushed rock application for the evaluation of enhanced rock weathering (ERW) for carbon dioxide removal (CDR). The 7-year average O₂:CO₂ exchange ratios for soil–air O₂ and CO₂ fluxes were 1.10±0.01 at TKY, whereas the two-year averages at the agricultural fields varied from 0.38 to 4.32. Assuming soil respiration yields an O₂:CO₂ ratio of 1.1, we partitioned CO₂ fluxes into soil respiration and abiotic reactions including CO₂ dissolution and dissociation. At TKB, the low CO₂ emissions from the basalt-applied plot were attributed to reduced soil respiration. In contrast, at MYK, both plots showed abiotic CO₂ uptake, comparable to modeled CDR rates for ERW, regardless of basalt/olivine application. These results suggest that inherent soil processes such as alkalinity-driven CO₂ absorption and leaching, rather than rock application, were responsible for the observed net CDR, questioning the impact of ERW at these sites. Our analytical approach effectively quantified CDR by accounting for both biotic and abiotic processes. The isotopic effect of soil respiration (ε_SR) from δ(O₂/N₂) and δ_atm(¹⁸O) was substantially lower at TKB and MYK than previously reported. Revising the plausible global average eSR to 14.6 ‰ would reduce the modeled Dole–Morita effect from 23.16 ‰ to 22.71 ‰.