O 2 :CO 2 exchange ratio and isotopic composition of atmospheric O 2 measured using soil chambers, and their application to evaluating enhanced rock weathering and the Dole–Morita effect

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Abstract. Soil chamber measurements of δ(O2/N2), CO2 amount fractions, and the δ(18O) of O2 in air (δatm(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 (εSR) from δ(O2/N2) and δatm(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 ‰.

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