The O-18 content of atmospheric O-2 is an important tracer for past changes in the biosphere and has been used to estimate changes in the balance between terrestrial and marine productivity. Its quantitative use depends on knowledge of the isotopic fractionations associated with the various O-2 production and consumption processes. Here we monitored oxygen concentration and delta O-18 of O-2 in sandy and clayey soils to evaluate in situ O-18 fractionation associated with soil respiration. In the clayey soil, O-2 concentrations decreased as low as 1% at 150 cm depth, and delta O-18 values ranged from 0 parts per thousand to -1.6 parts per thousand relative to atmospheric O-2. In the sandy soil the O-2 concentration was 20.38-20.53%, and delta O-18 values were -0.06 +/- 0.015 parts per thousand to 0.06 +/- 0.015 parts per thousand relative to atmospheric O-2. Using the observed [O-2] and delta O-18 profiles and their change with time, together with a one-box analytical model and a five-box numerical model, a mean discrimination of 12 +/- 1 parts per thousand was estimated for the two sites (including effects of concentration and temperature gradients). This low discrimination was consistent with that determined in closed-system soil incubation experiments (8.4-16.9 parts per thousand). The current understanding of the composition of air O-2 attributes the magnitude of the fractionation in soil respiration to biochemical mechanisms alone (about 18 parts per thousand and 25-30 parts per thousand in cyanide-sensitive and cyanide-resistant respiration, respectively). The low discrimination we report is significantly less than in dark respiration and is explained by diffusion limitation in soil aggregates and root tissues that results in low O-2 concentration in the consumption site. Soil respiration is a major component of the global oxygen uptake, and the potential contribution of low discrimination, such as observed here, to the global Dole effect should be considered in global-scale studies.
