Physical diffusion of isotopic tracers into and out of soil pores causes considerable uncertainty for the timing and magnitude of plant belowground allocation in pulse-labelling experiments.

Here, we partitioned soil CO2 isotopic fluxes into abiotic tracer flux (physical return), heterotrophic flux, and autotrophic flux contributions following 13CO2 labelling of a Swedish Pinus sylvestris forest. Soil CO2 efflux and its isotopic composition from a combination of deep and surface soil collars was monitored using a field-deployed mass spectrometer. Additionally, 13CO2 within the soil profile was monitored.

Physical (abiotic) efflux of 13CO2 from soil pore spaces was found to be significant for up to 48 h after pulse labelling, and equalled the amount of biotic label flux over 6 d. Measured and modelled changes in 13CO2 concentration throughout the soil profile corroborated these results.

Tracer return via soil CO2 efflux correlated significantly with the proximity of collars to trees, while daily amplitudes of total flux (including heterotrophic and autotrophic sources) showed surprising time shifts compared with heterotrophic fluxes.

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