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    Author(s): Claire L. Phillips; Nick Nickerson; David Risk; Zachary E. Kayler; Chris Andersen; Alan Mix; Barbara J. Bond
    Date: 2010
    Source: Rapid Communications in Mass Spectrometry. 24(9): 1271-1280
    Publication Series: Scientific Journal (JRNL)
    Station: Pacific Northwest Research Station
    PDF: Download Publication  (861.31 KB)


    The carbon isotopic composition (δ13C) of recently assimilated plant carbon is known to depend on water-stress, caused either by low soil moisture or by low atmospheric humidity. Air humidity has also been shown to correlate with the δ13C of soil respiration, which suggests indirectly that recently fixed photosynthates comprise a substantial component of substrates consumed by soil respiration. However, there are other reasons why the δ13CO2 of soil efflux may change with moisture conditions, which have not received as much attention. Using a combination of greenhouse experiments and modeling, we examined whether moisture can cause changes in fractionation associated with (1) nonsteady-state soil CO2 transport, and (2) heterotrophic soil-respired δ13CO2. In a first experiment, we examined the effects of soil moisture on total respired δ13CO2 by growing Douglas fir seedlings under high and low soil moisture conditions. The measured δ13C of soil respiration was 4.7% more enriched in the low-moisture treatment; however, subsequent investigation with an isotopologue-based gas diffusion model suggested that this result was probably influenced by gas transport effects. A second experiment examined the heterotrophic component of soil respiration by incubating plant-free soils, and showed no change in microbial-respired δ13CO2 across a large moisture range. Our results do not rule out the potential influence of recent photosynthates on soil-respired δ13CO2, but they indicate that the expected impacts of photosynthetic discrimination may be similar in direction and magnitude to those from gas transport-related fractionation. Gas transport-related fractionation may operate as an alternative or an additional factor to photosynthetic discrimination to explain moisture-related variation in soil-respired δ13CO2.

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    Phillips, Claire L.; Nickerson, Nick; Risk, David; Kayler, Zachary E.; Andersen, Chris; Mix, Alan; Bond, Barbara J. 2010. Soil moisture effects on the carbon isotope composition of soil respiration. Rapid Communications in Mass Spectrometry. 24(9): 1271-1280.


    decomposition, soil respiration, carbon dynamics, carbon dioxide

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