You are here:
Soil moisture effects on the carbon isotopic composition of soil respiration
Citation:
Phillips, C. L., N. Nickerson, D. Risk, Z. Kayler, C. P. ANDERSEN, A. Mix, AND B. J. Bond. Soil moisture effects on the carbon isotopic composition of soil respiration. RAPID COMMUNICATIONS IN MASS SPECTROMETRY. John Wiley & Sons, Ltd., Indianapolis, IN, 24:1271-1280, (2010).
Impact/Purpose:
The carbon isotopic composition ( 13C) 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 13C of soil respiration, which suggests indirectly that recently fixed photosynthates comprise a substantial component of substrates consumed by soil respiration.
Description:
The carbon isotopic composition ( 13C) 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 13C 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 13CO2 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) non-steady-state soil CO2 transport, and (2) heterotrophic soil-respired 13CO2. In a first experiment, we examined the effects of soil moisture on total respired 13CO2 by growing Douglas fir seedlings under high and low soil moisture conditions. The measured 13C 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 13CO2 across a large moisture range. Our results do not rule out the potential influence of recent photosynthates on soil-respired 13CO2, 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 13CO2.
