Soil moisture effects on the carbon isotope composition of soil respiration

The carbon isotopic composition (delta C-13) 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 delta C-13 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 delta(CO2)-C-13 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 delta(CO2)-C-13. In a first experiment, we examined the effects of soil moisture on total respired delta(CO2)-C-13 by growing Douglas fir seedlings under high and low soil moisture conditions. The measured delta C-13 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 delta(CO2)-C-13 across a large moisture range. Our results do not rule out the potential influence of recent photosynthates on soil-respired delta(CO2)-C-13, 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 delta(CO2)-C-13. Copyright (C) 2010 John Wiley & Sons, Ltd.