Soil moisture effects on the carbon isotope composition of soil respiration

Abstract

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.

Citation

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.