Response of δ¹³C in plant and soil respiration to a water pulse

Abstract. Stable carbon isotopes have been used to assess the coupling between changes in environmental conditions and the response of soil or ecosystem respiration, usually by studying the time-lagged response of δ¹³C of respired CO₂ (δ¹³C_R) to changes in photosynthetic carbon isotope discrimination (Δ_i). However, the lack of a systematic response of δ¹³C_R to environmental changes in field studies stresses the need to better understand the mechanisms to this response.

We experimentally created a wide range of carbon allocation and respiration conditions in Fagus sylvatica mesocosms, by growing saplings under different temperatures and girdling combinations. After a period of drought, a water pulse was applied and the short-term responses of δ¹³C in soil CO₂ efflux (δ¹³C_Rsoil) and δ¹³C in aboveground plant respiration (δ¹³C_Rabove) were measured, as well as leaf gas exchange rates and soil microbial biomass δ¹³C responses.

Both δ¹³C_Rsoil and δ ¹³C_Rabove values of all the trees decreased immediately after the water pulse. These responses were not driven by changes in Δ_i, but rather by a fast release of C stored in roots and shoots. Changes in δ¹³C_Rsoil associated with the water pulse were significantly positively correlated with changes in stomatal conductance, showing a strong impact of the plant component on δ¹³C_Rsoil. However, three days after the water pulse in girdled trees, changes in δ¹³C_Rsoil were related to changes in microbial biomass δ¹³C, suggesting that changes in the carbon source respired by soil microorganisms also contributed to the response of δ¹³C_Rsoil.

Our study shows that improving our mechanistic understanding of the responses of δ¹³C_R to changes in environmental conditions requires the understanding of not only the plant's physiological responses, but also the responses of soil microorganisms and of plant-microbial interactions.