No overall stimulation of soil respiration under mature deciduous forest trees after 7 years of CO₂ enrichment.

摘要

The anthropogenic rise in atmospheric CO₂ is expected to impact carbon (C) fluxes not only at ecosystem level but also at the global scale by altering C cycle processes in soils. At the Swiss Canopy Crane (SCC), we examined how 7 years of free air CO₂ enrichment (FACE) affected soil CO₂ dynamics in a ca. 100-year-old mixed deciduous forest. The use of 13C-depleted CO₂ for canopy enrichment allowed us to trace the flow of recently fixed C. In the 7th year of growth at ~550 ppm CO₂, soil respiratory CO₂ consisted of 39% labelled C. During the growing season, soil air CO₂ concentration was significantly enhanced under CO₂-exposed trees. However, elevated CO₂ failed to stimulate cumulative soil respiration (R_s) over the growing season. We found periodic reductions as well as increases in instantaneous rates of R_s in response to elevated CO₂, depending on soil temperature and soil volumetric water content (VWC; significant three-way interaction). During wet periods, soil water savings under CO₂-enriched trees led to excessive VWC (>45%) that suppressed R_s. Elevated CO₂ stimulated R_s only when VWC was <=40% and concurrent soil temperature was high (>15 degrees C). Seasonal Q₁₀ estimates of R_s were significantly lower under elevated (Q₁₀=3.30) compared with ambient CO₂ (Q₁₀=3.97). However, this effect disappeared when three consecutive sampling dates of extremely high VWC were disregarded. This suggests that elevated CO₂ affected Q₁₀ mainly indirectly through changes in VWC. Fine root respiration did not differ significantly between treatments but soil microbial biomass (C_mic) increased by 14% under elevated CO₂ (marginally significant). Our findings do not indicate enhanced soil C emissions in such stands under future atmospheric CO₂. It remains to be shown whether C losses via leaching of dissolved organic or inorganic C (DOC, DIC) help to balance the C budget in this forest.