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Seasonal and daily time course of the ¹³C composition in soil CO₂ efflux recorded with a tunable diode laser spectrophotometer (TDLS)

Marron, Nicolas, Plain, Caroline, Longdoz, Bernard, Epron, Daniel
Plant and soil 2009 v.318 no.1-2 pp. 137-151
absorption, canopy, carbon, climate, climate change, forests, growing season, isotopes, lasers, monitoring, roots, seasonal variation, soil, spectrometers, wavelengths
Temporal variations of carbon isotope composition of soil CO₂ efflux (FS and δ¹³CFS) at different time scales should reflect both temporal variations of the climate conditions that affect canopy functioning and temporal changes in the relative contribution of autotrophic respiration to total FS. A tunable diode laser spectrophotometer (TDLS) was installed in the Hesse forest (northeast of France) early during the 2007 growing season to determine the seasonal and daily variability in δ¹³CFS. This method, based on the measurement of the absorption of an infrared laser emission at specific wave lengths of the ¹³CO₂ and ¹²CO₂, allows the continuous monitoring of the two isotopologues. The concentrations of the two isotopologues in FS were continuously monitored from June to November 2007 using chamber method and Keeling plots drawn from nocturnal accumulation of CO₂ below the canopy. These TDLS measurements and isotope ratio mass spectrometer based Keeling plots gave very similar values of δ¹³CFS, showing the reliability of the TDLS system in this context. Results were analysed with regard to seasonal and daily changes in climatic and edaphic variables and compared with the δ¹³C of CO₂ respired by roots, litter and soil incubated under controlled conditions. Pronounced daily as well as seasonal variations in δ¹³CFS were recorded (up to 1.5[per thousand]). The range of variation of δ¹³CFS was of the same order of magnitude at both diurnal and seasonal scales. δ¹³CFS observed in the field fluctuated between values of litter and of root respiration recorded during incubation, suggesting that temporal (and probably spatial) variations were associated with changes in the relative contribution of the two compartments during the day and during the season.