Diurnal patterns of CO2 concentrations and stable isotopes in subalpine forest soils

Respiration of carbon dioxide (CO2) from soils contributes significantly to the carbon balance of ecosystems. Measurement of the stable isotopic composition (δ13C) of soil and soil-respired CO2 can provide insight into the physical and biological mechanisms controlling respiration. Very few studies examine diurnal variations in the isotopic composition of CO2 in soils because of the difficulty in making such measurements. We measured [CO2] and δ13C in and above soils in a subalpine forest near Niwot Ridge Colorado. Profile measurements were made every two hours of CO2 in air below the soil surface (-3 cm), on the soil surface, and up to 40 cm above the soil surface. Over the course of 20 days in July of 2007, the δ13C of respired CO2 (δR-soil) measured using these vertical profiles varied from -25.42 to -21.10‰ with a mean of -23.48 ± 1.11‰. Measurements in the air near the ground (δR-ground, 0.1 to 2 m) varied from -26.71 to -23.96‰ with a mean of - 25.49 ± 0.63‰. Values of δR-soil were enriched relative to δR-ground,. Diurnal patterns in δ13C of respired CO2 were not apparent even though soil temperatures varied by as much as 15°C over a 24 hour period. Soil moisture is also likely important as it affects the diffusion rate of CO2 soil and may stimulate microbial activity. Improved knowledge of the controls on soil respiration have the potential to increase process-level understanding of ecosystem respiratory fluxes and may also help better constrain terrestrial carbon cycling models.