On-Line Isotopic Analysis of Soil-Respired CO2

Soils from a replicated long-term litter manipulation study were collected from a Douglas fir stand in central Oregon and from a sugar maple-black cherry stand in northwestern Pennsylvania to determine if processing of labile and recalcitrant organic matter could be detected through changes in the isotopic signature of density-fractionated soil. Soils were density- fractionated at 1.6 g cm^-3 using sodium polytungstate and incubated under controlled conditions (40% volumetric water content, 21\deg C) for 65 days. The isotopic measurement of soil-respired CO₂ was accomplished with a Finnigan Gas Bench II coupled to a Delta Plus XL Continuous Flow Mass Spectrometer; external precision was 0.06\permil. We found significant isotopic (\delta¹³C) differences in respired CO₂ by density, but not among litter exclusion and addition treatments, even where the treatments have been in place for 11 years (PA). Microbially-mediated isotopic fractionation, expressed as the difference between substrate ¹³C and respired ¹³CO₂, was large and positive the day of wet-up, but fell to near zero by day 5. Isotopic composition and patterns of lability (measured as respiration rate per gram C) varied dramatically between the two sites. The deciduous forest soil (Alfisol) yielded the expected pattern of higher lability and lower \delta¹³CO₂ values from the low density fraction. In contrast, the coniferous forest soil (Andisol) revealed a complicated pattern of higher lability in the high density fraction, suggesting organic matter composition, soil mineralogy, and microbiology are interacting in as yet unexplained ways in this soil. Overall, our findings suggest that density alone is not a clear indicator of the recalcitrance of soil organic matter, and that isotopes of respired CO₂ may yield information about the processing of labile and recalcitrant organic matter.