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Final Activity Report Summary - ALICE (Advanced laser techniques to investigate C isotope discrimination during decomposition)

Most of the current research on the carbon cycle, which makes use of isotopic approaches, is based on the assumption that no discrimination occurs during respiration and decomposition processes. The verification of this assumption is needed to improve our understanding of soil carbon dynamics.

The objectives of this project were twofold:

1. to develop, implement and test state of the art ultra sensitive spectroscopic methods for laboratory and field measurements of isotope ratio in carbon dioxide (CO2) at ambient concentrations
2. to apply this new spectroscopic device to determine fractionation during respiration processes.

Optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS), which is one of the most advanced laser-based spectroscopic methods, was implemented and optimised for measuring the ratio of 13C to 12C in CO2 at ambient concentrations. The technique was based on measuring the transmission of a tuneable diode laser beam at 2 µm through a V-shaped high-finesse optical cavity with an effective path length of 30 km, in a volume of only 1 litre. The optical feedback effect was employed to narrow and lock the frequency of the laser beam for efficient coupling into the cavity. Owing to the long optical path, our prototype instrument was able to operate at CO2 concentrations as low as 200 ppm, yielding an accuracy of ±0.4 in delta-13C for a 100 seconds integration time and a 6 minute calibration interval. The instrument was compact, portable, cryogen-free and able to determine the isotopic ratio from a continuous flow in real time, without sample preparation. Thanks to the high temporal resolution and the non-invasive nature of the analysis, the diode laser spectrometer enabled us to investigate isotopic fractionation effects in soil respiration processes.

In particular, we measured 13C-CO2 derived from the microbial oxidation of simple organic compounds, mixed with silicon dioxide powder and subsequently subjected to laboratory incubations, with known values of the 13C/12C ratio. These investigations confirmed the validity of the hypothesis of no discrimination during respiration processes, at least at the accuracy level of the laser spectrometer. Simultaneously, leaf litter incubation experiments were performed in order to determine whether temporal changes in different carbon sources or direct microbial discrimination could be responsible for the observed changes in isotopic composition in 13C (delta13C) of litter respired CO2. The measurements showed a significant discrimination during litter decomposition. The temporal variations of this discrimination were mostly caused by the variations of the delta13C of respired CO2 than by those of the remaining bulk material.

Litter respired CO2 was 13C-enriched respective to the bulk litter material. This discrimination, however, seemed to be 'apparent'. In fact, our results argued in favour of a time-varying contribution of at least three different pools to the temporal variation of both respired CO2 and its delta13C. Further chemical and isotopic analyses using nuclear magnetic resonance (NMR) would allow for better constraining of our models. Moreover, these results strongly suggested taking into account the apparent discrimination occurring during organic matter decomposition in studies employing natural abundance isotope techniques.

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