Investigating the evolution of the chemical nature of SOM through stabilization thanks to long term field experiments and state-of-the-art synchrotron-based NEXAFS spectroscopy

Though it is of crucial importance in order to better understand carbon stabilization insoils, the chemical nature of stable soil organic matter (SOM) is poorly characterized. This is explained by two major difficulties: (1) there is still no successful experimental way toisolate the pool of SOM that has a pluri-decennial residence time and (2) SOM with highresidence time is often associated to minerals, which complicates its characterization.In this work, we overcame these two major difficulties by characterizing samples fromlong term bare fallow (LTBF) experiments using the state-of-the-art synchrotron-basedNEXAFS spectroscopy (Canadian Light Source, Saskatoon, Canada). Firstly, LTBF experiments offer a unique opportunity to study stable SOM, as without carbon inputs and with continuing biodegradation and mineralization, SOM becomes progressively enriched in its most stable components. Secondly, NEXAFS technique allows the characterization of Carbon speciation with no sample pre-treatments and no noise induced by the mineral part of the samples. In this work, the fluorescence emission spectra at the Carbon K-edge threshold (280 eV) were measured for samples taken up at the initiation of 5 different european LTBF experiments and several decades later. Results show that differences in chemical composition betweendifferent dates are subtlebut significant and spectra are highly reproducible between field replicates. More advanceddata treatment is ongoing and will be presented at SOM