## 1 Introduction
Although a slighter \({}^{13}\)C increase is expected due to the heterotrophic activity on the organic matter [27; 28], changes in particular OM components during the composting process may predominate in the observed \(\delta^{13}\)C shifts, i.e., preferential loses in the naturally \({}^{13}\)C enriched celluloses (labile polysaccharides) components during the first stages of the composting process, as well as the accumulation of the more recalcitrant lignin and waxes, which are comparatively \({}^{13}\)C depleted [29]. These effects may result in a net even or slight \(\delta^{13}\)C depletion of the final compost. A best and more accurate approach to the conventional isotope analysis in bulk samples to study the mechanisms of organic matter evolution during the composting process, is the analysis of \(\delta^{13}\)C in separate compounds. To the best of our knowledge, few studies have applied isotopic determinations to elucidate the degradation dynamics of organic amendments [30]. The compound-specific isotope analysis (CSIA) approach has been previously used to measure C stable isotope composition in major organic matter components, e.g., fatty acids [31; 32], \(n\)-alkanes [33], polysaccharides [34], lignin phenols [35], or sterols [36]. Isotope ratio mass spectrometry (IRMS) constitutes a helpful approach to differentiate several biogenic compounds from an organic, complex sample and to authenticate the origin of biomass. Determination of compound-specific isotopic composition can provide a fingerprinting that reflects decomposition process, technical process in food sciences, etc. [32]. The stable carbon isotope composition (\(\delta^{13}\)C) is informative of origin, transformation, utilization, and stabilization of carbon amendments in soils. Pyrolysis-gas chromatography coupled on-line with isotope ratio mass spectrometry (Py-GC-C/IRMS) has been previously used to study the \(\delta^{13}\)C values of the main compo nents of wood [37], polysaccharides [38], and essential oils [39], as well as to determine turnover rates of specific carbohydrates, lignin, lipids, and N-containing compounds from arable soils of a wheat C3/maize C4 rotation experiment [40]. These studies demonstrated that the technique did not produced isotopic fractionation and that the pyrolysis products represented the isotope composition of their precursors. This technique, named Py-CSIA [38], is a fast, reproducible, and robust method that inherits the advantages posed by the conventional analytical pyrolysis for the study of complex, poorly soluble organic matrices and is suitable for detailed studies on organic matter dynamics. In this work, the molecular composition and evolution during the composite process of maize biomass was recorded over a period of two and a half years. We hypothesize that the combination of Py-GC/MS and Py-CSIA can give accurate information of the humification degree of amenable compounds from compost and its evolution over time. Therefore, the objectives of this study were: (i) to characterize the molecular composition of the biomass and compost samples during the experimental trial using Py-GC/MS, (ii) to investigate whether the isotopic composition of specific compounds (Py-CSIA) can provide information on the origin and decay of certain compounds, supported by bulk isotopic analysis, and (iii) to test the suitability of coupling Py-GC/MS and Py-CSIA to monitor complex organic samples, avoiding any sample preparation, extraction, or derivatization.