The pyrolytical fingerprint of nitrogen compounds reflects the content and quality of soil organic carbon

The increasing land degradation is a problem that affects many soils in countries with a Mediterranean climate. In this aspect the soil organic matter (SOM) plays an important role, due to its progressive biodegradation parallels to desertification and the concomitant emissions of CO₂ to the atmosphere. These facts make basic research on the structure and composition of SOM important for soil conservation. Organic N-compounds in soil are of particular interest due to their chemical structure and speciation status in the SOM which can play an important role in soil N bioavailability and in the whole biogeochemical activity of the soil. For this reason, studying the possible relationships between the different N-compounds and soil properties, such as SOM content and its chemical characteristics, can provide new information on the stabilization and storage of organic C in soil. For this research, 30 soils from Spanish ecosystems with a wide range of SOM content were selected. The molecular composition of SOM in whole soil samples including N-compounds, was analyzed by analytical pyrolysis (Py-GC/MS). A parallel characterization of SOM quality was carried out using solid state ¹³C NMR and UV–vis spectroscopy. Based on their chemical structure, the N-compounds identified by Py-GC/MS were classified into seven main groups: indoles, pyridines, pyrazoles, benzonitriles, imidazoles, pyrroles and quinolines. Multivariate statistical analyses were used to explore the relationship between the distribution of the above compounds and the SOM content. A significant predictive model was obtained for the SOM using partial least squares (PLS) regression, which was used to predict SOM content using the pyrolytic N-compounds as descriptors. This would show that there is a relationship between the patterns of N-compounds and the biogeochemical mechanisms involved in the different C storage levels the soils. Also, multidimensional scaling (MDS) and principal components analysis (PCA) showed to what extent the individual N-compounds are informative of status and quality of the humic acid fraction of SOM. As a whole, the results obtained by Py-GC/MS suggest that indoles, alkylindoles, alkylbenzimidazoles and alkylpyridines could be indicators of SOM accumulation while unsubstituted benzonitrile and pyridine are related to SOM quality.