Maturation in composting process, an incipient humification-like step as multivariate statistical analysis of spectroscopic data shows

Humification is a process that plant and microbiota residues experiment in natural or agronomic soils under microorganisms action and environmental conditions. Under this process natural biomolecules - such as protein, carbohydrates or lignin – experience secondary biochemical and chemical reactions yielding to the formation of new organic biomolecules normally known as soil humus or humic substances (HS). In parallel, composting of fresh organic residues may be seen as an artificial process that involves many microorganism-induced secondary biochemical reactions that are probably also included in the first steps of natural humification in soils. In this context, we have applied multivariate statistical analysis to diverse and complementary analytical techniques (UV–Visible, synchronous fluorescence, FTIR, 13C- NMR and pyrolysis GS/MS) to follow the structural evolution of three groups of organic material: (i) fresh organic matter materials, (ii) compost of the fresh organic matter materials, and (iii) humic and fulvic acids including standards and references from the International Humic Substances Society. In order to discriminate among the three groups of organic materials, the set of data obtained from each analytical technique was analyzed using complementary statistical techniques: Correlations, Kolmogorov-Smirnov Test and Principal Component Analysis (PCA). The results showed positive correlations between UV–visible and fluorescence indexes and aromatic structures determined by 13C- NMR and pyrolysis GS/MS. However, these indexes were negatively correlated with polysaccharides and amides determined by FTIR, and lipids determined by pyrolysis GS/MS. The Kolmogorov-Smirnov Test showed that E4/E6, ε600, EEt/EBz, ε280 from UV/Visible; A440 from synchronous fluorescence; 1040/1400 and 1515/1715 by FTIR and, LIP from pyrolysis GS/MS were able to discriminate the samples in two different groups. The group formed by the transformed organic substances (humic, fulvic and composted materials) on the one hand, and the raw (fresh) organic materials on the other. These results, considered along with those obtained from the PCA analysis of spectroscopic data, indicated that composting could share secondary reactions and processes with the first steps of natural humification occurring in soil. Likewise, the results show that the organic molecules present in humic and composted materials are chemically different from the biomolecules present in fresh, no-transformed- materials.