An adapted sequential chemical fractionation coupled with UV and fluorescence spectroscopy for calcareous soil organic matter study after compost amendment

Extractions with pyrophosphate, and in a lesser amount with sodium hydroxide, represented the major part of DOC, for both control and treated soils. Yields of sequential extractions ranged from 16.1% to 16.4% of TOC for these two soils. Compost inputs resulted in evolution of quantitative and qualitative results, especially for water extractable organic matter (WEOM) and pyrophosphate extracts mainly during the first 3 months of this field experiment. Study of annual changes of DOC, SUVA254 and E2/E3 index values in treated soil indicated a priming effect during the weeks after compost spreading, in relation with a revitalizing effect of microbial activity. Contributions of organo-mineral interactions and rainfall regulated WEOM amount throughout the year, and the activity of microbial communities in soil seemed to play a significant role on WEOM production. FI index values demonstrated microbial origin for WEOM, and for OM extracted either with acid or with sodium hydroxide. At the contrary, pyrophosphate extracts revealed that OM came essentially from terrestrial origin, which corresponds to the most complex and aromatic compounds. It was found that sodium hydroxide extracted less aromatic SOM, but higher molecular weight compounds. 3D fluorescence spectra revealed the presence of four main types of fluorophores according to the different extracts: humic, fulvic and marine-like compounds (A, C and M) common to all extracts, and lignin-like compounds (H) specific to pyrophosphate extracts only in treated soil at the beginning of the experiment due to initial compost effect. PCA on quantitative and qualitative data for WOEM fraction revealed essentially that meteorological aspects, with temperature and precipitation variations, were an important parameter over the year of experimentation, and that compost input impacted significantly WEOM amounts at the beginning of the experiment.