Influence of non-crystalline minerals in the total amount, resilience and molecular composition of the organic matter in volcanic ash soils (Tenerife Island, Spain)

The influence of short-range ordered minerals, especially allophane or ferrihydrite, in the total amount, resilience and molecular structure of the soil organic matter (SOM) was determined in a series of volcanic ash soils from Tenerife Island (Spain), with a range of amorphous minerals, organic carbon (C) and clay minerals. Up to 120 main analytical descriptors of the SOM were processed by uni- and multivariate statistical analyses. Short-range ordered minerals were estimated from the (Al o + 1/2 Fe o) index, organo-metallic complexes were calculated from the Al p:Al o ratio and crystalline minerals were determined by X-ray diffraction. The humic acids (HAs) were isolated, purified and analysed by visible, infrared and 13C nuclear magnetic resonance spectroscopy and analytical pyrolysis. In addition, CO 2 release curves from whole soil samples were used to obtain indices of SOM resilience. The results revealed that the amount and resilience of the SOM depended on the presence of short-range ordered minerals: the greater the content of non-crystalline minerals, the greater the C stabilization. However, no significant relationships were found between these amorphous minerals and aromatic compounds, which are indicators of resilience in HAs. Conversely, certain molecular characteristics of the HAs, such as the optical density at 465 nm, the concentration of quinone-type fungal pigments, the yields of carbohydrate-derivatives and N-compounds, highlighted the role of allophane-sized nanoparticles in the protection of microbial metabolites produced during the decomposition, and are also likely to provide active sites for soil C stabilization at the colloidal scale.

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