Zulimar Hernández

Graphical statistical approach to soil organic matter resilience using analytical pyrolysis data

Pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) of humic acids (HAs) from 30 agricultural soils from a volcanic island (Tenerife, Spain) was used to discern the molecular characteristics of soil organic matter (SOM) associated to resilience. For faster perceptual identification of the results, the yields of the pyrolysis products in the form of surface density plots were compared in an update of the Van Krevelen graphical statistical method. This approach, with respect to data reduction and visualization, was also used to collectively represent statistical indices that were obtained after simple and partial least squares (PLS) regression. The resulting plots illustrate different SOM structural domains (for example, carbohydrate- and lignin-derived and condensed lipid). The content of SOM and total mineralization coefficient (TMC) values can be well estimated from the relative abundance of 57 major pyrolysis compounds: SOM content and composition parallels the accumulation of lignin- and carbohydrate-derived structures (lignocellulosic material) and the depletion of condensed polyalkyl structures. In other words, in the volcanic ash soils that were studied, we found that the higher the amount of SOM, the lower its quality in terms of resilience. Although no cause-and-effect is inferred from this fact, it is evident that the resistance to biodegradation of the SOM is related to its molecular composition.

Soil carbon storage predicted from the diversity of pyrolytic alkanes

Biogeochemical factors responsible of the highly variable content of soil organic matter (SOM) in the different types of soils are poorly known. In particular, the role of organo-mineral interactions has frequently been considered, but less attention has been paid to the molecular composition of the SOM. The aim of this work was to contribute to a better qualitative and quantitative assessment of the soil organic C (SOC) accumulation, using chemometric approaches that do not require the absolute knowledge of the structure and functioning of the whole system under study. For this reason, we monitored the n-alkanes released by analytical pyrolysis from 35 widely different Mediterranean soils. The H′ Shannon diversity index was calculated to evaluate the origin and transformations of the alkane homologous series (C9–C31). A series of multivariate data treatments succeeded in showing significant relationship between the diversity of alkanes and the SOC concentration, and additional indicators of SOM quality were also used. All statistical analyses pointed out the significant correlation (P < 0.01) between the H′ diversity of the pyrolytic alkanes and the amount of SOC. In particular, a significant relationship between SOC levels and the percentage of long-chain alkanes was found, whereas the percentage of short-chain alkanes was correlated with specific descriptors of SOM quality. Finally, the partial least squares (PLS) predicted the SOC content from the alkane patterns.