Reversed-phase high performance liquid chromatographic profile of organic fractions extracted by solvents with different polarity as a tool to evaluate the hydrophobic character of soil under different management

Abstract

In the past decade there has been increased scientific interest in organic farming, especially in comparison with conventional agriculture. The aim of this study was to investigate the effects of these two agricultural systems on the soil organic matter (SOM) quality evaluated according to a new methodological approach. In particular, a sequential solvent extraction and a linear gradient elution reversed-phase high performance liquid chromatography (RP-HPLC) were used to evaluate SOM hydrophobic/hydrophilic character. The selected areas for soil analysis were two adjacent fields located in Central Italy and cultivated for 8 years following organic and conventional management, respectively. This area was a cultivated arable land since last century. Soil was a Vertic Xerochrepts with sandy clay loam texture (USDA) and pH 7.6. The cation exchange capacity (CEC) was on the average 42 mequiv. 100 g −1. Each soil sample has been subjected to three cycles of extraction with organic solvents ( n-hexane, dichloromethane, ethyl acetate) using Soxhlet apparatus. The relative amounts of SOM extracted from conventional and organic soil by the most hydrophobic solvent ( n-hexane) were 45.4 and 28.9%, respectively; while dichloromethane was able to extract from the two soils 51.5 and 26.7% of SOM. The components of extracts have been separated by RP-HPLC using a linear acetonitrile gradient in water to elute organic compounds. The PCA analysis of RP-HPLC chromatographic profile has well discriminated the organic matter quality of soils under different managements on the basis of organic compounds polarity, evaluated by RP-HPLC. In organically managed soil, ethyl acetate extracts SOM components that span a wider range of hydrophobicity than the compounds extracted by this solvent from conventional managed soil, indicating that the organically managed soil had a more heterogeneous organic compounds, which could be originated from the continuous input of fresh plant residues. The wide variety of organic substances extracted with different solvents from this soil can also be due to the amphiphillic character of humic material. RP-HPLC technique may represent a powerful tool for a thorough analysis of SOM hydrophobic character, which is known to have substantial bearing on the formation of stable aggregates and the whole soil structural stability.