Cover plant functional types alter the abundance and composition of hydrophobic compounds: the relationship with soil water repellency on the Chinese Loess Plateau

Abstract

Background and aims It is widely accepted that soil water repellency (SWR) is mainly caused by plant-derived hydrophobic compounds in soils. The relation between these hydrophobic compounds, which are defined as SWR biomarkers, and SWR has been rarely known and the knowledge of their sources remains controversial. We aimed to select key SWR biomarkers predicting SWR and to trace their origin. Methods Topsoils under/around five dominant plant species (DPS) belonging to various plant functional types (PFTs) on the Chinese Loess Plateau were sampled, together with plant samples, i.e., plant leaves and roots. A sequential extraction procedure and hydrolysis approach was applied to obtain three organic fractions: dichloromethane (DCM)/MeOH soluble fraction (D), DCM/MeOH soluble fraction of isopropanol/ammonia solution (IPA/NH3) extract (AS), and DCM/MeOH insoluble fraction of IPA/NH3 extract (AI), which were analyzed by gas chromatography-mass spectrometry. Results The two-way hierarchical clustering analysis revealed a distinct division of soil organic carbon composition under different DPS, and the leaves of evergreen trees offered more cutin than those of other PFTs. In addition, structural equation modeling showed that AS cutin (path coefficient = 0.30) and AI cutin (path coefficient = 0.47) had direct and positive effects on SWR. Moreover, there was a strong link between SWR and the ratio of the two separate compound groups when AS cutin or AI cutin were taken as the numerators. Conclusion After considering the SWR behavior during extraction and the chemical composition of each fraction, we concluded that leaf-derived cutin appears to have the greatest effect.