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

It is widely accepted that soil water repellency (SWR) is mainly caused by hydrophobic compounds from plants and microbes. 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. 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 were applied to obtain three organic fractions: dichloromethane (DCM)/ methanol (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 (GC–MS). The cutin concentrations in leaves and soils of evergreen trees with higher WDPT values were 10–60 times and 35.39–78.77% higher than those of other PFTs, respectively. However, the root-derived suberin in the soils under evergreen trees, deciduous trees, and shrubs were 19.55–62.69% lower than that around the legumes and grasses. Correlation analysis further revealed that cutin was positively correlated with SWR (r2 = 0.24, p < 0.001), whereas suberin was not (p > 0.05). After considering the SWR behavior during extraction and the possible origin of the compounds, we concluded that leaf-derived cutin appears to have the greatest effect on SWR.