Predicting soil water repellency using hydrophobic organic compounds and their vegetation origin

J Mao,M Rietkerk,S C Dekker,K G J Nierop

doi:10.5194/soil-1-411-2015

Abstract

Abstract. It is widely accepted that soil water repellency (SWR) is mainly caused by plant-derived hydrophobic organic compounds in soils; such hydrophobic compounds are defined as SWR markers. However, the detailed influence of SWR markers on SWR is yet unclear and the knowledge of their original sources is still limited. The aims of this study are to select important SWR markers to predict SWR based on their correlation with SWR and to determine their origin. In our study, sandy soils with different SWR were collected, along with their covering vegetation, i.e. plant leaves/needles and roots. A sequential extraction procedure was applied to the soils to obtain three organic fractions: dichloromethane (DCM)/MeOH soluble fraction (D), DCM/MeOH insoluble fraction of isopropanol/ammonia solution (IPA/NH3) extract (AI) and DCM/MeOH soluble fraction of IPA/NH3 extract (AS), which were subdivided into 10 dominant SWR marker groups: D fatty acid, D alcohol, D alkane, AI fatty acid, AI alcohol, AI ω-hydroxy fatty acid, AI α,ω-dicarboxylic acid, AS fatty acid, AS alcohol and AS ω-hydroxy fatty acid. Waxes and biopolyesters of the vegetation were also sequentially extracted from plants. The soils with higher SWR have significantly higher relative concentrations of AS alcohols. A number of indications suggest that AS alcohols are mainly derived from roots and most likely produced by microbial hydrolysis of biopolyesters (mainly suberins). In addition, the strong correlation between the biomarkers of plant tissues and SWR markers in soils suggests that it is more accurate to predict SWR of topsoils using ester-bound alcohols from roots, and to predict SWR of subsoils using root-derived ω-hydroxy fatty acids and α,ω-dicarboxylic acids. Considering the sandy soils studied here, the relationships we obtained need to be tested for other types of soils. Our analysis indicates that plant roots have a primary role influencing SWR relative to plant leaves.

Highlights

The prediction of soil water repellency (SWR) from the quantity of the SWR markers follows the relationship between soil total organic carbon (TOC) and SWR
The relative amounts of most single short-chain SWR markers negatively relate to SWR, while the long-chain markers have positive but insignificant relationships with SWR
This implies that a single SWR marker is not suitable to explain and predict the behaviour of SWR

Summary

Introduction

Soil water repellency (SWR) is one of the important properties that can interrupt soil water infiltration and potentially lead to soil erosion, and occurs globally in a wide range of soil types under various kinds of vegetation (Franco et al, 1995, 2000; Doerr et al, 2000, 2005; Michel et al, 2001; Poulenard et al, 2004; Hansel et al, 2008; de Blas et al, 2010). SWR is caused by hydrophobic organic compounds in soils. Different groups of SWR markers have been isolated from water repellent soils by a number of extraction techniques with selective organic solvents and have been identified by using several types of analytical instruments in previous research (Ma’shum et al, 1988; Franco et al, 1995, 2000; Hansel et al, 2008; Atanassova and Doerr, 2010; de Blas et al, 2010; Mao et al, 2014). Mao et al.: Predicting soil water repellency using hydrophobic organic compounds

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