Understanding soil water repellency under native vegetation in Australia: An empirical and molecular dynamics approach

Soil water repellency (SWR) is a major land management issue across southern Australia and inhibits the infiltration of water into soil with effects on the germination of crops and pastures and run-off in agricultural and forested land. SWR is a natural phenomenon and generally occurs in the surface layers of sandy soils where hydrophobic materials mostly of plant origin occur as particulate organic matter and as waxy coatings on soil particles. Although its incidence and severity have been anecdotally related to the previous native vegetation little is known about the specific organic compounds that may derive from this vegetation. Previous studies have concentrated on characterizing the contribution of soil organic matter, and this and the fact that soils are complex mixtures of a broad range of organic components may have obscured the contribution of a small concentration of compounds from the native species to SWR. Moreover, the precise distribution of these compounds (e.g., as coatings or in interstices between soil particles) and their contribution to SWR has not been quantified. Although analysis of soil solvent extracts can identify the relative amounts of individual compounds, it is often difficult to determine the relative importance of all components in inducing SWR. It is even harder to determine synergistic effects from combinations of components. Experimental studies provide a broad understanding of the bulk effects of hydrophobic molecular coatings but do not provide a molecular level understanding of the coating structure or of its kinetic and/or thermodynamic stability. The molecular arrangement of those organic compounds on the soil particles have been shown to have implications for both the incidence and dynamics of SWR. Thus the present study employed a range of approches to understand SWR in native vegetation: i) quantifying the contribution of canopy derived exudates from native tree species to SWR, ii) discerning the contribution of organic matter in different pools (e.g., on particle surfaces, as interstitial matter) to SWR and iii) examining the physical interaction of the organic molecules (without and with water) with soil mineral surfaces. Canopy derived exudates were captured using an improvised technique, and organic compounds were extracted using nonpolar and polar solvents. In order to discern the contribution of different carbon pools to SWR, soils were separated into mineral and interstitial matter. Soils were extracted using a sonication technique. Individual and combinations of compounds were loaded onto acid washed sand (AWS) using a rotary evaporator. This experimental loading and measurement was complemented by fully atomistic molecular dynamics simulations with Materials Studio v 7.0 to understand the physical interaction of the molecules with mineral surfaces. A range of different organic compounds that are widely reported in the incidence of SWR were detected in the canopy derived exudates and soil extracts. They were mostly long-chain alkanes, fatty acids, phytols, phytanols, amides, aldehydes, ketones, terpenoids, steroids, and some complex ring containing structures. Though there was similarity in the composition of both the nonpolar and polar compounds of exudates of Banksia menziesii, Eucalyptus marginata, Xanthorrhoea preissii and Allocasuarina fraseriana, the concentration level of the chemical species was found to be significantly different. The concentrations were found to differ over the sampling period. For example, docosanoic acid was found to be dominant in the summer for B. menziesii and X. preissii. while in E. marginata and A. fraseriana it was evident in the winter. Moreover, the concentration level of the chemical species derived from the soil extracts were also found to be significantly different between the species of A. fraseriana, E. marginata, E. wandoo and B. menziesii. Notably, the main difference between A. fraseriana and the other three species (E. marginata, E. wandoo and B. menziesii) was the presence of long chain fatty acids and fatty alcohols. The concentration of the compounds was even different in different soil components (e.g., minerals and interstitial matter) of the soil matrix. For example, the concentration level of the individual compound derived from the bulk soil was similar to the total concentration level of the compound derived from both the coarse mineral materials and interstitial matters. The molecular level arrangement of various hydrophobic molecules with mineral surfaces also varied. For example, amphiphilic molecules at surface density of 2.3 molecs/nm2 were found in a tilted arrangement on kaolinite while on quartz surfaces they formed layered arrangements. However, silica did not favor a certain order of arrangement of the molecules due to its amorphous surface. The surface characteristics and polarity (abundance of OH groups) of the substrate (kaolinite > silica) were found to significantly modify the organo-mineral interactions. Different biogenic volatile organic compounds (BVOCs) or terpenoids commonly observed in vegetation smoke or forest fire, were not found to induce SWR. Moreover, the long chain amphiphilic molecules exhibited a balance between molecule-molecule and molecule-surface interactions on the quartz surface that supported surface adhesion, which in turn led to the formation of a hydrophobic layer. Furthermore, soil moisture or the addition of water molecules was found to significantly modify the conformation of molecules at the organo-mineral interfaces. For example, the polar kaolinite and silica surfaces were found to easily wet-up and reorganization of amphiphilic molecules on the surfaces took place following aquation. The stronger interaction between water and organic molecules can thus be inversely correlated to organo-mineral interaction on soil particles.