Key messages

• Soil mineral chemistry is equally as important as particle size in determining soil water repellency (SWR) susceptibility
• Rapid SWR screening techniques will aid in deployment of amendments
• A small change in soil organic matter content has a large impact on SWR
• Soil organic matter composition also strongly affects SWR

Aims

• Develop improved methods for the rapid detection of soil compounds that induce soil water repellency (SWR)
• Identify the molecular mechanisms that lead to SWR and use these to develop novel amendments for long-term and sustainable amelioration

Introduction

Soil water repellency (SWR) has been observed around the world, under different climates and land uses.^[i] It is estimated that southern Australia alone has two to five million hectares affected by hydrophobic or SWR soils.[ii] This is a concern for the farming regions of Western Australia where many farms are reliant on rainfall for crop growth².

Non-wettable soils cause both environmental and economic problems. Environmental implications include increased surface runoff, enhanced erosion rates and chemical leaching.^[iii] Additionally, SWR leads to decreased nutrient storage and plant-available water⁴. This results in diminished soil quality, and therefore, less plant growth.^[iv] Expanding on this, the economic implications of reduced soil quality and plant growth for non-irrigated farming include lessened and irregular germination of crop seed and grasslands, resulting in lower yields and harder to manage weed control.² The bulk of weed seeds in the soil will not germinate at the same time if the soil is not wetted evenly, resulting in staggered germination,² reducing the efficacy of herbicide applications and requiring farmers to invest in methods to combat SWR.

Soil water repellency is an impermanent property of soils,⁴ preventing soils wetting from seconds through to weeks³ and sandy soils are particularly susceptible. Soils in southwest Australia are dominated by silicate sands containing quartz, and sheet silicate clay minerals like kaolinite and iron and aluminium oxides.^[v]

The primary component of silica is silicon dioxide (SiO2), which is a polar compound that interacts with water easily via dipole intermolecular forces. Thus, SWR is not associated with water-surface interactions but instead with the presence of organic material in the soil. Organic compounds can enter soil by being released by living plants, the decomposition of plants, and microorganism interaction.² It is accepted that amphiphilic compounds (containing both a polar hydrophilic and a non-polar hydrophobic functional group) cause SWR.²

The aim of our research at Murdoch University is to develop improved ways to detect soil compounds that induce SWR and identify the molecular mechanisms that cause SWR with a view to developing new amendments.

Methods

Materials

The organic compounds selected for this study (1-hexadecanol, palmitic acid, isopropyl myristate and hexadecane) were obtained from Sigma-Aldrich. The mineral substrates considered in this study were acid-washed sand (AWS) (Chem-supply) and kaolinite (Fluka). Field samples were collected from a wheat (Triticum aestivum)-lupin (Lupinus angustifolius) paddock near Mingenew, Western Australia.

Soxhlet extraction^[vi]

A few anti-bumping granules were added to a 200mL round-bottom flask with 175mL of 7:3 isopropanol /ammonia (IPA/NH3). 2–30g of accurately weighed soil was added to a glass-fibre thimble and placed into a Soxhlet, which was connected to the round-bottom flask with the solvent. An additional 25mL of the IPA/NH3 solvent mixture was added to the Soxhlet and left for 15 minutes to soak the soil in the thimble. The solvent was heated under reflux for 24 hours then the anti-bumping granules were removed. The isopropanol was evaporated using a rotary evaporator (45oC) under reduced pressure (260mm Hg). Extraction of each soil was carried out in triplicate.

Accelerated solvent extraction (ASE)

Extraction was carried out using a Dionex ASE 150. Soil (10g) was placed into a stainless steel Dionex extraction cell (10mL) with a glass-fibre filter. Extraction was performed using isopropanol/ammonia (95.5:4.5). The samples were preheated to 75oC for 5 minutes and then extracted twice (20 minutes each at 75oC) at a pressure of 1500 psi. The extracts were cooled then transferred into unsealed centrifuge tubes in a fume cupboard overnight to remove any remaining ammonia. The extracts were dried using a Labconco Centrivap (50oC) under reduced pressure (100–300mm Hg). Samples were then prepared for GCMS analysis using 2:1 dichloromethane /methanol (DCM/MeOH) spiked with dodecane (6.6 x 10-4 mol/L) added as an internal standard. GCMS analysis was carried out using a Shimadzu GC-2010 and GCMS-QP2010S, equipped with an SGE GC BPX5 column (Trajan).

Soil water repellency testing

Soil water repellency was quantified using the Molarity of Ethanol Drop test. A stock solution of 4.0 mol/L ethanol in water was made up and used to prepare diluted solutions of ethanol in water for carrying out MED tests. Ethanol concentrations above 4.0 mol/L were not needed. Drops were added to each soil sample using an auto-pipette to maintain a constant drop size of 20µL. Solutions of increasing ethanol concentration (0.0–4.0 mol/L in 0.2 mol/L increments) were added to each sample until surface penetration times were £ 3 s.MED testing carried out prior to extraction was done with laboratory conditions of 19oC and 48% relative humidity.

Compound loading^[viii]

Organic compounds were applied directly on soil or mineral substrates at different loading ranges (0.25×10-6– 2.0 ×10-6 mol/g). The standard procedure involved preparing a solution of the compounds in tetrahydrofuran (THF) in a 50ml volumetric flask to achieve specific loadings. The solutions (50mL) were then applied to acid washed sand (40g) and thoroughly mixed for 10 minutes. The solvent was then removed in a rotary evaporator at 45°C under reduced pressure.

Computational procedures and models

Interactions of long chain carboxylic acids with different soil surfaces were modelled using molecular dynamics under periodic boundary conditions. The modelled surfaces include kaolinite and silica. The cell dimensions for the kaolinite surface cells were 30.894 x 35.736 x 78.000 Å in a rhombohedrum shape. The silica surface cell was rectangular with dimensions of 28.510 x 28.510 x 60.000 Å. The height of each cell was increased, beyond that necessary to contain the surface and organic acid chains, to provide an area of vacuum space. This is done to prevent interactions with adjacent cells in the z-direction, due to established 3D period boundary conditions.

Interatomic potentials were modelled using the COMPASS force field.^[ix] Coulomb forces were calculated using the Ewald method. Van der Waals interactions were calculated using the atom-based summation with a cut-off distance of 12.5 Å, a spline width of 1.00 Å and a buffer of 0.5 Å. Energy minimisation of chemical structures was achieved using conjugate gradient procedures. Simulations were carried out under NVT conditions, with 1ns equilibration, 1ns data acquisition and 1fs time steps. Temperature was set at 298K, monitored using the Anderson thermostat, and a collision ratio of 1.0.

Results and Discussion

The ASE method removed similar amounts of alkane compounds (0.834 ± 0.062 x 10-7 mol/g) compared to the Soxhlet method, but only minor amounts of acid species (0.207 ± 0.009 x 10-7 mol/g). Extraction of alcohol species was also comparable between the Soxhlet and ASE methods (0.095 ± 0.008 x 10-7 mol/g). However, the concentration of steroid species (0.127 ± 0.005 x 10-7 mol/g) identified in the ASE extract was approximately half that obtained from the Soxhlet and sonication methods. Soxhlet extraction has the advantage of achieving in situ derivatisation of carboxylic acid species, which makes these easier to detect by routine GC-MS. The lower concentrations obtained for acids using ASE are likely due to poorer detection rather than poorer extraction. We are currently developing and optimising post-extraction derivatisation to improve detection of acids, alcohols, and sterols. Accelerated Solvent Extraction coupled with Gas-Chromatography Mass-Spectrometry allows for assessment of soil organic matter in field samples within ~2 hours, compared with ~2 days using the Soxhlet method.

Extraction of water repellent soils identified many naturally occurring compounds that are responsible for inducing SWR on sandy soils. Adding these compounds individually to acid washed sand provided a method to assess the relevant contribution each make to SWR.  Palmitic acid has been identified as one of the most effective compounds at inducing SWR with severe water repellency observed at loadings of 0.25x 10-6 mol/g of soil. Hexadecanol can also induce severe water repellency but only at higher loadings (³2.0x 10-6 mol/g). Many other compounds including alkanes and esters do not individually induce SWR but have synergistic relationships with other compounds to either enhance or diminish SWR. These results show that the composition of soil organic matter has a significant bearing on the induction of SWR and that only very small amounts of specific classes of compounds are required to induce SWR. This opens the question around how to counteract the action of these compounds.

Complementary computer simulations provided molecular level information about the interactions and arrangement of the organic compounds on soil particle surfaces. H-bonding interactions of fatty acids and fatty alcohols drive the orientation of the molecules into layers that induce SWR on sand (Figure 2a). The arrangement of the same organic molecules on clay (Figure 2b) is different due to the contrasting surface chemistry of the mineral. Heat is also an important factor in dispersing compounds from plant matter into the soil and contributes to the formation of these layers.

The contrast in the interactions and arrangement of the organic molecules is emphasised when water is included in the simulations. The anchoring of the amphiphilic molecules to sand particle surfaces is strong and resists disruption by water (Figure 3a), thereby maintaining water repellency. However, the organic molecules undergo rearrangement on the clay (Figure 3b) that enables water to access and wet the mineral surface. Amendments that disrupt the H-bonding of the organic acid and alcohol molecules from soil particles lower SWR.

Conclusion

Accelerated solvent extraction coupled with GC-MS provides faster assessment of field samples for SWR and, when coupled with derivatisation, improves detectability of compounds that induce SWR. Analysis reveals that only tiny amounts of specific classes of compounds are responsible for SWR. Amendments that disrupt the interactions and organisation of specific types of organic compounds with soil particles can significantly reduce SWR.

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. We are grateful to the Australian National Computational Infrastructure (NCI) facility for computer time. We also gratefully acknowledge the provision of funding and a PhD scholarship to Dr Nick Daniel by the Grains Research and Development Corporation of Australia (DAW00244), to Dr Mijan Uddin by Murdoch International Postgraduate Scholarship, to Ms Mai Dao for a VIED Scholarship from the Vietnamese Government and Ms Maria Then from the CRC for High Performance Soils.

References

[i]Doerr, S. H.; Ritsema, C. J.; Dekker, L. W.; Scott, D. F.; Cater, D. Hydrological Processes. 2007, 21, 2223-2228.

[ii] Harper, R. J.; McKissock, I.; Gilkes, R. J.; Carter, D. J.; Blackwell, P. S. J. Hydrology. 2000, 231-232, 371-383.

[iii]Doerr, S. H.; Shakesby, R. A.; Walsh, R. P. D. Earth-Science Reviews. 2000, 51, 33-65.

[iv]Deurer, M.; Muller, K.; Van Den Dijssel, C.; Mason, K.; Carter, J.; Clothier, B. E. European J. Soil Science. 2011, 62, 765 – 779.

[v]Clarke, C. J. Introduction to Geology: Rocks and Minerals. 1997.

[vi]Dao, M. T. T., Henry, D. J., Dell, B., Daniel, N. R. R., Harper, R. J.Plant Soil 2022,478, 505–517.

[vii]Daniel, N. R. R., Uddin, S. M. M., Harper, R. J., Henry, D. J.Geoderma 2019, 338, 56–66.

[viii]Uddin, S. M. M., Harper, R. J., Henry, D. J.J. Phys. Chem. A2019, 123, 7518–7527.

[ix]Sun, H., J. Phys. Chem. B1998, 102, 7338-7364.

Contact details

David Henry
Murdoch University
90 South St, Murdoch, WA, 6150
Ph: 08 9360 2681
Email: d.henry@murdoch.edu.au