References

1. Dynamic Contact Angle vs. Water Content 100 Dynamic Contact Angle (degrees) 95-3 90 Hydrophobic Soils: Exploring the Mechanism for Reversible Hydrophobicity Priscilla Woolverton, Maria Dragila, Markus Kleber, Don Horneck Department of Crop and Soil Science, Oregon State University, Oregon, USA 80 Circle An 70 Circle Cf2 60 Circle 1 50 Circle 2 40 Circle 3 30 Circle E 20 10 Problem 0 Quincy soils 0 2 4 6 8 10 12 14 16 Water Content (%) A serious consequence of irrigation agriculture, in high-stress environments is that, in time, soil will degrade and develop water repellency, i.e., hydrophobic soil. Water repellency is a worldwide problem. An 1 2 As 4 E 3 Repercussions of hydrophobicity: Decreased crop productivity, increased water and fertilizer use, run-off and soil erosion, preferential flow and transport of contaminants to groundwater. Associated soils Cf3 Conceptual model The New View Study approach Confirmation Cf2 Csw (Based on Horne & McIntosh 2000; Kleber et al. 2007; Diehl et al. 2009) Present understanding: Water repellencyisthought to becaused by organichydrophobic compounds, which are present as coatings on soilparticles or as interstitialmatterbetweensoilparticles.  http://www.water-repellency.alterra.nl/ • To test this model we selected 10 125-acre circles (with same soil texture and climate) in agricultural production and representing a range of soil management strategies. We are quantifying the impact of management on OM structure and resulting laboratory and field scale hydrophobicity. • Soil sample analysis: surface energy, C and N content and OM structure. 1. 2. The new view: Water repellency can occur in any high stress environment. Depending on the ability of organic matter to reorient upon rewetting, hydrophobicity can be permanent or reversible. High input b. 3. 4. High H2O holding capacity Figure 2: a. Location of Quincy (red) and associated soils in Morrow County, Oregon. White circles indicate fields sampled and analyzed. b. Study site location in E. Oregon (45”47’15.21 N, 119”31’03.47 W). Soil texture Soil management a. Low H2O holding capacity Figure 5. Graph of CA (measure of severity of hydrophobic phenomenon) vs. water content. Low input • If hydrophobic coatings were the only reason for poor wettability, all contact angles should be high (above 60 degrees) • If water content was the only parameter controlling hydrophobicity, all data would fall within the blue region of the graph. • However, two distinct responses to water content are observed: • soils that retain hydrophobic behavior even at higher water contents (orange region) and • a dynamic hydrophobic characteristic of soil that changes in response to soil water content (reversible hydrophobicity). Arid Marginal soils, i.e. sandy soils in arid climates under low OM input systems. Climate • Scientific goal: • Determine the precise mechanisms that link molecular structure of soil OM to particulate surface energy (wettability). • Hypothesis: • The orientation of individual amphiphilic molecular fragments of soil organic matter (OM) determines the occurrence and the extent of water repellent properties in the soil, • this orientation changes as a function of moisture content. • Irrigation treatments enhance OM decomposition and preferentially deplete the soil of mobile amphiphilics. • As a consequence, the amphiphiles in the kinetic zone are lost and permanent hydrophobicity may develop. b. Acknowledgements This work is supported by the AFRI-USDA program on Soil Processes grant #2009-65107-05928, and National Science Foundation Hydrologic Sciences Program grant # 0449928. Moist References Bachmann, J., Woche, S., Goebel, M. –O., Kirkham, M. & Horton, R. 2003. Extended methodology for determining wetting properties of porous media. Water Resources Research, 39, 1353-1367. Diehl, D., Ellerbrock R.H. & Schaumann, G.E. 2009. Influence of drying conditions on wettability and DRIFT spectroscopic C–H band of soil samples. European Journal of Soil Science, 60, 557-566. Doerr, S.H. & Thomas, A.D. 2000. The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal. Journal of Hydrology, 231-232, 134-147. Horne, D.J. & McIntosh, J.C. 2000. Hydrophobic compounds in sands in New Zealand—extraction, characterization and proposed mechanisms for repellency expression. Journal of Hydrology, 231-232, 35-46. Kleber, M., Sollins, P. & Sutton, R. 2007. A conceptual model of organo-mineral interactions in soils: self-assembly of organic molecular fragments into zonal structures on mineral surfaces. Biogeochemistry, 85, 9-24. Figure 4. Definition of high-stress soils. Schematic of conditions leading to permanent hydrophobicity in soils. Figure 3 a. Even though, this is sandy soil with high hydraulic conductivity, water sits on surface of non-wettable soil and ponds between plants b. Schematic of preferential water infiltration between rows that is unavailable for use by plants c. Soil erosion as an effect of water- ponding and subsequent run-off a. c. q > 90 Three parameters drive soil hydrophobicity: Soil texture; Climate; OM management. While reversible hydrophobicity can occur in many environments, it is the particular intersection of conditions shown above (triangle) that lead to permanent hydrophobicity. Of the three conditions, growers can only control soil OM.