Water Repellency of Aggregate Size Fractions of a Volcanic Ash Soil

Abstract

Water repellency (WR) of soils can induce hydrological problems such as reduced water infiltration, enhanced surface runoff and erosion, and the forming of preferential flow patterns in soil. Although soil organic matter (SOM) may cause both soil aggregation and a hydrophobic‐material‐coating of aggregates, little is known about WR in aggregated soils. We investigated the degree of WR as functions of volumetric water content (θ) and pF [= log (‐ψ; soil‐water potential)] for sieved fractions of a volcanic ash soil samples from different depths with varying soil organic carbon (SOC) contents of between 1.1 and 12.3%. Water repellency was determined by the molarity of ethanol droplet (MED) test. Water repellency was observed in the samples with SOC ≥ 4.6%, and the effects of sample pretreatments (pressure chamber desorption, air‐drying at 20°C, and oven‐drying at 60°C) on the degree of WR were small. The degree of WR varied greatly with both SOC content, θ, and pF. Peaks of WR were observed when the water retained in intra‐aggregate pores was drained to a moderate extent with the corresponding pF values located in a relatively narrow range from 3.2 to 3.6. This indicates that the hydrophobicity of high‐SOC aggregate surfaces might be enhanced the most at a specific soil‐water potential. Examining relations between water repellency parameters, the integrated areas below the WR‐θ and WR‐pF curves were useful indexes for characterizing WR, and linear relationships between the integrated areas and both SOC and water contents at maximum repellency were found.

Hydro-physical processes and soil properties correlated with origin of soil hydrophobicity

“…This is closely related to the distribution of organic matter on soil surface, as described by JARAMILLO, 2006 andDe GRYZE et al (2006), who found in smaller fractions of the soil the highest degree of hydrophobicity, given the highest organic material content associated with these fractions. However, despite the susceptibility of soils with sandy texture to water repellency, there are several records of hydrophobicity in clay or fi ne texture-soils (TÄUMER et al, 2005;KAWAMOTO et al, 2007). The high repellency in clay soils occurs because clay particles form aggregates, reducing the surface area which might be covered with a thin layer of hydrophobic substances (KAWAMOTO et al, 2007;VOGELMANN et al, 2010).…”

“…However, despite the susceptibility of soils with sandy texture to water repellency, there are several records of hydrophobicity in clay or fi ne texture-soils (TÄUMER et al, 2005;KAWAMOTO et al, 2007). The high repellency in clay soils occurs because clay particles form aggregates, reducing the surface area which might be covered with a thin layer of hydrophobic substances (KAWAMOTO et al, 2007;VOGELMANN et al, 2010). Other studies showed that in certain environments, the contribution of hydrophobic material can be so high in addition to coating coarser particles, also coating by organic material part of the fi ne particles (WOCHE et al, 2005;KAWAMOTO et al, 2007;FOX et al, 2007).…”

“…But probably the major infl uence on water repellency is linked to the production of microbial biomass (hyphae) and exudates that change the characteristics of the hydraulic connection between soil and particles . This reduction in damping caused by increased hydrophobicity has serious implications for soil management, affecting growing conditions and enhancing the fl ow of water between aggregates (KAWAMOTO et al, 2007). The soil around plant roots, called the rhizosphere, may also have a higher rate of soil hydrophobicity alone (HALLET et al, 2003).…”

“…The local vegetation, depending on their chemical composition, may contribute to hydrophobic organic compounds, by deposition or decomposition (MATAIX-SOLERA et al, 2007). It may also be associated with other factors such as soil moisture conditioned by rain and drought (ROBINSON et al, 2010), the granulometric composition (WOCHE et al, 2005;KAWAMOTO et al, 2007;VOGELMANN et al, 2010), soil pH (BAYER & SCHAUMANN, 2007) and presence of some species of fungi WAHL et al, 2008) and burnings (MADSEN et al, 2011.…”

Soil Infrastructure, Interfaces & Translocation Processes in Inner Space ("Soil-it-is"): towards a road map for the constraints and crossroads of soil architecture and biophysical processes

“…The WR is closely related to soil-water content , OM quantity/quality (Kawamoto et al, 2007;Regalado et al, 2008), microbial activity (Hallett and Young, 1999), and soil management . Water repellency will at the same time control the onset of unstable (preferential or finger-like (Fig.…”