RESEARCH LETTERClays Are Not Created Equal: How Clay Mineral TypeAffects Soil ParameterizationKey Points:P. Lehmann1D. Or1,510.1029/2021GL095311 V ariations in clay mineral typesignificantly affect soil hydromechanical properties for soils withsimilar clay fractions Recent global maps of clay mineraltype permit spatially resolved andphysically based corrections ofpedotransfer functions The new clay mineral-informed soilproperties are in agreement withmeasurements and resolve biases ofundifferentiated use of clay factionSupporting Information:Supporting Information may be foundin the online version of this article.Correspondence to:P. Lehmann,firstname.lastname@example.orgCitation:Lehmann, P., Leshchinsky, B., Gupta,S., Mirus, B. B., Bickel, S., Lu, N., & Or,D. (2021). Clays are not created equal:How clay mineral type affects soilparameterization. Geophysical ResearchLetters, 48, e2021GL095311. https://doi.org/10.1029/2021GL095311Received 17 JUL 2021Accepted 6 SEP 2021, B. Leshchinsky2, S. Gupta1, B. B. Mirus3, S. Bickel1, N. Lu4, and1Soil and Terrestrial Environmental Physics, ETH Zurich, Zurich, Switzerland, 2College of Forestry, Oregon StateUniversity, Corvallis, OR, USA, 3U.S. Geological Survey, Landslides Hazards Program, Denver, CO, USA, 4ColoradoSchool of Mines, Golden, CO, USA, 5Division of Hydrologic Sciences, Desert Research Institute, Reno, NV, USAAbstract Clay minerals dominate the soil colloidal fraction and its specific surface area. Differencesamong clay mineral types significantly influence their effects on soil hydrological and mechanicalbehavior. Presently, the soil clay content is used to parameterize soil hydraulic and mechanical properties(SHMP) for land surface models while disregarding the type of clay mineral. This undifferentiated useof clay leads to inconsistent parameterization, particularly between tropical and temperate soils, asshown herein. We capitalize on recent global maps of clay minerals that exhibit strong climatic andspatial segregation of active and inactive clays to consider spatially resolved clay mineral types in SHMPestimation. Clay mineral-informed pedotransfer functions and machine learning algorithms trainedwith datasets including different clay types and soil structure formation processes improve SHMPrepresentation regionally with broad implications for hydrological and geomechanical Earth surfaceprocesses.Plain Language SummaryUnderstanding and predicting Earth's surface water and energymovement rely on accurate representation of soil properties, which are often estimated using simple-tomeasure information such as soil particle sizes (soil texture) and other attributes, such as vegetation. Soilclay content is an important trait that affects the rate by which rainwater infiltrates the soil and potentiallythe stability of soil on steep hillslopes. Most models make no distinction between clay minerals despitelarge differences in clay mineral and soil properties between tropical and temperate regions. New globalmaps of clay minerals show strong climatic and spatial segregation of clay minerals that allow betterestimates of fundamental soil properties. We developed models that use these clay mineral maps forestimating hydrological and mechanical properties for most soil types. These new equations will help toimprove global predictions of water availability, erosion, and natural hazards.1. IntroductionSoil clay minerals dominate the colloidal fraction and specific surface Earea (SSA) of soil. Clay minerals exhibit a wide range of microstructures and hydration responses that affect soil macroscopic hydraulic, chemical, and mechanical properties. We focus on kaolinite and smectite as end members of the clay mineralfamily due to their contrasting surface areas, differences in activity (shrink-swell behavior), high abundancein natural soils, and their general separation among climatic regions (e.g., kaolinite dominates in tropicalsoils that receive high values of mean annual precipitation, see Figure 1). Due to differences in their basicbuilding blocks and properties, kaolinite comprises tightly bound clay platelets that form large tactoids (aggregated stacks of platelets) and is considered inactive, whereas smectite is considered a highly active claymineral (Jefferson & Smalley, 1997; Skempton, 1953). 2021. The Authors.This is an open access article underthe terms of the Creative CommonsAttribution-NonCommercial-NoDerivsLicense, which permits use anddistribution in any medium, providedthe original work is properly cited,the use is non-commercial and nomodifications or adaptations are made.LEHMANN ET AL.In many land surface and Earth system models (the latter term includes simulations of aspects of the Earthsystem affecting the occurrence of natural hazards), information on ‘clay’ is often used in an undifferentiated manner as soil ‘clay content’ (defined as the mass fraction of soil particles smaller than 2 μm in diameter)to derive spatially distributed soil hydraulic and mechanical properties using pedotransfer functions (Gutmann & Small, 2007; Van Looy et al., 2017). The large differences in microstructure and hydration responsebetween kaolinite and smectite, along with their remarkable spatial segregation in climatic regions, havea large effect on the regional clay mineral-dependent soil hydraulic and mechanical properties. We seek to1 of 10
Geophysical Research Letters10.1029/2021GL095311Figure 1. Global distribution of kaolinite clay in tropical regions controlled by precipitation patterns (MSWEP,Beck et al. ). Kaolinite-rich regions (map adapted from Ito & Wagai, 2017) coincide with high mean annualprecipitation ( 1,000 mm) shown in (a) and high mean annual air temperatures ( 12 C). The spatial distribution ofkaolinite clay (b) is expressed as clay mineral mass per total mass of claysized soil particles ( 2 μm).capitalize on the recent mapping of clay minerals and the strong spatial segregation of these two dominantclay mineral types to improve the representation of soil hydraulic and mechanical properties (SHMP) inland surface models.Hodnett and Tomasella (2002) showed that two important soil hydraulic parameters, the water retentionparameterE [LE 1] ( is linked to the inverse capillary head for air entry pressure) and the saturated watercontentE sat [L3 L 3] are considerably higher in tropical regions for the same clay fraction. A recent review byLuijendijk and Gleeson (2015) also reported higher saturated hydraulic conductivity valuesEK sat [L T 1] forkaolinite clays relative to smectite.An important aspect of clay mineral effects is their impact on soil mechanical properties, which plays a significant role in natural hazards (Regmi et al., 2013; Skempton, 1985) and soil erosion (Blattmann et al., 2019;Ramezanpour et al., 2010). As shown in an illustrative example from Tiwari and Ajmera (2011) in the Figure S1, various mechanical properties change with clay mineral type. For kaolinite, the peak and residualfriction angle [ ] is much larger compared to smectite for the same clay content. Friction angle is a key mechanical property and is used to define the internal shear stress that is required to produce ‘displacement’(i.e., the onset of plastic shear strain). For soils with low clay fraction, the residual friction angle is high andis controlled primarily by the angularity of sand and silt particles that make up the soil matrix. The presenceof even small fractions of smectite drastically reduces the shear strength behavior of soil, particularly witha precipitous decrease in residual friction angle (relative to only a modest decrease in kaolinite-dominatedsoil). This sensitivity of friction angle to clay content and clay mineral type provides a potential avenue todefine pedotransfer functions (PTFs) for mechanical properties including clay mineral information. Thefew existing PTFs for friction angle neglect differences in clay mineral type and include only informationon soil textural fractions (Havaee et al., 2015) or other parameters describing the particle size distribution(Schjønning et al., 2020) or consistency limits (Lupini et al., 1981; Tiwari & Ajmera, 2011).The differences in SHMPs between smectite and kaolinite are attributed to differences in the microstructures of these clay minerals characterized by weak bonding between clay platelets in smectite, with numerous isomorphic substitutions that promote significant swelling and ease the onset of mechanicalLEHMANN ET AL.2 of 10
Geophysical Research Letters10.1029/2021GL095311deformation. The swelling and accessibility between clay platelets result in a large (internal) surface area forsmectite. The basic structure of the kaolinite building blocks results in strong bonds between clay plateletsthat promote stable tactoids and give rise to larger, often randomly oriented, microaggregates (Bourg andFranklin, 2017; Dor et al., 2020) with limited accessibility to the internal surface area. In addition to effectson nano- and mesopores in kaolinite, different soil formation processes in tropical regions with kaoliniteas the dominant clay mineral play a critical role in the SHMPs of tropical soils. As noted by Tomasella andHodnett (1996), clay soils from tropical regions often show ‘hybrid’ properties (a hybrid of ‘temperate’ clayand sandy soil properties) with high numbers of both large pores of biogenic origin and small pores betweenclay particles (Chauvel et al., 1991).Despite these well-known soil characteristics imparted by clay mineralogy (and the different soil formationprocesses in tropical and temperate regions), soil hydraulic properties estimated by PTFs often use clayfraction irrespective of dominant clay mineral type (Puhlmann & von Wilpert, 2012; Schaap et al., 2001;Vereecken et al., 1990; Weynants et al., 2009; Wösten et al., 1999; Zacharias & Wessolek, 2007; Zhang &Schaap, 2017). Several attempts have been made to include potential effects of clay mineral type by considering soil cation exchange capacity (Bruand, 2004; Tóth et al., 2015) or the cation exchange capacity per clayfraction (Pachepsky & Rawls, 1999; Rawls et al., 2001) in the training of PTFs. However, these approacheswere not systematically applied (or tested) for geographical regions with different dominant clay mineralsor soil formation processes.In light of the significant differences in SHMP for soils dominated by different clay mineral types, we expectthat modeling hydrologic (and mechanical) processes for tropical regions (with kaolinite clay type) usingPTFs trained with samples with more active clay types from temperate regions may lead to incorrect results.For example, Du et al. (2016) noted that land surface models underestimate subsurface runoff in the Amazon and attributed this difference to neglecting aggregates and macropores.The availability of recent global soils maps of clay mineral composition (developed at resolutions of 2' to2 grid cells; Ito & Wagai, 2017) offers the potential for a quantitative approach toward incorporating claymineral information in the spatial context to improve SHMP used for land surface parameterization inglobal models. The primary objective of this study is to propose methods for considering differences amongclay minerals at a large spatial scale and to define new types of PTFs. WeE use SSA and information on clayaggregation to guide the clay mineral-informed SHMP estimation. In addition, we will show how spatialmapping of SHMP is improved when using machine learning algorithms that are trained with data measured for different types of clay minerals.2. Theoretical Considerations to Estimate Clay Mineral-Informed SHMPs2.1. Modeling Soil Hydraulic PropertiesEELEHMANN ET AL.E [L2]) is an important water transport soilThe saturated hydraulic conductivityEK sat [L T 1] (or permeabilityproperty that varies with characteristics of soil (texture, porosity, surface area, and soil structure). As discussed in Text S2 and Figure S3, theE and SSA based onE soil K sat has been estimated from m the hydraulic properties are more controlled by soil structure (i.e., the spatial arrangement of the constituents) than by soil textural properties. Weattribute the higherEK sat values in tropical regions to structure formation with larger aggregates and largerpores. The joint effect of clay micro-aggregate formation (Jozefaciuk, 2009; Mansa et al., 2017) and bindingof sand and silt particles by clay (Wilson et al., 2014) affect the ability of soil to transmit water.ELEHMANN ET AL.E the parameter and theConsistent with the findings of Hodnett and Tomasella (2002), also the values ofE of corsaturated water contentE sat are larger in soils dominated by kaolinite clay (Figure S4). Large valuesrespond to weaker capillary forces and larger voids between stable aggregates. Note that the trend of larger values for tropical soils was also found for modeling the soil water retention with a bimodal distributionE values for tropical soils are attrib(Durner, 1994; see captions of Figure S4e). As with higherEK sat , largeruted to the formation of stable hierarchical structures in kaolinite-dominated soil. The observed trendsin valuesE of K satE and as a function of clay content were represented using the hierarchical mechanisticmodel presented in Section 2.1 and Text file S1. The packing of permeable aggregates and sand/silt particlessimultaneously satisfies porosity, permeability, and air entry pressure across the entire range of clay fractionand follows cues of soil structure formation and clay mineral substructures. To test if the hierarchical mechanistic model also reproduces soil hydraulic properties for regions rich in kaolinite that were not includedin the HYBRAS database, we made a literature searchE on K sat in tropical Africa (the references are listed inText S4). Figure 2b presents the 120 collected values and confirms the significantly higherEK sat values in soilscontaining kaolinite clay minerals.6 of 10
Geophysical Research Letters10.1029/2021GL095311Figure 3. A proposed pedotransfer function (PTF) for friction angle of soils using specific surface area (SSA) as asurrogate for clay type and content. (a) SSA as a function of clay fraction for smectite (active, swelling) compared tomoderately active illite and inactive (i.e., non-swelling) kaolinite clay. The lines show the relationships used for spatialmapping as discussed in Section 4.3 and Figure S6a in the Supporting Information and different symbols are usedfor different references. (b) Residual friction angle as a function of SSA. Red points represent admixtures with purekaolinite whereas blue points represent mixtures with pure smectite (black triangles for soil samples without claymineral information). The black solid line shows the PTF proposed in Equation 3.4.2. Clay Mineral-Dependent Soil Shear Strength PropertiesWe applied the PTF for soil macroscopic friction as a functionE of SSA (Equation 3). As depicted in Figure 3a,E the SSA is highly sensitive to the clay fraction and the clay mineral type. Generally, inactive and aggregatedclay minerals such as kaolinite exhibit higher strength (friction angle) through interparticle friction androlling, while the strength of smectite strongly depends on cation type and on the sliding resistance withinthe mineral interplatelet structure (Müller-Vonmoos & Løken, 1989).E Figure 3b illustrates the relationship betweenE ResESSA and residual internal friction angle. The residualfriction angle as well as peak friction angle (not shown) decreases precipitously with increasingESSA (thusproviding a means to normalize the influence of specific clay mineralE via SSA). Results show that the soilshear strength (friction angle) for soil mixtures of high clay fractions of kaolinite and smectite tend to col E for ResE(and Peaklapse to similar values on the curve in Figure 3b as a functionE of SSA. The PTF) based onSSA captures the measured relationship.Significant scatter exists at low surface areas that can be attributed to variability in cations of active clayminerals (depending on hydration radius of the cation and its charge; Kirchhof, 2017), as well as void structure and roundedness of grains. Similar PTFs could be used to evaluate upper- and lower-bounds of frictionto constrain the scatter at lower SSA, but are not shown herein. Overall, the trends of the PTF demonstrateE that SSA serves as a unifying means of assessing shear strength when clay minerals are present and the influence of coarse material (e.g., gravel) is minimal.4.3. Spatial Applications of Clay Mineral-Informed PTFsELEHMANN ET AL.Combining the clay mineral-informed PTFs for soil strengthE and K sat with the global clay mineral maps ofIto and Wagai (2017) enabled estimation of the spatial distribution of global SHMPs with a resolution of1/4 . The global maps are presented in the Supporting Information Figure S6Ewith SSA as a primary variable that carries clay mineral-type information (Figure ES6a), K sat (Figure S6b), and soil friction Eangle Resand Peak (Figures S6c and S6d). However, these simple maps only reproduce the main effects of clay mineraltype on SHMPs (represented by solid lines in Figures 2a and 3b) and do not represent local variations of other soil properties (e.g., bulk density and organic content, as shown in Figure S5E for K sat ) and soil formationprocesses (climate, topography, and vegetation). Thus, the results depict effective values for a given pixel,which should still be interpreted based on knowledge of local variations in terrain and soil type. To consider7 of 10
Geophysical Research Letters10.1029/2021GL095311Figure 4. Effects of clay mineral type on th
clay minerals at a large spatial scale and to define new types of PTFs. We use & 44" and information on clay aggregation to guide the clay mineral-informed SHMP estimation. In addition, we will show how spatial mapping of SHMP is improved when using machine learning algorithms that are trained with data meas-ured for different types of clay .
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