A proposed master V horizon for near surface horizons with vesicular porosity Judith Turk University of California, Riverside Carrie-Ann Houdeshell USDA-NRCS, Victorville Robert Graham

Vesicular Horizons

Vesicular horizons Silt (%) 10 15 20 25 30 35 40 45 Depth (cm) 60 80 60 80 V horizon V horizon (Rossi, 2009)

Vesicular Horizon Formation

Formation in Laboratory Miller, 1971

Why we need a master V horizon: V horizons are critical to hydrologic and ecological function of arid and semi-arid lands V horizons are abundant and widespread Currently there is no NCSS-approved terminology for V horizons V horizons are designated as A horizons in NCSS terminology, but do not fit the main concept of an A horizon

Definition of proposed horizon V horizons: Mineral horizons that have formed at the soil surface, or below a layer of rock fragments (e.g., desert pavement), physical or biological crust, or recently deposited eolian material. They are characterized by the predominance of vesicular pores and have platy, prismatic, or columnar structure.

Hydrologic and ecological function: Vesicular horizons have low infiltration rates Young et al., 2004

Absent or weak V horizon Occur primarily in intercanopy spaces Intercanopy: Strong V horizon Undercanopy: Absent or weak V horizon

V horizon formation in intercanopy leads to heterogeneity of hydraulic properties Schafer et al., 2007

Infiltration Rate (cm hr-1) Infiltration rates in soils with V horizons are consistently lower than those with non-vesicular A horizons Region Infiltration Rate (cm hr-1) Reference V horizon A horizon Sonoran Desert 0.8 6.0-9.6 Musick, 1975 Central Mojave 0.3-0.8 6.8-15 Young et al., 2004 1.3-4.6 8.9 Miller et al., 2009 Northern Mojave 1.2-4.5 5.5-17 Schafer et al., 2007 0.4-1.4 3.1-3.2 Eckert et al., 1975 Great Basin 1.7-3.2 5.8-7.2 Blackburn, 1975 Patagonia 0.6 4.1 Rostagno, 1989

Runon from intercanopy soils helps to support plant growth in shrub islands

Distribution of V horizons V horizons occur on every continent In the United States there are 1460 soil series with vesicular horizons These soil series include Aridisols, Mollisols, Entisols, Alfisols, Inceptisols, Andisols, and Vertisols The total mapped area of these soil series is 200,000 km2 (52 million acres)

Basin and range and intermontane plateaus

Current vesicular horizon nomenclature Terms used in scientific literature Av horizon Vesicular layer Schaumböden (foam soil) FAO-WRB Yermic diagnostic horizon (desert pavement and vesicular layer) USDA-NRCS Usually designated as A horizons No terminology specific to the vesicular horizon

Does not fit the main concept of A horizon Limitations of current NRCS nomenclature: Vesicular horizons as A horizons Does not fit the main concept of A horizon Not enriched in humified organic matter Designated as A horizons because they are at the surface Does not distinguish between surface horizons that support rapid infiltration and those that impede infiltration (V horizons) in desert landscapes

Summary Currently there is no NCSS terminology to distinguish the vesicular horizon from other types of surface horizons. Adoption of a master V horizon by the NCSS will highlight the presence of a widespread and ecologically important soil feature in arid and semi-arid lands.

Acknowledgments Supporting research funded by: USDA-NRCS Helpful comments on the proposal were provided by: Joe Chiaretti Craig Ditzler Brenda Buck Patrick Drohan Les McFadden