Post on 02-Jan-2016
description
A proposed master V horizon for near surface horizons with vesicular porosity
Judith TurkUniversity of California, Riverside
Carrie-Ann HoudeshellUSDA-NRCS, Victorville
Robert GrahamUniversity of California, Riverside
Silt (%)10 15 20 25 30 35 40 45
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V horizon
V horizon
Vesicular horizons
(Rossi, 2009)
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
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
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
Central Mojave 1.3-4.6 8.9 Miller et al., 2009
Northern Mojave 1.2-4.5 5.5-17 Schafer et al., 2007
Northern Mojave 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
Infiltration rates in soils with V horizons are consistently lower than those with
non-vesicular A horizons
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)
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
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.