Pedogenesis and Soil-Geomorphic Relationships in an Arid Mountain Range, Mojave Desert, California
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Pedology Pedogenesis and Soil-Geomorphic Relationships in an Arid Mountain Range, Mojave Desert, California Mountains are impressive features of many desert landscapes because of their elevation, complex topography, and Daniel R. Hirmas* sheer extent. Soil genesis and landscape processes were studied in the southern Fry Mountains, Mojave Desert, Dep. of Geography California. Our aim was to better understand the processes responsible for the distribution of soil properties in Univ. of Kansas this landscape. Measured properties in 65 soil pits across the study site show that dust, soluble salt, NO −–N, and Lawrence, KS 66045-7613 3 carbonate distributions are correlated with the prevailing wind direction. Th is fi nding suggests that the mountain range eff ectively traps eolian sediment. Soils mantling these mountains have accumulated, on average, 41 kg m−2 Robert C. Graham −2 −2 − −2 Soil and Water Sciences Program silicate dust, 172 g m soluble salts, 3.3 g m NO3 –N, and 79 kg m carbonate and reached maximum −2 −2 −2 −2 Dep. of Environmental Sciences concentrations of 156 kg m , 1800 g m , 43 g m , and 398 kg m , respectively, on windward sides of the range. Univ. of California Th e basin fl oor encompassing Soggy Lake, an upwind playa, is the probable primary source of these materials. Riverside, CA 92521-0424 Soil morphology and land surface characteristics from four major mountain landforms were used to interpret the pedogenic and soil-geomorphic processes that have led to the distribution patterns of these accumulations. Our − study demonstrates that arid mountains accumulate and store appreciable quantities of dust, soluble salts, NO3 , and carbonate and are therefore important to the overall geomorphic evolution and biogeochemical cycling of the region. Th e previously unaccounted storage of pedogenic carbonate in similar mountain ranges could increase the global soil inorganic C pool estimate by as much as 15 to 174 Pg C. Abbreviations: CCE, calcium carbonate equivalent; EC, electrical conductivity; SAR, sodium adsorption ratio; TDS, total dissolved solids. ountains are distinct and conspicuous parts of the desert landscape. Th ey Mcomprise approximately 38% of desert lands of the southwestern United States (Clements et al., 1957) and are characterized by extreme topography, com- plex slopes with steep gradients, and distinct geomorphic junctions at the moun- tain–piedmont boundary (Cooke et al., 1993). Because of their extent and setting, mountains probably control or strongly infl uence surfi cial processes in the desert landscape, yet few studies have directly explored pedogenic and soil-geomorphic processes of these landforms. Previous work has shown that, for desert soils generally, eolian dust accumulation and the amount and pattern of water infi ltration are key determinants of pedogenesis (Brown and Dunkerley, 1996; Shafer et al., 2007). In turn, dust trapping and infi ltration are controlled by land surface characteristics and near- surface soil horizons. For example, desert pavements and vesicular horizons are the products of those surface characteristics that are able to trap and store considerable quantities of dust (Wells et al., 1985). Th us, the process of dust fl ux across and into the surface is immensely important to the landscape hydrology and geomorphology in these arid systems. Although dust is a major contributor to soils of desert piedmonts (Simonson, 1995), several studies have shown that dust also accumulates in arid mountains. Soils under steep bouldery talus slopes of the Buckskin Range, Nevada, are deep and well developed (Blank et al., 1996). Th ese soils have resulted from a combination Soil Sci. Soc. Am. J. 75:192–206 Posted online 17 Nov. 2010 doi:10.2136/sssaj2010.0152 Received 29 Mar. 2010. *Corresponding author ([email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. 192 SSSAJ: Volume 75: Number 1 • January–February 2011 of eolian deposition, subsequent mineral transformation to of Los Angeles. Th e study area is a bolson (i.e., an internally drained smectites, and the upward raft ing of talus. Dust-fi lled pockets intermontane basin; Peterson, 1981) and includes Soggy Lake playa and crevices in the bedrock of mountain peaks and isolated ridges (Fig. 1). Bedrock exposures in the Fry Mountains include Mesozoic occur across the Mojave Desert (Reynolds et al., 2006). Th e diorites, monzonites, and interspersed high-grade metamorphic rocks propensity for dust accumulation in the soils of arid mountains (Nash, 1988). Soil ages on the bajada, estimated from luminescence has been linked to the distinctive land surface characteristics of dates at nearby trenches, range from 4400 to 28,000 yr (Rockwell et the uplands (Hirmas, 2008), yet few studies have investigated al., 2000). Annual rainfall commonly varies between 76 and 127 mm this connection. and occurs predominantly during the winter months (Howell et al., In this study, we examined the relationships between soil 2007), which yields an aridic soil moisture regime that borders on genesis and geomorphologically controlled dust fl ux in an arid xeric. Creosote-bush [Larrea tridentata (DC.) Coville] and burro-weed mountain range by integrating multiple techniques. Th e ultimate [Ambrosia dumosa (A. Gray) W.W. Payne] dominate the vegetation, goal of this work was to understand and isolate the relevant with brittlebush (Encelia farinosa A. Gray ex Torr.) locally prominent in soil-geomorphic processes responsible for the distribution of soil the mountains and saltbush (Atriplex spp.) common at the playa margin morphological, textural, and chemical properties in this landscape. (Hirmas et al., 2010). Elevations within the ?430-ha study area range from 875 m in the playa to 1200 m at the mountaintop (Fig. 1). MATERIALS AND METHODS Soggy Lake playa is intermittently fl ooded by runoff from the Study Area surrounding piedmont and uplands during fl ash fl oods (French, Th is study was conducted in the southern Fry Mountains, 1977). Studies on paleolake levels and alluvial fans in the Mojave Mojave Desert, California (Fig. 1), approximately 150 km northeast Desert suggest that the region experienced several periods of increased moisture since the last glacial maximum (e.g., Enzel et al., 1989, 2003; Ely et al., 1993; Wells et al., 2003). Th ese wetter periods are known to be associated with intermittent lake stands of the nearby Mojave River basin. We are, however, unaware of any studies documenting the paleohydrology of Soggy Lake. Th e site included six watersheds of the southern Fry Mountains, the associated piedmont, and the basin fl oor. Th e mountains were subdivided using the taxonomic logic of Peterson (1981) and terms consistent with Wysocki et al. (2000) where applicable. Previous work at the site revealed four major landforms within the southern Fry Mountains: mountaintop, mountainfl at, mountainfl ank, and mountainbase (Hirmas, 2008). Briefl y, mountaintops are defi ned as crest or ridgeline positions on a mountain and are characterized by gentle ridge slopes oft en interrupted by crags or knolls, which may form small isolated peaks. Mountainfl ats are broad, open expanses of subdued, low-relief topography containing hills, colluvial aprons, and pediments that are located within the mountain range and are elevated above the surrounding piedmont by at least 50 to 100 m. Mountainfl anks are long Fig. 1. Location and soil classifi cation of sampling sites in the southern Fry Mountain bolson study complex sideslopes of a mountain. area. Location of the Lenwood fault is approximate; H = Haplo, L = Lithic, P = Petro, N = nodic, S = Mountainbase landforms occur at Sodic, X = Xeric, THS = Typic Haplosalid, CA = Calciargid, HA = Haplargid, TO = Torriorthent, TPSA = the base of mountain slopes and are Torripsamment, CAL = calcid, and CAM = cambid. Elevation contours are given in 5-m intervals. SSSAJ: Volume 75: Number 1 • January–February 2011 193 composed largely of thick wedges of colluvium. Bench landforms, which was collected from each location. Th e particle size distribution was occurred within the mountainfl anks, were grouped with mountaintops determined on suspensions of soil, dust trap, and natural trap samples on the basis of similar land surface characteristics, such as the frequency, using a Horiba LA 930 laser scattering particle-size and distribution size, and sorting of surface clasts. Th e locations of these landforms analyzer (Horiba Instruments, Irvine, CA), which yielded data in 73 within the study area are shown in Fig. 1. bins between 0.115 and 2000 μm (Hirmas et al., 2010). Saturated soil pastes were extracted with the procedure outlined Soil Sampling and Analyses in Soil Survey Laboratory Staff (1996). Soil weights and water volumes Th e number of sampling sites was approximately proportional to for each paste were recorded. Th ese values were combined with bulk the map area of each landform (Fig. 1) and sampling locations were density to convert concentration data from each horizon to a mass-per- randomly distributed within each landform. At each sampling site, the volume basis according to following land surface characteristics were measured: percentage of land CC f f surface covered by clasts, mean clast width, clast frequency, clast sorting, spRR sp 11 ad gr S b 1! 76 [1] percentage of land surface covered by overlapping clasts, and land C −3 C surface roughness. Mean values and procedures used to measure these where is concentration (g m ), sp is concentration of the saturation −1 variables are given by Hirmas (2008). Briefl y, a 1-m tape transect was paste extract (mg L ), θsp is the gravimetric water content of the placed on the ground in a random orientation.