See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236670364 Biological Soil Crusts in the Mojave Desert, USA: Micromorphology and Pedogenesis Article in Soil Science Society of America Journal · September 2012 DOI: 10.2136/sssaj2012.0021 CITATIONS READS 25 446 3 authors: Amanda (Mandy) Williams Brenda Buck SWCA Environmental Consultants University of Nevada, Las Vegas 35 PUBLICATIONS 162 CITATIONS 102 PUBLICATIONS 970 CITATIONS SEE PROFILE SEE PROFILE Mengesha Beyene U.S. Department of Transportation 11 PUBLICATIONS 50 CITATIONS SEE PROFILE All content following this page was uploaded by Amanda (Mandy) Williams on 13 January 2017. The user has requested enhancement of the downloaded file. Pedology Biological Soil Crusts in the Mojave Desert, USA: Micromorphology and Pedogenesis Biological soil crusts (BSCs) are bio-sedimentary complexes that play criti- Amanda J. Williams* cal ecological roles in arid landscapes; however, the interactions between formerly at component biota and sediments are poorly understood. A detailed micro- Dep. of Geoscience morphological investigation of BSC development and crust microstructure in currently at School of Life Sciences the Muddy Mountains Wilderness Area, Nevada, examined features in thin Univ. of Nevada, Las Vegas section using petrographic microscopy, light microscopy, scanning electron 4505 S. Maryland Pkwy. microscopy, and energy dispersive x-ray spectroscopy. The >1800 microscopic Las Vegas, NV 89154-4004 observations were linked to crust macroscale features and soil geomorphology. Complex bio-sedimentary structures of BSCs reflect a dynamic genetic histo- Brenda J. Buck ry and diverse formative processes, including: (i) stabilization and authigenic Dep. of Geoscience mineral precipitation; (ii) wetting–drying and expansion–contraction; (iii) dust Univ. of Nevada, Las Vegas capture; (iv) microscale mass wasting; and (v) vesicular (Av) horizon forma- 4505 S. Maryland Pkwy. tion. A new conceptual model for hot deserts illustrates how these processes Las Vegas, NV 89154-4010 co-develop with BSC succession, during countless wet–dry cycles, to build up pinnacle microtopography while simultaneously forming Av horizons in the Mengesha A. Beyene bio-rich and bio-poor zones. Complex surficial and internal bio-sedimentary formerly at structures, which vary as a function of crust morphology, trap surface water Dep. of Geoscience for uptake by crust organisms, while dust influx provides a source of nutrients. Univ. of Nevada, Las Vegas 4505 S. Maryland Pkwy. These phenomena influence landscape-scale water dynamics and biogeochemi- Las Vegas, NV 89154-4010 cal cycling, increasing the availability of soil resources during times of biotic stress. Biological soil crusts uniquely facilitate the accumulation, morphology, currently at and ecosystem function of dust and should, therefore, be considered critical SES Group & Associates, LLC agents in arid pedogenesis and landscape development. Turner-Fairbank Highway Research Center/FHWA Abbreviations: BSC, biological soil crust; EPS, extracellular polymeric secretions; XPL, 6300 Georgetown Pike cross-polarized light. McLean, VA 22101 iological soil crusts are complex matrices of cyanobacteria, mosses, lichens, bacteria, algae, and fungi that fuse around soil particles to create a living, protective membrane in arid soils (Eldridge and Greene, 1994). Biological Bsoil crusts control the movement of water, gases, and solutes across soil surfaces (Belnap et al., 2003) and prevent desertification by impacting particle detachment and transport (McKenna Neuman et al., 1996; Miralles-Mellado et al., 2011), wa- ter and energy balances (Belnap, 2006), soil fertility (Kleiner and Harper, 1977; Evans and Belnap, 1999), and plant community establishment (Li et al., 2005; Escudero et al., 2007). Biological soil crusts are extremely fragile and sensitive to physical impacts such as off-road vehicles, hiking, and grazing, making them excel- lent indicators of disturbance (Belnap, 1998). Biological soil crust development follows a natural succession (Belnap, 2001), beginning with the development of smooth cyanobacterial-algal crusts and, un- der favorable conditions, is followed by formation of short moss-lichen crusts and eventually tall moss-lichen pinnacled crusts (Fig. 1). Estimated rates of crust forma- tion, or recovery after disturbance, range from 10s to 1000s of years depending on climate, soil texture, and geomorphic stability as well as metabolic adaptations and doi:10.2136/sssaj2012.0021 Received 13 Jan. 2012 *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. Soil Science Society of America Journal Fig. 1. (A) Cyanobacteria crusts display slightly darkened surfaces (arrow) and (B) knob features (arrow; marker for scale); (C) tall, pinnacled crusts with extensive lichens (l) have summits cemented in authigenic mineral precipitates (p); and (D) short, moss-dominated crusts accumulate eolian sand grains. reproductive strategies of the component organisms (Belnap and MATERIALS AND METHODS Warren, 2002; Thomas and Dougill, 2007; Kidron et al., 2008; The Hidden Valley portion of the Muddy Mountains Williams et al., 2008; Langhans et al., 2010). Previous studies Wilderness Area in Nevada is an ideal natural laboratory to in- have identified recovery times of individual species but not res- vestigate BSC microstructure. This semi-enclosed basin contains toration of bio-sedimentary features. Despite the conspicuous a variety of BSC types that inhabit a diverse sequence of geomor- surface microtopography of moss-lichen pinnacles (Fig. 1C), few phic surfaces and soils formed from mixed limestone and sand- have critically analyzed their internal structure (e.g., Campbell, stone parent materials. The study area lies within the northern 1979; Danin and Ganor, 1991; Belnap, 2001; Miralles-Mellado reaches of the Mojave Desert at approximately 1000-m elevation, et al., 2011) or the genesis of their unique morphology in hot, where mean annual temperatures are 27°C and mean annual pre- arid deserts. These microfeatures potentially control distribu- cipitation is 114 mm (Gorelow and Skrbac, 2005). Most pre- tions of soil water, nutrients, and energy and should, therefore, cipitation falls from January to March, when average highs range be investigated to maximize restoration efforts and to under- from 14 to 21°C, while some rainfall occurs in July and August, stand their role in pedogenesis and landscape development. when average highs range from 39 to 40°C (Gorelow and Skrbac, To better understand the structure, function, and genetic 2005). Given its proximity to the Las Vegas, NV, metropolitan history of tall moss-lichen pinnacled crusts in hot environments, area and its designation as a wilderness area, the valley is of criti- we completed a micromorphological investigation of the biotic cal concern to the Bureau of Land Management. and mineral components of several BSC types from Hidden In June 2009, 70 BSC samples were collected, which includ- Valley, Muddy Mountains Wilderness Area, in the Mojave ed (i) tall, moss-lichen pinnacled crusts and (ii) cyanobacteria Desert of southern Nevada (Fig. 2). This work provides insight crusts, which were collected adjacent to (iii) short moss-lichen into BSCs’ role in dust accretion and pedogenesis and identifies crusts (Williams, 2011, p. 10–103). Crust surfaces and cross- crust microstructures that potentially influence soil resource al- sections were examined for macroscopic features, such as pores, location and water dynamics. cracks, voids, microscopic surface relief, pedogenic structure, 6 Soil Science Society of America Journal and soil aggregation. Ten randomly distributed sites were sampled, including eolian and alluvial sur- faces that ranged in age from recent Holocene to Pleistocene. Corresponding soil profiles and surfac- es were described and characterized in two compan- ion studies (Williams, 2011, p. 104–350). Twenty-eight select BSC specimens were im- mersed in Spurr low-viscosity embedding medium and placed in a vacuum desiccator for 4 to 6 h under increasing pressure (1.03–4.14 MPa) and subse- quently in an oven at 70°C for 16 h. Embedded sam- ples were cut into 39 vertically oriented billets using a rock saw equipped with a diamond ISOMET wafering blade and an oil-based lubricant to pre- vent salt dissolution. Billets were ground into thin sections in lubricating oil (National Petrographic Service). Initially, thin sections were imaged under a digital flatbed scanner. Micromorphological fea- tures were then characterized at 4´, 10´, and 20´ magnification under a Nikon Eclipse LV100POL polarized light microscope and under a light mi- croscope at 0.8´ to 3.2´. Microscopic photos were taken using digital still cameras. Lastly, thin sections were sputter coated in Au for 30 s with a Cressington 108 Auto Sputter and examined and Fig. 2. The study area, the Hidden Valley Area of Critical Environmental Concern imaged in the University of Nevada, Las Vegas (ACEC), lies within the Muddy Mountains Wilderness Area, Nevada.