Contrasting effects of microbiotic crusts on runoff in desert surfaces Giora J. Kidron a, ⁎, H. Curtis Monger b , Ahuva Vonshak c , William Conrod d a Institute of Earth Sciences, The Hebrew University, Givat Ram Campus, Jerusalem 91904, Israel b Dept. of Plant and Environmental Sciences, New Mexico State University, MSC 3Q, Las Cruces, NM 88003-8003, USA c Dept of Dryland Biotechnologies, The Jacob Blaustein Institute for Desert Research, Ben Gurion University of the Negev, Sede Boqer Campus 84993, Israel d White Sands National Monument, National Park Service, United States abstract article info Article history: Received 6 October 2010 Received in revised form 16 November 2011 Accepted 20 November 2011 Available online 1 December 2011 Keywords: Biological soil crusts Physical crust Infiltration Microrelief Exopolysaccharides Chihuahuan Desert Microbiotic crusts (MCs) play an important role in surface hydrology by altering runoff yield. In order to study the crust's role on water redistribution, rainfall and runoff were measured during 1998–2000 at three sites within the northern Chihuahuan Desert, New Mexico, USA: the Sevilleta National Wildlife Refuge (SEV), the White Sands National Monument (WS), and the Jornada Experimental Range (JER). Whereas quartz and gypsum sand characterize the SEV and WS sites, respectively, both of which have high infiltration rates, silty alluvial deposits characterize the JER site. Runoff was measured in four pairs of 1.8–6.4 m 2 plots having MCs, one of which was scalped in each pair. No runoff was generated at WS, whether on the crusted or the scalped plots. Runoff was however generated at SEV and JER, being higher on the crusted plots at SEV and lower on the JER plots. The results were explained by the combined effect of (a) parent material and (b) the crust properties, such as species compo- sition, microrelief (surface roughness) and exopolysaccharide (EPS) content (reflected in the ratio of carbo- hydrates to chlorophyll). Whereas the effective rainfall, the fines and the EPS content were found to explain runoff initiation, the effective rainfall and the crust microrelief were found to explain the amount of runoff at SEV and JER where runoff generation took place. The findings attest to the fundamental role of the parent ma- terial and the crust's species composition and properties on runoff and hence to the complex interactions and the variable effects that MCs have on dryland hydrology. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Microbiotic crusts (MCs) abound in arid and semi-arid regions where they may have an important role in the ecosystem structure. Microbiotic crusts have been reported to stabilize the surface (McKenna Neuman et al., 1996; Belnap and Gillette, 1997; Marticorena et al., 1997), fix sub- stantial amounts of carbon (Lange et al., 1992) and nitrogen (Harper and Marble, 1988; Evans and Lange, 2001; Malam Issa et al., 2001), to in- fluence soil evaporation (Verrecchia et al., 1995) and plant germination (Prasse and Bornkamm, 2000). The crusts have been also reported to af- fect the surface hydrology. Nevertheless, although runoff was measured in tropical (Valentin and d'Herbès, 1999; van de Giesen et al., 2000), humid (Rutin, 1983; Jungerius and ten Harkel, 1994), temperate (Bergkamp et al., 1999; Knapen et al., 2007), and arid and semi-arid zones (Booth, 1941; Bergkamp et al., 1996; Li et al., 2008), the role of MCs as impacting surface hydrology is not yet clear (Eldridge, 2001; Warren, 2001a). Whereas some researchers observed that MCs promote runoff (Loope and Gifford, 1972; Brotherson and Rushforth, 1983; Graetz and Tongway, 1986; Kidron and Yair, 1997; Kidron et al., 2003), others maintained that MCs impede runoff (Finlayson et al., 1987; Harper and Marble, 1988; Greene and Tongway, 1989; Alexander and Calvo, 1990; Williams et al., 1995, 1999). Mazor et al. (1996), for instance, showed a substantial clogging of the surface by the crust sheath and the matrix of extracellular slime. As sheaths and slime are composed of extracellular polysaccharides, known as exo- polysaccharides, EPS, i.e., mainly carbohydrates excreted beyond the cyanobacteria cell boundary, high ratios of carbohydrates to chloro- phyll or carbohydrates to protein were interpreted as indicating high amounts of EPS (Kidron et al., 1999). Capable of absorbing copi- ous amounts of water upon wetting (up to 12–13 times their own weight, Durrell and Shields, 1961; Campbell, 1979), EPS swelling ap- pears to result in substantial clogging of the surface, thus hindering water infiltration through the surface crust following high intensity rains. Yet, the contrasting results published in the literature regarding the effect of crusts on runoff (or infiltration) may have stemmed from the different parent materials. Thus, while silty (loessic) soils will readily generate runoff, usually as a result of the formation of a physical crust, PC (Blackburn, 1975; Tarchitzky et al., 1984; Warren, 2001a; Wuest et al., 2006), sandy soils do not always form a PC (used herein as a generic term to describe all non-biological crusts) Geomorphology 139–140 (2012) 484–494 ⁎ Corresponding author. Tel.: + 972 544 967 271. E-mail address: kidron@vms.huji.ac.il (G.J. Kidron). 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.11.013 Contents lists available at SciVerse ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph