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Geology; March 2008; v. 36; no. 3; p. 259-262; DOI: 10.1130/G24343A.1
© 2008 Geological Society of America
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Large near-surface nitrate pools in soils capped by desert pavement in the Mojave Desert, California

Robert C. Graham*,1, Daniel R. Hirmas1, Yvonne A. Wood2 and Christopher Amrhein3

1 Soil & Water Sciences Program, Department of Environmental Sciences, University of California–Riverside, Riverside, California 92521-0424, USA
2 University of California Cooperative Extension, Inyo-Mono Counties, 207 West South Street, Bishop, California 93514, USA
3 Soil & Water Sciences Program, Department of Environmental Sciences, University of California–Riverside, Riverside, California 92521-0424, USA


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
 REFERENCES CITED
 
We found exceptionally high nitrate levels (up to 12,750 kg ha–1) at shallow depths (≤1 m) in soils mantled by desert pavement, a common land-surface feature in arid regions. Nearby soils without desert pavement had nitrate contents that were one to two orders of magnitude lower. The soil conditions coincident with desert pavement (i.e., stability, antiquity, and virtually no leaching) favor the retention and accumulation of nitrate delivered by atmospheric deposition or in situ fixation. The nitrate stored in soils under desert pavement is a previously unrecognized pool of nitrogen that has the potential to increase the global nitrogen inventory for near-surface desert soils to five times previous estimates. Its near-surface occurrence makes this labile nitrogen pool particularly susceptible to mobilization by climate change or human disturbance, risking contamination of surface and groundwaters.

Key Words: desert pavement • soil • nitrogen • nitrate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
METHODS
 RESULTS
 DISCUSSION
 REFERENCES CITED
 
Nitrogen (N) is a nutrient element essential to life. As such, there is considerable interest in understanding its sources, forms, cycles, and reservoirs in Earth's diverse ecosystems (Schlesinger, 1997). Deserts account for about one-third of Earth's land area (Post et al., 1985; Abrahams and Parsons, 1994), yet the N stores and processes of these arid lands are incompletely understood. Nitrogen is delivered to desert landscapes by wet or dry atmospheric deposition (Böhlke et al., 1997; Michalski et al., 2004) and by biological N2 fixation (Evans and Ehleringer, 1993; Walvoord et al., 2003; Johnson et al., 2007), and, after water, it is the factor that most limits net primary productivity in desert ecosystems (Vitousek et al., 2002). On the other hand, deposition of excess N can be detrimental to ecosystem functions and water quality (Fenn et al., 2003).

Soil nitrate appears to be the primary pool of available nitrogen in desert regions, and its characterization is particularly important for understanding nitrogen cycling (Evans and Ehleringer, 1993). Because nitrate is highly soluble, it is readily taken up by plants and microbes in moist soils, and excess nitrate is deeply leached when soils are flushed by infiltrated water (Schlesinger, 1997; Groffman, 2000). Nitrate is reduced to N2O or N2 in suboxic and anoxic water-saturated soils. While the distribution of nitrate within desert soils has been shown to be highly variable (Hunter et al., 1982), it can only accumulate to significant levels in exceptionally dry soils where water infiltration and leaching are minimal. For example, the most concentrated surficial nitrate deposits are in the Atacama Desert of Chile, the driest location on Earth. They occur as isolated high-grade caliche-type deposits with nitrate-N concentrations estimated at 160,000 kg ha–1 (Ewing et al., 2006). These nitrate deposits are the result of long-term accumulation from atmospheric deposition under conditions of hyperaridity and virtually no leaching (Böhlke et al., 1997; Michalski et al., 2004). In the United States, concentrations of vadosezone nitrate are higher and occur nearer the surface in arid regions compared to semiarid areas, which have greater mean annual precipitation (Walvoord et al., 2003). These nitrate concentrations in the southwestern United States have been leached to the zone of accumulation during the past 10,000–16,000 yr.

Across desert landscapes, infiltration rates and leaching are spatially variable, depending on surface conditions (Wood et al., 2005). Desert pavement, a natural concentration of a more-or-less single layer of closely fitted clasts on the surface of desert soils (Figs. 1A and 1B), is particularly effective at impeding the infiltration of water (Cooke et al., 1993). The primary mechanism by which desert pavements form involves the trapping of eolian dust by the rough pavement surface, translocation of the dust under surface clasts, and the simultaneous rafting upward of the clasts on the accumulating mantle of eolian sediment (Wells et al., 1985; McFadden et al., 1987; Valentine and Harrington, 2006). Desert pavement surfaces are often elongate features because they occur on interfluves of dissected alluvial fans or on lava flows. They are generally on the order of 102 to 104 m2 in size. Nondesert pavement areas are found in shrub islands (≤100 m2) surrounded by desert pavement and on younger geomorphic surfaces associated with ephemeral washes that dissect the alluvial fans (Wood et al., 2005; McAuliffe and McDonald, 2006). Formation of well-developed desert pavement requires thousands of years. As desert pavements get older, surface clasts are comminuted by various processes, such as thermal expansion and salt crystallization. This yields greater clast density and surface cover, which, along with increased soil development, decreases infiltration and increases runoff (Wells et al., 1985). Thus, the soils beneath desert pavement are much more arid than the general climatic conditions suggest and are known to accumulate large quantities of soluble salts (Wood et al., 2005). We initiated this study to determine if nitrate is a significant component of those salt species.


Figure 01
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  		Figure 1. Photographs of (A) desert pavement landscape in Pinto Basin including nondesert pavement patches around shrubs ("shrub islands") and sampling excavations at PB1 and PB2 (PB—Pinto Basin), (B) desert pavement overlying soil at PB1, and (C) soil without desert pavement near shrub at PB2. Desert pavement forms a tight mosaic that severely impedes infiltration, while lack of pavement and extensive animal burrowing under shrubs (creosote bush, Larrea tridentata) produce a highly permeable soil. In (A), PB1 and PB2 are 38 m apart and the creosote bush is 1 m high. In (B) and (C), scale tape is marked in 10 cm increments.

 

   METHODS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
RESULTS
 DISCUSSION
 REFERENCES CITED
 
Three widely separated sampling locations with well-developed desert pavement were selected in the Mojave Desert of southern California (Fig. 2; Table DR1 in the GSA Data Repository1). These locations represent a variety of landforms common in the Mojave: lava flow (CV), alluvial fan (PB), and mountain range (FM). The CV site is on the 580 ka basalt flow at the Cima volcanic field where key soilgeomorphic research on desert pavement has been conducted (e.g., Wells et al., 1985; McFadden et al., 1987; Anderson et al., 2002; Wood et al., 2005). The CV soils sampled were from the desert pavement (DP2) and bare ground (BG3) surface mosaic units defined by Wood et al. (2002). The FM site is in granitic terrain of the southern Fry Mountains. We sampled two geomorphic surfaces there: a broad summit position (desert pavement) and a debris-flow deposit (nonpavement). The PB site (Fig. 1) is on a dissected Pleistocene alluvial fan containing gravels from metamorphic and granitic sources. All sites are on surfaces with slope gradients ≤25%. At each location, we excavated pits in a desert pavement soil and a nearby non-pavement soil. Desert pavement clasts exhibited varying degrees of dorsal desert varnish, ranging from patchy coatings on the rocks at the FM site to continuous coatings at the CV site. The desert pavements were all underlain by vesicular (Av) horizons on the order of 3–10 cm thick. Subsoil horizons under desert pavement included accumulations of calcium carbonate (calcic horizons) and clay (argillic or natric horizons). The nonpavement soils were disrupted by burrowing rodents and lacked Av horizons or had very thin (<1 cm), discontinuous ones. Soils were sampled by morphologic horizon and sieved to separate the fine earth (<2 mm diameter) and coarse fragment fractions. The fine earth fraction was extracted with deionized water, and the leachates were analyzed for Cl– by automated coulometric titration and for NO3– colorimetrically using an auto-analyzer (see GSA Data Repository [see footnote 1]). Nitrate-N was calculated on an aerial basis representing a 1 m depth by multiplying the concentration of nitrate-N in the fine earth fraction, the ratio of the fine earth to whole soil (including coarse fragments), and the bulk density of the whole soil. Soil properties are more completely reported in Table DR2 (see footnote 1).


Figure 02
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  		Figure 2. Map showing sampled locations in southeastern California. FM—Fry Mountains, CV—Cima volcanic field, PB—Pinto Basin.

 

   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
DISCUSSION
 REFERENCES CITED
 
We found that the soils under desert pavement (e.g., Fig. 1B) have high concentrations of nitrate close to the surface, reaching a maximum within 0.1–0.6 m depth (Fig. 3). At each location, the soils without desert pavement (e.g., Fig. 1C) were found to have low nitrate concentrations throughout the upper meter (Fig. 3). Chloride is commonly used as a conservative solute tracer to interpret long-term water flux in arid soils (Scanlon, 1991; Phillips, 1994; Wood et al., 2006). The chloride concentrations and distribution profiles in the nonpavement soils are indicative of well-leached soils, whereas the desert pavement soils have characteristics that indicate minimal leaching within the upper half meter. The depth distribution trends of nitrate, for the most part, correspond closely with those of chloride. The solute transport behavior of these two ions is nearly identical, such that chloride is generally used as a proxy for nitrate in solute transport studies (Stagnitti et al., 1998). It is unclear why nitrate and chloride distribution trends of the FM1 soil deviate below 0.4 m (Fig. 3), although nitrate responds to biological activity in addition to the hydraulic influences that control chloride distribution (Walvoord et al., 2003).


Figure 03
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  		Figure 3. Nitrate-N (NO3-N) and chloride (Cl–) concentration profiles for soils with (CV1, PB1, FM1) and without (CV2, PB2, FM2) desert pavement at locations indicated in Figure 2. Values are for water extracts of fine earth fraction (<2 mm diameter). Sites represent a variety of common geomorphic settings in southeastern California, including a 580,000-yr-old lava flow (CV), an alluvial fan (PB), and a mountain range (FM).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 METHODS
 RESULTS
 DISCUSSION
REFERENCES CITED
 
Desert Pavement Hydrology
The surface hydrology of desert pavement landscapes is key to the disposition of soluble compounds, including nitrate, in the underlying soils. Both the proportion of the land surface covered by desert pavement and the nature of the underlying soil affect the fate of water falling on the surface. Infiltration rates decrease with increased clast cover (Abrahams and Parsons, 1991) and in progressively older soils with increased development of the vesicular layer that underlies desert pavement (Young et al., 2004). Minimal infiltration results in minimal leaching, so salts that are delivered to the soil surface with the dust (Reheis and Kihl, 1995; Reynolds et al., 2006) are moved into the upper part of the soil and retained there. Desert pavement soils become saltier with time, and the distribution of salts within the profile is an excellent indicator of long-term behavior with regard to water infiltration and deep percolation (Young et al., 2004). Reduced infiltration also increases runoff, the flow of which may infiltrate in adjacent areas that lack desert pavement but support desert shrubs (Abrahams and Parsons, 1991), thereby leaching those soils and more deeply recharging them with water.

While soil hydrologic function has been closely linked to desert pavement age, single-aged geomorphic surfaces with desert pavement do not have homogeneous land-surface conditions. They are typically a mosaic of desert pavement areas that are virtually devoid of vegetation and other areas with more exposed bare soil that support clumps of shrubs and burrowing rodents (Figs. 1 and 4) (Wood et al., 2002; McAuliffe and McDonald, 2006). Desert pavement surfaces (Fig. 1B) effectively shed rainwater and are immediately adjacent to nonpavement areas (Fig. 1C) where infiltration is rapid and percolation is relatively deep (Fig. 4). Soils of the runoff areas (desert pavement) accumulate high concentrations of salts, while soils of the infiltration areas (bare ground) are leached of salts (Wood et al., 2005). Salt concentrations in soils under similar desert pavement types have been shown to be remarkably consistent across the single geomorphic surface of the CV site (Wood et al., 2005). Our data indicate that nitrate is an abundant component of the accumulated salts under desert pavement.


Figure 04
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  		Figure 4. Conceptual model of near-surface hydrology leading to NO3– distribution patterns in desert pavement landscape.

 
Soil Nitrate Inventory
The inventory of nitrate-N within the upper meter of soil under desert pavement is remarkably high, ranging from 8900 to 12,750 kg ha–1. These values are close to the highest values reported for subsoil (1–30 m depth) inventories (Walvoord et al., 2003). Nearby soils without desert pavement have much lower nitrate-N pools, ranging from 80 to 1500 kg ha–1, although these values are also within the range reported for the deeper subsoil inventories in the arid and semiarid southwestern United States (Walvoord et al., 2003). Apparently, the two pools of nitrate are nearly equal in magnitude, but the near-surface pool under desert pavement is concentrated within the uppermost meter of soil, whereas the subsoil pool is spread through a 29-m-thick zone. If desert pavement areas have both upper and lower pools of soil nitrate-N, their sum may exceed 25,000 kg ha–1.

The nitrate observed in association with desert pavement was consistent across landforms (lava flow, alluvial fan, mountain range) and at widely separated locations within the Mojave Desert (Fig. 2), suggesting a common and general association. If it occurs globally, the correspondence of high soil nitrate levels with desert pavement may substantially increase estimates of soil nitrogen inventories. Soils of warm temperate deserts worldwide have been estimated to average 1065 kg ha–1 of total nitrogen within the upper meter (Post et al., 1985), compared to the mean value of 10,500 kg ha–1 that we determined for nitrate-N under desert pavement. Desert pavement has been estimated to cover ~50% of desert landscapes in North America (Evenari et al., 1985). If this proportion is consistent for deserts worldwide, the order of magnitude increase in soil nitrogen inventory under desert pavement elevates the estimate of nitrogen stored in near-surface soils of warm deserts from 0.2 Pg (Post et al., 1985) to 1.1 Pg. Total nitrogen storage for soils worldwide is estimated at 95 Pg (Post et al., 1985), so recognition of additional nitrate-N in soils under desert pavement may, by itself, account for 1% of the global inventory.

Sources of Nitrate
Atmospheric deposition and biological nitrogen fixation have been recognized as sources of nitrate in arid soils (Böhlke et al., 1997; Walvoord et al., 2003; Michalski et al., 2004). Both of these sources likely contribute nitrate to desert pavement soils. Cyanobacteria associated with biological soil crusts in deserts are known to fix nitrogen and export excess nitrogen, including nitrate, to the soils on which they occur (Evans and Ehleringer, 1993; Johnson et al., 2007). Biological soil crusts were not prominent at the desert pavement sites we sampled, but nitrogenfixing cyanobacteria have been found in desert pavement landscapes (Schlesinger et al., 2003). Dry deposition rates of nitrogen are 4 kg ha–1 yr–1 near site PB (in Joshua Tree National Park) (Sullivan et al., 2001), and we measured 6–16 kg nitrate-N ha–1 yr–1 in dust traps at the FM site. These indications of dual sources are consistent with results of isotopic analyses of soil nitrate samples from the Mojave Desert, which implicate both atmospheric deposition and biological fixation as sources (Michalski et al., 2004).

Implications for Ecosystems and Environmental Quality
Desert ecosystems are predominately N-limited (Schlesinger, 1997; Vitousek et al., 2002), yet nitrate is concentrated in desert pavement soils. We speculate that high salinity and lack of water beneath desert pavement impede root access to the nitrate stored there. The extent to which this nitrate is accessible to plants around the margins of desert pavement, as in shrub islands (Fig. 1A), is unknown.

Desert land use (e.g., roads, off-road vehicle use, military training) often disrupts fragile land surfaces, increasing surface erosion by rain and wind (Lovich and Bainbridge, 1999). Eolian transport of N-laden dust from disturbed desert pavement soils may impact distant N-limited ecosystems, such as alpine lakes. Desert dust has been found in high elevation snow packs in the western United States (Painter et al., 2007). Increased water input by land use (e.g., irrigation, wastewater disposal), flooding, or climate change may transport high nitrate levels to groundwater or surface waters. Elevated nitrate levels in drinking water have been associated with serious health issues (Bouchard et al., 1992). Increased soil moisture also increases the potential for denitrification and the production of nitrous oxide, a major greenhouse gas (Groffman, 2000). Overall, the high concentrations of nitrate under desert pavement represent a large, near-surface pool of labile N that can be mobilized adversely within the environment in a variety of ways.


   ACKNOWLEDGMENTS
 
We thank the U.S. National Park Service and Bureau of Land Management for permission to sample on land administered by those agencies, and Brad Hewitt, Brandi Reese, Rebecca Lawson, Ed Betty, and Paul Sternberg for assistance with analyses. Comments by K. Anderson and two anonymous reviewers, and by K. Kendrick, D. Parker, and J. Sickman on an earlier draft, improved the manuscript. This research was funded by the University of California Kearney Foundation of Soil Science


   FOOTNOTES
 
GSA Data Repository item 2008062, references for analytical methods and Tables DR1 and DR2, is available online at www.geosociety.org/pubs/ft2008.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. Back

*robert.graham{at}ucr.edu Back


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Received for publication 6 August 2007

Revised manuscript received 21 November 2007

Manuscript accepted 27 November 2007





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