2006-2011 Mission Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem Functions Across Spatial and Temporal Scales Progress Report: 2006022, 1/1/2007-12/31/2007 Linking Arid Land Surface Characteristics to Soil Hydrologic and Ecosystem Functions in Mojave Desert Landscapes Robert C. Graham *1 and Daniel R. Hirmas 1 Introduction Desert lands comprise more than 6.5 million ha in Southern California and are increasingly impacted by human activities. This increased land use poses significant environmental challenges, including hazardous dust, runoff and erosion, degradation of habitats, and invasion of exotic species. Previous research and our own observations suggest that most of these issues are directly related to soil surface conditions. Our overarching hypothesis is that the physical characteristics of the land surface, such as desert pavement, the amount of bare soil, and the structure and texture of A horizons, closely control key soil processes related to ecosystem function in the Mojave Desert. Objectives (1) Develop a classification of land surface conditions within the Mojave Desert landscape by distinguishing and defining mesoscale (several to tens of meters) land surface units within mountain, piedmont, and playa components of the landscape. (2) Characterize the soils associated with the land surface units in order to interpret the hydrologic and ecosystem functions of the soils. (3) Develop a conceptual model of how the identified mesoscale units function in the broader landscape-scale context. Approaches and Procedures Work thus far has concentrated on expanding the landform classification system of Peterson (1981) to include desert mountain ranges, testing the general application of the system in the Mojave Desert, and sampling delineated units to determine which land surface features and soil characteristics have the most utility for interpreting processes. Aerial photographs (1:24,000 scale) were viewed stereoscopically to subdivide and delineate the mountains, piedmont slope, and basin floor into landform components. Sites within these landform components were characterized with respect to surface conditions (e.g., clast cover, bare soil, vegetation, surface roughness) and soil properties (e.g., particle-size distribution, soluble salts). Results Sixteen landform units were identified in the mountain-piedmont-playa landscape of the southern Fry Mountains east of Victorville (table 1). Units defined by Peterson (1981) worked well for the piedmont and basin floor, but seven new units were developed for the mountains. Mean 1 University of California, Riverside * Principal Investigator
values of five selected surface characteristics, from the >25 measured, are presented in table 1. Mean slope gradients in the mountains ranged from 15 to 49%, with an overall average of 32%, compared to overall averages of 7% for the piedmont and 0% slope for the basin floor. Mean clast cover was nearly twice as great in the mountains as on the piedmont, as was clast width. The basin floor had no clasts. Surface roughness also was much greater in the mountains than on the piedmont. Shrub cover was variable across the landscape, ranging from 0% on the playa to 11% on some fan units of the piedmont. Based on this classification (table 1) and a previous finer scale classification by Wood et al. (2002), we sampled to determine if the land surface-soil relationships discovered by Wood et al. (2005) were broadly applicable in the Mojave. We sampled paired pedons at three geographically and geomorphically distinct areas in the Mojave Desert, as described in the caption to figure 1. Each pair consisted of a soil under desert pavement and a soil without desert pavement. The soils without desert pavement had low concentrations of chloride (<150 mg kg -1 ) and nitrate-n (<25 mg kg -1 ) in saturation paste extracts (fig. 1). In contrast, those soils under desert pavement had exceptionally high concentrations of both chloride (1,900 2,800 mg kg -1 ) and nitrate (200-800 mg kg -1 ). The highest concentrations of both occurred within the 10- to 50-cm depth. Discussion The landform classification system is easily applicable at the 1:24,000 scale. The landform units have distinctly different surface characteristics that are visible in the field and have been quantified by this research (table 1). We expect that these surface characteristics will serve to further define mesoscale units within the landform units presented in table 1. The results of our soil investigations (fig. 1) are consistent with the results of Wood et al. (2005), which showed that soils under desert pavement contain much higher levels of soluble salts than adjacent non-pavement soils. These salts are part of the eolian dust input derived from playas and trapped by the desert pavement. Desert pavement and the attendant vesicular A horizons have exceptionally low infiltration rates (Young et al. 2004; Wood et al. 2005), so the salts are leached into the soil, but accumulate at a shallow depth. Most water from intense rainfall runs off desert pavement rather than infiltrating, leaving the underlying soils drier than the general climate would suggest. Conversely, downslope soils that lack desert pavement, such as those around shrubs where burrowing animals disrupt the surface, receive runoff from desert pavement and are more strongly leached than the climate suggests. The result is a landscape with strongly contrasting hydrologic environments in immediate proximity to each other: desert pavement soils that have very low infiltration rates and accumulate salts, and non-pavement soils that have high infiltration rates and are leached of salts (fig. 2). The discovery of high nitrate concentrations in the soluble salt load of desert pavement soils is a new development (Graham et al. 2008). The source of the nitrate is unknown, but may be both atmospheric and biological. The role of this nitrate in ecosystem function remains to be explored. We plan to continue to refine our understanding of the different kinds of desert surface conditions, including pavements, their spatial distributions, and their effects on the distribution of water and the disposition of soluble salts. 2
References Graham, R.C., D.R. Hirmas, Y.A. Wood and C. Amrhein. 2008. Large near-surface nitrate pools in soils capped by desert pavement in the Mojave Desert, California. Geology 36:259-262. Peterson, F.F. 1981. Landforms of the basin & range province defined for soil survey. Nevada Agricultural Experiment Station Technical Bulletin 28. University of Nevada, Reno. Wood, Y.A., R.C. Graham and S.G. Wells. 2002. Surface mosaic map unit development for a desert pavement surface. Journal of Arid Environments 52:305-317. Wood, Y.A., R.C. Graham and S.G. Wells. 2005. Surface control of desert pavement pedologic process and landscape function, Cima Volcanic Field, Mojave Desert, California. Catena 59:205-230. Young, M.H., E.V. McDonald, T.G. Caldwell, S.G. Benner and D.G. Meadows. 2004. Hydraulic properties of a desert soil chronosequence in the Mojave Desert, USA. Vadose Zone Journal 3:956-963. Table 1. Landform units and selected characteristics for a test area (southern Fry Mountains) in the Mojave Desert. Landform unit Slope (%) Clast cover (%) Clast width (mm) Roughness (mm) Shrub (%) Bounding mountains 32.0 80.7 12.6 5.1 7.3 Mountaintop 16.1 85.5 8.3 3.4 5.6 Mountainflank 48.8 85.1 15.0 6.8 8.3 Bench 15.0 81.5 9.6 3.1 5.6 Mountainflat Colluvial apron 20.5 68.0 7.3 3.6 7.8 Hill 15.3 63.0 11.4 4.1 9.4 Pediment 19.0 63.5 11.9 3.6 9.0 Mountainbase 27.4 80.8 15.4 5.0 5.3 Piedmont slope 6.8 46.2 7.2 2.2 6.1 Alluvial fan 4.5 13.0 4.4 1.3 6.3 Erosional fan remnant 6.0 65.5 12.8 3.2 10.5 Inset fan 11.5 58.5 6.8 2.5 10.5 Fan piedmont Nonburied fan remnant 4.0 34.0 6.1 2.2 0.8 Erosional fan remnant 5.0 74.8 9.1 3.0 4.0 Inset fan 10.0 16.0 5.9 1.5 10.5 Fan apron 8.7 27.3 4.4 1.3 5.2 Basin floor 0.0 0.0 0.0 0.3 4.5 Playa 0.0 0.0 0.0 0.3 0.0 Floodplain playa 0.0 0.0 0.0 0.3 9.0 3
Figure 1. Chloride and nitrate-n concentration profiles for soils with desert pavement (CV1, PB1, FM1) and without desert pavement (CV2, PB2, FM2) at locations representing a variety of common geomorphic settings throughout the desert of southeastern California (Graham et al., 2008). CV = a 580,000 year old basalt flow in the Cima Volcanic Field south of Baker; PB = a Pleistocene alluvial fan in Pinto Basin, Joshua Tree National Park; FM = a mountainflat in the southern Fry Mountains east of Victorville. 4
Figure 2. Conceptual model relating land surface characteristics to near surface hydrology and the distribution of nitrate in desert pavement landscapes (Graham et al. 2008). This research was funded by the Kearney Foundation of Soil Science: Understanding and Managing Soil-Ecosystem Functions Across Spatial and Temporal Scales, 2006-2011 Mission (http://kearney.ucdavis.edu). The Kearney Foundation is an endowed research program created to encourage and support research in the fields of soil, plant nutrition, and water science within the Division of Agriculture and Natural Resources of the University of California. 5