Fig. 1.

Location of the Amargosa Desert Research Site (ADRS) near Beatty, NV, and Death Valley, CA. Atmospheric deposition was collected at the ADRS weather station, and soil and plant sampling was done within approximately 3000 m of the weather station.


Fig. 2.

Local-scale sampling areas for two of three geomorphic settings studied (Table 1) wherein soil samples were collected with respect to shrub proximity. Shoulder slope (SS) and footslope (FS) sampling areas are shown; not shown are valley floor (VF) sampling areas, which were located away from knoll depicted above. Also shown are the hillslope-scale grid-sampling area (300 × 300 m) and shallow-soil and creosote bush leaf sample points. For hillslope segment analysis, each sample point was assigned one of three segments based on its land-surface elevation (Z, m): lower, 842 < Z ≤ 845; middle, 845 < Z ≤ 848; upper, 848 < Z ≤ 851.


Fig. 3.

Local-scale means comparisons for shallow-soil (0–30 cm) variables with respect to shrub proximity in three geomorphic settings: (A) perchlorate (ClO4); (B) chloride (Cl); (C) nitrate as N (NO3–N); (D–E) Cl/ClO4 and NO3/ClO4 molar ratios; (F–I) gravel, sand, silt, and clay contents; (J) total organic carbon (TOC); and (K) carbonate C (CO3–C). For significant ANOVA interactions (Table 2), paired comparisons used the S × P LSD test (0.05 level); for significant main effects, averages were compared using S or P LSD (0.05 level).


Fig. 4.

Variations in (A) precipitation, (B) atmospheric (wet plus dry, including dust) deposition ClO4 concentrations, and (C) atmospheric deposition ClO4 fluxes during 6-yr monitoring period (3 Aug. 2005 to 19 Aug. 2011). Samples for collection periods with <1.31 mm of precipitation were not analyzed.


Fig. 5.

Local-scale means comparisons for shallow-soil (0–30-cm) nitrate stable isotope ratios with respect to shrub proximity in three geomorphic settings: (A) δ15N and (B) δ18O. Means were calculated from four replications except for δ18O Shoulder slope–Near, Footslope–Near, and Valley floor–Near (n = 3). For (A), the ANOVA interaction was significant, and paired comparisons used the S × P least significant difference (LSD) test (0.05 level). For (B), the main effects were significant, and averages were compared using S or P LSD (0.05 level).


Fig. 6.

Conceptual diagrams of interactive factors and processes influencing perchlorate (ClO4) distribution in a desert landscape. (A) Topographic, soil, and plant effects on ClO4 deposition, transport, and soil–plant cycling. Smaller hillslope-segment leaf and shallow-soil (0–30 cm) ClO4/chloride (Cl) molar ratios (Tables 3 and 4) reflect greater cycling. (B) Soil–plant ClO4 cycle depicted for lower-slope position where features contributed to increases in available mass and cycling (cycle includes selective plant uptake, bioaccumulation, and leaf-drop return to the soil surface that results in soil ClO4 enrichment and recycling).