Abstract Experiments were conducted before and during spring snowmelt in 1993 and 1994 at Niwot Ridge in the Colorado Front Range to assess the degree of ...
Biogeochemistry of Seasonally Snow-Covered Catchments (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 228, 1995.
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Snowpack controls on soil nitrogen dynamics in the Colorado alpine PAUL D. BROOKS Department of EPO Biology and Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309-0450, USA
MARK W. WILLIAMS Department of Geography and Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309-0450, USA
STEVEN K. SCHMIDT Department of EPO Biology, University of Colorado, Boulder, Colorado 80309-0340, USA
Abstract Experiments were conducted before and during spring snowmelt in 1993 and 1994 at Niwot Ridge in the Colorado Front Range to assess the degree of interaction between inorganic nitrogen (N) deposited in seasonal snowpacks and soil N pools in alpine environments. Soils typically froze in early winter with minimum soil temperatures inversely related to the depth of early season snowpacks. Minimum soil temperatures under late-accumulating, shallow snowpacks reached -10 to -14°C, while soils under deeper, earlier snowpacks reached minimum temperatures of - 5 to -6°C. Mineralization and nitrification inputs to the soil inorganic N pool were an order of magnitude higher than snowmelt inputs and were controlled by the timing and depth of snowpack accumulation. Ion exchange resin bags located at the soil surface indicated that the actual N inputs at any location were highly variable. About 90% of isotopically labelled 15NH4+ applied to the snow surface before melt was recovered in soil pools. Nitrogen mineralization in 1994 was generally higher (1712-1960 mg N mf2) and exhibited relatively little spatial variability (CV 0.04-0.26) under deeper, earlier accumulating snowpacks. In contrast, N mineralization under shallower, late-accumulating snowpacks was lower (511-1440 mg N mf2) and much more variable (CV 0.42-0.83). The lowest nitrification rates were found under deep/early snowpacks (8-18% of mineralized N); the highest were found under shallow/late snowpacks (16-58% mineralized N). These results indicate the timing and depth of snowpack accumulation plays a key role in nitrogen cycling in alpine ecosystems and may control inorganic nitrogen export in surface waters.
INTRODUCTION Elevated nitrate (N0 3 ) concentrations in high-elevation surface waters throughout the Colorado Front Range (Baron, 1991; Williams et ai, 1993) appear to be related to increased atmospheric N deposition (Grant & Lewis, 1982; Lewis et al, 1984;
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Bowman, 1992; Sievering et al., 1992). Increases in N concentrations in surface waters along the Colorado Front Range have been attributed to nitrogen saturation (Baron et al., 1994), and may serve as an early indicator of surface water acidification. In highelevation basins, a significant fraction of atmospheric N deposition is stored in seasonal snowpacks and released in an ionic pulse during the first portion of snowmelt (Williams & Melack, 1991a). Recent work has suggested that most of this snowmelt enters soil and has the potential to interact with both inorganic and organic soil N pools (Williams & Melack, 1991b; Williams et al., 1995). Biologically mediated soil processes, therefore, have the potential to affect the magnitude of the surface water N03" pulse observed during snowmelt. Soil microbial processes under seasonal snowpacks have been shown to affect nitrogen export in surface waters. Mineralization of organic matter followed by nitrification under seasonal snowpacks is apparently responsible for elevated stream water N03" concentrations in a northeastern US watershed (Rascher et al., 1987). Nitrate concentrations in stream water from an alpine/montane watershed in Colorado were inversely related to winter snow depths within the catchment (Lewis & Grant, 1980), presumably due to a change in the balance between N release and immobilization in snow covered basin soils. The factors controlling this balance between mineralization, nitrification, and immobilization in subnivian soils are unknown. This study examines the relationship between the timing and depth of snow cover, atmospheric N loading, and mineralization and nitrification in alpine soils at Niwot Ridge in the Colorado Front Range.
STUDY SITE All experiments were conducted on Niwot Ridge, Colorado (40°03'N, 105°35'W) located in the Front Range of the Rocky Mountains 5 km east of the Continental Divide. This site is an UNESCO Biosphere Reserve and has been the location of extensive research by the University of Colorado's Long-Term Ecological Research program. The climate is characterized by long, cold winters and short, cool growing seasons. Mean annual temperature is - 3 °C, annual precipitation is 900 mm, the majority of which falls as snow (Greenland, 1989). Sites were located at an elevation of 3510 m. Soils are cryochrepts and vary in depth from approximately 0.3 to 2.0 m overlying granitic parent material (Bums, 1980). Soil pH in the autumn of 1992 ranged from 4.6 to 5.0.
METHODS Experimental design To evaluate the interaction between snowmelt N and soil, isotopically labelled 15N-NH4+ was applied to the snowpack before melt in 1993 and followed throughout the spring and summer. N fluxes and pool sizes in soil and snow were monitored at two additional sites in 1993 to identify the major biogeochemical processes affecting N cycling under the snowpack. Measurements at unamended sites were continued and expanded during the winter and spring of 1994 to a total of six sites. These manipulations are detailed below.
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1993 Three experimental plots were established in October 1992, one for the addition of labelled 15N, and two for measurements in unamended soils. Each site was approximately 10 m2 and was representative of moist meadow communities. All sites were characterized by relatively shallow snowpacks (approximately 1 m maximum accumulation). Labelled 15N-NH4+ (99 atom%) was applied at a loading of 2 g N m"2 to one site 2 week before melt and followed into soil and litter pools throughout melt and into the growing season. Measurements at the unamended sites included dissolved inorganic nitrogen (DIN) in the snowpack, inorganic N inputs to soil, surface water inorganic N concentrations, leached inorganic N at a depth of 100 mm, N mineralization and nitrification, and soil physical characteristics. 1994 A total of six unamended experimental plots were established in October 1993. Three were located in areas with shallow snowpacks and included a winter 1993 site, and three were located in areas with deeper, earlier accumulating snowpacks (~2 m). All measurements performed at the three unamended plots used in 1993 were repeated with one exception: subsurface N outputs were not measured. Nitrogen inputs and outputs Snow samples for chemical content and snowpack loading were collected using the protocol of Williams & Melack (1991a). Vertical, contiguous cores were collected, in increments of 400-mm, from the snow-air interface to the snow-ground interface. Snow density measurements were made using a 1 liter stainless steel cutter (Elder et al., 1991). Snow depth was measured manually using a 2-m rule. Surface water samples were collected from the edge of melting snowpacks at about weekly intervals during snowpack runoff in acid-washed, polyethylene bottles. Snowpack N inputs to the soil surface were estimated based on inputs to PVC tubes containing mixed bed ion exchange resins in permeable nylon bags. Resin bags were similar to those used by in other systems (DiStefano & Gohlz, 1986; Hart & Gunther, 1989) and are described in detail in Brooks et al. (submitted) and Fisk & Schmidt (1995). Resin bags were placed in 50-mm-long, 35-mm-diameter PVC tubes located at the soil surface designed to be open to the snowmelt inputs, but to exclude soil water and overland flow. Tubes were placed at the soil surface in late October of each year and were collected immediately after the sites became snow free. Subsurface hydrologie losses were estimated from ion exchange resin bags placed to intercept leachate at a depth of 100 mm in 35 x 100 mm PVC tubes containing intact soil cores and left in place over the winter. 15
N in soil and litter
At each sampling date, three 50-mm-diameter soil samples were collected from thawed soils to a maximum depth of 100 mm. Concurrently, three surface litter samples were collected from a 50 X 50 mm area. Soil and litter each were dried at 60°C, ground to 200 mesh, and subsampled for analysis by mass spectrometry.
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Mineralization and nitrification Mineralization tubes (35 X 100 mm), similar to those described by DiSteffano & Gholz (1986) and Hart & Gunther (1989), were used to estimate mineralization inputs in 1993. Because soils within these tubes remained frozen below 50 mm, this method was modified in 1994 by placing resin bags within acid washed radiator hose directly in the soil at a depth of 50 mm. These bags were installed in December while the ground was frozen in an attempt to minimize soil disturbance. Mineralization was calculated as the increase in KC1 extractable soil inorganic N (to a depth of 50 mm) plus buried resin bag N, minus snowmelt inputs (estimated from surface resin bags) throughout the season. Nitrification was calculated as the increase in soil N03" plus buried resin bag N03", minus snowmelt inputs. Soil temperatures were measured manually in 1993 at the bottom of snowpits dug to obtain soil samples. Soil temperatures in 1994 were measured both manually and from thermisters installed at the soil surface in October 1993.
Laboratory analyses The chemical content of snow was measured using the methods of Williams & Melack (1991a). Snow samples were transferred to new polyethylene bags after collection and stored frozen (—20°C) until analysis. Samples were melted in polyethylene buckets at 4°C to minimize chemical changes. Subsamples were filtered through pre-rinsed glass fiber filters (1 jj.m) and stored at 4°C until analyzed. Ammonium was determined colorimetrically within 24 h of melting, on a Flow Injection Analyzer (FIA) (LACHAT Instruments, Mequon WI; detection limit 0.7 /xeq l"1, precision 2.7%), nitrate was measured by ion chromatography (Dionex Model 2010i, detection limit 0.1 /^eq l"1, precision 1.5%) for both snow and surface water samples. Soil samples were processed within 12 h of returning from the field using standard methods (Hart & Binkley, 1984; Davidson et al, 1989) and described in detail in Brooks et al. (submitted). Fresh soils were sieved and homogenized using a 2-mm sieve. Soil moisture was determined gravimetrically (water weight/dry weight X 100%) following drying to constant weight in a 60°C oven. Subsamples of sieved soil were extracted with 2N KC1 by shaking at 250 rpm for 1 h. Ion exchange resin bags were air dried in the laboratory and extracted with 2N KC1. All extracts were filtered through Whatman No. 1 filter paper and analyzed colorimetrically on an FIA.
RESULTS Site characteristics 1993 Shallow snowpack alpine sites were characterized by discontinuous snow cover from October through December 1992. Maximum snow depth of 0.8 m occurred in the end of April 1993 (Fig. 1). Minimum soil surface temperatures of — 14°C occurred during the first week of January. Soil temperatures began to warm as a consistent snowpack began to accumulate and snow depth increased (Fig. 1). By early March, soil temperatures had risen above -5°C and soils began to thaw under the snowpack.
Snowpack controls on soil nitrogen dynamics
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Gravimetric soil moisture before snowmelt was lower at Alpine 1 (64-88%), which is on a slight southerly aspect, than at Alpine 2 (112-126%) which is comparatively flat and remained constant until melt. Soil moisture increased rapidly at all sites with the onset of snowmelt to 160-170% at all sites. 1994 In contrast to 1993 data, a consistent snow cover at the shallow snowpack alpine sites did not develop until early April 1994 (Fig. 2). Maximum snow depth of
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0.30 to 0.50 m for the shallow alpine sites, and 1.55 to 1.60 m for the deep alpine sites occurred in the end of April 1994. Minimum soil surface temperatures reached - 10°C for shallow alpine sites and - 6 to -8°C for deep snowpack sites during December and January. Soil temperatures had risen above -5°C at all deep snowpack sites by February and soils began to thaw under the snowpack. A decrease in snow depth at the shallow sites in March resulted in decreased soil temperatures. Soils at these sites did not thaw until April. 15
N label
The background content of !5N in soil and litter collected well before snowmelt was 0.360 atom%. Bulk soil and surface litter samples collected at the beginning of snowmelt (15 May 1993) had values of 0.361 and 0.362 atom% 15N, respectively (Fig. 3). The 15N content of litter remained unchanged from the initiation of snowmelt until the site was snow free. In contrast, the 15N signature of soil gradually increased throughout the snowmelt season, reaching approximately 0.364 atom% on the 15 June. Concurrent with this increase in the 15N concentration in soil was a decrease in the coefficient of variation (CV) from 0.32 to 0.13. This decrease in CV suggests a more homogeneous distribution of labelled N within the soil pool, potentially due to rapid cycling through an active microbial pool. The increase in 15N content was sufficient to account for approximately 90% of the added label in soil. Nitrogen inputs 1993 Inorganic N in the snowpack at maximum accumulation was 146 mg N m"2. Estimates of N inputs at each plot obtained from resin bag inputs at the soil surface were of a similar magnitude, ranging from 104 to 306 mg N m"2. Mineralization inputs from 0.366 0.365 0.364 uo
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Fig. 3 Isotopic N values from soil and litter under an alpine snowpack which received 2 g m"2 99 atom % 15NH4+ applied to snow surface two weeks before snowmelt (n = 3). Approximately 90% of added label was recovered in soil.
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4 March through 3 May were 1116 mg N m"2 and 3313 mg N m 2 for the two alpine sites. Nitrification estimates could not be made because buried resin bags failed to accumulate a significant amount of N, apparently due to ice lenses at 50 to 70 mm depth which were present in the tubes when collected. 1994 The combination of shallower snowpacks and lower N concentrations within the snow resulted in lower N loadings for the sites in 1994. Mean inorganic N loadings from the snow were 33 mg N m"2 for the shallow snowpack sites and 106 mg N m"2 for deep snowpack sites. Estimates of inputs using the surface resin bags ranged from 165 to 643 mg N m"2. Soil inorganic N concentrations showed that, each of the snowpack regimes exhibited a 2 week period with high net N mineralization inputs. These inputs to the soil inorganic N pool were more than an order of magnitude greater than DIN inputs from snowmelt. Mineralization was lower, but much more variable, at the shallow sites (511-1440 mg N m"2) than at the deep sites (1712-1960 mg N m"2) (Table 1). The fraction of mineralized N subsequently nitrified was higher at the shallow sites (16-58%), than at the deep sites (8-18%). The absolute amount of nitrified N ranged from 81 to 793 mg N m"2 at the shallow sites and from 145 to 346 mg N m"2 at the deep sites (Table 1). Table 1 Physical characteristics and nitrogen inputs. Mean (std) (# = 5). Site
Minimum Maximum Snowpack soil T snow depth loading
(m8Nm'2)(mgNm-2)
(mg N m 2 )
206 (124) 14 (10)
100 (9) 90 (79)
1116(804) 3313(877)
25 41 33
112(91) 525 (639) 246 (200)
69 (37) 112 (77) 106 (67)
106 103 109
423 (324) 87 (22) 330(131)
220 (126) 78 (14) 77 (13)
(cm)
1993: Shallow 1 Shallow 2
-13 -13
80 77
148 142
1994: Shallow 1 Shallow 2 Shallow 3
-10 -10 -10
30 50 40 155 150 160
-6 -7.5 -8
Mineralization Nitrification Percent inputs inputs nitrified
(mg N W 2 )
(°C)
Deep 1 Deep 2 Deep 3
Resin bag inputs
(mg N m 2 ) a a
1440(1191) 793(1109) 1211 (780) 703 (441) 511 (212) 81 (161) 1712(448) 1792 (66) 1960 (245)
195 (324) 180 (25) 346 (178)
a a 55 58 16 8 10 18
a = nitrification estimate not made due to ice lens over resin bag.
Hydrologie outputs The high recovery of 15N label in 1993 suggests that the majority of snowmelt DIN entered the soil and was immobilized by a combination of physical and/or biological processes. Concentrations of inorganic N in surface waters collected in this study remained at or below detection limits throughout the spring and summer in 1993 and 1994. This suggests no export of N from the system occurred in surface water draining from the alpine sites during snowmelt. Similarly, no subsurface N loss was measured in 1993. Resin bags located under 100 mm soil cores in the PVC mineralization tubes failed to accumulate any leached N in 1993. Apparently, ice lenses developed in the soil isolating microbial activity at the surface and preventing the export of N in subsurface
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flow. These PVC tubes were not used in 1994 because of this icing problem. Although no direct measurements of leaching were performed in 1994, the presence of ice lenses within the soil again suggested little or no subsurface export of N. DISCUSSION The depth and timing of snow cover exerts significant controls on soil surface temperatures. A consistent snow cover insulated the soil surface from extreme air temperatures and allowed soils to warm at the shallow sites in 1993 and the deep sites in 1994 well before snowmelt began. Inconsistent snow cover in 1994 at the shallow sites greatly slowed the rate of soil warming at these sites. Thawed soils first appeared under seasonal sno wpacks as temperatures approached or exceeded — 5 ° C. Observations in both 1993 and 1994 indicated that soils thawed first at the soil surface, and later at lower depths. This suggested that the majority of the microbial activity at these sites was occurring in the upper portion of the soil. The completion of a freeze/thaw cycle as soils warmed apparently released labile carbon and nitrogen compounds from ruptured cell membranes. While the carbon was probably utilized by the surviving microbial population, a significant portion of the nitrogen was mineralized. A similar process has been proposed to account for a flush of mineral N as arctic soils thaw (Schimel et al., 1995). The amount of N mineralized under winter/spring snowpacks appeared to be related both to the severity with which the soils froze and the length of time thawed soils exist under the snow before melt began. The highest mineralization inputs were under a shallow snowpack in 1993 which experienced a severe freeze followed by an extended period of snow cover. The lowest mineralization inputs were measured at a shallow site in 1994 when there was no extended period of snow cover after the freeze. Mean mineralization inputs under deeper snowpacks in 1994 were intermediate between the shallow sites, but exhibited much lower variability. Although all of the inorganic N released from these snowpacks was retained in the soil, mineralized N dominated soil inorganic N pools at all sites. Dissolved inorganic N inputs from snowmelt to the soil inorganic pool ranged from 1 to 14% of the inputs from mineralization of organic matter. The higher values identified in this study are within the range of 5% reported for arctic tundra (Schimel et al., 1995), and 25% reported for an alpine basin with relatively poor soil development (Williams et al., 1995). The lower values, which occurred in the shallow snowpack sites in 1994, underscore the effect of snowpack regime on N cycling in these systems. Nitrification was not calculated for the 1993 data since the buried resin bags were isolated from the biological active soils by ice and failed to accumulate any N. In general, nitrate production was higher under shallow snowpacks then under the deeper snowpacks in 1994. A possible explanation for this difference is reduced oxygen availability resulting from a longer period of heterotrophic respiration under the deeper snowpacks. This is supported by other work which has shown that winter C0 2 production from soils under deeper snowpacks is significantly greater than from soils under shallower snowpacks (Brooks et al., 1995, this volume). The inverse relationship between snow depth and nitrification provides a potential mechanism to explain a portion of the surface water pulse of N03" observed at other sites in the Colorado Front Range. A similar relationship between surface water N03" export and seasonal snowpack depth
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was reported by Lewis and Grant (1980) who suggested extreme soil frosts interfered with biological retention of N. This study suggests that shallower snowpacks also may result in increased N export by increasing the pool of mobile N species in these systems. The processes identified in this study provide a new perspective for the evaluation of elevated stream water N concentrations in spring reported along the Colorado Front Range (Baron, 1991; Williams et al, 1993). In this study, all of the N stored in the seasonal snowpack apparently entered a much larger, biologically active, inorganic soil N pool during snowmelt. The size and composition of this soil pool appeared to be directly related to the depth and timing of snowpack accumulation. In spite of the large soil inorganic N pools, no export of N was observed in surface or ground water. In contrast, a large pulse of N03" has been observed in surface waters during snowmelt a short distance away in Green Lakes Valley (Williams et al., 1993). The reasons for this difference are unclear, but may be related to a lack of well-developed soils in the upper regions of Green Lakes Valley. Experiments designed to determine if the relationships between snow cover and soil N cycling identified by this work hold for other soil and ecosystem types within the watershed are needed. CONCLUSIONS Soil biological activity at Niwot Ridge begins before snowmelt, with the depth and timing of snow cover the most important factors regulating soil thaw and the onset of activity. Over-winter N mineralization inputs to the soil inorganic N pool were more than an order of magnitude higher than atmospheric inputs and appeared to be related to the insulating effect of continuous snowpacks which allowed microbial activity to begin well before snowmelt. Taken together, the large soil N inputs, a high degree of spatial heterogeneity in snowmelt N inputs, and a biologically mediated retention mechanism suggest that the timing of soil thaw may be an important factor controlling N export during snowmelt. More research is needed in this area to quantify the importance of subnivian N cycling in these environments, both with respect to gaining a better understanding of system biogeochemical cycles, and with respect to assessing potential system impacts from atmospheric N deposition.
Acknowledgments We thank C. Seibold, T. Bardsley, B. Cress, and M. Fisk for assistance with laboratory analyses and field work; and M. Fisk and an anonymous reviewer for comments on the manuscript. Funding was provided by the National Biological Service, NASA/EOS (NAGW-2602), the Niwot Ridge Long-Term Ecological Research Program (NSF DEB 9211776) and EPA Grant R819448-01-1.
REFERENCES Baron, J. S. (1991) Biogeochemistry of a Subalpine Ecosystem: Loch Vale Watershed. Ecological Studies Series 90. Springer-Verlag, New York. Baron, J. S., Ojima, D. S., Holland, E. A. & Parton, W. J. (1994) Analysis of nitrogen saturation potential in Rocky Mountain tundra and forest: implications for aquatic systems. Biogeochemistry 27, 61-82. Bowman, W. D. (1992) Inputs and storage of nitrogen in winter snowpack in an alpine ecosystem. Arc. Alp. Res. 24, 211215.
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Brooks, P. D., Williams, M. W. & Schmidt, S. K. (submitted) Microbial activity under alpine snowpacks: Implications for immobilization of atmospheric N inputs. Biogeochemistry. Brooks, P. D., Williams, M. W., Walker, D. A. & Schmidt, S. K. (1995) The Niwot Ridge snow fence experiment: biogeochemical responses to changes in seasonal snowpack. In: Biogeochemistry of Seasonally Snow-Covered Catchments (ed. by K. Tonnessen, M. W. Williams & M. Tranter) (Proc. Boulder Symp., July 1995). IAHS Publ. no. 228. Burns, S. F. (1980) Alpine soil distribution and development, Indian Peaks, Colorado Front Range. Ph.D. dissertation, University of Colorado, Boulder. Davidson, E. A., Eckert, R. W., Hart, S. C. & Firestone, M. K. (1989) Direct extraction of microbial biomass nitrogen from forest and grassland soils of California. SoilBiol. Biochem. 21, 773-778. DiStephano, J. F. & Gholz, H. L. (1986) A proposed use of ion exchange resins to measure nitrogen mineralization and nitrification in intact soil cores. Commun. Soil Sci. Plant Anal. 17, 989-998. Elder, K., Dozier, J. &Michaelsen,J. (1991) Snow accumulation and distribution in an alpine watershed. Wat. Resour.Res. 27, 1541-1552. Fisk, M. C. & Schmidt, S. K. (1995) Nitrogen mineralization and microbial biomass N dynamics in three alpine tundra communities. Soil Sci. Soc. Am. J. (in press). Grant, M. C. & Lewis, W. M. (1982) Chemical loading rates from precipitation in the Colorado Rockies. Tellus 34, 74-88. Greenland, D. (1989) The climate of Niwot Ridge, Front Range, Colorado. Arc. Alp. Res. 21, 380-391. Hart, S. C. & Binkley, D. (1984) Colorimetric interference and recovery of adsorbed ions from ion exchange resins. Commun. Soil Sci. Plant Anal. 15, 893-902. Hart, S. C. & Gunther, A. T. (1989) In situ estimates of annual net mineralization and nitrification in a subarctic watershed. Oecologia 80, 284-288. Lewis, W. M. & Grant, M. C. (1980) Relationship between snow cover and winter losses of dissolved substances from a mountain watershed. Arc. Alp. Res. 12, 11-17. Lewis, W. M., Jr., Grant, M. C. & Saunders, J. F., Ill (1984) Chemical patterns of bulk atmospheric deposition in the State of Colorado. Wat. Resour. Res. 20, 1691-1704. Rascher, C. M., Driscoll.C. T. & Peters, N. E. (1987) Concentration and flux of solutes from snow and forest floor during snowmelt in the west-central Adirondack region of New York. Biogeochemistry 3, 209-224. Schimel, J. P., Kielland, K. & Chapin, F. S., Ill (1995) Nutrient availability and uptake by tundra plants. In: Landscape Function: Implications for Ecosystem Response to Disturbance: A Case Study in Arctic Tundra (ed. by J. F. Reynolds & J. D. Tenhunen). Springer-Verlag, New York (in press). Sievering, H., Burton, D. & Caine, N. (1992) Atmospheric loading of nitrogen to alpine tundra in the Colorado Front Range. Global Biogeochem. Cyc. 6, 339-346. Williams, M. W. &Melack, J. M. (1991a) Precipitation chemistry and ionic loading in an alpine basin, Sierra Nevada. Wat. Resour. Res. 27, 1563-1574. Williams, M. W. &Melack, J. M. (1991b) Solute chemistry of snowmelt and runoff in an alpine basin, Sierra Nevada. Wat. Resour. Res. 27, 1575-1588. Williams, M. W., Caine, N., Baron, J., Sommerfeld, R. A. & Sanford, R. L. (1993) Regional assessment of nitrogen saturation in the Rocky Mountains. EOS (Trans. AGU) Suppl. 74, 257. Williams, M. W., Bales, R. C , Brown, A. D. & Melack, J. M. (1995) Fluxes and transformations of nitrogen in a highelevation catchment, Sierra Nevada. Biogeochemistry 28, 1-31.
