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DIVISION S-7—FOREST & RANGE SOILS

Soil Freezing and the Acid-Base Chemistry of Soil Solutions in a Northern Hardwood Forest

^a Dep. of Plant Biology, Univ. of Illinois, 505 S. Goodwin Ave., Urbana, IL 61801
^b Dep. of Civil and Environmental Engineering, Syracuse Univ., 220 Hinds Hall, Syracuse, NY 13244
^c Institute of Ecosystem Studies, P.O. Box AB, Millbrook, NY 12545
^d Dep. of Natural Resources, Cornell Univ., Ithaca, NY 14853
^e Cold Regions Research and Engineering Laboratory, U.S. Army, Hanover, NH 03755

^* Corresponding author (fitzhugh{at}life.uiuc.edu).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Reductions in the depth and duration of snow cover under a warmer climate may cause soil freezing events to become more frequent, severe, and spatially extensive in northern temperate forest ecosystems. In this experiment, snow cover was manipulated to simulate the late development of snowpack and to induce soil freezing at sugar maple (Acer saccharum) and yellow birch (Betula alleghaniensis) stands at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire. The objective of this manipulation was to elucidate the effects of soil freezing on the concentrations and fluxes of soil solution H⁺, Ca²⁺, Mg²⁺, K⁺, and Na⁺, as well as values of acid neutralizing capacity (ANC). Mild soil freezing events (soil temperatures never decreased below -5°C) resulted in pronounced acidification of soil solutions, driven primarily by nitrification, in the forest floor of sugar maple stands during the growing season. This mobilization of NO^-₃ from the forest floor of maple stands was accompanied by the leaching of Ca²⁺ and Mg²⁺ in Oa horizon solutions. Responses of soil solution acid-base chemistry to soil freezing were not evident in yellow birch stands or in the Bs horizon of either vegetation type, emphasizing the importance of vegetation type and the mineral soil in determining the effects of climatic disturbance on drainage water chemistry and nutrient loss. These results suggest that models of soil biogeochemistry in temperate forest ecosystems should consider soil-freezing events when simulating the acid-base chemistry of soil solutions and the translocation of nutrient base cations between soil horizons.

Abbreviations: ANC, acid neutralizing capacity • ANOVA, analysis of variance • AA, atomic absorption • DIC, dissolved inorganic C • DIP, dissolved inorganic P • DOC, dissolved organic C • HBEF, Hubbard Brook Experimental Forest • IC, ion chromatography • SRP, soluble reactive P

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
THE SOIL SOLUTION is an integral component of element cycling and loss in forest ecosystems, being a source of water and nutrients for plant uptake and the primary vector of loss for most nutrients (Bormann and Likens, 1967). As pH is a master variable influencing the extent and rates of a variety of biological and geochemical processes (Binkley and Richter, 1987), the acid-base chemistry of soil solutions is a key variable affecting nutrient cycling in soils. By influencing cation exchange and mineral weathering reactions (Hyman et al., 1998), as well as microbial (Klein et al., 1983) and plant activity (Finlay, 1995), the acid-base chemistry of soil solutions affects the sizes of pools of available nutrients in the soil and rates of loss for nutrients such as Ca²⁺ and for elements potentially toxic to terrestrial and aquatic biota, such as Al (Krám et al., 1995; Krám et al., 1997). To the extent that surface waters are derived from soil solutions, the acid-base chemistry of soil solutions is an important factor in determining the acid-base chemistry of surface waters.

The development and melting of snowpacks strongly influence the hydrology and biogeochemistry of northern temperate ecosystems (Brooks et al., 1999; Stottlemyer and Toczydlowski, 1999). As the result of the insulating quality of snow cover, the development of the seasonal snowpack in northern latitudes decreases the loss of heat from the soil to the atmosphere, thereby preventing or mitigating the freezing of soils during the over-winter period (Brooks et al., 1995; Hardy and Albert, 1995). The depth and duration of snow cover are therefore critical factors determining the over-winter soil temperature regime; shallow, ephemeral snowpacks tend to promote soil freezing while deep, persistent snow cover may keep the soil free of frost throughout the winter (Shanley and Chalmers, 1999; Stadler et al., 1996). Reductions in the depth and annual duration of snow cover may be a regional scale effect of climate warming in northern hardwood forests of North America, potentially leading to increases in the frequency, severity, and spatial extent of soil freezing events. While there are no detailed predictions of snow depth changes with climate warming, both field and modeling experiments suggest that snowpack dynamics are highly responsive to climate change (Baron et al., 2000; Moore and McKendry, 1996; Williams et al., 1996).

Changes in the over-winter snow cover and soil temperature regimes induced by climate change may have significant effects on soil biogeochemical processes (Brooks and Williams, 1999; Brooks et al., 1998). We hypothesized that soil freezing may significantly disturb the belowground environment and the normally tight linkages between mineralization and uptake processes that are thought to be important to nutrient conservation in forest ecosystems (Bormann and Likens, 1979). Previous studies have indicated that soil freezing can result in enhanced soil NO^-₃ concentrations, either by stimulating nitrification rates or by decreased root uptake (Biederbeck and Campbell, 1973; Boutin and Robitaille, 1995; Campbell et al., 1971). At the watershed-scale, soil freezing has been suggested to cause the enhanced transport of NO^-₃ from soils to stream waters (Edwards et al., 1986; Mitchell et al., 1996). The accelerated loss of NO^-₃ from soils can increase leaching of base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) (Foster et al., 1989). As nitrification and plant NO^-₃ uptake produce and consume acidity, respectively, any disturbance that increases nitrification and/or decreases NO^-₃ uptake has the potential to result in the acidification of soil solutions. Study of the effects of changes in snow cover on soil freezing and soil solution chemistry is therefore needed to better understand the potential responses of the biogeochemistry of the belowground environment in the northern forest to climate change.

In this paper, data are presented from a snow manipulation study at the HBEF in the White Mountains of New Hampshire. Over-winter snow cover and soil temperature regimes were manipulated during two consecutive winters at four experimental stands (two dominated by sugar maple and two by yellow birch). Snow cover was removed from the first snowfall through early February to simulate the effects of a late accumulating snowpack on the over-winter soil temperature regime and on biogeochemical processes. Our objectives were to elucidate the effects of soil freezing on the acid-base chemistry of soil solutions and to evaluate the effects relative to the response of soil solution N chemistry reported in an earlier paper (Fitzhugh et al., 2001).

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Study Site
The HBEF is located in the White Mountains of New Hampshire, USA (43° 56' N lat., 71° 45' W long.). The climate of the HBEF is characterized by long cold winters and short cool summers; precipitation is distributed relatively evenly throughout the year (annual mean = 140 cm), with 25 to 33% of precipitation occurring as snow (Federer et al., 1990). Mean air temperatures range from -10°C in January to 17°C in July. A continuous snowpack typically begins to develop in late autumn so that soils usually remain unfrozen during the over-winter period (Likens and Bormann, 1995). Canopy vegetation is dominantly mixed northern hardwoods and conifers. Sugar maple and yellow birch comprise approximately 36 and 28%, respectively, of the basal area of trees between 21 and 70 cm diameter at breast height (dbh) at the biogeochemical reference catchment at the HBEF, Watershed 6 (Bormann et al., 1970). Soils are predominantly well drained, coarse-loamy, mixed frigid Typic Haplorthods (pH approximately 3.9) developed in shallow glacial till (mean thickness = 2 m) overlying metamorphosed sedimentary and igneous rocks (Dahlgren and Driscoll, 1994).

Experimental Stands
Four experimental stands were established at the HBEF (Groffman et al., 1999), two in sugar maple and two in yellow birch stands. The elevation, aspect, and slope of the stands are given by Hardy et al. (2001). The dominant tree species contributed over 80% of the basal area at each stand. These species were chosen because the elevation range of birch typically exceeds maple, and thus birch would be expected to be more frost hardy. Two 10 by 10 m plots (one reference, one treatment) were located in each experimental stand (Groffman et al., 1999). In the fall and winter of 1996, minor amounts of understory vegetation were cleared from both treatment and reference plots for plot installations and to facilitate shoveling. Snow was removed by shoveling through early February at the treatment plots to induce soil freezing and to simulate the biogeochemical effects of a late accumulating snowpack as may be expected under a warmer climate. Snow removal occurred during two consecutive winters (1997-1998 and 1998-1999). The treatment was initiated in November 1997. Between 5 and 10 cm of snow from early winter storms was manually compacted at the treatment plots to protect plot installations and the forest floor from shovel damage and to increase the albedo of the forest floor, promoting soil freezing. The smooth backside of the shovel was used to carefully compact the snow and protect the soil from disruption before its freezing. This compacted snow layer was maintained throughout the entire treatment period and observations each spring confirmed that the protective compact layer of snow was effective in minimizing compaction of the forest floor. The reference plots accumulated snow at natural rates all winter, while the treatment plots accumulated snow at natural rates after snow removal ceased in early February.

Soil pits (approximately 1 m wide by 0.6 m deep) were excavated at each reference and treatment plot during the autumn of 1996. During excavation of the pits and installation of lysimeters and thermistor probes, extreme care was taken not to disturb the area upslope of the vertical pit wall. Duplicate zero-tension lysimeters, similar to the design of Driscoll et al. (1988), were installed horizontally into the vertical, upslope pit wall below the Oa and within the Bs soil horizons at each pit, a total of four lysimeters per plot (Groffman et al., 1999). An additional set of four lysimeters was installed at the first birch treatment plot during the summer of 1998 because the original set of lysimeters at this plot tended to collect relatively low sample volumes. Lysimeters below the Oa horizon were installed between 3 and 8 cm below the surface of the forest floor, while lysimeters in the Bs horizon were installed between 10 and 22 cm below the forest floor surface. After the manipulation of snow cover was initiated in November 1997, soil solutions were collected on 37 dates at weekly to monthly intervals from December 1997 through November 1999 and shipped on ice to Syracuse University for chemical analyses.

Additionally, five thermistor probes were installed horizontally into the vertical, upslope wall at each soil pit at 0.1-m depth intervals to a depth of 0.5 m below the surface of the forest floor. A Campbell CR10 data logger recorded soil temperatures every minute and stored hourly averages on a storage module. After installation of thermistor probes and lysimeters, the soil pits were carefully backfilled.

Laboratory Methods
Soil solutions were stored at approximately 4°C in a constant-temperature room until analysis. Soil solution pH was determined at the HBEF potentiometrically with a glass electrode within 24 h of sample collection. Analysis of dissolved inorganic C (DIC) was via phosphoric acid addition to convert DIC to CO₂, followed by infrared detection (Dohrmann Corp., 1984). After filtration, dissolved organic C (DOC) was analyzed by persulfate and ultraviolet enhanced oxidation, followed by infrared detection of CO₂ (McDowell et al., 1987). Ammonium was analyzed with an autoanalyzer via phenate colorimetry (American Public Health Association [APHA], 1981). Acid neutralizing capacity was measured by automated titration with strong acid and Gran plot analysis (Gran, 1950). Acid neutralizing capacity could not be measured on samples collected after August 1999 as the result of failure of the titration apparatus. Acid-neutralizing capacity values during the last 2 mo of the experiment were therefore missing. Total monomeric (Al_m) and organic monomeric Al (Al_o) were determined colorimetrically by a technique utilizing pyrocatechol violet (McAvoy et al., 1992). The Al_o was separated from the inorganic monomeric Al fraction (Al_i) by cation-exchange chromatography (Driscoll, 1984). Concentrations of Al_i were calculated as the difference between Al_m and Al_o. Inorganic strong acid anions (Cl^-, NO^-₃, SO^2-₄) were analyzed by ion chromatography (Tabatabai and Dick, 1983). Soluble reactive phosphorus (SRP) was measured through the formation of a blue antimony-phospho-molybdate complex and measurement on a UV-VIS spectrophotometer at 880 nm. Dissolved inorganic phosphorus (DIP) was assumed to equal SRP. Sodium, K⁺, Mg²⁺, and Ca²⁺ were analyzed by ion chromatography (IC) on samples collected through January 1998 and by atomic absorption (AA) thereafter (Slavin, 1968). Comparisons between results from IC and AA indicated that there were no significant differences between these methods in the estimated concentrations. Total F^- was determined with an ion selective electrode after addition of a total ionic strength adjustor and buffer (Orion, 1976).

Computation of Chemical Speciation
For each soil solution sample with sufficient volume for the full suite of solute analyses (60% of samples), the organic anion (A^n-) concentration was calculated by an electroneutrality approach as the anion deficit (Driscoll et al., 1989; 1994; Eshleman and Hemond, 1985). The anion deficit was calculated as the difference between the sum of inorganic cations and inorganic anions:

[1]

[2]

[3]

where the brackets indicate solute concentrations in µmol_c L^-1. With this approach it was assumed that all inorganic solutes significantly contributing to charge balance have been measured so that any discrepancy in electroneutrality was attributable to unanalyzed A^n-. The A^n- concentrations were used to estimate the contributions to acidity from organic solutes. All chemical speciation calculations were performed using the assumption of a dilute solution (i.e., that activities equaled concentrations). Hydrogen ion concentrations were calculated from pH. The Al_i charge (Alⁿ⁺) was determined as the sum: 3[Al³⁺] + 2[AlOH²⁺] + + 2[AlF²⁺]. These inorganic Al species were calculated by an approach combining mass balance and chemical equilibrium using the thermodynamic data presented in Driscoll (1984) and measurements of Al_i, total F, and pH. Other Al species were assumed to be negligible. The bicarbonate concentration was determined by chemical equilibrium calculations using measurements of DIC and pH (Stumm and Morgan, 1981). Fluoride ion was calculated as the difference between total F and AlF²⁺. The DIP species were determined by chemical equilibrium calculations using measurements of pH and DIP (Stumm and Morgan, 1981).

Calculation of Contributions of Solutes to Changes in Acid Neutralizing Capacity
Contributions of individual solutes to declines in soil solution ANC in the forest floor of maple treatment plots were estimated by an approach commonly used to determine sources of episodic acidification during hydrological events in surface waters (e.g., DeWalle and Swistock, 1994; Schaefer et al., 1990). Acidification was defined as declines in ANC below an initial ANC. For the soil freezing experiment, the initial ANC was taken to be the soil solution ANC during snowmelt in early April (1 Apr. 1998 and 5 Apr. 1999) at the treatment plots. The acidification events were defined to occur from May through November 1998 and from May through August 1999 based on visual inspection of the temporal patterns of soil solution ANC (Fig. 1) . Changes in soil solution ANC in response to soil freezing events were defined as:

[4]

where the subscript ‘i’ indicates initial and the subscript ‘e’ denotes event (responding to soil freezing disturbance). Positive values of [ANC] indicate ANC depression. Contributions of cations to [ANC] were calculated as the ratio:

[5]

where [C] indicates the cation concentration of interest in µmol_c L^-1. The cations considered were Alⁿ⁺, NH⁺₄, Na⁺, K⁺, Mg²⁺, and Ca²⁺. Contributions of anions to [ANC] were calculated as the ratio:

[6]

where [A] indicates the anion concentration of interest in µmol_c L^-1. The anions considered were NO^-₃, A^n-, Cl^-, SO^2-₄, F^-, and DIP. Positive values of the ratios d[C]/d[ANC] and d[A]/d[ANC] indicated contributions to ANC declines (acidification) while negative ratios occurred for solutes that neutralized acidity. Theoretically, the sum of the ratios among solutes should equal 1.0 to entirely explain ANC changes. Only samples with [ANC] >20 µmol_c L^-1 and a complete suite of solute analyses were included in this analysis.

View larger version (32K): [in this window] [in a new window]   Fig. 1. Time series of (a) acid neutralizing capacity (ANC) values and (b) H+ concentrations for soil solutions draining the Oa horizon of reference and treatment plots in sugar maple stands. Soils in treatment plots were frozen from December through March. Error bars represent ±one standard error of the mean.

Snow removal and the development of ice lenses likely resulted in lower rates of snowmelt infiltration into treatment than reference plots (Hardy et al., 2001). Decreased infiltration of snowmelt at treatment plots likely caused decreased soil water fluxes relative to reference plots during the snowmelt period (March and April). Reductions in infiltration at treatment plots were approximately proportional to decreases in snow water equivalence (SWE) of the snowpack at peak accumulation (early March), with treatment SWE being 41 and 30% of the reference SWE during 1998 and 1999, respectively (Hardy et al., 2001). The effects of snow removal and soil freezing on soil water fluxes at treatment plots were therefore estimated by decreasing the modeled daily hydrological fluxes during March and April by 59 and 70% during 1998 and 1999, respectively. Although water fluxes through the soil during snowmelt were reduced significantly by our snow cover manipulation, water fluxes at treatment plots were only reduced modestly on an annual basis (annual water fluxes at treatment plots were 81 and 91% of fluxes at reference plots in the Oa and Bs horizons, respectively). Water fluxes were not adjusted at treatment plots during the treatment period (December through February) because the volume of leachate collected in lysimeters during this period was slightly greater at treatment plots (102 L) than at reference plots (82 L), and thus water fluxes did not appear to be reduced at treatment plots during the treatment period. Trimble et al. (1958) examined different types of frost in New Hampshire and found that only concrete frost reduced infiltration and that concrete frost was rare in forest soils.

Statistical Analyses
All statistical analyses were performed using SAS software (SAS Institute, 1989). Annual volume-weighted mean concentrations were calculated separately for the 2 yr of the snow manipulation experiment (December 1997 through November 1998 and December 1998 through November 1999) for each plot (n = 8) and soil horizon (n = 2) by dividing the sum of the daily solute fluxes by the sum of the daily water fluxes. Two-way analysis of variance (ANOVA) was then performed to test for the effects of soil freezing treatment (two levels: reference and snow cover manipulation), vegetation type (two levels: maple and birch) and the interaction between soil freezing and vegetation type on the annual volume-weighted mean soil solution ANC, H⁺, Ca²⁺, Mg²⁺, Na⁺, K⁺, NO^-₃, and DOC concentrations separately for each soil horizon (Oa, Bs). For those solutes with a significant interaction between soil freezing and vegetation type, one-way ANOVA was performed separately for each vegetation type to test for effects of soil freezing. Statistical analyses were not performed on soil solution solute fluxes because water fluxes were not directly measured. Linear regression was performed to predict soil solution ANC, Ca²⁺, and Mg²⁺ concentrations as functions of NO^-₃, DOC, and A^n- for each vegetation type, soil horizon, and treatment. Patterns of correlation were used to identify linkages among soil solution solute concentrations as well as differences between reference and treatment plots within single species.

	RESULTS

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Our experimental manipulation of snow cover had significant effects on the over-winter soil temperature regime (Hardy et al., 2001). The maximum depth of soil freezing (<0°C) was variable among treatment plots, ranging between 10- and 50-cm soil depths, while soils at the reference plots generally remained unfrozen (>0°C) throughout the over-winter period. Soils at the treatment plots typically froze at 10-cm depth beginning in mid- to late December and had thawed at that depth by early to mid-April (Fig. 2) .

View larger version (32K): [in this window] [in a new window]   Fig. 2. Time series showing daily reference (solid lines) and treatment (dotted lines) soil temperatures averaged among all plots at (a) 10-, (b) 20-, (c) 30-, (d) 40-, and (e) 50-cm depth during the over-winter period. The dashed lines in each panel are references for 0°C.

View larger version (30K): [in this window] [in a new window]   Fig. 3. Time series of nitrate concentrations for soil solutions draining the Oa horizon of reference and treatment plots in (a) sugar maple and (b) yellow birch stands. Soils in treatment plots were frozen from December through March. Error bars represent ±one standard error of the mean.

View larger version (31K): [in this window] [in a new window]   Fig. 4. Time series of Ca2+ concentrations for soil solutions draining the Oa horizon of reference and treatment plots in (a) sugar maple and (b) yellow birch stands. Soils in treatment plots were frozen from December through March. Error bars represent ±one standard error of the mean. Note that the scales of the y-axes are different between panels (a) and (b).

View larger version (31K): [in this window] [in a new window]   Fig. 5. Time series of Mg2+ concentrations for soil solutions draining the Oa horizon of reference and treatment plots in (a) sugar maple and (b) yellow birch stands. Soils in treatment plots were frozen from December through March. Error bars represent ±one standard error of the mean. Note that the scales of the y-axes are different between panels (a) and (b).

View larger version (28K): [in this window] [in a new window]   Fig. 6. Soil solution Ca2+ as a function of (a) NO-3 and (b) dissolved organic C (DOC) concentrations in the Oa horizon of sugar maple stands. Linear regression results are: (a) reference: Ca2+ = 0.18 x NO-3 + 22, R2 = 0.09, p = 0.0019, n = 106; treatment: Ca2+ = 0.25 x NO-3 + 14, R2 = 0.90, p < 0.0001, n = 105; and (b) reference: Ca2+ = 0.018 x DOC + 9.6, R2 = 0.82, p < 0.0001, n = 102; treatment: Ca2+ = 0.035 x DOC + 9.5, R2 = 0.57, p < 0.0001, n = 101.

View larger version (29K): [in this window] [in a new window]   Fig. 7. Soil solution Mg2+ as a function of (a) NO-3 and (b) dissolved organic C (DOC) concentrations in the Oa horizon of sugar maple stands. Linear regression results are: (a) reference: Mg2+ = 0.033 x NO-3 + 6.9, R2 = 0.06, p = 0.011, n = 106; treatment: Mg2+ = 0.050 x NO-3 + 6.0, R2 = 0.85, p < 0.0001, n = 105; and (b) reference: Mg2+ = 0.0034 x DOC + 4.6, R2 = 0.64, p < 0.0001, n = 102; treatment: Mg2+ = 0.0059 x DOC + 5.8, R2 = 0.41, p < 0.0001, n = 101.

Sources of Soil Solution Acidity
Significant acidification of soil solutions draining the forest floor of treatment maple stands was observed during the growing season of both years of the experiment. The ratios of changes in solute concentrations to changes in ANC during the acidification events indicated that nitrification was the dominant source of acidity during both years of the experiment, with smaller but significant contributions from organic acids (Table 4). During 1998, the mean d/d ratio comprised 72% of the sum of the ratios which contributed to acidification (i.e., the sum of the solutes with positive average ratios), indicating that on average nitrification was responsible for 72% of the decline in ANC while organic acids contributed 21% of the acidification. During 1999, nitrification and organic acids contributed 64 and 27%, respectively, of the acidification of soil solutions. In contrast with NO^-₃, temporal patterns and concentrations of DOC and A^n- were generally similar between treatment and reference plots for soil solutions draining the Oa horizon of maple stands (Fig. 8) . Concentrations of DOC and A^n-, however, were elevated during the growing season, the period of soil solution acidification, relative to the initial ANC (April) and thus contributed to the acidification of soil solutions draining the Oa horizon of treatment plots at maple stands. Exchange of H⁺ for nutrient base cations at surfaces of soil particles was a likely mechanism neutralizing acidity, as indicated by the negative average ratios for d[Ca²⁺]/d[ANC], d[Mg²⁺]/d[ANC], and d[K⁺]/d[ANC] (Table 4). Exchange of H⁺ for Ca²⁺ appeared to be the dominant mechanism of neutralization, with smaller contributions from K⁺ and Mg²⁺ exchange (Table 4). During 1998, the mean d[Ca²⁺]/d[ANC], d[K⁺]/d[ANC], and d[Mg²⁺]/d[ANC] ratios comprised 66, 19, and 12%, respectively, of the changes in solutes associated with the neutralization of acidity while during 1999, the mean d[Ca²⁺]/d[ANC], d[K⁺]/d[ANC], and d[Mg²⁺]/d[ANC] comprised 54, 15, and 9%, respectively. The sum of the mean ratios of changes in solute concentrations to changes in ANC was close to 1 (0.93 in 1998 and 1.01 in 1999), suggesting that most of the sources contributing to changes in ANC were quantified.

View larger version (43K): [in this window] [in a new window]   Fig. 8. Time series of (a) dissolved organic C (DOC) and (b) organic anion (An-) concentrations for soil solutions draining the Oa horizon of reference and treatment plots in sugar maple stands. Soils in treatment plots were frozen from December through March. Error bars represent ±one standard error of the mean.

View larger version (25K): [in this window] [in a new window]   Fig. 9. Soil solution ANC as a function of (a) NO-3 and (b) organic anion (An-) concentrations in the Oa horizon of sugar maple stands. Linear regression results are: (a) reference: ANC = -0.23 x NO-3 + 2.9, R2 = 0.10, p = 0.0028, n = 87; treatment: ANC = -0.43 x NO-3 + 3.9, R2 = 0.79, p < 0.0001, n = 82; and (b) reference: ANC = -0.057 x An- - 4.1, R2 = 0.042, p = 0.10, n = 65 (line not shown); treatment: ANC = -1.1 x An- - 4.6, R2 = 0.55, p < 0.0001, n = 71.

View larger version (32K): [in this window] [in a new window]   Fig. 10. Soil solution ANC as a function of (a) Ca2+, (b) Mg2+, and (c) K+ concentrations in the Oa horizon of sugar maple stands. Linear regression results are: (a) reference: ANC = -0.25 x Ca2+ + 1.8, R2 = 0.041, p = 0.06, n = 86 (line not shown); treatment: ANC = -1.8 x Ca2+ + 28, R2 = 0.84, p < 0.0001, n = 82; (b) reference: ANC = -0.58 x Mg2+ - 0.2, R2 = 0.0096, p = 0.37, n = 86 (line not shown); treatment: ANC = -7.4 x Mg2+ + 43, r2 = 0.68, p < 0.0001, n = 82; (c) reference: ANC = -0.20 x K+- 1.1, R2 = 0.028, p = 0.13, n = 86 (line not shown); treatment: ANC = -1.5 x K+ - 8.8, r2 = 0.40, p < 0.0001, n = 82.

	DISCUSSION

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Acidification of Soil Solutions by Soil Freezing Disturbance
Mild soil freezing resulted in significant acidification of soil solutions draining the forest floor of maple stands during the following growing season (Fig. 1). While soils at treatment maple plots had thawed by early April, soil solution ANC and H⁺ at treatment plots began to deviate from reference levels in June, a lag of approximately 2 mo. The elevated flux of soil solution H⁺ in the Oa horizon solutions following soil freezing at maple stands (mean treatment minus reference flux = 680 mol ha^-1 over the 2-yr study; Table 3) was significant relative to the exchangeable H⁺ pool for the Oa horizon measured in Watershed 5 of the HBEF in 1983 (3000 mol ha^-1) by Johnson et al. (1997).

Evidence for the dominant sources of soil solution acidity at the treatment maple plots being nitrification and dissociation of organic acids was provided by:( i) the synchronous pattern between soil solution H⁺ and NO^-₃ concentrations (Fig. 1 and 3), (ii) the strong negative correlations between soil solution ANC and NO^-₃ and A^n- (Fig. 9), and (iii) analysis of changes in solute concentrations relative to decreases in ANC following soil freezing disturbance (Table 4). Potential causes of the accelerated leaching of NO^-₃ from the forest floor of maple plots were: (i) increased inputs of labile organic N via fine root mortality, physical disruption of soil aggregates, and fragmentation of fresh litter, (ii) disruption of the synchrony between biotic mineralization and uptake processes, and (iii) a decreased capacity of trees to assimilate N as the result of fine root mortality and stress (Fitzhugh et al., 2001; Tierney et al., 2001). As elevated fine root mortality was potentially the source of up to approximately 20% of the increased NO^-₃ leaching at maple treatment plots (Fitzhugh et al., 2001) and rates of net N mineralization and nitrification were not significantly affected by the soil freezing treatment in our study (Groffman et al., 2001), reduced N uptake by roots and physical disturbance of root-soil-microbe interactions were mechanisms that likely contributed to the increased rates of NO^-₃ leaching.

Increases in the ionic strength of soil solutions may have contributed to the acidification of soil solutions at maple treatment plots (e.g., Skyllberg et al., 2001). While removal of snow cover affected soil moisture values at our treatment plots during the spring snowmelt (March and April), soil moisture values and the volumes of leachate collected in lysimeters were similar between reference and treatment plots during the summer (May through November; Hardy et al., 2001), indicating that reductions in soil moisture were not responsible for the increases in ionic strength and acidification of soil solutions at our treatment plots. Increases in ionic strength at maple treatment plots were disproportionately driven by increases in soil solution NO^-₃ concentrations (Table 4), rather than a proportional increase in the concentrations of all solutes, suggesting that NO^-₃ dynamics strongly influenced the acidification of soil solutions. This influence may have been caused by release of H⁺ from the soil exchange complex as the result of increases in ionic strength and/or generation of acidity via nitrification.

Soil exchange reactions between H⁺ and nutrient base cations (Ca²⁺, Mg²⁺, K⁺) appeared to neutralize a significant fraction of the soil solution acidity in the forest floor of maple treatment plots, as indicated by the strong correlations between ANC and base cations (Fig. 10) as well as by the analysis of changes in solute concentrations relative to decreases in ANC (Table 4). These patterns suggest that soil solution acidification would have been more severe in the absence of elevated leaching of base cations.

Results from the current experiment at the HBEF complement and enrich previous research on the effects of soil freezing on soil solution acid-base chemistry. The season-long removal of snow cover in Canadian sugar maple stands by Boutin and Robitaille (1995) increased soil solution H⁺ during the following growing season, a pattern the authors attributed to increases in nitrification. Thus, temporal patterns of soil solution H⁺ responses to soil freezing were similar between the Boutin and Robitaille (1995) and the HBEF studies. The pH decline observed by Boutin and Robitaille (1995) was much smaller (pH 4.5 in soil-freezing treatment plots compared with pH 4.7 in control plots), than the decline observed in the current study, likely because lysimeters were placed only in the mineral soil during the Canadian study. As Boutin and Robitaille (1995) sampled soil solutions at one depth, it was previously unclear if the responses of soil solution ANC to soil freezing varied within soil profiles. In the current experiment, soil solution acidity generated by soil freezing in the forest floor of maple stands was neutralized on reaching the mineral soil Bs horizon. Pronounced decreases in soil solution NO^-₃ and DOC concentrations and fluxes from the Oa to the Bs horizon occurred concurrently with this neutralization of acidity (Tables 2 and 3). Removal of soil solution NO^-₃ during percolation below the Oa horizon via denitrification and the uptake of NO^-₃ by roots and microbes were potential mechanisms that contributed to the neutralization of acidity. Root uptake, however, was considered unlikely to explain the patterns of soil solution NO^-₃ between the Oa and Bs horizons because most nutrient uptake occurs in the forest floor at the HBEF (Yanai, 1992). Adsorption of DOC to positively charged particle surfaces in the mineral soil (McDowell and Wood, 1984) likely neutralized organic acidity generated in the forest floor.

We are currently unable to predict whether repeated soil freezing over more than 2 yr would result in enhanced leaching of NO^-₃ from the soil profile and the potential acidification of drainage waters. It is possible that the long-term (>5 yr) responses to soil freezing would differ from the short-term responses observed in the current study if pools of root biomass and labile soil organic matter change with repeated freezing. A longer-term experiment monitoring soil solution chemistry, root biomass, and pools of labile organic matter would be necessary to make accurate predictions of the effects of soil freezing on NO^-₃ loss and the acidification of drainage waters over several years.

As Boutin and Robitaille (1995) sampled soil solutions under a single species (sugar maple), it was previously unclear if the response of soil solution acid-base chemistry to soil freezing varied between forest stands of different species. As reflected in the significant interactions between the soil freezing treatment and vegetation type for soil solution NO^-₃ and ANC (Table 1), NO^-₃ was the primary form of N mobilized after soil freezing at maple stands, resulting in significant acidification of soil solutions, while NH⁺₄ and DON dominated N leaching in birch stands (Fitzhugh et al., 2001), with no net effect on soil solution ANC. These results demonstrate the important role of the N species liberated by disturbance on the consequent soil solution acid-base chemistry. Thus, the composition of the canopy tree stratum may be an important variable influencing spatial patterns of drainage water acid-base chemistry in response to soil freezing events.

Previous studies have suggested that soil freezing may affect the acid-base chemistry of stream water. Soil freezing increased mobilization of H⁺ to stream water in an upland Scottish catchment (Edwards et al., 1986), whereas stream H⁺ concentration was lower during years with low snowpack accumulation and likely soil freezing than during years with normal snowpack accumulation and unfrozen soils at a high elevation catchment in Colorado (Lewis and Grant, 1980). A synchronous pulse of elevated stream NO^-₃ concentrations was observed at five catchments spanning the northeastern USA, including the HBEF, during the snowmelt of 1990 (Mitchell et al., 1996). This pattern was attributed to a regional scale soil-freezing event during December 1989 when the seasonal snow cover accumulated relatively late and air temperatures were unusually low.

Effects of Soil Freezing on Calcium and Magnesium Leaching
Although soil solution Ca²⁺ and Mg²⁺ concentrations tended to be greater at treatment than reference plots during the growing season in Oa horizons of maple stands (Fig. 4 and 5), there was no statistically significant effect of soil freezing on the leaching of Ca²⁺ and Mg²⁺. The mean treatment minus reference fluxes in Oa horizon solutions at maple stands over the 2-yr study were 300 and 52 mol ha^-1 for Ca²⁺ and Mg²⁺, respectively (Table 3). In comparison, Johnson et al. (1997) measured soil exchangeable Ca²⁺ and Mg²⁺ pools of 1900 and 340 mol ha^-1, respectively, for the Oa horizon in Watershed 5 of the HBEF in 1983. There were no effects of soil freezing on soil solution Ca²⁺ and Mg²⁺ in the Bs horizon of maple stands or in either horizon of the birch stands (Table 3). Relative to the reference plots of maple stands, variation in soil solution Ca²⁺ and Mg²⁺ concentrations in the Oa horizon of treatment maple plots was more strongly correlated with NO^-₃ (Fig. 6 and 7), suggesting that NO^-₃ mobility strongly influenced the transport of Ca²⁺ and Mg²⁺ following soil freezing via leaching from cation-exchange sites. The ratio of annual volume-weighted mean soil solution Ca/Mg was similar between reference (3.6) and treatment (3.8) plots for the Oa horizon of maple stands; these ratios were lower than the ratio of soil exchangeable Ca/Mg (5.5) for the Oa horizon of Watershed 5 at the HBEF in 1983 reported by Johnson et al. (1997). The contrast in the controls on base cation leaching between maple and birch stands following soil freezing was the result of a smaller NO^-₃ response at birch stands (Fig. 3). Fitzhugh et al. (2001) suggested several potential reasons for this difference in NO^-₃ response between tree species, including: (i) significantly greater rates of potential net nitrification in soils under maple than birch stands as the result of differences in litter quality and decomposition dynamics (mean [SE] rates of potential net nitrification in the forest floor were 3.8 [0.6] and 2.0 [0.4] mg N kg^-1 d^-1 in maple and birch stands, respectively) (Groffman et al., 2001), and (ii) differences in mycorrhizal association between these tree species (maple is endomycorrhizal while birch is ectomycorrhizal). As decomposition of fine roots releases significant quantities of K⁺ (Fahey and Arthur, 1994), soil solution K⁺ concentrations would be expected to increase following soil freezing if fine root mortality was a significant source of base cation leaching. The lack of a treatment response for K⁺, therefore, suggested that fine root mortality was a relatively unimportant source for the mobilization of base cations following the soil-freezing disturbance. We were unable to reconcile the lack of a soil solution K⁺ response with the significantly greater mortality of fine roots observed at treatment plots (Tierney et al., 2001), which potentially contributed up to approximately 20% of the excess soil solution N flux from the forest floor of treatment plots (Fitzhugh et al., 2001).

The Ca²⁺ and Mg²⁺ mobilized in the forest floor by soil freezing in maple stands were largely removed from the soil solution before reaching the Bs horizon (Table 3). The effective retention of these base cations was likely the result of significantly diminished leaching of NO^-₃ in the Bs horizon, as discussed above. Subtracting the soil solution fluxes of the Bs from the Oa horizon, the quantities of Ca²⁺ and Mg²⁺ retained in the Bs horizon of treatment plots in maple stands over the 2-yr study were estimated to be 500 and 82 mol ha^-1, respectively. In comparison, Johnson et al. (1997) measured soil exchangeable Ca²⁺ and Mg²⁺ pools of 4800 and 1100 mol ha^-1, respectively, in the mineral soil of Watershed 5 at the HBEF in 1983.

A previous study found that stream Ca²⁺ and Mg²⁺ concentrations were significantly greater during years with likely soil freezing than during years where the soil remained unfrozen at a Colorado catchment (Lewis and Grant, 1980). Edwards et al. (1986) suggested that soil freezing enhanced Ca²⁺ transport to stream water in a Scottish catchment. Snow cover manipulation in Canadian sugar maple stands demonstrated that soil freezing significantly increased soil solution Ca²⁺ and Mg²⁺ concentrations; these increases were hypothesized to be driven by NO^-₃ leaching (Boutin and Robitaille, 1995).

Mild soil freezing events resulted in significant acidification of soil solutions in the forest floor of maple stands. In contrast, soil freezing did not affect the acid-base chemistry of soil solutions in the Bs horizon of maple stands or in either horizon of birch stands. These contrasts indicate the important roles of vegetation type and the mineral soil in the responses of soil solution acid-base chemistry to soil freezing. Although rates of net nitrification were not affected by our snow cover manipulation, greater nitrification rates in the forest floor of maple than birch stands (Groffman et al., 2001) likely reflected differences in the quality of soil organic matter derived from litter of these two tree species and led to greater leaching of NO^-₃ from the forest floor of sugar maple stands, which was the primary factor determining differences in the responses of soil solution ANC, H⁺, Ca²⁺, and Mg²⁺ between vegetation types. As soil solutions percolated through the soil profile, considerable neutralization of acidity and retention of Ca²⁺ and Mg²⁺ were evident in the mineral soil. This vertical pattern within the soil profile emphasized the importance of mineral soil horizons in retaining NO^-₃ and thereby neutralizing the acidity generated in the forest floor as well as preventing increased drainage loss of base cations from the soil profile.

	ACKNOWLEDGMENTS

 
The hard work of Jason Demers and Adam Welman in the field and laboratory were critical to the success of this study. We thank Scott Nolan and Wayne Martin for help with site location and establishment. This research was supported by the National Science Foundation (DEB-9652678) and a Syracuse University Graduate Fellowship to RDF. The Hubbard Brook Experimental Forest is operated and maintained by the Northeastern Research Station, USDA, Newtown Square, PA. This is a contribution to the Hubbard Brook Ecosystem Study.

Received for publication August 7, 2002.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

• APHA. 1981. Standard methods for the examination of water and wastewater. 15th ed. American Public Health Association, Washington, D. C.
• Baron, J.S., M.D. Hartman, L.E. Band, and R.B. Lammers. 2000. Sensitivity of a high-elevation Rocky Mountain watershed to altered climate and CO₂. Water Resour. Res. 36:89–99.
• Biederbeck, V.O., and C.A. Campbell. 1973. Soil microbial activity as influenced by temperature trends and fluctuations. Can. J. Soil Sci. 53:363–376.
• Binkley, D., and D. Richter. 1987. Nutrient cycles and H⁺ budgets of forest ecosystems, p. 1–51. In A. MacFayden and E.D. Ford (ed.) Advances in ecological research. Vol. 16. Academic Press Inc., London.
• Bormann, F.H., and G.E. Likens. 1967. Nutrient cycling. Science 155:424–429.[Abstract/Free Full Text]
• Bormann, F.H., and G.E. Likens. 1979. Pattern and process in a forested ecosystem. Springer-Verlag, New York.
• Bormann, F.H., T.G. Siccama, G.E. Likens, and R.H. Whittaker. 1970. The Hubbard Brook ecosystem study: Composition and dynamics of the tree stratum. Ecol. Monogr. 40:373–388.
• Boutin, R., and G. Robitaille. 1995. Increased soil nitrate losses under mature sugar maple trees affected by experimentally induced deep frost. Can. J. For. Res. 25:588–602.
• Brooks, P.D., D.H. Campbell, K.A. Tonnessen, and K. Heuer. 1999. Natural variability in N export from headwater catchments: Snow cover controls on ecosystem N retention. Hydrol. Process. 13:2191–2201.
• Brooks, P.D., and M.W. Williams. 1999. Snowpack controls on nitrogen cycling and export from seasonally snow-covered catchments. Hydrol. Process. 13:2177–2190.
• Brooks, P.D., M.W. Williams, and S.K. Schmidt. 1998. Inorganic nitrogen and microbial biomass dynamics before and during spring snowmelt. Biogeochemistry 43:1–15.
• Brooks, P.D., M.W. Williams, D.J. Walker, and S.K. Schmidt. 1995. The Niwot Ridge snow fence experiment: Biogeochemical responses to changes in the seasonal snowpack, p. 293–302. In K. Tonnessen et al. (ed.) Biogeochemistry of seasonally snow-covered catchments. Vol. 228. International Association of Hydrological Sciences, Wallingford, UK.
• Campbell, C.A., V.O. Biederbeck, and F.G. Warder. 1971. Influence of simulated fall and spring conditions on the soil system: II. Effect on soil nitrogen. Soil Sci. Soc. Am. Proc. 35:480–483.
• Dahlgren, R.A., and C.T. Driscoll. 1994. The effects of whole-tree clear-cutting on soil processes at the Hubbard Brook Experimental Forest, New Hampshire, USA. Plant Soil 158:239–262.
• DeWalle, D.R., and B.R. Swistock. 1994. Causes of episodic acidification in five Pennsylvania streams on the northern Appalachian Plateau. Water Resour. Res. 30:1955–1963.
• Dohrmann Corp. 1984. Total organic carbon systems manual. Dohrmann Corp., Santa Clara, CA.
• Driscoll, C.T. 1984. A procedure for the fractionation of aqueous aluminum in dilute acidic waters. Int. J. Environ. Anal. Chem. 16:267–283.
• Driscoll, C.T., R.D. Fuller, and W.D. Schecher. 1989. The role of organic acids in the acidification of surface waters in the eastern U.S. Water Air Soil Pollut. 43:21–40.
• Driscoll, C.T., R.D. Fuller, and D.M. Simone. 1988. Longitudinal variations in trace metal concentrations in a northern forested ecosystem. J. Environ. Qual. 17:101–107.[Abstract/Free Full Text]
• Driscoll, C.T., M.D. Lehtinen, and T.J. Sullivan. 1994. Modeling the acid-base chemistry of organic solutes in Adirondack, New York, lakes. Water Resour. Res. 30:297–306.
• Edwards, A.C., J. Creasey, and M.S. Cresser. 1986. Soil freezing effects on upland stream solute chemistry. Water Res. 20:831–834.
• Edwards, A.C., and M.S. Cresser. 1992. Freezing and its effect on chemical and biological properties of soil. Adv. Soil Sci. 18:59–79.
• Eshleman, K.N., and H.F. Hemond. 1985. The role of organic acids in the acid-base status of surface waters at Bickford Watershed, Massachusetts. Water Resour. Res. 21:1503–1510.
• Fahey, T.J., and M.A. Arthur. 1994. Further studies of root decomposition following harvest of a northern hardwoods forest. For. Sci. 40:618–629.
• Federer, C.A. 1995. BROOK90: A simulation model for evaporation, soil water, and streamflow. Computer freeware and documentation,Version 3.24. USDA Forest Service, Durham, NH.
• Federer, C.A., L.D. Flynn, C.W. Martin, J.W. Hornbeck, and R.S. Pierce. 1990. Thirty years of hydrometerologic data at the Hubbard Brook Experimental Forest, New Hampshire. General Tech. Rep. NE-141. USDA Forest Service, Northeastern Forest Exp. Stn., Radnor, PA.
• Finlay, R.D. 1995. Interactions between soil acidification, plant growth and nutrient uptake in ectomycorrhizal associations of forest trees. Ecol. Bull. 44:197–214.
• Fitzhugh, R.D., C.T. Driscoll, P.M. Groffman, G.L. Tierney, T.J. Fahey, and J.P. Hardy. 2001. Effects of soil freezing disturbance on soil solution nitrogen, phosphorus, and carbon chemistry in a northern hardwood ecosystem. Biogeochemistry 56:215–238.
• Foster, N.W., J.A. Nicolson, and P.W. Hazlett. 1989. Temporal variation in nitrate and nutrient cations in drainage waters from a deciduous forest. J. Environ. Qual. 18:238–244.[Abstract/Free Full Text]
• Gran, G. 1950. Determination of the equivalence point in potentiometric titrations. Acta Chim. Scand. 4:559–577.
• Groffman, P.M., C.T. Driscoll, T.J. Fahey, J.P. Hardy, R.D. Fitzhugh, and G.L. Tierney. 2001. Effects of mild winter freezing on soil nitrogen and carbon dynamics in a northern hardwood forest. Biogeochemistry 56:191–213.[ISI]
• Groffman, P.M., J.P. Hardy, S. Nolan, R.D. Fitzhugh, C.T. Driscoll, and T.J. Fahey. 1999. Snow depth, soil frost and nutrient loss in a northern hardwood forest. Hydrol. Process. 13:2275–2286.
• Hardy, J.P., and M.R. Albert. 1995. Snow-induced thermal variations around a single conifer tree. Hydrol. Process. 9:923–933.
• Hardy, J.P., P.M. Groffman, R.D. Fitzhugh, K.S. Henry, A.T. Welman, J.D. Demers, T.J. Fahey, C.T. Driscoll, G.L. Tierney, and S. Nolan. 2001. Snow depth manipulation and its influence on soil frost and water dynamics in a northern hardwood forest. Biogeochemistry 56:151–174.
• Hyman, M.E., C.E. Johnson, S.W. Bailey, R.H. April, and J.W. Hornbeck. 1998. Chemical weathering and cation loss in a base-poor watershed. Geol. Soc. Am. Bull. 110:85–95.[Abstract/Free Full Text]
• Johnson, C.E., R.B. Romanowicz, and T.G. Siccama. 1997. Conservation of exchangeable cations after clear-cutting of a northern hardwood forest. Can. J. For. Res. 27:859–868.
• Klein, T.M., J.P. Kreitinger, and M. Alexander. 1983. Nitrate formation in acid forest soils from the Adirondacks. Soil Sci. Soc. Am. J. 47:506–508.[Abstract/Free Full Text]
• Krám, P., J. Hruska, C.T. Driscoll, and C.E. Johnson. 1995. Biogeochemistry of aluminum in a forest catchment in the Czech Republic impacted by atmospheric inputs of strong acids. Water Air Soil Pollut. 85:1831–1836.
• Krám, P., J. Hruska, B.S. Wenner, C.T. Driscoll, and C.E. Johnson. 1997. The biogeochemistry of basic cations in two forest catchments with contrasting lithology in the Czech Republic. Biogeochemistry 37:173–202.
• Lewis, W.M., Jr., and M.C. Grant. 1980. Relationships between snow cover and winter losses of dissolved substances from a mountain watershed. Arct. Alp. Res. 12:11–17.
• Likens, G.E., and F.H. Bormann. 1995. Biogeochemistry of a forested ecosystem. 2nd ed. Springer-Verlag, New York.
• McAvoy, D.C., R.C. Santore, J.D. Shosa, and C.T. Driscoll. 1992. Comparison between pyrocatechol violet and 8-hydroxyquinoline procedures for determining aluminum fractions. Soil Sci. Soc. Am. J. 56:449–455.[Abstract/Free Full Text]
• McDowell, W.H., J.J. Cole, and C.T. Driscoll. 1987. Simplified version of the ampoule-persulfate method for determination of dissolved organic carbon. Can. J. Fish. Aquat. Sci. 44:214–218.
• McDowell, W.H., and T. Wood. 1984. Podzolization: Soil processes control dissolved organic carbon concentrations in stream water. Soil Sci. 137:23–32.
• Mitchell, M.J., C.T. Driscoll, J.S. Kahl, G.E. Likens, P.S. Murdoch, and L.H. Pardo. 1996. Climatic control of nitrate loss from forested watersheds in the northeast United States. Environ. Sci. Technol. 30:2609–2612.
• Moore, R.D., and I.G. McKendry. 1996. Spring snowpack anomaly patterns and winter climatic variability, British Columbia. Canada. Water Resour. Res. 32:623–632.
• Orion. 1976. Instruction manual: Fluoride eectrodes. Orion Research Inc., Cambridge, MA.
• Sakai, A., and W. Larcher. 1987. Frost survival of plants: Responses and adaptation to freezing stress. Springer-Verlag, New York.
• SAS Institute. 1989. SAS/STAT user's guide. 4th ed. SAS Institute Inc., Cary, NC.
• Schaefer, D.A., C.T. Driscoll, Jr., R. Van Dreason, and C.P. Yatsko. 1990. The episodic acidification of Adirondack lakes during snowmelt. Water Resour. Res. 26:1639–1647.
• Shanley, J.B., and A. Chalmers. 1999. The effect of frozen soil on snowmelt runoff at Sleepers River, Vermont. Hydrol. Process. 13:1843–1857.
• Skyllberg, U., K. Raulund-Rasmussen, and O.K. Borggaard. 2001. pH buffering in acidic soils developed under Picea abies and Quercus robur—Effects of soil organic matter, adsorbed cations and soil solution ionic strength. Biogeochemistry 56:51–74.
• Slavin, W. 1968. Atomic absorption spectroscopy. Wiley Interscience, New York.
• Stadler, D., H. Wunderli, A. Auckenthaler, and H. Fluhler. 1996. Measurement of frost-induced snowmelt runoff in a forest soil. Hydrol. Process. 10:1293–1304.
• Stottlemyer, R., and D. Toczydlowski. 1999. Seasonal changes in precipitation, snowpack, snowmelt, soil water and streamwater chemistry, northern Michigan. Hydrol. Process. 13:2215–2231.
• Stumm, W., and J.J. Morgan. 1981. Aquatic chemistry: An introduction emphasizing chemical equilibria in natural waters. 2nd ed. John Wiley and Sons, New York.
• Tabatabai, M.A., and W.A. Dick. 1983. Simultaneous determination of nitrate, chloride, sulfate, and phosphate in natural waters by ion chromatography. J. Environ. Qual. 12:209–213.[Abstract/Free Full Text]
• Tierney, G.L., T.J. Fahey, P.M. Groffman, J.P. Hardy, R.D. Fitzhugh, and C.T. Driscoll. 2001. Soil freezing alters fine root dynamics in a northern hardwood forest. Biogeochemistry 56:175–190.
• Trimble, G.R., Jr., R.S