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Published online 28 June 2005
Published in Soil Sci Soc Am J 69:1298-1313 (2005)
DOI: 10.2136/sssaj2003.0129
© 2005 Soil Science Society of America
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Forest, Range & Wildland Soils

Recent Changes in Soil Chemistry in a Forested Ecosystem of Southern Québec, Canada

François Courchesnea,*, B. Côtéb, J. W. Fylesb, W. H. Hendershotb, P. M. Bironc, A. G. Roya and M.-C. Turmela

a Dép. de Géographie, Univ. de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, QC, Canada, H3C 3J7
b Dep. of Natural Resource Sciences, Macdonald Campus, McGill Univ., 21 111 Lakeshore Road, Ste.-Anne de Bellevue, QC, Canada, H9X 3V9
c Dep. of Geography, Concordia Univ., 1455 de Maisonneuve W., Montréal, QC, Canada, H3G 1M8

* Corresponding author (francois.courchesne{at}umontreal.ca)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We analyzed the temporal trends of elemental changes in the soil of the Hermine, a 5.1-ha watershed of the Lower Laurentians, Quebec, Canada, from 1993 to 2002. The forest canopy of the Hermine is dominated by sugar maple (Acer saccharum Marshall) growing on Podzols formed in a shallow (<2 m) anorthositic till. The results show a significant long-term decrease of SO4 concentration in the solution of both the LFH (–1.33 µmol L–1 yr–1; {alpha} = 0.05) and B (–0.78 µmol L–1 yr–1; {alpha} = 0.01) horizons. This SO4 decline is associated with a reduction in dissolved Ca and Mg in the B (–1.83 and –0.38 µmol L–1 yr–1 for Ca and Mg, respectively; {alpha} = 0.001) and, to a lesser extent, in the LFH horizons (–2.09 and –0.69 µmol L–1 yr–1 for Ca and Mg, respectively; {alpha} = 0.01). Thus, the combined change in dissolved Ca and Mg in the B horizon not only follows that of SO4 but it proceeds, on an equivalent basis, at a rate almost three times faster than that of SO4. For SO4, the concentration changes in solution are accompanied from 1994 to 2002 by a moderate depletion of the H2O-soluble SO4 pool in the podzolic B horizon of Zones A and C (mean rate for Zones A and C of –3.5 µmol kg–1 yr–1; {alpha} = 0.05). Indeed, SO4 desorption from the B horizon occurs even under constant S deposition levels and seems to respond to changes in atmospheric deposition that occurred decades ago. In the case of exchangeable calcium and magnesium (Caex and Mgex), decreasing trends are present in the FH horizons but they are statistically significant only for Mgex in the B horizons of Zones A and C. A decrease of up to 50% of the Mnex pool is observed in the FH and B horizons, a decline that is partly balanced by an increase in Mn uptake by sugar maple since 1994. A tendency toward acidification is also noted in the solution (mean rate of +3.48 µmol H+ L–1 yr–1, {alpha} = 0.01) and in the solid phase (mean rate of +36.4 µmol H+ kg–1 yr–1, {alpha} = 0.001) of the organic horizons. The increase in exchangeable aluminum (Alex) in the FH horizons of zones B and C (mean rate for zones B and C of +0.16 cmol(+) kg–1 yr–1; {alpha} = 0.01) could reflect these decreasing pH trends. Finally, a recurrent seasonal pattern in exchangeable cations is observed where Alex increases at the expense of Caex, Mgex, and exchangeable manganese and potassium (Mnex and Kex) during the growing season. This study shows that long-term, seasonal, and episodic trends in soil properties create a complex temporal pattern that needs to be recognized and partitioned when assessing the response of soil materials to changes in environmental conditions.

Abbreviations: Ala, amorphous inorganic aluminum • Alex, exchangeable aluminum • BS, base saturation • Caex, exchangeable calcium • CEC, cation exchange capacity • DOC, dissolved organic carbon • Fea, amorphous inorganic iron • Kex, exchangeable potassium • Mgex, exchangeable magnesium • Mnex, exchangeable manganese


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
TEMPORAL CHANGES in elemental concentration and distribution in soils, particularly of soluble and exchangeable macronutrients, are of great interest because of their impact on plant nutrition, on the health of terrestrial and aquatic ecosystems, on the transfer of solutes to surface waters, and because these changes are good indicators of the response of ecosystems to environmental stresses such as the atmospheric deposition of pollutants, fluctuations in climatic conditions, and human disturbances.

Contrasting and sometimes conflicting results exist in the literature on the nature and magnitude of the recent chemical changes measured in the composition of surface soil horizons (LFH, A, and upper B horizons) and, as a corollary, on the processes considered to be responsible for the observed changes. In northeastern North America, long-term monitoring studies conducted in forested ecosystems that have not recently been either harvested or burned, have revealed a severe depletion in the dissolved and exchangeable base cation content of surface horizons, notably Ca and Mg (Foster et al., 1989; Lawrence et al., 1995; Houle et al. 1997; Likens et al., 1998). Similar results were reported for forested watersheds located in western Europe (Ulrich et al., 1980; Johnson et al., 1991), southeastern USA (Knoepp and Swank, 1994; Markewitz et al., 1998), and Scandinavia (Hallbåcken and Tamm, 1986; Kirchner and Lydersen, 1995). Hence, decreases in nutrient cations are geographically widespread. Yet, other similar studies have concluded, to the contrary, that forest soils were currently not experiencing a marked depletion in base cations (Yanai et al., 1999) or that the cation efflux from the surface horizons was quantitatively balanced by active biological processes (e.g., litterfall input, decomposition of fine roots) with the consequence that no net base cation changes were detected (Morrison and Foster, 2001). This research hypothesis on the lack of net cation depletion challenges current concepts and still needs to be tested in other forested ecosystems.

Declines in dissolved SO4 in the upper soil have also been observed in relatively pristine forest ecosystems (Wessenlink et al., 1995; Lazerte and Scott, 1996) and in experimental catchments where ambient acidic inputs were artificially excluded by means of a roof (Giesler et al., 1996; Alewell et al., 1997) or where chemical manipulations (i.e., SO4 and/or NO3 additions) were performed at the plot scale (Rustad et al., 1996). However, direct measurements of changes in the sorbed SO4 pools of soils are few despite the fact that the release of previously sorbed SO4 has been invoked to explain the delay in the recovery of dissolved SO4 and nutrient cations in soils and surface waters. Markewitz et al. (1998) reported a significant decrease in VO3–extractable SO4 from the upper B horizon of an Ultisol for the period 1982 to 1990. Except for data from manipulated watersheds or plots (Wright et al., 1988; Rustad et al., 1996), no such report exists for the podzolic soils typical of northeastern North America where acid-sensitive watersheds abound.

The decrease in soil nutrient cation reserves in unmanaged and undisturbed forests have been attributed to a range of ecosystem processes, including recent changes in the concentration of base cations in atmospheric deposition (Driscoll et al., 1989; Wessenlink et al., 1995), soil leaching by acidic compounds of anthropogenic or natural origin (Richter et al., 1994; Kirchner and Lydersen, 1995; Markewitz et al., 1998), and elemental sequestration in biomass (Binkley et al., 1989; Knoepp and Swank, 1994; Trettin et al., 1999). Changes in soil SO4 pools are mostly considered to be a consequence of decreased S deposition from the atmosphere (Likens et al., 2002), although short-term fluctuations in climatic conditions have also been noted to impact on SO4 fluxes from upland soils and associated wetlands (Eimers and Dillon, 2002). These processes can operate individually or in combination and at various spatial or temporal scales, thus creating a complex response in time and a mosaic of spatial patterns in soil SO4 and cation concentrations.

A decade ago, we initiated a biogeochemical study of nutrient pools and fluxes in the Hermine, a forested catchment of the Lower Laurentians (Quebec), in an effort to describe and quantify the rate of its response, specifically, changes in the chemical composition of Podzolic soils, to fluctuations in environmental conditions. Here, we report on the temporal and spatial patterns of change in the concentration of major elements both in the liquid (H+, Ca, Mg, K, Na, and SO4) and the exchangeable (H+, Ca, Mg, Mn, Al, K, and SO4) phases of the Hermine soil with the objective of elucidating the processes responsible for the observed chemical changes and the time scales of their specific impact. Our approach was to sample soil materials repeatedly during a long period of time (9 to 10 yr) and at a high temporal frequency (three to seven times per year) to assess their response to changes in environmental conditions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Site
The Hermine watershed is located at the Station de Biologie de Laurentides, operated by the Université de Montréal, near St-Hippolyte, QC (45°59' N, 74°01' W, altitude {approx}400 m), 80 km north of Montréal. The 5.1-ha catchment is drained by an intermittent first-order stream. The total annual water input to the watershed averages 1150 mm (± 136 mm) for the last 30 yr, of which about 30% falls as snow. The mean air temperature for the same period is 3.9°C (± 0.7°C). The forest canopy (mean age: 85 yr, mean basal area: 28 m2 ha–1) is dominated by sugar maple (78% of total basal area) with some American beech (Fagus grandifolia Ehrh., 9%) and yellow birch (Betula alleghaniensis Britton, 6%). The last major perturbation in the Hermine is a fire that occurred in 1925. Still, large sections of the watershed were spared and the afforestation of these zones dates back to about 100 to 150 yr (Bélanger et al., 2002). No harvesting operations have been conducted since the turn of the 20th century. Soils are classified as sandy orthic or gleyed humo-ferric and ferro-humic Podzols (Soil Classification Working Group, 1998) that developed in a shallow (<2 m) till derived from the anorthositic bedrock (Precambrian anorthosite of the Morin series). The mineralogy of the soil is dominated by quartz, plagioclases, and K-feldspars with small amounts of amphiboles and magnetite. Interstratified minerals are present in the eluviated A and in the B horizons.

Sample Collection and Chemical Analyses
Soil (1993–2002) and soil solution (1994–2002) sampling was conducted in nine 300-m2 plots representing the range of elevations, tree species, and forest history found in the watershed (Biron et al., 1999). The plots are grouped into three zones. Zone A (Plots 1, 2, 3) is located in the downstream portion of the watershed, a few meters above the stream channel on a northwestern-facing slope; Zone B (Plots 4, 5, 6) is situated at the upstream end of the watershed; while Zone C (Plots 7, 8, 9) is located uphill on the same slope as Zone A but close to the watershed's divide. Within a given zone, the different plots are separated by less than 100 m and are considered as field replicates. Tree coring indicated that the stand in Zone C (mostly beech and birch) was initiated by a fire around 1925, while zones A and B (dominated by sugar maple) were spared.

Each plot is equipped with one throughfall collector, two zero-tension lysimeters, one at 0 cm (under the LFH horizons) and one at 50 cm (in the B horizon below the rooting zone), a water table well and a time domain reflectometry soil moisture probe at 25 cm. Since 1993, the watershed has a weir with continuous flow monitoring (90° v-notch weir with a sensor bubbler) and automatic daily water sampling. Bulk deposition volume and chemical composition are determined at a weather station located 1 km north of the Hermine and operated by the Quebec Ministry of the Environment. These data are available for 1982 to 2002.

Water samples from the soil lysimeters were collected every 2 wk from May to November of each year, except for 1998, when solutions were not sampled. Solutions were analyzed for pH and electrical conductivity (EC) before filtration using appropriate meters. After filtration through a 0.45-µm polycarbonate membrane, Ca and Mg concentrations were determined by atomic absorption spectrophotometry, while K, Na, NH4, Cl, NO3, and SO4 were measured by ion chromatography. Dissolved organic carbon (DOC) is measured using a carbon analyzer on 5% of the samples. For the remaining unfiltered solutions, DOC concentration is estimated based on a linear relationship between absorbance at 254 nm and DOC (Turmel and Courchesne, 2005). The soil solution data were validated by the ionic balance method. For solutions collected under the FH horizon, the ionic balance was high and heterogeneous (45 ± 20%) because of the high organic anion concentration (average DOC concentration of 28.5 ± 11.5 mg L–1). For solutions from the B horizon, a balance of 7.5 ± 6.5% or less was considered acceptable. Reference water samples from the National Water Research Institute (Burlington, ON, Canada) were used as quality controls.

Composite samples (n = 16 subsamples per plot) of the forest floor (FH horizons where each subsample was 25 cm2 x depth of FH) and of the top 10 cm of the B horizon (mostly Bhf with some Bhf + top of Bf; each subsample was 250 cm3) were collected with an auger in each plot. For years 1994 to 1997, the soils were sampled monthly from May to November. In 1993 and from 1999 to 2002, the soils were collected three times (every other month) during the growing season. Soils were sampled once (November) in 1998. The soil materials were air-dried and analyzed for pH in H2O, and cation exchange capacity (CEC) determined as the sum of BaCl2–exchangeable Al, Ca, Mg, K, Na, Fe, and Mn (Hendershot and Duquette, 1986). Organic C by the dichromate oxidation method (Nelson and Sommers, 1982) and pyrophosphate and acid-ammonium oxalate extractable Al and Fe (Ross and Wang, 1993) were also measured in the B horizon samples. The amounts of amorphous inorganic aluminum and iron (Ala, Fea) were estimated as the difference between the oxalate and the pyrophosphate extracts. The inorganic SO4 pools (H2O-soluble and PO4–extractable SO4) were determined in the B horizon samples using a batch extraction and a 1:10 soil-to-solution ratio (Courchesne and Landry, 1994). Reference soil samples from the Centre for Land and Biological Resources Research (Ottawa, ON, Canada) were used as quality controls for soil analyses.

Statistical Analyses
The detection of temporal patterns in chemical variables first involved the establisment of the normality of the distributions (Kolmogorov-Smirnov test) and of the equality of variances (Levene test). Because these assumptions of parametric tests (normality or equality of variance) are not always met by our datasets, the nonparametric Kruskal–Wallis test was used to compare the distribution of each chemical variable across all years and for each of the three zones. It uses the ranks of the data rather than their raw values to calculate the statistic and to establish if at least one of the years differs from the others. The slope values obtained from simple linear regression analysis were then used to represent the annual rate of change in soil chemical properties for the period 1993 (soil) or 1994 (soil solution) to 2002. For a given soil property, the slopes were calculated for each of the three zones on time series containing the data of all individual plots and of all sampling dates. One of the limitations of regression analysis for estimating rates of change is sample autocorrelation in time and space. The autocorrelation in time exists for lysimeter solutions since each sample comes from the same location in space over the years. It is, however, not the case for composite soil samples that were collected at different locations within a given plot across time. As for autocorrelation in space between the three replicate plots of a given zone, this constraint is small because the distance between plots exceeds the length of functional links between sites and because the spatial heterogeneity of soil properties is elevated in forest ecosystems like the Hermine, even in somewhat homogeneous areas. Data were analyzed with SPSS (Norusis, 1993).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The soils are acidic with pH in water values below 5.15, and have a low base saturation (BS), except in the organic horizons where the BS values exceed 90% (Table 1). The CEC averages close to 40 cmol(+) kg–1 in the FH horizon but decreases to 6 cmol(+) kg–1 or less in the B horizon. The CEC of the Hermine soils is closely related to the organic matter content, here represented by organic C, the amount of which decreases with depth in the mineral section of the profile after a peak in the upper B horizon. These podzolic soils contain 45 to 70% sand, whereas the amount of clay in any horizon is always below 20%. As reflected by bulk density data, these shallow soil profiles present a compact, rather impervious layer at a depth of about 65 to 75 cm that restricts root penetration and slows water infiltration (Table 1).


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  		Table 1. Soil properties for the three zones of the Hermine (Adapted from Savoie, 1988).

 
Soil Solution Chemistry
Organic Horizons
The SO4 concentration of the solution collected below the organic horizons of the Hermine soil decreased significantly during the period 1994 to 2002 (Fig. 1) . This temporal trend is most pronounced in Zones A and C, where the rate of decrease reaches a value of –1.61 and –1.47 µmol L–1 yr–1 (Table 2) a rate fast enough to reduce the mean SO4 concentrations of the LFH by almost 30% over the last decade (Fig. 1). A seasonal pattern characterized by increasing dissolved SO4 in autumn is sometimes superimposed on this multiyear trend as evidenced by the data for Zone C in Fig. 1. Here, the highest SO4 concentrations in organic horizons are synchronous with the large litterfall inputs (close to 3 Mg ha–1 yr–1 of leaf litter, dry weight) measured in September and October (Fyles et al., 1994). Except for K in Zone B, the slopes for Ca, Mg, and K are always negative and, thus, generally follow those of SO4 (Table 2). These negative trends are all significant except for Ca in Zone A and for Mg in Zone B (Table 2).



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  		Fig. 1. Sulfate, Ca, Mg, H+, K, and Na concentrations in the solutions collected under the LFH horizons of the Hermine watershed for the period 1994 to 2002, inclusively. The lines represent the simple linear regression for the data of each of the three zones (Zone A: dashed line; Zone B: dotted line; and Zone C: full line).

 

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  		Table 2. Results of simple linear regression analyses and for the Kruskal–Wallis test for SO4, Ca, Mg, H+, K, and Na concentrations in soil solutions collected under the LFH and the B horizons at the Hermine during the period 1994 to 2002.

 
The organic horizon leachates are also more acidic in 2002 than in 1994, as indicated by the positive slope for H+ ions (Fig. 1). Interestingly, the strongest acidification rate is measured in Zone C (Table 2), the zone that initially had the most acidic solution of the three zones, with a mean pH of 4.35. The pH of the LFH solution averages about 4.75 in both Zones A and B. Moreover, the large difference in H+ slopes observed among the nine plots (CV of 83%) emphasizes the strong spatial heterogeneity in the response of soils. This variability is a common feature of small, forested catchments like the Hermine. The data also show changes in Na with a mean concentration increasing from 11.3 µmol L–1 in 1994 to 20.3 µmol L–1 in 2002 (Fig. 1). This Na trend is significant (Table 2) despite the large variability in the data (CV for Na of 145%). The mean NH4, Cl, NO3, and DOC concentrations are presented in Table 3 together with EC values. Temporal changes for EC are apparent, notably in the LFH horizons, but could not be tested statistically because of a lack of data. No significant temporal patterns are detected for NH4, Cl, NO3, and DOC concentrations in the solutions collected from either the LFH or the B horizons. Hence, these solutes will not be discussed further unless required for the interpretation of SO4, Ca, Mg, H+, K, and Na data.


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  		Table 3. Mean concentration (standard deviation in parentheses) of selected dissolved substances in the LFH and B horizons at the Hermine for the period 1994 to 2002.

 
Mineral Horizons
In the B horizon, a significant reduction of dissolved SO4 echoes the marked downward SO4 trend detected in the LFH horizon (Fig. 2 and Table 2). Contrary to the organic layers, the SO4 pattern in the B horizon is associated with a generalized and highly significant decline in Ca and Mg concentrations (Table 2). The rate of dissolved Ca and Mg changes is quite remarkable and the concentration decrease reaches about 33% of the levels measured in 1994. For example, the mean Ca level in Zone A fell sharply from above 45 µmol L–1 in 1994 to less than 30 µmol L–1 by the year 2002. For the same period, the absolute decrease in dissolved Mg (from 9 to 6 µmol L–1) is not as pronounced as for Ca but represents a similar relative loss (Fig. 2). When the nine plots are taken into consideration, the rates of change of Ca and Mg concentrations are more homogeneous in the B (CV of 21 and 23% for Ca and Mg, respectively) than in the LFH horizon (CV of 51 and 52% for Ca and Mg, respectively). The spatial variability in rates of dissolved SO4 decline is also higher in the LFH (CV of 60%) than in the B horizon (CV of 14%).



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  		Fig. 2. Sulfate, Ca, Mg, H+, K, and Na concentrations in the solutions collected in the B horizons of the Hermine watershed for the period 1994 to 2002, inclusively. The lines represent the simple linear regression for the data of each of the three zones (Zone A: dashed line; Zone B: dotted line; and Zone C: full line).

 
In the solution from the B horizon of the Hermine soil, the combined decrease rate for Ca and Mg (Ca + Mg) far exceeds the rate of SO4 decline, on the basis of charge equivalents (Table 2). Since long-term downward temporal trends are absent for Cl, NO3, and DOC, only an increase in the concentrations of cations other than Ca and Mg is likely to ensure the balance of charges in B horizon solutions. Our data indicate that the discrepancy between Ca + Mg vs. SO4 rates is best explained by the existence of frequent episodic increases in dissolved H+, K, and Na (Fig. 2). Yet, Na is the only element showing a significant concentration increase (from a 28.7 µmol L–1 mean in 1994 to 37.3 µmol L–1 in 2002) for all three zones (Table 2). In agreement with the tendencies observed in the forest floor, the apparent rate of increase in dissolved H+and K in the podzolic B horizon shows a strong spatial heterogeneity (CV of 96 and 360%, respectively).

Soil Chemistry
Organic Horizons
The concentration of exchangeable manganese (Mnex) decreased in all three zones for the period 1994 to 2002 (Fig. 3) . This represents the main significant interannual trend observed for exchangeable cations in the solid phase of the organic horizons (Table 4). Indeed, and except for Ca in Zone B, the trends for exchangeable Ca and Mg (Caex and Mgex) are neither systematic or significant (Table 4), whereas exchangeable K (Kex) increases slightly in two of the three zones. Some increases in CEC values are also measured, notably in Zone B (Table 4).



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  		Fig. 3. Soil pH in water (pH H2O), and exchangeable Ca (Caex), Mg (Mgex), Mn (Mnex), Al (Alex), and K (Kex) in the FH horizons of the Hermine watershed for the period 1994 to 2002, inclusively. The lines represent the simple linear regression for the data of each of the three zones (Zone A: dashed line; Zone B: dotted line; and Zone C: full line).

 

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  		Table 4. Results of simple linear regression analyses and for the Kruskal–Wallis test on soil properties in the FH (1994–2002) and B horizons (1993–2002) at the Hermine.{dagger}

 
At the watershed scale, the general tendency is toward the acidification of the organic horizons as indicated by the significant decrease in pH, here expressed in terms of the H+ ion content per unit mass of soil calculated from the pH in H2O values, between 1994 and 2002 (Table 4). During that period, the magnitude of change in the FH horizons reaches 0.2 to 0.4 pH units (Fig. 3). For example, in Zone A, the mean pH decreased from 4.50 in 1994 to 4.20 in 2002. In zones B and C, the 2002 pH levels are 4.00 and 3.90, respectively, for initial mean pH values of 4.20. This apparent acidification of the organic horizons nicely follows the trend detected in the solutions sampled from the same horizons, notably in Zone C (Table 2). It is also accompanied by a significant and concomitant long-term increase in Alex in zones B and C (Fig. 3 and Table 4).

Mineral Horizons
The B horizon of the Hermine is not the site of pronounced interannual changes in the magnitude and chemical composition of the cation exchange complex (Fig. 4) . Indeed, although rates of CEC changes are negative in two of the three zones, none are significant (Table 4). Moreover, except for Mnex and, to a lesser extent, Mgex, no other significant trends are recorded for exchangeable cations (Table 4).



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  		Fig. 4. Soil pH in water (pH H2O); exchangeable Ca (Caex), Mg (Mgex), Mn (Mnex) and Al (Alex); and H2O-soluble SO4 (SO4–H2O) in the B horizons of the Hermine watershed for the period 1993 to 2002, inclusively. The lines represent the simple linear regression for the data of each of the three zones (Zone A: dashed line; Zone B: dotted line; and Zone C: full line).

 
A systematic seasonal pattern is, however, observed for the chemical composition of the cation exchange complex from 1994 to 1997. This trend is not identified in 1993, and from 1998 to 2002 data, because soils were then sampled only three times or less per year. The high frequency soil data presented in Fig. 5 show that the mean fraction of the exchange complex occupied by Ca, Mg, Mn, or K decreases markedly (Caex = –6 to –9% of CEC; Mgex = –0.6 to –1.0% of CEC; Mnex = 0.0 to –1.0% of CEC; Kex = 0.0 to –1.0% of CEC) during the growing season, whereas Alex increases simultaneously by 7 to 10% of the CEC. The comparable magnitude of these fractional changes suggests that exchange mechanisms are activated where Al progressively, although temporarily, replaces some Ca, Mg, Mn, and K at the soil-solution interface of the B horizon.



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  		Fig. 5. Mean exchangeable Ca (Caex), Mg (Mgex), Mn (Mnex), Al (Alex), and K (Kex) expressed as a percentage of the cation exchange capacity (CEC) in the B horizons of the Hermine watershed for the period 1993 to 1997, inclusively. The mean values for a given sampling date are for the nine plots. The lines are displayed to underline the seasonal trends. Error bars are standard deviations.

 
The slope values for H2O-soluble SO4 are all negatives (Fig. 4 and Table 4). This pool of labile SO4 declines significantly from 1994 to 2002 in the soils of zones A and C (Table 4). The decrease in H2O-soluble SO4 is slow but it generally follows the negative and significant trends documented for the soil solutions of the organic and B horizons of all zones (Table 2). It is noteworthy that the B horizons of the watershed with the highest amounts of H2O-soluble SO4, like in Zone A (Fig. 4), are also poor in Ala and Fea, two pedogenic oxides that are strongly associated to SO4 retention in these soils (Courchesne and Hendershot, 1989). For example, the lowland sites of Zone A only contain 4.5 ± 1.6 g Ala kg–1 and 2.7 ± 1.2 g Fea kg–1, whereas in Zone C, Ala and Fea average 11.1 ± 1.8 and 5.9 ± 1.2 g kg–1, respectively. The amount of PO4–extractable SO4 averages close to 1.5 mmol kg–1, which is almost one order of magnitude higher than that of H2O-soluble SO4. The time series for PO4–extractable SO4 are not shown because data are unavailable for the period 2000 to 2002.

Bulk Precipitation Chemistry
The fluxes of elements in bulk precipitations at the Hermine are presented for the period 1982 to 2002 (Fig. 6) . The level of water input to the watershed fluctuates considerably from year to year, but shows no significant temporal trend during this period. Consequently, the yearly variations in elemental influx closely follow changes in volume-weighted mean concentrations. Significant correlations between solute fluxes are observed for SO4 and H+ ions (r2 = 0.79; P = 0.01), and for Ca and Mg inputs (r2 = 0.78; P = 0.01). As for temporal patterns, the trend for the period 1982 to 1993 is characterized by a general decrease in deposition for all solutes, except for Na, which is relatively stable through time (Fig. 6). The following period, 1993 to 2002, however, reveals no systematic changes in atmospheric inputs of SO4, Ca, Mg, and Na, while H+, and to a lesser extent K, fluxes decrease slightly.



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  		Fig. 6. Sulfate, Ca, Mg, H+, K, and Na fluxes, and H2O depth in the incident bulk precipitation collected at the Hermine watershed for the period 1982 to 2002, inclusively.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Depletion of Soil Nutrient Pools
The declining SO4 concentrations in soil solutions and H2O-soluble SO4 pools in the mineral soil, together with the decreasing Ca and Mg concentrations in the soil solutions of the B horizons measured in this study, are consistent with the downward concentration trends observed in the Hermine stream for Ca and SO4 (Courchesne et al., 2001). The concomitant and equivalent response of Ca and Mg to a decline in SO4 is commonly observed in stream (Clow and Mast, 1999), lake (Driscoll et al., 1995), and, to a lesser extent, precipitation (Hedin et al., 1994) waters. Temporal variations in streamwater composition reflect the net change in the relative contribution of all the processes controlling the storage and flux of solutes and, as such, integrate the elemental dynamics at the watershed scale. In forested watersheds of northeastern North America, the dominant processes involved in the terrestrial segment of the Ca, Mg, and S cycles are organic matter decomposition, uptake by biota, mineral weathering, and ion exchange or sorption reactions (Likens et al., 1998, 2002).

The decomposition of organic substances is a recurrent seasonal phenomenon, and therefore is not likely to contribute significantly to a long-term change in a relatively mature forest like the Hermine, where tree species composition is rather stable. Forest monitoring at the Hermine showed that the total annual tree biomass production was either constant in time or slightly declining for the period 1994 to 1997 (Courchesne et al., 2001). Accordingly, the uptake and sequestration of Ca and Mg in the biomass followed a downward trend, a trend that is not likely to result in the recent decrease in Ca and Mg concentrations measured in the soil solutions of the Hermine. Indeed, a decrease in biomass storage would be expected, all other things being equal, to increase available Ca and Mg soil pools. Finally, if we accept the reasonable assumption that the mineral weathering rate was constant in the Hermine during the last decade, then a decline in dissolved Ca, Mg, or SO4 in streamwater points toward a potential reduction in Caex and Mgex, and in the sorbed SO4 pools. The trends observed in H2O-soluble SO4 in the soils of the Hermine (Fig. 4 and Table 4) are moderate, but nonetheless in agreement with this line of reasoning. However, few significant trends were measured for exchangeable Caex and Mgex pools in organic and in mineral horizons (Table 4), contrarily to results from similar forested catchments (Kirchner and Lydersen, 1995).

In the Hermine soil, the total Caex and Mgex pools for a 1.2-m-deep profile are estimated at 58 and 7 kmol ha–1, respectively (Savoie, 1988; Hendershot and Courchesne, 1994). These values are relatively high compared with data reported by Houle et al. (1997) for forest soils of southern Quebec. This may be due in part to the abundance of Ca-plagioclases in the parent material of the Hermine. Mean precipitation input is 1200 mm yr–1, while streamflow averages about 600 mm yr–1 (Courchesne et al., 2001). Assuming that all of the streamwater has leached through the soil and using the dissolved Ca and Mg decline rates estimated in this study for the B horizon (–1.6 to –2.0 µmol Ca L–1 yr–1 and –0.3 to –0.4 µmol Mg L–1 yr–1), we can estimate the recent change in soil Ca leaching to be in the order of –9.6 to –12 mol ha–1 yr–1, while Mg leaching changed by about –1.8 to –2.4 mol ha–1 yr–1. Rapid exchange reactions occur between the solid and the liquid phases of sandy podzolic soils and any elemental decrease in one of the two phases should be accompanied by a concurrent decline in the other (Sposito, 1984). Accordingly, the observed changes in dissolved fluxes should represent a potential annual depletion of less than 0.15% of the Caex or Mgex pools in a 25-cm B horizon. Using the same rate of change and projecting across a 10-yr period (since the initiation of this study) yields an expected pool reduction of 0.56 to 0.70% and 1.08 to 1.43% of the exchangeable reservoirs for Ca and Mg, respectively. In short, because the recent reductions in dissolved Ca and Mg flux values in the Hermine are relatively small compared with the soil exchangeable reserves, significant temporal trends for the latter pools are difficult to detect at the catchment scale. The inherent spatial heterogeneity of soils also complicates the detection of temporal trends in chemical properties (Trettin et al., 1999; Yanai et al., 1999), an observation that further stresses the inescapable need for long-term studies to document slow environmental changes.

If we undertake similar calculations for soil SO4 in the B horizon using a mean decline in dissolved SO4 of –0.80 µmol L–1 yr–1 (Table 2), the expected reduction in SO4 leaching rate should approximate –4.8 mol ha–1 yr–1. Assuming that the main mechanisms regulating SO4 mobility in the mineral portion of the soil profile are concentration-dependent adsorption and desorption reactions (Courchesne and Hendershot, 1989; Dhamala and Mitchell, 1995) and that pyrite weathering is insignificant in the Hermine, this reduction of the dissolved SO4 flux should translate annually into a 0.15% depletion of the PO4–extractable SO4 reservoir for a 25-cm deep B horizon (Courchesne et al., 2001). For the 10-yr duration of this study, a value of about 1.5% of the PO4–extractable SO4 reservoir would be anticipated, or 11% of the H2O-soluble pool. An elemental pool depletion of this magnitude is detectable, although only marginally in some zones, as seen by the data in Table 4 and Fig. 4. These findings nonetheless suggest that the bulk of the sorbed SO4 in the Hermine soil is reversibly bound and that this SO4 pool responds to declining SO4 concentrations in the soil solution. In turn, the desorption of this previously accumulated SO4 delays the response of soil solutions and surface waters to decreasing S deposition from the atmosphere, as observed in a variety of forested watersheds (Likens et al., 1996; Giesler et al., 1996; Manderscheid et al., 2000).

The Mnex pools significantly decreased in the Hermine, notably in the organic horizons. This pool is estimated at 720 and 175 mol Mn ha–1 in the FH (8 cm thick) and upper B (first 10 cm) horizons, respectively. The measured total Mn decline of 0.58 cmol(+) kg–1 in the FH translates into a total loss of 415 mol Mn ha–1 or 58% of the Mnex pool. This loss mostly occurred during the 1994 to 1999 5-yr period (Fig. 3). In the upper B, calculations based on a 0.022 cmol(+) kg–1 total decline for the same period represent a 96 mol Mn ha–1 loss or a 55% depletion of the exchangeable pool (Fig. 4). Similar soil Mn losses were measured in areas close to the Hermine (N. Gingras, 2005, unpublished data). Between 1994 and 1999, we documented a marked increase in the Mn, and only for Mn, concentration of sugar maple leaves in the Hermine (Côté et al., 2003). Available data on annual biomass production at the Hermine allow us to estimate the cumulative increase in Mn uptake by sugar maple leaves at about 43 mol ha–1 for 1994 to 1999 (Courchesne et al., 2001). Moreover, the best estimate available for the cumulative increase in Mn storage in the total biomass for 1994 to 1999 yields a value of about 170 mol ha–1. Thus, the measured soil losses of Mnex (about 511 mol Mn ha–1) are, at least in part, balanced by an increased uptake of Mn, about 213 mol ha–1, by the dominant tree species of the Hermine. Yet, part of this flux is cycled back to the soil surface as litterfall. Further work is needed to better quantify these biogeochemical fluxes and to identify the mechanisms responsible for the increased Mn uptake by trees. Of particular interest is the strong biocycling effect of yellow birch on Mn recently documented in the Hermine (Manna et al., 2002).

The recurrent seasonal pattern in the cationic composition of the exchange complex of the B horizon (Fig. 5) is likely a response of the soil system to the uptake by the biota of nutrient cations like Ca, Mg, Mn, and K during the growing season (Blaser et al., 1999). During that period, the release of H+ ions associated with cation uptake temporarily acidifies the soil solution, hence favoring the increase of dissolved Al through the dissolution of Al-rich secondary minerals. Such solids abound in the Hermine soils (Courchesne and Hendershot, 1989). Once in solution, the Al ions compete for exchange sites and replace a measurable fraction of the Ca, Mg, Mn, and K retained on the complex because of the higher selectivity of the exchanger for Al. The trend in cation exchange reactions apparently reverts when the dormant season resumes and the root-induced acidification of the soil solution slows down (Fig. 5). In addition, the replenishment of the cation exchange complex in Ca, Mg, Mn, and K probably benefits from the release of these cations from decomposing leaves and organic matter during late fall and winter. Seasonal changes in the cationic composition of the soil solution and of the cation exchange complex were observed by others (Johnson et al., 1991) and were attributed to the uptake of nutrient cations from the soil solution and to their subsequent sequestration in biomass.

Response of Soils to Changes in Atmospheric Deposition
Since the end of the 1970s, the overall reduction of S emissions to the atmosphere has led to a progressive decrease in S deposition to terrestrial ecosystems in northwestern Europe and northeastern North America (Matzner and Meiwes, 1994; Driscoll et al., 1995; Houle et al., 1997; Lynch et al., 2000; Likens et al., 2001). In contrast, NO3 concentrations declined only slightly or remained constant. Most regions also exhibited a concomitant and unanticipated decrease in Ca, Mg, K, and Na concentrations, which offsets some of the expected reduction in the free acidity of the precipitation (Hedin et al., 1994). Surface water bodies responded quickly to changes in the chemistry of atmospheric deposition. Indeed, SO4 concentrations generally decreased during the last two decades in Europe and North America, with the downward trends in lakes and streams being even more pronounced in the 1990s than in the 1980s (Stoddard et al., 1999). Moreover, the rates of SO4 decline in streams often are of a magnitude similar to those measured in precipitation (Clow and Mast, 1999). The recovery in the alkalinity of surface waters was also documented in some areas of Europe. However, only some regions of eastern North America showed a similar trend, with stream pH being relatively invariant while alkalinity often remained negative (Jeffries et al., 1995; Clow and Mast, 1999; Stoddard et al., 1999).

Boulet and Pinard (1997) showed that weighted mean SO4 concentrations in wet precipitation were decreasing for the period 1985 to 1993 at the meteorological station of the Quebec Ministry of the Environment located in the vicinity of the Hermine. This is in general agreement with the regional analysis of Stoddard et al. (1999), who found that SO4 and base cations were decreasing from 1980 to 1995 in the precipitation of the Vermont and Quebec region. Yet, in the Hermine area, the decreasing trend for Ca, Mg, and K concentrations, although pronounced, was not significant from 1985 to 1993 (Boulet and Pinard, 1997). For the period investigated in this study (1993–2002), only K presents a significant decreasing trend in precipitation inputs (–0.56 mol ha–1 yr–1). The absence of a significant trend for Ca and Mg concentrations in precipitation was also observed by LaZerte and Scott (1996) in southern Ontario from 1982 to 1992. According to Lynch et al. (2000), the major part of the decline in cation deposition indeed occurred before 1995.

The repeated measurement of soil properties allowed us to detect a significant decline in H2O-soluble SO4 from 1993 to 2002 in two of the three zones of the Hermine and to demonstrate that the soils indeed released some of the previously sorbed SO4 (Fig. 4). The approach provides a direct assessment of the evolution of this key soil pool, while most ecosystem studies rely on indirect estimates from elemental budget (Alewell et al., 2000). The present downward trends in dissolved and H2O-soluble SO4 strongly coincide with the documented decrease in SO4 deposition in eastern North America, but they are somewhat out of line with the deposition fluxes measured at the Hermine. Indeed, our soil data show that the desorption of SO4 proceeds even under the constant atmospheric SO4 deposition levels prevailing from 1993 to 2002 (Fig. 6). If we assume that SO4 desorbs from the B horizon in response to a reduction of atmospheric deposition, then it responds to changes that occurred in the 1980s. These observations indicate that the SO4 desorption mechanism can lag far behind changes in the chemical composition of atmospheric deposition, and that the duration of this lag can reach the scale of decades. The decrease in the H2O-soluble SO4 reserves apparently controls the associated downward trend in dissolved SO4. Finally, the moderate signals of intense climatic episodes, like the drought of the summer of 1995, are superimposed on the long-term impact of decreasing S deposition levels and contribute to the time-series recorded at the Hermine. Indeed, the growing season of 1995 was extremely dry because of a conjunction of low rainfall and high air temperature (Courchesne et al., 2001). Consequently, soil water content was very low and water fluxes in soils were nonexistent from mid-June to the end of October. Organic matter decomposition and mineral weathering products together with atmospheric inputs were then stored in the soils in amounts larger than usual (Biron et al., 1999). These conditions might have favored the buildup of a large pool of inorganic SO4, a soil pool that is since being depleted (Fig. 4).

In this context, and in the absence of systematic temporal changes in exchangeable cations, except for Mn, the sustained decrease in dissolved Ca and Mg concentrations observed in the B horizon cannot be attributed solely to the decrease in cation inputs from the atmosphere since the 1980s and is, most probably, also associated to the slow and continuous reduction of SO4 concentrations and fluxes in the soil. Indeed, a lowering of the total anionic charge in the percolating soil solution or of the equivalent contribution of SO4 to the total anionic charge are expected to lead to a concomitant decrease in cation concentrations, as observed in the Hermine. Calcium and Mg nonetheless remain the dominant dissolved cations linked to SO4 fluxes in the soil. In the case of K, the significant trends measured in the soil solution or in the exchangeable phase of the LFH horizons are often contrary to the sustained decrease in deposition since the late 1980s (Fig. 6). This apparent resilience probably reflects the tight K cycling operating between the vegetation and the surface horizons of the soils in this forested ecosystem.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
In the forested watershed of the Hermine, the assessment of temporal changes in soil properties indicates a broad pattern of SO4, Ca, and Mg depletion in the soil solution, with the absolute rate of change being generally faster in the organic than in the mineral horizons. For the period 1994 to 2002, there is a significant decrease in dissolved SO4 in both the organic and mineral horizons. This decline is associated with a significant reduction in Ca and Mg concentrations in the B and, to a lesser extent, in the organic horizons. For SO4, the changes in solution composition are associated to a significant and moderate depletion of the H2O-soluble SO4 pool in the podzolic B horizon. In the case of Caex and Mgex, the apparent trends are statistically significant only for Mgex in the B horizons but not in the FH. A strong decline in Mnex is, however, measured in the LFH and B horizons. It appears to be, at least in part, linked to a recent increase in Mn uptake by trees, although the driving force behind this trend is not known with certainty. A tendency toward acidification is also noted in the solution and the solid phase of the LFH, but not in the B horizon. The increase in Alex in the FH horizons probably reflects the decreasing pH. Finally, the absolute Kex values augment in the FH horizons, a trend that is in contrast with the sustained and significant decrease in K inputs in atmospheric deposition at the Hermine.

The direction of changes observed in the soils of the Hermine watershed for the period 1994 to 2002 is in general agreement with the decline in the atmospheric deposition of S compounds documented for eastern North America. Indeed, the decrease in SO4 inputs has progressively reduced the amount of dissolved and sorbed SO4 in soils, and, consequently, Ca and Mg fluxes in the soil. However, SO4 desorption from the B horizon presently occurs even under constant S deposition levels and, thus, seems to be responding to changes in atmospheric deposition that occurred decades ago. This lag in the soil response to environmental changes appears to be more pronounced for the solid than the liquid phase, notably for nutrient cations.

Our sampling strategy further allowed for the detection of episodic patterns (e.g., occasional severe drought effect on SO4 retention as in 1995), of seasonal oscillations (e.g., nutrient cation uptake and Al release during the growing season in the B horizon; litterfall SO4 inputs to the LFH solution) and of multiyear sequences (e.g., increased Mn uptake by sugar maple) in the soil's response. These are superimposed on the long-term trends apparently associated with changes in the chemical composition of atmospheric deposition. The combination of all these cycles creates a complex temporal pattern that integrates the impact of a series of internal and external factors on the chemical composition of the liquid and the solid phases of soils. We suggest that these multiple cycles, and the specific time scale of their impact, need to be considered when attempting to establish the detailed response of soils, soil solutions, and of surface waters to temporal changes in environmental conditions.


   ACKNOWLEDGMENTS
 
This research was funded by the Fonds FCAR (now FQRNT), the Ministère des Ressources Naturelles du Québec (MRN), and the Natural Sciences and Engineering Research Council of Canada (NSERC). We thank Alain Dufresne, Hélène Lalande, Pascale Legrand, Sylvie Manna, Sylvain Savoie, and Sandrine Solignac for laboratory analyses and field assistance, Sophie Roberge and Thomas Buffin-Bélanger for their help with the statistical analysis of the data, and Gilles Boulet and Henri Durocher from the Ministère de l'Environnement du Québec for access to precipitation data.

Received for publication May 7, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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