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222 San Lorenzo Blvd., Santa Cruz, CA 95060
* Corresponding author (callen{at}cruzio.com).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: CEC, cation exchange capacity • subscript [d], dithionite-citrate-bicarbonate extractable • DCB, dithionite-citrate-bicarbonate • MSST, mean summer soil temperature • subscript [o], oxalate extractable
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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In one of the classic studies of arctic–alpine geomorphic processes, Rapp (1960) stated that the stabilizing action of plant roots and the protection provided by vegetation cover influenced mass movements and periglacial processes in Kärkevagge, Swedish Lapland. He commented that vegetational zonation was an important factor in the distribution of soils in the Kärkevagge watershed, and also concluded, more importantly perhaps and somewhat more controversially, that chemical weathering was the most effective geomorphic process operating in the region (Rapp, 1960).
Other than the brief association of soils with vegetation distribution, soils were overlooked in Rapp's (1960) paper, and it has been left to recent pedological studies in Kärkevagge to provide discussion on the soils in the region. Darmody et al. (2000) have characterized the soils in the watershed and revealed ample evidence to exemplify the efficacy of chemical weathering. They further described the plant–rock–soil associations in a meadow on a colluvial slope (Darmody et al., 2001), and the influence of vegetation on soil properties and soil distribution has been confirmed (Darmody et al., 2004). Each of these recent studies is part of an ongoing investigation into weathering and soil development in the watershed (Thorn et al., 2001). Darmody et al. (2000) provide the most comprehensive discussion and classification of soils in Kärkevagge, with one exception: the soils that are forming on the large garnet-mica-schist and mica-schist boulders evident throughout the valley and giving Kärkevagge its name, the "Valley of the Boulders" (Fig. 1) , have been neglected.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Laboratory Methods
The soil samples were oven-dried at 100°C overnight in the lab, and run through a 2-mm sieve. Gravel content was weighed, recorded and then discarded. The <2-mm fraction was sieved for the sands (<0.05 mm), weighed, and saved. Percentages of silt and clay were determined using the hydrometer method (Gee and Bauder, 1986).
Soil nutrients Ca2+, Mg2+, K+, Na+, and available S, were obtained through inductively coupled plasma spectroscopy on 1:10 soil/Mehlich-3 extracts (Mehlich, 1984), and CEC was estimated through the summation of the Mehlich-3 extracted cations (Darmody et al., 2000). In a study of Alaskan soils, Michaelson and Ping (1986) found that there is a good correlation between the Mehlich-3 extractant and other extractant methods used to measure extractable bases in acidic soils. Base saturation was calculated by estimation of exchangeable H from SMP buffer pH at pH 7.8 (McLean, 1982). Soil pH was determined on a 1:1 soil/distilled water suspension (McLean, 1982). Organic matter was estimated by loss on ignition at 430°C (Davies, 1974), and was divided by 1.7 to estimate organic C (Birkeland, 1999), since there were no free carbonates in the soils. Total N was not measured for it was considered closely related to organic C (Weih, 1998; Broll et al., 1999; Darmody et al., 2004).
In an effort to characterize the range of soil properties on the boulder tops, soil samples from six pedons, M1, M2, U1, L3, G1, and G4, were selected based on elevation, mineralogy, horizonation, and chemistry for selective dissolution analysis of Fe and Al. Dithionite-citrate-bicarbonate (DCB) and acid oxalate extractions were used to analyze pedogenic Fe and Al (McKeague and Day, 1966; Soil Survey Laboratory Staff, 1996). Concentrations of DCB extractable Fe (Fed) and Al (Ald), and oxalate extractable Fe (Feo), and Al (Alo) were measured by atomic absorption. The soils that had formed on the boulder tops were classified according to U.S. Soil Taxonomy (Soil Survey Staff, 1999).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The mineralogy of the boulders was predominantly quartz, feldspar, and muscovite; with the only difference between the garnet-mica-schist and mica-schist boulders the presence of garnet in the former rock type (Allen, 2001). Trace amounts of chlorite and 2:1 layer interstratified minerals were present in the partially weathered rock (parent material) immediately underlying the soil (Allen, 2001). Clay mineralogy of the boulder-top soils was predominantly muscovite, chlorite, and 2:1 layer interstratified minerals (likely mica-smectite), with lesser quantities of quartz and feldspar (Allen, 2001). The clay mineralogy also indicated that there was little to distinguish the garnet-mica-schist from the mica-schist, and therefore the influence of a perceived difference in parent material composition on pedogenesis had to be discounted.
Soil Chemical Properties
The soils forming on the boulder tops were acidic, with soil pH ranging from 3.7 to 5.3 (Table 2). Soil pH was influenced in part by organic acid production from the vegetation cover as indicated by acid pH near the surface of the soil, and organic C content decreasing with depth. The soil pH was also partially influenced by the composition of the parent rock as abrasion pH for parent rock ranged from 4.0 to 5.0 (Allen, 2001). The CEC in the boulder-top soils averaged 22 cmolc kg–1, and for the most part decreased with depth. Since both soil organic matter and CEC decreased with depth, and the clay concentration was fairly low and constant with depth (Table 1), it is likely that the soil organic matter provides most of the exchange sites.
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Micaceous schists provided much of the extractable K in the soils, and K was evident in all boulder-top soils. The extractable K was greatest in the surface horizons of all of the soils, and decreased with depth. The relatively high K concentration in the near-surface horizons of pedons U5, G3, M1, and L5 coincided with the presence of O and O/A horizons, and was also suggestive of biocycling. Pedons G2, G5, and M2 also had O horizons, but extractable K was no different than in soils without O horizons. The mean S content for the boulder-top soils was 70 mg kg–1. The S content is highest near the soil surface and, when excluding O horizons, decreased with depth; a further indication of the importance of biocycling in the boulder-top soils.
The chemical properties for boulder-top soils did not range as widely as for soils in the rest of the watershed (Darmody et al., 2000). The CEC values for the boulder-top soils were comparable with the other soils in Kärkevagge, which ranged from 2 to 31 cmolc kg–1 (Darmody et al., 2000, 2004). Mehlich extractable bases were similar to the quantities determined for other soils in Kärkevagge in that relative concentrations of extractable cations were Ca > Mg > K > Na (Darmody et al., 2000, 2004), albeit values were substantially lower overall for the boulder-top soils.
Pedogenic Iron and Aluminum
An increase in the concentration of free Fed; that is, organically bound, crystalline and poorly crystalline Fe oxides, in the soil invariably corresponds with increased in situ weathering (Buol et al., 1997). Therefore, the quantity of Fed in the soil additionally provides an estimate of the degree of soil development, since Fed represents the total pedogenic Fe (Blume and Schwertmann, 1969; Birkeland, 1999). The Fed in the Kärkevagge boulder-top soils ranged from 1.0 to 6.7%, had a mean of 2.9%, and increased with depth in all of the pedons (Table 3). Of the six soils analyzed for Fe and Al, Pedon U1 had the highest amount of Fed; this pedon also had the greatest number of horizons and was the deepest.
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Oxalate extractable Fe is essentially a measure of poorly crystalline and organically bound Fe in the soil (Parfitt and Childs, 1988). The Feo in the boulder-top soils ranged from 0.10 to 1.32%, had a mean of 0.54%, and increased with depth. It is possible to estimate pedogenic, inorganic Fe that is the Fe sequestered in crystalline Fe oxides such as goethite and hematite by the difference between Fed and Feo (Fed–Feo) (Birkeland, 1999). Estimated inorganic Fe values increased with depth in the boulder-top pedons, thereby signifying an increase in crystalline Fe with depth. Active Fe ratios determined using Feo/Fed, (McKeague and Day, 1966) were low (
0.4). Since this ratio is used as a relative measure of the degree of crystallinity of free Fe oxides (Blume and Schwertmann, 1969), the ratios confirmed that most of the Fe in these soils was crystalline. The crystalline Fe oxide was likely the secondary Fe oxide goethite, a common Fe mineral in well-drained soils undergoing oxidation, typical of the boulder-top soils.
The quantity of Ald; that is, Al substitution in Fe oxides and organic matter-bound Al (Parfitt and Childs, 1988), in the soils was lower than Fe, with Ald ranging from 0.10 to 0.44% and having a mean of 0.25%. The Ald values were comparable with those calculated for soils forming in tills in Baffin Island (Birkeland et al., 1989). The total pedogenic Al was estimated by Alo (Birkeland et al., 1989), which is an estimate of the poorly crystalline Al and Al associated with humus (Parfitt and Childs, 1988). The Alo ranged from 0.12 to 0.34%, and had a mean of 0.17%. The amount of inorganic secondary Al present, possibly as incipient gibbsite, did not exceed 0.3%. On this occasion, the amount of Alo is lower than in other incipient soils (Graham et al., 1988), although the boulder-top soils exhibit some similarities, with inorganically bound Al where organic matter is high (increased Alo values nearer the soil surface). It was concluded that given the low pH, coarse texture, and the high organic matter content of the boulder-top soils, almost all of the Al is in complex with organic matter, such as in Pedon M1.
Pedogenesis and Soil Classification
The soils that are forming on the boulder tops are weakly developed; yet they display a number of interesting attributes worthy of consideration. Textural characteristics of the soils suggested that silt content could be attributed to eolian influx. However, the selection of boulders that were on topographic highs and had relatively horizontal surfaces, minimized the possibility of eolian additions, and soil morphology did not indicate material translocation. Clay skins and silt caps were absent, there was no evidence of cryoturbation and/or other mixing, the soil profiles had clear boundaries, and there was no evidence of allochtonous mineralogy. Silt content did not decrease with depth in all profiles, which would have illustrated the upward-fining sequence ascribed to eolian input (Munn and Spackman, 1990; Dixon, 1991; Bockheim and Koerner, 1997). In part, the lack of an upward-fining sequence was undoubtedly influenced by the shallow depth of the soil, but the conservative interpretation was that silt-enrichment in the soils was due to parent rock weathering into silt-size particles, and presumably clay-size particles as well. The mineralogy of the boulders and interstratified clay minerals in the boulder soils supported this interpretation (Allen, 2001). In lacking eolian input, the boulder-top soils therefore offer a unique perspective in the context of other arctic–alpine and mountain soils (Ellis, 1980; Burns and Tonkin, 1982; Litaor, 1987; Birkeland et al., 1989; Dixon, 1991; Blank et al., 1996; Bockheim and Koerner, 1997; Bockheim et al., 2000).
The interstratified clay minerals in conjunction with the presence of short-range order Fe oxyhydroxides, that is, the formation of secondary products, exemplifies that chemical weathering is an effective agent of pedogenesis on these boulder tops. Weathered rock at the soil–rock interface exhibited thin, reddish-colored weathering rinds, indicative of secondary Fe accumulation, which is probably the result of the weathering of ferrous iron in chlorite as part of the oxidation process (Buol et al., 1997). Both crystalline and poorly crystalline Fe, exemplified by the increase in Fed and Feo values with depth, indicated Fe accumulation, despite the shallow depth and absence of B horizons in these boulder-top soils. This indicates that chemical activity appears to be concentrated at the soil–rock interface. I hypothesize that moisture influx to the soil is able to percolate down through the soil and eventually collect at the soil–rock interface where the rock is foliated, thereby promoting chemical weathering in this zone influencing pedogenesis.
The chemical activity in the boulder-top soils is also almost certainly bolstered by plant roots, which in the boulder-top soils exhibit similar density and extend to the lithic contact, or soil–rock interface. Frazier and Graham (2000) illustrated the biochemical activity of roots at the soil–weathered rock interface and noted the influential role of fracture zones in parent rock, particularly vertically oriented joints, on pedogenic processes. More recently, Certini et al. (2002) determined that beneficial orientation of cavities in parent rock abetted the collection of water, mineral, and organic debris, thereby enabling "embryonic" soil formation.
Despite the apparent effectiveness of chemical weathering, the boulder-top soils are relatively infertile, as signified by their shallow depth and coarse texture. Nutrient availability is controlled by vegetation, which in arctic–alpine soil formation is regularly equated with organic matter contribution (Howell and Harris, 1978; Gensac, 1990). The O horizons and the relatively high organic matter content in the boulder-top soils are typical of sites with a stable vegetation cover, albeit only patchy and localized in this circumstance. Heath vegetation, lichens, and mosses are all important contributors to the pedogenic processes since they are strong proton donors, that is, acidification agents. In well-drained conditions the acids can also weather materials and mobilize the Fe and Al (Ugolini, 1986). The organic matter underlying the heath vegetation on the boulder tops provides most of the exchange sites in the soil.
It was deemed likely that the slightly higher quantities of Ca and Mg concentrations in surface horizons were associated with the vegetation cover, which is an established association that has been documented in arctic soils (Gíslason et al., 1996; Moulton and Berner, 1998). However, while Ca was the highest concentration extractable base cation in the boulder soils, it was present in smaller amounts than in the rest of the watershed (Darmody et al., 2000) implying that it has been removed through leaching and/or by the vegetation and/or not supplied by eolian deposition.
Despite the overall weak soil formation, biochemical weathering appears to be the effective pedogenic process in the boulder-top soils. The effectiveness is borne out by four of the soils, U1, L3, G1, and G4 that display incipient spodic properties, for example, poorly crystalline Fe (Parfitt and Childs, 1988). However, while spodic-like materials are present (Alo + 1/2 Feo > 0.5%), the soils lack the increase relative to overlying ochric epipedons. Oxidation of Fe is a process prevalent in soils with high quantities of organic matter, and often leads to the formation of spodic horizons (Buol et al., 1997). Furthermore, the thinness of the soil horizons does not necessarily inhibit Spodosol classification criteria, since "...some spodic horizons are thin or otherwise weakly developed" (Soil Survey Laboratory Staff, 1996). Spodosols have been previously documented in arctic and alpine regions (Burns, 1990; Dahlgren and Marrett, 1991), and Haplocryods have already been characterized for the lower reaches of Kärkevagge (Darmody et al., 2000). Extractable Fe and Al concentrations in the boulder-top soils compared well with B horizons in arctic and subalpine Spodosols (Dahlgren and Marrett, 1991), while extractable Al concentrations were similar to those recorded for the chemically pedogenic yet poorly developed soils in Baffin Island soils forming in till (Birkeland et al., 1989). However, Feo/Fed ratios would have to be higher to signify podzolization (Mckeague and Day, 1966), and the boulder-top soils lacked all of the requisite criteria, such as illuvial accumulation of spodic materials, to meet the Spodosol classification (Soil Survey Staff, 1999); and therefore, the incipient boulder-top soils were not classified as Spodosols.
Gelisol classification of the soils was also ruled out. Mean soil temperature at approximately the 10-cm depth in three of the study boulders, U1, U2, and U3, was –0.37°C (Thorn et al., 2002). The soil temperature regime was classified as cryic (Soil Survey Staff, 1999). According to Soil Taxonomy (Soil Survey Staff, 1999), soil temperature is measured at 50 cm or at a lithic, paralithic, or densic contact, whichever is shallower. In addition, cryic soils have a mean annual soil temperature of higher than 0°C but less than 8°C, lack permafrost, and have relatively cool summers, with O horizons <6°C MSST (mean summer soil temperature) and without O horizons <13°C MSST (Soil Survey Staff, 1999). The 50-cm depths were unattainable due to the shallow depth of soil caused by lithic contact. In addition to the temperature criteria, there was no evidence of cryoturbation, or gelic material underlain by permafrost (Bockheim and Tarnocai, 1998), which in part illustrated the complexity of the definitive criteria, that is, lacking permafrost (Thorn and Darmody, 2002).
The soils forming on top of the boulders were eventually classified as loamy-skeletal, micaceous, acid Lithic Cryorthents. In grouping all of the boulder-top soils in the Entisol category, this meant that the boulder-top soils were most comparable with two other watershed communities: low-alpine (with regard to heath attributes especially) and highest elevations (with regard to cryptogams), that is, the alpine soils (Darmody et al., 2000). The comparison with the alpine soils is appropriate even though the boulder soils were not as deep as the alpine soils, they did not exhibit cryoturbation, and nor were they affected by cold-based ice. Boulder-top soils did however form in residuum, and despite their coarse texture, minimal structural development and horizonation, they merited classification. The boulder-top soils exhibited horizons, pedogenic processes were active, the soil was clearly distinguishable from the weathered material at the soil–rock interface, and they supported rooted plants in a natural setting.
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ACKNOWLEDGMENTS |
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Received for publication January 1, 2003.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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