Published in Soil Sci. Soc. Am. J. 67:1544-1550 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
DIVISION S-7—FOREST & RANGE SOILS
Natural Isotopic Distribution in Soil Surface Horizons Differentiated by Vegetation
S. A. Quideau*,a,
R. C. Grahamb,
X. Fengc and
O. A. Chadwickd
a Dep. of Renewable Resources, Univ. of Alberta, Edmonton AB, Canada T6G 2E3
b Soil and Water Sciences Program, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521
c Dep. of Earth Sciences, Dartmouth College, Hanover, NH 03755
d Dep. of Geography, Univ. of California, Santa Barbara, CA 93106
* Corresponding author (sylvie.quideau{at}ualberta.ca).
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ABSTRACT
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The isotopic composition of soil organic matter (SOM) is a useful tool for deciphering the different mechanisms underlying decomposition processes in soils. The objective of this study was to quantify the influence of oak (Quercus dumosa Nutt.) and pine (Pinus coulteri D. Don) vegetation on the isotopic variation occurring during decomposition by measuring 
13C and 15N in selected litter and soil fractions. Soil samples obtained from A horizons of two lysimeter soils were separated by density and mineral size to isolate the floatable, fine silt, and clay fractions. These fractions as well as the litter samples were subjected to sequential chemical extractions to differentiate between polar and nonpolar extractives, acid-soluble carbohydrates, and acid-insoluble residues. The physical fractions varied by up to 3.5
for 13C and 4.7 for 15N, while acid-insoluble residues were depleted by 0.9 to 2.1 13C as compared with the samples before extraction. Under oak, 13C and 15N content progressively increased from the litter to the floatable, fine silt, and clay fractions (by 4.7 for 13C and 4.9 for 15N). By comparison, under pine, enrichment of the clay fraction was 1.7 for 13C and 1.7 for 15N as compared with the initial litter. The greater enrichment in heavy isotopes under oak vegetation as compared with the pine could not be explained based on differences in litter inputs. Results suggested instead that variation in decomposition processes by vegetation type caused the differences in heavy isotope enrichment.
Abbreviations: SDEF, San Dimas Experimental Forest • SOM, soil organic matter
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INTRODUCTION
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VARIATIONS IN THE NATURAL ABUNDANCES of soil C and N isotopes provide a useful test for determining the mechanisms underlying organic matter processes in terrestrial ecosystems. Distinct photosynthetic pathways cause C3 and C4 plants to exhibit different C isotopic signatures (Rundel et al., 1989). Introduction of C4 plants in areas dominated by native C3 vegetation, or vice versa, induces a variation in the 13C/12C ratio of SOM that is used as a label to reconstruct paleoecological vegetation shifts (e.g., Connin et al., 1996), or estimate C turnover rates in situations where the time of vegetation shift is known (Balesdent et al., 1988; Vitorello et al., 1989; Skjemstad et al., 1994). Similarly, variations in the natural abundance of 15N between atmospheric N2 and soil N have been used to estimate the contribution of biologically fixed N2 to plant N (Högberg, 1997). A basic assumption in these studies is that isotope fractionations occurring within the soil system can be ignored in light of the differences associated with the distinct sources of C and N. Yet, SOM isotopic signatures are not conservative tracers, and a simple mixing model may not be adequate to describe organic matter processes.
Physical separation techniques have proven useful to isolate SOM pools at different stages of decomposition (Khanna et al., 2001). Humification processes can be followed by comparing the partially decomposed organic matter associated with the lighter and coarser soil fractions to the more decomposed or humified products associated with the finer mineral particles (Kögel-Knabner, 1995). A number of investigators have measured 13C and 15N content in physical separates of soils. For instance, density separation yielded a 13C difference of 2 between the light (<1.6 Mg m-3) and heavy (>1.6 Mg m-3) soil fractions (Skjemstad et al., 1994), and differences ranging from 2 to 5 were found for 13C and 15N between distinct size fractions (Tiessen et al., 1984; Vitorello et al., 1989; Bonde et al., 1992). Taken together, these results illustrate the enrichment of SOM in the heavier isotopes with increasing degree of humification. Alternatively, studies based on chemical extractions have documented distinct isotopic signatures for different plant fractions (e.g., Benner et al., 1987). In theory, the preferential degradation of polysaccharides that are enriched in 13C relative to recalcitrant components such as lignins should leave a residual material depleted in 13C. This mechanism is in apparent conflict with the 13C enrichment observed along the decomposition sequence, and questions remain to be answered before all available data can be reconciled (Ehleringer et al., 2000).
Proposed mechanisms of 13C enrichment during decomposition include microbial fractionation, differential litter decomposition, and soil mixing (Ågren et al., 1996; Ehleringer et al., 2000). Examples of N processes associated with isotope effects are ammonia volatilization, nitrification, and denitrification (Shearer and Kohl, 1986). While laboratory incubations of microbial populations are useful to quantify the isotopic fractionation effects of specific metabolic reactions (Högberg, 1997), extrapolation of these results to field conditions often constitutes a leap of faith. To date, relatively little is known about the importance of environmental factors (climate, biota, etc.) in controlling the level of isotope fractionations within soil systems, and whether these fractionations should be expected to vary among ecosystems.
The lysimeter installation at the San Dimas Experimental Forest (SDEF) hosts a unique long-term experiment that quantifies vegetation influence on pedogenic processes in a highly controlled environment that is less artificial than in laboratory or greenhouse studies. Past research has addressed soil morphological development (Graham and Wood, 1991), clay mineralogy (Tice et al., 1996), cation release by weathering (Quideau et al., 1996), and organic C accumulation (Quideau et al., 1998; Feng et al., 1999) in lysimeter soils planted to different native plant species. Vegetation-induced differences in C turnover and SOM chemistry have been documented (Quideau et al., 2001a and 2001b). The objective of this particular study was to assess the influence of vegetation on organic matter isotopic composition within the lysimeter soils. Specifically, we compared natural 13C and 15N distributions in litter layers and in physical and chemical fractions of A horizons sampled from two lysimeters planted to scrub oak and Coulter pine to follow isotope changes along the decomposition sequence.
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MATERIALS AND METHODS
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Experimental Area
The SDEF is a U.S. Forest Service Pacific Southwest Research Station facility located within the San Gabriel Mountains of southern California. The SDEF has a typical Mediterranean climate, with a mean annual precipitation of 670 mm, and a mean annual air temperature of 14.3°C (Dunn et al., 1988). Vegetation is dominated by chaparral, including chamise (Adenostoma fasciculatum Hook. and Arn.), ceanothus (Ceanothus spp.), manzanita (Arctostaphylos spp.), and oak (Quercus spp).
The lysimeter installation was constructed in 1937 in an area known as Tanbark Flats, located at an elevation of 830 m within the SDEF (Patric, 1961). Large (5.3 by 5.3 m horizontally and 2.1 m deep) earthen-walled lysimeters were filled with a fine sandy loam (58% sand, 31% silt, 11% clay) derived on site from the weathering of diorite. The fill material was thoroughly mixed before filling to ensure homogeneity, and analysis at the time showed no statistical textural difference in 100 randomly collected samples. The lysimeters were planted in 1946 with monocultures of native woody species, including buckwheat (Eriogonum fasciculatum Benth.), chamise, hoaryleaf ceanothus (Ceanothus crassifolius Torr.), scrub oak, and Coulter pine. Although design of the installation did not allow for lysimeter replication, homogeneity of the original fill material ensures that differences between lysimeters solely reflect differences in vegetation.
By 1955, all lysimeters supported pure stands and had complete litter covers (Patric, 1961). In 1960, a wildfire burned the buckwheat, chamise, and ceanothus stands to the ground but, except for some patchy burning of the oak litter, did not affect the oak and pine lysimeters. By 1972, vegetation was again vigorous in all stands, and for the most part persisted as such until time of sampling for this study. Above ground biomass as measured in 1993 was similar for the pine (304 Mg ha-1) and the oak (301 Mg ha-1) stands (Quideau et al., 1996).
Soil Sampling
Soils on each vegetation plot were described and sampled in 1987 as reported by Graham and Wood (1991). In this study we limited our investigations to the soils under oak and pine. The soil under oak, a coarse-loamy, mixed, superactive, mesic Typic Xerorthent, showed a dark-colored, 7-cm thick A horizon overlain by a 6-cm thick litter layer. In comparison, the soil under pine had a 10-cm thick litter layer, and a thin (1 cm thick) A horizon underlain by a Bt horizon with sufficient clay increase for this soil to classify as an Alfisol (coarse-loamy, mixed, superactive, mesic Typic Haploxeralf). Litter layers (i.e., the entire O horizons to the top of the mineral soil) and soil A horizons were sampled in triplicate at the two lysimeters. Litter samples were dried at 65°C, and ground in a Wiley mill (Thomas Scientific, Swedesboro, NJ). Soil samples were air-dried and sieved to remove coarse fragments (>2 mm).
Soil Physical Fractionation
Soil samples (20 g, <2 mm) were ultrasonically dispersed in water and fractionated using a combination of density and sedimentation techniques to yield four distinct SOM pools (Quideau et al., 1998): (i) the floatables, isolated from the sand fraction (>50–2000 µm) by flotation in water, (ii) the sand + coarse and medium silt (5–2000 µm), (iii) the fine silt (2–5 µm), and (iv) the clay fraction (<2 µm). The floatables and the sand + coarse and medium silt fraction were dried to constant weight at 65°C. The fine silt and clay fractions were flocculated with KCl, dialyzed against water until free of salt, and freeze-dried. Weight of the fractions was recorded. Recovery of the original 20 g was excellent, with an average recovery of 97.7%.
Litter samples, unfractionated soils and all density and size fractions were analyzed for total C and N by dry combustion using a Carlo Erba 1106 CN analyzer (Thermo Finnigan, San Jose, CA). The litter, floatables, fine silt, and clay fractions were further analyzed for 13C/12C and 15N/14N ratios using either a Finnigan MAT 252 Ratio Mass Spectrometer (Thermo Finnigan, San Jose, CA), or a Europa Scientific ANCA G/S/L Preparation Module coupled to a Europa Scientific Tracer/20 Mass Spectrometer (PDZ Europa, Northwich, England). Isotopic results were expressed in the -notation, representing the variation from the standard reference material:
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where R is the ratio of 13C/12C or 15N/14N, the standard is Pee Dee Beleminite (PDB) limestone for C and atmospheric N2 for N. The precision (1
) of the 13C analysis was better than 0.15, and for the 15N analysis, was better than 0.25. Isotopic compositions of the sand + coarse and medium silt fraction were calculated from the measured 13C/12C and 15N/14N ratios of the other fractions and whole soil samples using the following equation:
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where represents isotopic composition, f is elemental distribution (as % of total C or N contained in the unfractionated or whole soil sample), and subscripts correspond to the following fractions: SA = sand + coarse and medium silt; WS = whole soil sample; FL = floatables; FS = fine silt; and CL = clay.
Litter and Soil Chemical Extraction
All samples containing >30 g kg-1 C, including the litter, floatables, fine silt, and clay fractions, were moistened with distilled water, shaken under N2 at room temperature, and extracted with a mixture of chloroform and ethanol (1:1) to isolate the nonpolar (fats, oils, and waxes) and polar (simple sugars and amino acids) extractives (Ryan et al., 1990; Kögel-Knabner, 1995). Residues from this first extraction (i.e., the nonextractives) were separated into acid soluble and acid-insoluble fractions using a two-stage sulfuric acid digestion. This method effectively hydrolyses crystalline (cellulose) and noncrystalline polysaccharides such as plant hemicelluloses (Kögel-Knabner, 1995). Samples were incubated with 72% (v/v) H2SO4 for 1 h at 30°C, and then autoclaved at 120°C in 2.5%(v/v) H2SO4. Residues were analyzed after each extraction for total C and N concentration, and for 13C and 15N values. Also for the litter samples and floatables, weight of the initial and postextraction residual materials was recorded to calculate the amount of material removed by each extraction. For the fine silt and clay fractions, the amount of extracted material was estimated from C concentration measurements in the initial and post-extraction materials. Statistical comparison of means was conducted using a t test for unpaired variables assuming unequal population variances.
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RESULTS
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Physical Fractionation
Results of C distribution with particle size have been described in detail elsewhere (Quideau et al., 1998). The original fill material of the lysimeters contained very little C and no floatable fraction was recovered following physical separation (Table 1). In comparison, C concentrations in the 1987-sampled A horizons were at least four times greater than those of the fill material, depending on the particle-size fraction, and the floatables accounted for at least 30% of the total C. The A horizons from both the oak and pine lysimeters were enriched in 13C relative to the litter materials. The 13C value for the oak litter was 2.7 more negative than whole SOM, while the litter and A horizon sampled at the pine lysimeter differed by 1.0. Further differences were observed between the isolated soil fractions (Table 1). The unfractionated soil was enriched in 13C as compared with the coarser fraction (i.e., the sand + coarse and medium silt), while the clay was the most enriched in the heavier isotope (Fig. 1a)
. The difference in 13C values between the clay and the sand + coarse and medium silt fraction was 3.5 for the oak and 1.2 for the pine. These data clearly demonstrate the potential of the physical fractionation method in separating SOM pools with distinct isotopic composition. Furthermore, the variability in C isotopic composition among size separates was larger than the difference between litter and whole SOM (Table 1).
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Table 1. Carbon concentration and distribution, C/N ratios, and isotopic composition in the original fill material, and in the litter layers and A horizons sampled under oak and pine vegetation. Numbers in parentheses are an estimate of the sampling error, and represent one standard error from the mean (n = 3).
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The 15N values under oak showed a trend similar to that of the 13C values and increased in the following order: sand + coarse and medium silt < floatables < fine silt < clay (Table 1 and Fig. 1b). The relationship between fractions, however, was not as clear as for the C isotopes, and the fine silt was the only fraction showing a statistical difference (p = 0.10) as compared with the whole soil samples. In contrast, for the 13C values, differences between fractions and whole soil samples were statistically different at p = 0.05 for all fractions except the floatables (Fig. 1a). There was no apparent trend with decreasing particle size in the A horizon sampled under pine vegetation, but some of the soil fractions also differed in N isotopic composition. In particular, 15N was significantly (p = 0.05) less abundant in the floatables than in the whole soil samples (Fig. 1b).
Even though there is no standard method for physical fractionation of soil, which complicates comparison between studies, differences in the natural isotopic abundance among size separates as observed in this study are consistent with other results reported in the literature. For instance, coarser fractions exhibited lower 13C values as compared with whole soil in a Typic Haplorthox under natural C3 forest vegetation in southeastern Brazil (Vitorello et al., 1989; Bonde et al., 1992), as well as in a native North American prairie dominated by C4 grasses (Balesdent et al., 1988). The clay fraction from these different soils was enriched in 13C by 0.6 to 0.9 as compared with whole SOM. Similar results have been reported for 15N values in size separates from cultivated and uncultivated prairie soils, with the finer size particles being enriched in the heavier isotope (Shearer and Kohl, 1986). In A horizons from Canadian prairie soils (Cryoborolls), the sand + coarse and medium silt fraction (5–2000 µm) showed the lowest 15N abundance (-2 as compared with whole soil) while the clay (<2 µm) possessed the highest enrichment (+3). The fine clay (<0.2 µm) was further enriched in 15N by 5 as compared with the coarse clay (Tiessen et al., 1984).
Chemical Fractionation
The litter and floatables nonextractives showed a lower C content (g kg-1) than the original materials, indicating a high C concentration in the extracted substances (Tables 1 and 2). Biomolecules soluble in chloroform or other organic solvents (i.e., lipids encompassing waxes and fats) typically contain long carbonated chains, and may be expected to be highly concentrated in C. Following the first extraction, the sulfuric acid hydrolysis yielded litter and floatables residues with a high C concentration of 466 to 529 g kg-1 (Table 2). The hydrolysis, which leaves as a residue lignins and other acid-insoluble aromatics, releases cellulose and hemicelluloses (Kögel-Knabner, 1995). The building blocks of these carbohydrates have the general formula Cn(H2O)n and contain a relatively low C concentration (i.e., 400 g kg-1).
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Table 2. Carbon concentration and distribution, C/N ratios, and isotopic composition in the chemical fractions of the litter layers and the A horizons sampled under oak and pine vegetation. Nonextractives correspond to residues after chloroform/ethanol (1:1) extraction, while acid-insolubles are residues after sulfuric acid digestion. Numbers in parentheses are an estimate of the sampling error, and represent one standard error from the mean (n = 3).
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The litter nonextractives were enriched in 13C by about 1 as compared with the original materials (Fig. 2)
. The floatables and mineral soil fractions also yielded 13C-enriched residues following extraction, while the fine silt and clay fractions only showed a slight enrichment and, in the case of the clay isolated under oak, even a depletion in 13C as compared with the original materials. In contrast to the nonextractives, all residues of the sulfuric acid hydrolysis presented a significantly lower 13C abundance as compared with the unfractionated samples (Fig. 2). The difference in 13C was between 1 and 2 for organic matter recovered in the different soil fractions. Carbon isotopic distribution has not been reported for SOM chemical fractions comparable with those utilized in our study, but our results agree with those obtained for plant tissues and litter layers. In particular, Benner et al. (1987) conducted a comprehensive study of 13C variability in plant components and reported a 2 to 6 depletion for lignin relative to 13C values of whole-plant tissues. Similarly, lignin fractions of several C3 perennial grass species were consistently lighter than bulk plant tissues, with an average difference between lignin and bulk samples of 3.6 (Wedin et al., 1995).
The litter nonextractives had a higher C/N ratio than the initial samples, demonstrating the removal of nitrogenous compounds from the original litter material (Tables 1 and 2). Also, the 15N of the litter nonextractives were 1.0 to 2.1 lower than those of the original samples, indicating that the extracts were proportionally enriched in 15N (Tables 1 and 2). Although this first extraction did not cause any clear changes in the C/N ratios of the floatables, fine silt or clay fractions, the residues left after the subsequent H2SO4 treatment exhibited comparatively higher C/N ratios that demonstrate the release of nitrogenous compounds from these samples by acid hydrolysis (Table 2). Simple amino sugars and amino acids may be solubilized in ethanol, but hot acid hydrolysis is required to liberate nitrogenated compounds that occur in soils in the form of chitin or proteins either physically or chemically retained by humic materials (Schulten and Schnitzer, 1998). It is worthwhile noting that a portion of the soil N was resistant to acid hydrolysis. Heterocyclics, which constitute the most prominent soil N components after proteinaceous materials, are not typically hydrolyzed by acid (Schulten and Schnitzer, 1998), suggesting that the residual N may be largely present as heterocyclic N. With the exception of the pine floatables, residues of the floatables, fine silt, and clay fractions following acid hydrolysis were 15N depleted as compared with the samples before extraction (Tables 1 and 2). Overall these results indicate a higher 15N content for the extracted compounds such as amino acids and proteinaceous moieties as compared with the postextraction residual materials.
Differences between Lysimeters
The oak and pine litter materials differed in isotopic composition, with the pine litter being significantly enriched in 13C relative to the oak (Table 1 and Fig. 3)
. In a similar fashion, the floatables and all mineral fractions under pine were enriched in 13C relative to the soil under oak, indicative of the original litter influence on SOM isotopic composition. Under oak vegetation, there was a progressive enrichment in 13C and 15N from the litter to the floatable, the fine silt, and the clay fractions, indicating an increase in the concentration of the heavier isotopes with increasing decomposition (Table 1 and Fig. 1). Furthermore, the 13C and 15N enrichment along the decomposition sequence from the litter to the clay fraction was greater under oak than under pine. As a result, differences in 13C between the oak and pine decreased along this sequence to the point that the clay fractions did not show any statistical differences between the two vegetation types (Fig. 3).
The differences in 13C values between the oak and pine lysimeters increased following each chemical extraction (Fig. 3). This was true for all fractions and may be exemplified by the clay fractions, which were not significantly different before extraction, but showed a statistical difference at p = 0.05 after the first extraction, and at p = 0.001 after the second extraction. The acid-insolubles thus contributed to a greater extent than the other fractions to the observed differences in 13C values between the oak and the pine vegetation (Table 2). Also noteworthy was the similar 13C enrichment of the two chemical fractions along the decomposition sequence. For instance, the 13C values of the oak litter and clay fractions, which represent the end two members of the decomposition sequence, differed by 4.7 for the original materials and by 4.4 for the acid-insolubles (Tables 1 and 2). As a result the greater differences in isotopic composition observed in the litter and floatable acid-insolubles as compared with the initial materials was preserved in the more humified soil fractions (i.e., the fine silt and clay fractions).
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DISCUSSION
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The highly controlled experimental conditions at the San Dimas lysimeter installation allowed us to precisely quantify the influence of oak and pine on SOM isotopic composition in a situation where vegetation was the only environmental variable. By using a combination of density and size fractionation techniques, as well as some chemical extractions, we were able to differentiate between organic matter pools with distinct isotopic signatures. All organic matter pools obtained from the A horizon under pine vegetation were enriched in the heavier isotopes (13C and 15N) relative to those obtained under oak, and paralleled the enrichment in heavier isotopes of the pine litter as compared with the oak (Tables 1 and 2). Balesdent et al. (1993) related the isotopic composition of SOM to that of tree leaves according to the following equation: 13C of soil C = 13C of leaves - 0.7log10(C), where C was the proportion of C in the soil sample. This equation basically described a linear relationship between foliar and SOM 13C content, with the decimal logarithm term defining a correction factor that reflects the dilution effect of the litter material with low 13C content and is negatively correlated to soil C content. In our study, the C content was greater under oak than under pine vegetation, yet the difference in 13C between surficial litter and SOM was larger for the oak (Table 1). Thus, it appears that in addition to the dilution effect of the litter materials, measured differences in SOM 13C values between the two lysimeters may be attributable to additional factors such as different rates of changes in isotopic composition during decomposition processes.
Important potential controls of SOM isotopic compositions are: 1. plant litter inputs, 2. preferential decomposition of litter, 3. microbial fractionation during decomposition, and 4. soil C mixing (Nadelhoffer and Fry, 1988; Ehleringer et al., 2000):
- The greater enrichment in heavy isotopes under oak could be potentially due to a reduced amount of oak litterfall as compared with the pine since the pine litter was more enriched in heavy isotopes than the oak. Litterfall as measured in 1997 through 1998 was 225 g C m-2 yr-1 for the oak, and 135 g C m-2 yr-1 for the pine lysimeter (Quideau et al., 2001b). Consequently, differences in litter amounts are unlikely to explain the observed differences in SOM isotopic composition at the two lysimeters. In addition to surficial litter, roots also may constitute an important source of organic matter for mineral soils. This would be particularly true for the A horizon under pine, where many very fine roots were found (Graham and Wood, 1991). However, roots are generally enriched in 13C as compared with aboveground biomass and more specifically foliage (Gebauer and Schulze, 1991; Yeh and Wang, 2001). Again, this does not constitute a valid explanation for the observed differences between lysimeters. Taken together, these results imply that the comparatively greater 13C enrichment observed in the SOM under oak as compared with the pine cannot be explained based on differences in litter inputs.
- The selective preservation of recalcitrant litter components such as lignin and waxes should result in a residual material becoming progressively more and more depleted in 13C (Ehleringer et al., 2000). Clearly, this mechanism is acting in the opposite direction of the heavy isotope enrichment observed during the course of decomposition, demonstrating that even though there might be some preservation of 13C-depleted litter components, it cannot be the driving variable responsible for the observed changes in the isotopic composition of the remaining substrate (Fig. 1). Furthermore, our data indicated a similar 13C increase with increasing decomposition for all chemical fractions, including the acid-insolubles, which encompass lignins and other recalcitrant constituents (Fig. 3). Therefore, these results provide no evidence that the observed changes in 13C with decomposition should be related to preferential litter decomposition.
- and 4. Microbial discrimination against 13C during catabolic processes, corresponding to the emission of 13C-depleted CO2 by microbial respiration, would increase 13C concentration in the residual SOM and produce the expected patterns of changes in SOM isotopic composition (Ågren et al., 1996). Alternatively, Ehleringer et al. (2000) proposed that soil C mixing between plant and microbial or fungal residues is responsible for the observed shift in 13C values during decomposition. The authors base their hypothesis on literature evidence that microbes and fungi are enriched in heavy isotopes as compared with their substrates. For instance, saprotrophic fungi had a highly enriched 13C signature (-22.6 ) relative to the organic and even the mineral (-25.6) soil (Hobbie et al., 1999), while decomposing basidiomycetes showed an accumulation of 13C by 4 relative to their substrates in wood (Gleixner et al., 1993). In this case, the isotopic enrichment would be explained by an increase in fungal and microbial-derived constituents with increasing decomposition. Results from prior 13C CPMAS NMR analysis of the fine silt fractions at the San Dimas lysimeter installation evidenced the predominance of carbonyl C under oak, which suggested extensive oxidation reactions (Quideau et al., 2001a). Further, turnover rates of SOM were estimated using total C and 14C content to be twice as fast under oak as under pine (Quideau et al., 2001b). These results are indicative of a greater microbial activity under oak than under pine but do not allow us to differentiate between microbial fractionation and soil mixing. More detailed isotopic studies, particularly compound-specific measurements for chosen sugar moieties, could test this hypothesis and clarify organic matter processes in the oak lysimeter.
In conclusion, our results should be taken as a caution for studies that rely on stable isotope analysis of SOM. The shift in 13C during decomposition processes does not reach the same extent for different vegetation types and should be considered when using 13C as a tracer following vegetation changes.
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ACKNOWLEDGMENTS
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The authors thank Dave Larson, manager of the San Dimas Experimental Forest, for providing access to the lysimeter installation. This research was supported in part by the Kearney Foundation of Soil Science.
Received for publication July 24, 2002.
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ABSTRACT
INTRODUCTION
