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Published in Soil Sci. Soc. Am. J. 68:175-184 (2004).
© 2004 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-5—PEDOLOGY

Carbon Distribution in a Hummocky Landscape from Saskatchewan, Canada

A. Landib, A. R. Mermut*,a and D. W. Andersona

a Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, SK S7N 5A8, Canada
b Shahid Chamran University, Faculty of Agriculture, Dep. of Soil Science, Ahwaz, Iran

* Corresponding author (mermut{at}sask.usask.ca).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Changes in the topography influence organic and inorganic C contents and {delta}13C values of soil C across a landscape. The objectives of this research were to: (i) study the effect of landscape on the formation and distribution of pedogenic carbonate and organic matter distribution in a hummocky landscape, and (ii) estimate the amount of organic C and pedogenic carbonate accumulation in local scale in comparison with regional scale using the stable isotope geochemistry techniques and standard characterization analyses. A hummocky landscape, typical of 38% of Saskatchewan's land, with glacial till parent material under virgin grassland, was studied. Organic C content of A horizons range between 20 to 98 g kg–1. Both extremes occurred in level positions of the south-facing and north-facing slopes. The lowest {delta}13C value of organic C (–29.6{per thousand}) was measured in a depression and the highest (more positive) was obtained on a shoulder (–21.7{per thousand}). The {delta}13C values of carbonate ranged from –0.9{per thousand} (carbonated parent material) at the 114-cm depth in level complex to –7.9{per thousand} at depth of 100 cm in footslope complex and depression. The amount and percentage of pedogenic carbonate was higher in north-facing slopes than in southward slopes. The highest proportion and amount of pedogenic carbonate up to 1-m depth was found in Calcicryolls in footslope complex position in the north-facing slope, and likely represents a gain in carbonate through lateral flows. The lowest proportion and amount (34.4% and 33.9 kg m–2) was found in the shoulder complex segment of west-facing slope and in footslope complex position in east-west direction. On average, the rate of accumulation is about 1.25 g C m–2 yr–1 of inorganic C (pedogenic carbonate) and 1.25 g C m–2 yr–1 as organic C. These are close to the calculated rate of 1.4 g C m–2 yr–1 for Dark Brown, and 1.3 g C m–2 yr–1 for Black soils (Mollisols) in Saskatchewan.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SOIL PROPERTIES VARY with topographic settings due to aspect (i.e., north facing vs. south facing) and slope shape (Birkeland, 1999). The influence of slope aspect on microclimate and soils is greatest between 40 to 60° latitudes, and less important in both equatorial and polar latitudes (Hunkler and Schaetzl, 1997). Knowledge about the quantitative C circulation (i.e., gains, distribution, and losses of C that circulates within the soil system) is needed to understand soil development and ecosystem function (Warembourg and Kummerow, 1991).

The amount and distribution of pedogenic carbonates in soils within a toposequence are controlled mainly by soil moisture. Downward movement of percolating water, capillary rise of ground water, and lateral moisture flow, play different roles in the different landform segments. Generally, noncarbonatic and leached soils occur in concave recharge areas, and carbonatic soils occur in ground water discharge areas, particularly on lower slopes adjacent to recharge depressions (Miller et al., 1985).

Landscape position influences water movement and the nature and extent of erosion or deposition processes occurring at any given location in a field (Mermut et al., 1983; Pennock and de Jong, 1987). The degree of past erosion affects the distribution of organic matter in the soil profile and among various aggregates and primary particles (Bajracharya et al., 1998). Shoulder slope complexes lose soil and organic C and footslope complexes gain more soil and organic C under long-term cultivation (Pennock et al., 1994). The thickness of Ap or Ah horizons follows the same trend. Depressional areas with strongly Eluviated Luvic Gleysols (Argiaquolls) had much less carbonate, and little or no pedogenic carbonate, whereas Rego Black (Calcicryolls) soils just above the depression had marked gains of pedogenic carbonate (Wang and Anderson, 2000).

Depth to calcic horizon, horizon thickness, density, and percentage of calcium carbonate in the calcic horizon are closely associated with parent material, permeability, texture, structure, and to the age or length of time the soil has been developing (Harper, 1957). Honeycutt et al. (1990a) found that the shoulder and upper backslope had much shallower depth to carbonate accumulation zone when compared with the summit. The possible explanation for this observation was that greater erosion on the shoulder and backslope segment resulted in shallower depths to secondary carbonate accumulation zone.

Soil C, either organic or inorganic (soil carbonates) reflects the plant source C. For soil organic C the {delta}13C values are close to those of the original plants, with small variations due to partial bacterial oxidation. Oxidation of labile components (lipids, cellulose, etc.) has the effect of raising {delta}13C values of the residual organic matter. Soil carbonates derive their C from CO2 in the soil, and is related to organic matter source (Deines, 1980; Boutton, 1996). A large component of this comes from the roots of plants, so that the {delta}13C values of soil carbonates are an indicator of plant type (C3 versus C4).

The objectives of this work were to: (i) study the effect of landscape on the formation and distribution of pedogenic carbonate and organic matter in a virgin hummocky grassland, and (ii) estimate the amount of organic C and pedogenic carbonate accumulation in local scale in comparison to regional scale using the stable isotope geochemistry technique and standard characterization analyses.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Area and Sampling
A noncultivated hummocky landscape with glacial till parent material was chosen for the study. The location of the site, in the context of North America is shown in Fig. 1 . The site, 12 km east of Saskatoon (52° 20' N, 106° 38' W), had mostly Dark Brown Chernozem (Typic Haplocryolls) soils (Weyburn Association) with some Black Chernozem (Udic Haplocryolls) soils (Oxbow association) (Acton and Ellis, 1978). The site is in a transition between the Moist Mixed Grassland and Aspen Parkland ecoregions. A grid-sampling design with 10-m cell spacing was constructed, and topographic observations were taken at each node using a laser-based Total Station by Leica (Heerbrugg, Switzerland). Grid dimension was 110 by 100 m (Fig. 2) .



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  		Fig. 1. Location of the study area in North America.

 


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  		Fig. 2. Block diagram of the study area landscape. Numbers indicated in x and y axes are in meters.

 
Four transects across the grid were sampled, two south-north and two east-west, sampled to a depth of 1 m with an auger 7.5 cm in diameter. Samples were taken from the center of each (10 by 10 m) grid cell. However, two representative profiles were excavated for depression and shoulder positions. The rest of the sampling was carefully done with auger, not to mix up soils from different horizon. In many cases small pits up to 40 cm were opened and then augured. In soils where there were pebbles then mini profiles were opened. During preliminary studies, carbonate accumulation was found above 1-m depth in almost all of the landscape positions.

Also, two profiles (Tables 1 and 2), one on the shoulder complex position and the other at the depression, were excavated, described in detail, and sampled. The Topo Suite software developed in the Department of Soil Science, University of Saskatchewan in Saskatoon SK, Canada by Pennock and Elliott (2000) was used to specify landform elements and landform complexes for each grid cell, based on slope morphological and positional characteristics as defined by Pennock et al. (1987)(1994). A three-dimensional map was produced, by using Surfer (Golden Software, Golden, CO) (Fig. 2).


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  		Table 1. Carbon isotopes in pedogenic carbonate in the Orthic Humic Gleysol (Cryoaquolls) soil in the large depression (at the end of AA' transect).

 

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  		Table 2. Carbon isotopes in organic C (OC) and pedogenic carbonate in the Orthic Dark Brown soil (Haplocryolls) on the shoulder complex (at the beginning of DD' transect).

 
Sample Preparation and Analytical Methods
Soil samples were dried at room temperature and ground to pass a 2-mm sieve. A representative subsample was further ground to pass a 250-µm sieve for measuring the amount of organic and inorganic C. Organic C was measured by combustion at 840°C, and total C was measured at 1140°C using a Leco CR-12 C analyzer (Leco Corp., St. Joseph, MI) (Wang and Anderson, 1998). Inorganic C was calculated by subtracting organic C from total C, and reported as calcium carbonate equivalent (C x 8.33).

The {delta}13C value of organic C was measured on samples from all horizons with >0.2% organic C. Samples containing both organic C and carbonate were treated with 3 M HCl to remove carbonates, then washed with deionized water using 0.22-µm Millipore filters (Millipore Corp., Bedford, MA) to remove excess HCl, until a negative test was obtained for Cl in the filtrate using AgNO3. The carbonate-free soil samples were dried and then ground to a fine powder by using a ball mill.

The isotopic composition ({delta}13C value) of soil organic matter was determined using Europa Scientific Instruments elemental analyzer coupled to a Europa Tracer/20 mass spectrometer (Crewe, UK) in continuous-flow mode. The reproducibility of this method for C ratio ({delta}13C) is ±0.2{per thousand}.

To measure the {delta}13C value of carbonates, bulk soils were ground to pass a 250-µm (60-mesh) sieve. Then samples were treated with sodium hypochlorite 5.25% (commercial bleach) for 15 d to remove organic matter (when >0.1% organic C). Bleach is the most effective reagent to remove organic materials (Gaffey and Bronnimann, 1993), and far less reactive with calcium carbonate in comparison with deionized water, sodium hydroxide, Alconox, and hydrogen peroxide (Pingitore et al., 1993). Samples were shaken and bleach was added daily. After all reactions ceased, the soils were washed with deionized water and centrifuged to remove extra bleach. Treated samples were powdered and analyzed using Thermo-Quest-Finnigan-GasBench-II coupled to a Finnigen Mat Delta Plus mass spectrometer (ThermoFinnigan, Bremen, Germany) with continuous flow technology. The reproducibility of this method for C ratio is ±0.1. The results of the isotope analyses were expressed as {delta} value ({per thousand}):

where Rs = 13C/12C in sample, and Rst corresponding stable isotope ratio in the reference standard (Friedman and O'Neil, 1977). The {delta} values for the {delta}13C are reported relative to Vienna Pee Dee Belemnite (VPDB). The amount of pedogenic carbonate to 1-m depth of soil was calculated from the C isotopic composition using Salomons and Mook (1976) equation:

where {delta}13C (soil), {delta}13C (pm), {delta}13C (new) represent the stable C isotopic composition of the carbonate in the bulk soil, pm (original carbonate rock), and the newly formed carbonate. The {delta}13C value of newly formed carbonate in the equation is calculated by subtracting the fractionation for Dark Brown soil (Haplocryoll) (18.6{per thousand}, calculated by Landi et al., 2003b) from the mean {delta}13C value of organic C for each profile. When the fractionation value of 18.6{per thousand} is added to the average {delta}13C values of organic C for the top 30 cm of the profile (Orthic Humic Gleysol; Cryaquoll used as a secondary reference material and working standard) in Table 1, we get the same {delta}13C value for pedogenic carbonate (pendant) that was found and measured in this profile. This means that the fractionation value 18.6{per thousand} is quite suitable for the area we studied.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soils
Soils in the study area are classified as Dark Brown and Black Chernozems (Haplocryolls), Orthic Regosol (Cryorthents), and Orthic Humic Gleysol (Cryoaquolls) great groups in soil taxonomy (Soil Survey Staff, 1996; Soil Classification Working Group, 1998). Four major soil-landform element complexes (footslope, level, shoulder, and backslope complexes) were identified. The Chernozomic soils (Mollisols) are distributed throughout most of the landscape with Orthic Black Chernozems (Haplocryolls) dominating in the footslope and level complexes and Orthic Dark Brown Chernozems (Haplocryolls) in the shoulder complex. Regosolic soil (Cryorthents) was found on the upper part of shoulder complex (on the knoll), and Gleysolic (Cryoaqoulls) occurred in poorly drained depressions (Fig. 3 and 4) .



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  		Fig. 3. Cross-section of the north-south transect in the study area with measured associated characteristics.

 


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  		Fig. 4. Cross-section of the west-east transect in the study area measured with associated characteristics.

 
The thickness of the A horizons ranged from 3 cm in an Orthic Regosol (Cryorthents) soil on a sharply convex shoulder position to 26 cm in a level complex position (Fig. 3 and 4). This was consistent with Kleiss (1970) findings from northeast Iowa, and Pennock et al. (1987) from southern Saskatchewan. Pennock and De Jong (1987) reported that both thickness of A horizons and depths to calcium carbonate accumulation zone overall increase in the sequence of shoulder < backslope < level < footslope positions. Similarly, depth to calcic horizon in the study area increases from almost near the surface on the knoll to more than 120 cm in depression.

Organic Carbon Contents and Rate of Accumulation
Organic C content of the A horizons of the soils ranged between 19.6 and 97.8 g kg–1 (Fig. 3 and 4). The average amount of organic C in this horizon increases from footslope complex to backslope complex position (footslope complex < shoulder complex < level complex < backslope position) (Table 3). In Transect A' to A along a north facing slope, the organic C contents of the A horizons of the soils ranged from 44.6 to 68.7 g kg–1 in the lower slope and level complex landscape elements (Fig. 3). A horizons in the shoulder complex of south-facing slopes have organic C contents between 19.6 to 30.5 g kg–1. This reflects an average value of total organic C storage of 16.7 ± 1.0 kg m–2 in north-facing slopes in comparison with 13.9 kg m–2 ± 1.5 in the south slopes in Transect A' to A (Table 4).


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  		Table 3. Average values of soil characteristics measured (n = 40).

 

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  		Table 4. Mean values of pedogenic carbonate, organic C contents, and {delta}13C value of organic C in two different landscape aspects.

 
The A horizons in south-facing footslope soils are comparatively low in organic C, but A horizons and sola are thicker, resulting in reasonably high storage of organic C. Similar findings were reported by other workers (Pennock and Corre, 2001; Aguilar et al., 1988, Schimel et al., 1985; Malo et al., 1974; Aandahl, 1948). A study by Honeycutt et al. (1990b) in the Great Plains shows that organic C content increases from the upper part of the landscape to footslope by as much as 23%. Kleiss (1970) reported that the amount of organic C increases from 0.9% in upper slope to 3.5% in lower slope positions. Many researchers relate this kind of trend to long-term transport of fine organic matter and lateral movement of clay from higher elevations (Reiners, 1983). It may also be related to the biomass productivity as it is affected by moisture redistribution in the landscape.

The largest amount of organic C to a depth of 1 m (25.6 kg C m–2) was found in the level complex landform element (Profile C4) in west-facing slope and the lowest amount (8.2 kg C m–2) was found in the same landform element (Profile A11) on a south-facing slope (Fig. 3). However, the average values for each landscape elements were rather closer and changed between 12.5 and 16.4 kg C m–2 with the sequence level complex < backslope position < shoulder complex < foot slope complex (Table 3). The strong relationship between landscape element and organic C storage observed for cultivated landscapes is not evident in this study of virgin grassland. Erosion, particularly tillage erosion, likely accentuates differences in organic C, with convex upper slopes losing soil and organic C and gaining them in lower concave areas.

The average amount of organic C down to 1-m depth in the entire landscape was 15.0 kg m–2, which is higher than the mean for Dark Brown soils and equivalent to the average of 14.9 kg C m–2 for Black soils (Landi et al., 2003a). The studied landscape does occur in the northern part of the Dark Brown soil zone, and contains Black Chernozem soils. Using Christiansen's (1978) average age since deglaciation for this area (12000 yr), the rate of C accumulation in the area studied is about 1.25 g C m–2 yr–1, which is higher than Dark Brown (1.0 g m–2 yr–1) and similar to Black soils (1.2 g m–2 yr–1) calculated by Landi et al. (2003a) for level upland soils.

Increase in available moisture results in greater production of plant biomass and increased inputs to the soil organic C pool (Peterson et al., 1988). Higher clay content may also account for slower decomposition rates in downslope positions (Burke et al., 1995). A study in southern Ohio by Finney et al. (1962) showed higher amounts of organic matter in soils facing northeast and lower landscape position than soils on south-facing slopes. In our study, northeast-facing slopes were moister, with a more dense vegetation cover. This observation suggested that most probably organic matter decomposition rates on warmer southwest-facing slopes were higher and some organic C losses occurred through surface runoff.

Stable Isotopes of Soil Organic Carbon
The {delta}13C values of soil organic C in the A horizons range from –21.4{per thousand} in Profile A8, on a dry south-facing shoulder, to –29.6{per thousand} in the large depression area, in a profile studied near A1 with the LFH horizons (Table 1). The low values of {delta}13C are associated with the level complex and footslope, and significantly higher values in the upper level and shoulder landscape positions, and it is related to environmental controls such as moisture distribution in the landscape (Van Kessel et al., 1994). When averages for each landscape position were taken, level complex and backslope position were more negative than footslope and shoulder complexes (Table 3).

A report by Ehleringer and Cooper (1988) shows that {delta}13C values of C3 plants increases from –26.5{per thousand} in moist lower areas to –24{per thousand} on dry upper slopes. In North America and Europe the {delta}13C value of C3 plants in deciduous forests increases by 1 to 2{per thousand} from mesic, lower positions to drier upland positions (Balesdent et al., 1993; Garten and Tylor, 1992). A study by Longpre (1986) from the Manito Sand Hills in west-central Saskatchewan indicate that organic matter in north-facing slope has a more negative {delta}13C than south-facing slope (–26.1 and –24.2{per thousand}, respectively), and attributed the difference to a greater proportion of C4 plants on south-facing slopes. In profile near A1, the Orthic Humic Gleysol (Cryoaquolls) at 70- to 120-cm depth, the {delta} 13C value of organic C was –18.6{per thousand} (Table 1). It is likely that this reflects enrichment from organic matter decomposition.

The {delta}13C value of organic C in the soils of the north-facing slopes were slightly more negative than those facing south, especially in AA' transect (Fig. 3 and Table 4). Wang and Anderson (2000) drew similar conclusion in their study from southern Saskatchewan. In northern direction and close to large depression (Profiles A1 and A2), there was an enrichment of 13C in organic C. As mentioned before, this could be the result of more humification and release of organic gases such as methane, a consequence of generally more moist conditions (boggy conditions).

The {delta}13C value of organic C in most of the sampling cores generally becomes more positive with depth. Several researchers have reported similar results (Khademi and Mermut, 1999; Balesdent et al., 1993; Skjemstad et al., 1990; Natelhoffer and Fry, 1988; and Volkoff and Cerri, 1987). The 13C enrichment with depth may be due to more positive values for roots, or an increasing degree of humification. Natelhoffer and Fry (1988) have provided two explanations for the enrichment with depth: (i) discrimination against 13C during humification or the preferential preservation of litter and organic material enriched in 13C, and (ii) changes through time from litter inputs with high {delta}13C values (due to more positive value of atmospheric CO2 in the past) to litter inputs with lower {delta}13C values. Variation in {delta}13C values of organic C in east-west direction is minimal. Slight changes in the {delta}13C value of organic C may well be related to differences in soil moisture in slight depressions or convex areas.

Pedogenic Carbonate
The highest {delta}13C values of carbonates (–0.9{per thousand}) was found in a sample from the 114-cm depth of the C horizon of Profile A2 (level complex), which was equal to lithogenic carbonate pebbles in the same horizon. The lowest {delta}13C value was –7.9{per thousand} in profile B8 at depth of 100 cm in the footslope complex near the depression (Fig. 3). Average {delta}13C values of the A and B horizons for each landscape position were not much different from each other (Table 3) and statistical analyses also confirmed this. In some profiles there is not a clear change (decreasing or increasing) in {delta}13C value of carbonate with depth. This irregularity has been observed in shoulder complexes and in east to west direction, where there is a complexity of water flow.

The highest proportion and amount of pedogenic carbonate up to 1-m depth were found in footslope complex position in the north-facing slope (95.8% in Profile D3 and 222.1 kg m–2 in Profile A1). Lowest proportion and amount were found in the shoulder complex position in west-facing slope (34.4% in Profile C2), and in footslope complex position in east west direction (33.9 kg m–2 in Profile C7) (Fig. 3 and 4). Average values for both kilograms per meter and percentage for each landscape position were footslope complex > Level complex > shoulder complex > back slope position (Table 3). While the overall average amount of pedogenic carbonate in all profiles facing north was 153.0 ± 14.4 kg m–2, whereas south-facing slope this value was 97.9 ± 12.9 kg m–2 (Table 4). Tables 1 and 2 show that there is pedogenic carbonates beyond 100 cm, but the amounts are not significant.

Gains and losses of carbonates in the landscape were calculated based on the assumption that all parent material initially contained 16% CaCO3 (average of carbonate content in C horizons) or 224 kg m–2 to 1-m depth (bulk density 1400 Mg m–3) as the area is very small (110 by 100 m). We made this assumption as lithogenic carbonates (the glacial till, unaffected by soil formation) were all leached beyond 1-m depth, and it was practically not possible to obtain proper samples. With the available data we calculated an average value, which we felt the best assumption to be used for our calculations.

The results show that the soils in Transect AA' have lost substantial amounts of lithogenic carbonates (>55%); nevertheless, the soils on footslope complexes (Profiles A1 and A2) and shoulder complex (A7) gained more pedogenic carbonates than lithogenic carbonates lost (Fig. 5) . Accumulation of carbonate in both footslope and shoulder complex landscape positions may well be related to the upward movement of carbonate from a shallow water table or lateral flow of water from upper slope carrying dissolved biogenic carbonates (Richardson et al., 1992).



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  		Fig. 5. Distribution of lithogenic and pedogenic carbonates in soil on AA' transect. The solid horizontal line indicates the calculated average lithogenic carbonate value.

 
In the CC' transect, there is a gain of pedogenic carbonates in the shoulder complex (C1, and C2 profiles of both side of the hill, see Fig. 2), but in small depressions (Profiles C4 and C5) all or nearly all the pedogenic and lithogenic carbonates were lost from the soil. We concluded that: (i) large amount of pedogenic carbonate can accumulate within a meter of the soils occurring in higher elevations and rather stable land surface at a given landscape, (ii) pedogenic carbonates can precipitate deeper than 1 m in closed depressions, and (iii) carbonates, in ionic form, can move by lateral transfer to adjacent soils and ground and surface waters. Carbonate balance in the majority of the soil profiles studied indicate that the total pedogenic carbonates and current lithogenic carbonates are less than the initial carbonates, at least when considering the upper 1 m (Fig. 5 and 6) .



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  		Fig. 6. Distribution of lithogenic and pedogenic carbonates in soil on CC' transect. The solid horizontal line indicates the calculated average lithogenic carbonate value.

 
A study by West et al. (1988) explains the formation of strong calcic and petrocalcic horizons in lower hillslope position of Texas by overland movement of particulate or dissolved carbonates, or downslope movement of dissolved carbonate below the soil surface. Research by Sobecki and Wilding (1983) in Texas Coast Prairie suggests that the formation of calcic horizons in the upper slope is related to lateral carbonate distribution. In their conceptual model, carbonate moves from wetter concave area (with strongly leached soils) and precipitate in relatively drier upper slopes (Calciaquolls, Haplustolls). Hall (1983) attributes the formation of strong calcic horizons in lower hillslope to different stability of landscape elements.

Usually, there is more moisture in footslope than middle slope positions and this moisture causes more biomass production, which in turn produces CO2 and weathering of Ca containing minerals. Therefore, deeper sampling could give a better idea of the amount and proportion of pedogenic carbonate in the landscape. St. Arnaud (1979) found no carbonates in depressions in a hummocky landscape of Saskatchewan and suggested that such a position of landscape receives more water; carbonates could move down and precipitate at greater depths.

The average amount of inorganic C as pedogenic carbonate within 1-m depth in the whole landscape was 15 kg C m–2, and considering two cores without carbonate at the depth of 1-m, the average was reduced to 14.1 kg C m–2. This is slightly lower than the mean value reported for Dark Brown (17.6 kg C m–2) and Black soils (16.6 kg C m–2) (Landi et al., 2003b), but represent a soil depth of 1 m rather than 1.2 m. Taking into account the age of the landscape as 12000 yr, the average rate of pedogenic carbonate accumulation was 1.25 g C m–2 yr–1, and with those two cores without pedogenic carbonate the rate was 1.2 g C m–2 yr–1. This is close to the calculated rate of 1.4 g C m–2 yr–1 for Dark Brown, and 1.3 g C m–2 yr–1 for Black soils by Landi et al. (2003b).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
In general, soils from knoll (upper part of shoulder complex) to footslope positions become deeper, and the thickness of Ah and B horizons and depth to secondary carbonate layer, increase. The amount of organic C and pedogenic carbonate increases from the upper landscape to lower positions. Organic C contents of Ah horizons range between 20 to 98 g kg–1. The amount of pedogenic carbonate to 1-m depth shows a very high variation, changing from 33.9 to 222.1 kg m–2 except for profiles B2, B3, and C4 in which all the carbonates, both pedogenic and lithogenic, were removed from upper 1-m segment. The amount and percentage of pedogenic carbonate is higher in north-facing slopes than those facing south. On average, the soils accumulated about 1.25 g C m–2 yr–1of C as pedogenic carbonate and 1.25 g C m–2 yr–1 as organic C, rates that are similar to those calculated for Dark Brown soils (Mollisols) in Saskatchewan.

Carbonate balance in the majority of the soil profiles indicate that the total pedogenic carbonates and current lithogenic carbonates are less than the initial carbonates, at least when considering the upper 1 m. This suggests the loss of carbonates from the profile to deeper layers or ground and surface waters, which is an aspect of C cycling in terrestrial ecosystem that need to be further explored.

The {delta}13C value of carbonate ranges from –0.9{per thousand} at the 114-cm depth (shoulder complex) to –7.9{per thousand} at depth of 100-cm in carbonate accumulation layer (footslope complex and depression). There is an irregularity in {delta}13C value of carbonate with depth in shoulder complexes, which may be due to vertical and lateral water flows.

There are distinct {delta}13C values of organic C with the following patterns: (i) soils in the shoulder complex, especially facing south were considerably enriched in {delta}13C values (–21.4{per thousand}); (ii) {delta}13C values were more negative in north-facing slopes than their south-facing counterparts; (iii) {delta}13C values varied insignificantly in the east-west direction due to little changes in microclimate and landscape; and (iv) {delta}13C values of organic C generally became more positive with depth.

Carbonate balance calculations showed that >55% of lithogenic carbonates have been removed from the soil profile. Some landscape positions gained more pedogenic carbonates than the amount of lithogenic carbonates lost. Carbonate balances in the majority of the soil profiles indicated that the total pedogenic carbonates and current lithogenic carbonates are less than the initial carbonates. This was probably due to vertical movement of carbonates below 1-m depth, in certain soils. Carbonate can also move laterally from the higher elevation to lower landscape and even reach to ground and surface waters.

Marked differences were found in the amount and rate of organic and inorganic C accumulation in landscape segments of the area studied. This may seem to complicate the estimation of soil C fluxes and accumulation. However, when averaged for the entire landscape, the amount and rates of organic and inorganic C accumulations become similar to the soil zone in which they occur. From the standpoint of C flux in terrestrial ecosystems, we conclude that carbonate losses and gains in soils deserve closer attention.

Received for publication May 16, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome