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Dep. of Soil Science, University of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8 Canada
* Corresponding author (pennock{at}sask.usask.ca)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: dbh, diameter at breast height • FS, low-catchment-area footslope • LL, Low-elevation level and high-catchment area footslope elements • SOC, soil organic C • UL, upper level and convex shoulder elements
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Estimates of SOC loss because of land conversion and agriculture in the northern Great Plains typically range from 15 to 30% (Pennock and van Kessel, 1997; Janzen et al., 1998). Losses of SOC are because of decreases in organic C inputs (Janzen et al., 1998), soil redistribution (Gregorich et al., 1998; Pennock et al., 1994), changes in mineralization because of altered soil moisture conditions (Janzen et al., 1997), and leaching of soluble organic C (Gregorich et al., 1998). Van Veen and Paul (1981) suggest that the large initial decrease in SOC levels is due, in part, to the mineralization of roots that accumulated under native grassland vegetation.
Root growth and its relationship to SOC dynamics is also sensitive to landscape position. Both soil moisture and fertility (Hanna et al., 1982; Pennock et al., 1994; Verity and Anderson, 1990) typically increase downslope in soils of the northern Great Plains. Low soil moisture levels stimulate root elongation (growth), provided that there is enough moisture available to support plant maintenance and minimal plant growth (Ellis et al., 1977). High moisture levels inhibit both root elongation (Johnson and Aguirre, 1991; Qian et al., 1997), and density (Plaut et al., 1997). High nutrient levels within the soil increase lateral root production, while low nutrient levels increase root length, provided that growth is not limited by some other factor (Garcia et al., 1988). In general, fertile soils support shallower denser root systems than their less fertile counterparts.
Weaver (1920) observed that although maximum rooting depth was always highest in upland prairie sites in comparison with lowland sites, the degree of root development (density) was always highest in lowland sites in the mixed grass prairie. Van Rees et al. (1994) found that footslope positions in the Dark Brown soil zone of Saskatchewan had a higher number, and therefore, density of roots compared with shoulder positions. Clearly if roots are sensitive to landscape position then the changes to root biomass after land clearing and cultivation may differ between landscape positions. Hence the most appropriate methodology for comparisons between sites is to segment the landscape into landform units with a defined range of terrain attributes (Pennock et al., 1994). If the observed changes in SOC levels because of cultivation differ among the landform positions, then the SOC process models used to simulate C dynamics should be sensitive to landscape position.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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500 m apart on the same geomorphic surface, northwest of St. Denis, Saskatchewan (approximately 52°10'30'' N lat., 106°10'00'' W long.). The grassland site was in the northeast quarter of section 10- township 37- range 2-W3 and the cultivated field in the northwest quarter of 11-37-2-W3. The parent material is unsorted glacial till, and slopes range from 6 to 15% in this hummocky landscape. This region is in the Dark Brown soil zone, and soils were mapped in the same map unit of the Weyburn association (Acton and Ellis, 1978). Given the proximity of the sites and their occurrence on a continuous geomorphic surface, we assumed that the research sites possessed similar soil properties (e.g., SOC) at time of breaking of the land for agriculture. The dominant graminoid species in the fescue prairie grassland were plains rough fescue, porcupine grass (Stipa curtiseta (A.S. Hitchc.) Barkworth), blunt-leaved sedge (Carex obtusata Lilj.), and Western wheatgrass (Agropyron smithii Rydb.). The dominant forb species were prairie crocus (Anemone patens L.), golden bean [Thermopsis rhombifolia (Nutt.) Richards], and American vetch (Vicia americana Muhl.), and the most common shrub species were Western snowberry (Symphoricarpos albus L.), and wolf-willow (Eleagnus commutata Bernh.). The primary tree species was trembling aspen (Populus tremuloides Michx.). Vegetation in the fescue prairie displayed a landscape trend. Upper level and shoulder complexes were primarily vegetated by graminoids and forbs, the concave footslopes by shrubs, and lower level positions by aspen.
The UL and FS positions in the cultivated field were broken in 1950 and the LL slope positions in 1981. On 8 June 1999, the cultivated field was sown to spring wheat (cv. AC Barrie) at a rate of 101 kg ha-1, and at a drill spacing of 17.8 cm. Phosphate was seed-placed (13 kg ha-1 of P), while N, K, and S were banded (67, 9, and 7 kg ha-1, respectively) at seeding. Herbicides were used to control weed populations.
Research Design and Field Sampling
Site topography was surveyed and a digital elevation model (10-m spacing) constructed. Each grid cell was classified as a specific landform element based upon slope morphological and positional attributes (Pennock et al., 1987, 1994). Three landform element groups were used in this study. This grouping of elements was adopted because of the high percentage of level elements (i.e., no significant profile curvature and slope gradients <3.0°) at the sites and the time-consuming root washing procedures. An initial distinction was made between level elements based on their relative elevation in the landscape. The UL group included all level elements at elevations greater than the mean elevation (i.e., 1.92 m in the fescue prairie and 1.68 m in the cultivated field) and all divergent and convergent shoulder elements. The FS element group contained all divergent and convergent footslope elements that had a global catchment area <250 m2. The LL group included all level elements at elevations less than the mean elevation, and all those footslope elements that had global catchment areas >250 m2. The terrain attributes of each landform class (UL, FS, LL) in the fescue prairie were very similar to those at the cultivated field. Landform classes were, therefore, directly comparable between the two research sites.
The experimental population at each site was defined as the set of all landform elements present and the specific experimental units sampled were randomly chosen from the full population. Soil properties were measured at ten specific experimental units of each of the three landform element groups, and above- and belowground biomass properties were measured at eight of the ten units selected for soil sampling.
Soils were sampled with a soil core (6.7-cm diam.) that was divided into the following segments: 0 to 10, 10 to 30, 30 to 60, 60 to 90, and 90 to 120 cm. The evaluation unit for belowground biomass was a composite of three soil cores (6.7-cm diam.) divided into the same depth increments as those for soil properties as well as two additional depth increments (120 to 150, and 150 to 180 cm).
Species composition was determined in the fescue prairie within a 0.25 m2 quadrat at each sampled experimental unit. Aboveground biomass was determined from a 1-m2 evaluation unit. The only exception was in the eight LL positions in the fescue prairie, which were dominantly vegetated by aspen. The evaluation unit for vegetation in these landform elements was 50 m2.
The soil cores were sampled on 27 May 1999 via a truck-mounted hydraulic corer. A second core at each sampling point was extracted to describe the soil profile. Soil samples were also obtained on 7 and 17 May, 9 July, and 17 August for gravimetric moisture contents. Thermocouple probes were installed in five randomly chosen experimental units for each landform element group, and manually read on a weekly basis from 16 June to 26 August at 10-, 30-, 60-, 90-, and 120-cm depths. Daily air temperature, and precipitation readings were obtained throughout the 1999 field season via a CR10 data logger (Campbell Scientific Canada Corp., Edmonton, AB, Canada) located 6 km east of the research sites.
Belowground biomass was sampled over the period of 17 to 24 August. Plant cover in the prairie was essentially continuous and cores were taken over the plants at the sampling point was located. Cores were taken directly over the plant row in the cultivated field. This may have lead to an overestimation of biomass in the wheat field. Belowground biomass samples included crown material, roots, and rhizomes. Samples were comprised of both living and dead plant tissue; we estimated dead plant tissue to contribute <5% of the total belowground biomass based on qualitative estimates during root washing.
Aboveground biomass was harvested with a sickle at the same time as belowground biomass sampling. The only exception occurred in the eight aspen-dominated LL positions in the prairie grassland. To estimate aboveground biomass, the height and diameter at breast height (dbh) was recorded for each tree within 3.99 m of the sample point. The wheat plants were postanthesis, and ranged from Zadoks developmental stage 65 to 70 (Zadoks et al., 1974).
Laboratory Methods and Data Analysis
Bulk density was measured (Blake and Hartge, 1986) on all soil samples. Gravimetric soil moisture was determined, and converted to a volumetric basis (Topp, 1993). Soil moisture contents at 1500 kPa pressure were determined, and utilized to determine plant available soil moisture levels for all samples. Soil organic C contents were quantified for all samples using a Leco CR 12 Carbon System (Wang and Anderson, 1998). Particle size (Indorante et al., 1990), and soil pH (2:1 soil/water paste) (Tan, 1996)) were measured 20% of soil samples from each landform element.
Belowground biomass samples were initially refrigerated at 4°C, to ensure that neither growth nor decomposition occurred after sampling (Sheng and Hunt, 1991) and were subsequently frozen. Samples were thawed, placed on a 1.5-mm2 mesh screen, and gently sprayed with warm water. The sample was enfolded in the screen, immersed in a tub of water and gently massaged to facilitate removal of soil particles.
To remove gravel held by the roots, samples were placed in a tub of water, stirred and then decanted into funnels (diam. of 5.1 cm), covered with stainless steel mesh (0.5 mm2). This procedure was repeated at least ten times to remove all gravel. Belowground biomass samples were then floated in water in clear pyrex (Corning Glassworks Scientific Products, Corning, NJ) dishes and aboveground plant material removed. Root biomass were then oven-dried at 60°C for 48 h, cooled in a dessicator, and weighed. Four of the belowground biomass samples from each of the three studied landform elements at each research site were also analyzed to determine organic C contents (Leco CR 12 Carbon System). On average, belowground biomass was 42.6% C in the prairie grassland and 36.8% C in the cultivated field. The C content of the belowground biomass in the cultivated field is slightly below other published values for wheat roots and may indicate incomplete removal of soil from the root systems. As well, root washing procedures tend to lead to loss of very fine roots from the samples, and the loss of these fine roots may be more pronounced for the wheat crop than the grassland soil.
Aboveground biomass samples were dried at 30°C for 15 d, and then weighed. The only exception occurred in eight aspen-dominated LL units in the prairie grassland. Oven-dried estimates of aspen biomass (stem wood, bark, live branches, twigs, and leaves) were obtained by placing field measurements of tree height and dbh into the following equation, developed by Singh (1986) for aspen in Western Canada.
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The majority of properties assessed in the study had statistical properties indicative of normal frequency distributions. Therefore, parametric statistics were used to analyze the data set. Comparisons between landform positions within a site were made using a one-way ANOVA and a multiple comparison test using the LSD. Comparison between sites were made with a t-test.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Soil Characteristics and Climatic Conditions
Several soil physical and chemical criteria were assessed to ensure that no major differences existed that could complicate comparisons between the sites. Soil pH was not limiting to plant growth at either site (Table 1). Soil pH was higher in UL landscape positions in comparison to FS and LL positions in the landscape. Overall, the prairie grassland had lower pH values than the cultivated field. The increase in pH after cultivation has been attributed to incorporation of calcium-carbonate rich subsoil into the surface soil (Pennock et al., 1994). Mean electrical conductivity measurements were below 1 mS cm-1 in the upper 60 cm of the soil at all positions at both sites (data not shown), indicating similar, nonsaline conditions.
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1°C and with soil depth by 4°C in both the prairie grassland and the cultivated field (Table 3). On average, the prairie grassland had significantly lower soil temperatures (p-value 
0.001) for all landform positions at every soil depth in comparison with the cultivated field. The differences are 2 to 3°C at most depths.
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With 49 yr of cultivation, the upper 60 cm of soil in the UL positions in the cultivated field had only 56.6% of the SOC present in the grassland (Table 4). The greatest loss (25.5 Mg ha-1) of SOC was associated with the 10- to 30-cm soil depth in these positions (Fig. 1) . Soil organic C changed little in the upper 10-cm of FS positions; however, as with UL positions, a significant loss (14.7 Mg ha-1) of SOC occurred within the 10- to 30-cm soil depth. Lower level landscape positions experienced an SOC increase of 9.1 Mg ha-1 within the top 60 cm of soil after 18 yr of cultivation but the difference between the two sites was not statistically significant.
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Plant Biomass—Landform Relationships
Mean aboveground biomass increased downslope in both sites (prairie: UL = 757 g m-2, FS = 1060 g m-2, LL = 3741 g m-2; cultivated field: UL = 922 g m-2, FS = 1028 g m-2, LL = 1021 g m-2). Mean biomass levels were not significantly different between the sites in either UL or FS positions. Low-level positions had significantly (p-value 0.05) higher levels of biomass in the prairie grassland in comparison with the cultivated field. This was largely because 97% of reported aboveground biomass in this landscape position in the prairie grassland came from aspen trees.
Belowground biomass also differed by landscape position within both sites. In the prairie grassland, belowground biomass increased downslope from UL landscape positions in the upper 60 cm of soil (Table 5). This increase downslope was probably because of the different specific vegetative cover in each landform position (i.e., UL = grasses, FS = shrubs, LL = aspen). In the cultivated field, UL and FS positions had significantly lower levels of belowground biomass than LL positions (Table 5). The aboveground/belowground biomass ratio strongly reflects the difference in vegetation. The grass- and shrub-dominated UL and FS positions at the prairie have similar ratios dominated by belowground biomass (0.41 and 0.42, respectively), whereas the aspen-dominated LL positions have a ratio of 1.02, indicating an approximate balance between above- and belowground biomass. In the cultivated field, the aboveground biomass dominates in all three positions. The aboveground/belowground biomass ratios were 4.1, 5.1, and 3.5 for the UL, FS, and LL positions respectively.
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Upper level positions in the prairie grassland had significantly higher levels of belowground biomass C in comparison with the cultivated field (Table 6). This decrease in organic C from belowground biomass in the UL position accounted for 17.6% of total SOC loss (Table 6). Footslope positions experienced lower total losses SOC but higher losses of belowground biomass C; in all, belowground biomass changes accounted for 71.7% of observed SOC losses in footslopes. Lower-level landscape positions lost 15.5 Mg ha-1 of belowground biomass C, but did not experience a statistically significant change in SOC from the native site (Table 6).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The soils of the cultivated field have undergone a considerable transformation from their native state. In the prairie, A horizon thickness is at a maximum in the FS position. In the cultivated field, significant truncation of the A horizon in UL positions occurs, coupled with over-thickening of the A horizon in the LL positions. This results in the loss of the Bk horizon from UL soils of the cultivated field.
The changes in soil morphology are also linked to major shifts in the spatial pattern of soil organic C (Table 4). In the prairie, the LL positions had the lowest SOC levels. The lower SOC content of LL positions has been previously noted in comparable landscapes, and has been attributed to reduced belowground C inputs in the tree-dominated lower slope positions (Pennock and van Kessel, 1997) and reduced productivity because of denitrification from these wetter positions (Corre et al., 1996).
In the cultivated field, SOC levels were significantly lower in the UL and FS positions, whereas the LL positions had no significant change from the native state. This pattern has been previously attributed to soil redistribution (Pennock, 1997), primarily through tillage. Losses of soil through tillage redistribution are greatest on convex slope segments and accumulation occurs in concave segments (Govers et al., 1999).
The belowground biomass in both fields is comparable with published estimates. Sims and Singh (1978b) reported mean root biomass measurements in the mixed-grass prairie ranging from 1070 to 1532 g m-2 and annual root biomass production ranged from 516 to 1062 g m-2. In comparison, wheat-root biomass yields presented by Campbell et al. (1977) and Sheng and Hunt (1991) ranged from 135 to 154 g m-2. Estimates of root biomass of aspen range from a minimum of 3600 g m-2 (15-yr-old stand) to a maximum of 4120 g m-2 (53-yr-old stand) (Peterson and Peterson, 1992). The perennial nature of many of the prairie grassland species allows root systems (and hence belowground biomass) to become more developed compared with those of annual crops, resulting in shoot:root ratios <1 (Acton, 1991). Plants in the grassland also experienced a longer growing season, and their root systems are not disturbed by tillage. Finally, the density of plants in the prairie is greater than in the cultivated field. All of these factors contribute to significantly higher belowground biomass in the prairie grassland.
The results show that changes in belowground biomass and C contribute significantly to the total SOC changes. Belowground biomass within the prairie grassland was significantly greater than that found within the cultivated field. The magnitude of the difference varied by landform position. The prairie grassland contained, respectively, 16.2, 23.3, and 33.7 Mg ha-1 more belowground biomass in UL, FS, and LL landscape positions in comparison with the cultivated field. Belowground biomass loss after cultivation caused C losses of 7.0, 10.0, and 14.5 Mg ha-1 in UL, FS, and LL landscape positions (0 to 60 cm) respectively.
In UL positions, changes in belowground biomass accounted for only 17.6% of total SOC loss. This position has been shown to have the highest levels of soil loss because of soil redistribution in these landscapes (Pennock 1997) and the contribution of losses of belowground biomass C to total SOC change is small.
In FS positions, loss of belowground biomass C accounted for 71.7% of the total change in SOC levels between the two sites. These losses dominantly occur in the 10- to 60-cm soil layer, which is beyond the layer most affected by surface soil redistribution, and in situ changes in belowground biomass C appear to be the major process in these positions.
The lack of change in mean SOC levels in LL positions despite a significant decrease in belowground biomass was most likely because of (i) deposition of organic rich topsoil and (ii) increased organic C inputs (Gregorich et al., 1998; Verity and Anderson, 1990). These positions were also converted to agriculture later (i.e., 1981 versus 1950 for the remainder of the landscape) and may still be in a period of more rapid adjustment of SOC levels. In addition, the clonal-root system of aspen acts as a long-term carbohydrate storage organ and does not contribute greatly to SOC levels. Soil organic C contributions in aspen dominated stands are primarily in the form of adventitious roots or fine laterals that have relatively fast turnover rates (Peterson and Peterson, 1992). The combined effect of soil deposition and increased plant productivity because of the addition of N fertilizer appear to more than compensate for the high C losses because of changes in belowground biomass.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The magnitude of changes observed in this study provide an estimate of potential contribution of root systems to SOC increases should management practices be adopted that increase belowground biomass. Clearly much of the SOC loss experienced in FS positions could be offset by increases in belowground biomass, but even complete reestablishment of the original grass community in UL positions would do little to restore their original SOC levels. The greatest potential for C increases from belowground biomass exists in the LL positions, but interactions between increased biomass C and the changes in the soil redistribution and productivity regimes because of management changes in these positions is likely to be complex.
Received for publication April 9, 2001.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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