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a Dep. of Natural Resources and Environmental Sciences, W-401 Turner Hall, 1102 S. Goodwin Ave., Univ. of Illinois, Urbana, IL 61801
b Faculty of Geography, Moscow State Univ., Russia
* Corresponding author (k-olson1{at}uiuc.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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60 to 80 yr ago. Deposition of fly ash derived from distant railway traffic started in or about 1851 and increased in 1870 because of construction of a closer railway. Considering the whole transect, the cultivated site had 12% less fly ash in the upper 20-cm soil layer as compared with the reforested site. The fly-ash results indicated that only 2.4 cm or 12% of the upper soil layer has been removed from the hillslope because of accelerated erosion associated with the last 60 to 80 yr of cultivation. Deposition of sediment rich in fly ash on the lower and upper footslopes suggests accelerated erosion has occurred at the cultivated site.
Abbreviations: IF, interfluve • LBS, lower backslope • LFS, lower footslope • SH, shoulder • UBS, upper backslope • UFS, upper footslope
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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In addition to soil movement during the erosion process, recent studies (Lindstrom et al., 1992; Guiresse and Revel, 1995; Lobb et al., 1995; Govers et al., 1996; and Quine et al., 1997) have shown that tillage affects soil redistribution of arable land. Tillage erosion results from the greater downhill displacement of soil during a downslope tillage operation than for the uphill displacement during a corresponding upslope tillage operation. Govers et al. (1994) and Poesen et al. (1997) have shown that slope gradient has a significant effect on the average soil displacement distance. They found soil translocation by tillage could be expressed by a proportionality coefficient between unit soil transport rate and slope gradient. This coefficient can be used to predict intensity of tillage erosion and soil redistribution rates (Lobb and Kachanoski, 1999). Tillage caused erosion on shoulders (convex shape) and deposition on footslopes (concave shape).
Researchers have determined the intensity of erosion by comparing soil properties (Olson et al., 1994a) and landscape position effects (Kreznor et al., 1989, 1992) on cultivated sites with uncultivated sites. In Southern Illinois, Olson et al. (1994b) found the removal of a 7.5-cm layer from a cultivated site during 80 yr because of accelerated erosion on a backslope with 5 to 10% gradient. They arrived at these results by comparing the soil properties of the cultivated site with a nearby 80-yr-old pasture and forested site with similar soil, slope, and landscape characteristics. Kreznor et al. (1989) determined the effects of accelerated erosion on a cultivated site by comparing with an uncultivated and uneroded cemetery site in Northwestern Illinois having similar slopes in the range of 6 to 10%. They found a decrease in A horizon thickness and organic C content because of erosion. Clay content increased as subsequent cultivation incorporated illuviated clay into the surface layer of soil. Among the landscape positions, the shoulder was slightly eroded, whereas because of greater slope lengths in the study area, lower backslope, and upper footslope were severely eroded.
The areal extent of erosion and the phase of eroded soil is identified on the basis of the properties of the soil profile that remain (Soil Survey Staff, 1993). Erosion classes are defined in chapter 4 of the Soil Survey Manual (Soil Survey Staff, 1993) on the basis of the percentage of loss of the original A horizon or the uppermost 20 cm of the soil.
Magnetic susceptibility is the magnetizing ratio of material to the magnetic field inducing it. For soil it is the result of contributions of susceptibilities from the large number of different substances in the soil. Diamagnetic substances like organic matter and quartz have low and negative mass magnetic susceptibilities ranging from -0.01 to -6 x 10-6 m3 kg-1. The mass magnetic susceptibility (
g) in paramagnetic substances, such as clay minerals with transitional elements in their structure, are weakly positive. Antiferrimagnetic substances like hematite, goethite, and lepidocrocite are examples of paramagnetic minerals with weak positive susceptibilities in the range 0.01 to 0.50 x 10-6 m3 kg-1. Magnetite and maghemite are ferrimagnetic substances with strong positive magnetic susceptibility on the order of 375 and 10050 x 10-6 m3 kg-1, respectively (McBride, 1986; Mullins, 1977).
The magnetic susceptibility of sediment would be different from that of the original land surface as in the case of a buried soil (Fine et al., 1992; Babanin et al., 1995). Singer and Fine (1989) found higher magnetic susceptibility in eluvial horizons than illuvial horizons. Magnetic susceptibilities of both eluvial and illuvial horizons were higher than parent materials, because of the crystallization of maghemite and its accumulation in these horizons. Soils in poor and somewhat poorly drained classes had lower magnetic susceptibilities than the well-drained soils. Wetter soils have lower magnetic susceptibility because of either dissolution of maghemite in reducing conditions following formation or because maghemite did not crystallize under these conditions (Mullins, 1977; Babanin et al., 1995). The age of a soil also contributed to its magnetic susceptibility. Older soils had higher magnetic susceptibilities than younger soils because of weathering of parent material and transformation of nonmagnetic Fe-bearing minerals to magnetic minerals. Magnitude and change in magnetic susceptibility are functions of soil-forming factors such as: topography, climate, parent material, and time (Singer and Fine, 1989; Babanin et al., 1995).
In a pilot study, Hussain et al. (1998) compared the fly-ash content of a cultivated site with an uncultivated (wooded) site in southern Illinois, USA. Fly-ash content on interfluve and shoulder landscape segments of the cultivated site was 50% less, 35% less on backslopes and 67% less on depositional lower footslope position. Considering the whole transect, the cultivated site had 47% less fly ash in the upper 22.5 cm of soil profile as compared with the uncultivated site. These results indicated that 10.6 cm or 47% of the upper soil layer has been eroded from the hillslope since 1855 (142 yr) because of accelerated erosion induced by cultivation. For the backslope, the presence of 65% or 15 cm of the original surface soil layers and 67% of the fly ash at the cultivated site places the soil in the moderately eroded phase of the Grantsburg soil (Fine-silty, mixed, mesic Oxyaquic Fragiudalfs).
Changes in magnetic susceptibility have been used to locate soil boundaries and to identify different soil horizons and parent materials (Williams and Cooper, 1990; Babanin et al., 1995). Magnetic susceptibility can also be used for detecting erosion or deposition on soil surfaces. Magnetic susceptibility (Hussain et al., 1998) was the highest on summits and sideslopes and lowest in drainage ways. Magnetic susceptibility decreased with lower elevations and was also lower on poorly drained soils as compared with well-drained soils.
Soil characteristics like organic C content and magnetic susceptibility are not only affected by erosion but also by the formation processes in soil that can result in either under- or overestimation of erosion. Using fly ash as time markers may overcome this problem. Fly ash could be a better indicator to estimate the soil erosion since European settlement (1850s) in Illinois (USA). Magnetic spherules of fly ash were deposited by wind as a result of the burning of coal and venting of combustion products by steam-powered locomotives and steam boilers beginning about 1855 in Illinois (Corliss, 1950). In Illinois, 16000 km of railroads were built from 1850 to the late 1920s, including the railroad lines that were the main source of fly ash deposited in the study area. Steam-powered farm machinery that used coal and was commonly used from 1880 to 1920 could have contributed small amounts of fly ash to the soil. Fly ash has been used in studies as a sedimentation marker (Jones and Olson, 1990), as a pollution tracer (Rose, 1996), and as a direct indicator for upland erosion estimation in one pilot study (Hussain et al., 1998).
Fly ash seems to be a rather stable material. One study showed no cracking, crystallization, or any deterioration of fly ash when exposed to weathering for more than 30 yr (Capp and Spencer, 1970). The magnetic minerals in fly ash are magnetite and wustite (Huffman and Huggins, 1986; Babanin et al., 1995). Jones and Olson (1990) reported that fly ash constitutes >75% of the magnetic fraction in the upper the 12.5-cm soil layer on an uncultivated stable ridgetop near Springfield, IL. At this site, the presence of <1% fly ash in deeper horizons was thought to be the result of mixing by soil fauna and illuviation in large channels. Fly ash was found to a 45-cm depth in the nearby Sangamon River floodplain where the deeper occurrence of fly ash was attributed to stream deposition of sediments rich in fly ash. Fly ash has the physical appearance of spheroids of siliceous glass, mostly with diameters <20 µm. Fly-ash particles are able to travel long distances in an airstream because of low settling velocities of particles ranging from 1 to 20 µm with normal wind speed (Wark and Warner, 1976). Fly ash was found 45 km from a coal-powered plant in Glasgow, Scotland (Rose, 1996).
This study was carried out in Pushkino, Russia with the objectives of: (i) determining of the extent and local variability of soil removal by accelerated erosion from a hillslope using duplicate transects, and (ii) determining the amount of subsequent deposition of sediments on a footslope using the presence, depth distribution, and concentration of fly ash, magnetic minerals, organic C, and magnetic susceptibility.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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10–15 Mg ha-1), continuous cultivation, intensive usage of organic and mineral fertilizer, and machine-induced compaction. The cultivated and reforested sites were located on land previously managed as a state-operated farm. Representative pedon descriptions of the Glossocryalfs (soddy-podzolic) taken on the backslope at the cultivated and reforested sites are presented in Table 1.
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In the study reported here, fly-ash deposition started between 1851 and 1870, about the time of significant settlement in this area 40 km north of Moscow. If fly-ash deposition on both study sites was uniform prior to 1930, redistribution of fly ash since then would be because of geological erosion at the wooded site and accelerated erosion at the cultivated site.
Sampling sites were at the Talitsa river basin. The area is an outwash plain located along a prequaternary valley eroded into Cretaceous sands with loam and siltstone bands. Altitudes are 160 to 180 m above sea level. The soil cover of interfluves generally consists of thick regolith of 2.5- to 3-m thick underlain by Pleistocene outwash sands. Both the cultivated site and the reforested site had an average slope of 4%. The hillside slopes on both cultivated and uncultivated sites are west facing and are located 10 m apart (Fig. 2) . Two 200-m long transects, located 10 m apart, were made at each site. The spacing between transect lines on Fig. 2 was enlarged by a factor of two to be cartographically legible to the reader. Glossocryalfs (soddy-podzolic) occur at the study site.
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The magnetic minerals were determined using procedures developed by Jones and Olson (1990). Fly-ash content was determined in the ferrimagnetic fraction of samples. This fraction was separated from 7 g of sample (<2 mm) that had been oxidized with sodium hypochlorite and dispersed by agitation on a shaker. The dispersed sample was slowly passed through a separatory funnel on the outer wall of which a strong permanent magnet was affixed. Ferrimagnetic particles (magnetic minerals) were attached to and held to the inner wall of the separatory funnel by the magnet. After all of the dispersed sample had passed through the separatory funnel, its sides were gently washed with water to remove nonmagnetic particles.
The ferrimagnetic fraction was collected by placing a porcelain dish under the separatory funnel, removing the magnet, and washing the ferrimagnetic minerals into the dish with a minimum of water. The minerals in the dish still contained substantial nonmagnetic minerals. Careful agitation by panning the minerals over a small cylindrical magnet resulted in the ferrimagnetic fraction being gathered on the side of and near the top of the dish. The dish with the ferrimagnetic minerals concentrated on the upper wall and nonmagnetic fraction in the dish's bottom was dried at 110°C.
The ferrimagnetic fraction was attracted to a rectangular (15 by 20 mm) piece of Lexan plastic (Cadilac Plastic, Boston, MA) by forcing the plastic held in forceps through the magnetic minerals. The minerals are attracted. These minerals often occur in rows or rays extending from the leading edge of the plastic forced through the ferrimagnetic. A drop of warm Canada balsam (mounting medium) was gently placed on plastic and then the Lexan mounted on a glass microscope slide. The slide is allowed to cool in an inverted position so that the particles remain fixed to the surface adjacent to the plastic.
Fly-ash was determined microscopically by choosing a random field using x400 magnification. A digital image of the field was made and this image was analyzed for spherical particles using ImagePro image-analysis software (Media Cybernetics, Silver Spring, MD). Each spherical particle was identified by the microscopist and counted as fly ash. The metric reported was area of fly ash as a percentage of the total area of opaque particles that were assumed to be ferrimagnetic. At least six contiguous fields were counted.
Subsamples of fine-earth samples previous passed through a 2-mm sieve were ground to <250 µm in a mortar and pestle to pass a 0.250-mm sieve. Magnetic susceptibility was subsequently determined (Hussain et al., 1998) with a modified Gouy balance (model MK I #6632, Johnson Matthey, Wayne, PA).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Organic C concentration also decreased regularly with depth in the cultivated site (Table 4). The IF, SH, UBS, LBS, UFS, and LFS landscape positions had organic C concentrations of >8 g kg-1 of soil to the 20-cm depth, reflecting mixing of three soil layers by cultivation practices prior to being left in hayland for the last few years. The thicker (0–30 cm) organic-rich horizons (>11 g kg-1) on the UFS and LFS supports the idea that organic-C enriched sediments from upslope were deposited on these positions. The LFS had at least 50 cm of organic-rich horizons.
At the reforested site (Table 3), magnetic susceptibility and concentrations of magnetic minerals were higher on all landscape positions than at respective positions of the cultivated site (Table 4). This is an indication that the reforested site was relatively stable and perhaps a more efficient collector of air-borne fly ash from 1930 to 1960. Magnetic susceptibility and concentration of magnetic minerals decreased with depth on all landscape positions at the reforested site. Magnetic susceptibility varied irregularly with depth on all landscape positions (Table 4).
At the reforested site, >90% of total fly ash in the soil profile was detected in the upper 0- to 20-cm soil layers (Table 3). In the 0- to 5-cm layer, >80% of total fly ash in the soil profile was found on IF, SH, UBS, and LBS landscape positions. Since sediment rich in fly ash increased the thickness of soil layers with high fly-ash content (Table 3), the 0- to 10-cm layer of UFS and LFS contained only 65% of the total fly-ash contents of the profile. The presence of higher fly-ash content in the 5- to 20-cm layer of the UFS and LFS suggests that some deposition of fly-ash-rich sediment occurred when the reforested site was previously cultivated between 1850 and 1930. After the trees were planted in 1930, little erosion would have occurred. Also, planting trees in rows along the contour contributed to stability of this landscape. Compared with the cultivated site, concentrations of fly ash in the 0- to 5-cm increment of the IF, SH, UBS, and LBS landscape positions at the reforested site reflects the absence of cultivation since 1930 and lack of mixing of fly ash to lower depth. At the reforested site, the surficial 5 cm of soil on the IF contained a lesser amount of fly ash (Table 3) than adjacent landscape positions. This difference reflected more accelerated erosion prior to tree planting in 1930 and, perhaps, more geologic erosion since 1930. Runoff from the IF at the reforested site prior to 1930 could remove soil and fly ash and deposit sediment on the other landscape positions. The higher amount of fly ash on the SH and LBS as compared with IF and UBS positions was attributed to a slight concavity across the LBS. The concave position may have trapped some fly-ash in sediments from the IF and UBS. In the 0- to 20-cm soil interval, more fly ash on LFS might be the result of deposition of fly ash containing sediments on this landscape position. Minute quantities of fly ash at 20 to 30 cm of all landscape positions could be attributed to the mixing of fly-ash by soil fauna and downward movement through channel and cracks (Jones and Olson, 1990).
On the cultivated site (Table 4), fly ash occurred in substantial amounts to 20- or 30-cm depth on all landscape positions. Fly ash was observed to 30-cm depth on the UBS and LFS. On the one LFS position sampled, fly ash was observed to be relatively abundant to 50 cm. In the 0- to 30-cm soil layer, UBS, LBS, UFS and LFS positions had higher amounts of fly ash as compared with the IF and SH. These differences are attributed to past tillage and erosion on the IF and SH with a 3% slope which has moved sediments and fly ash onto the UBS, LBS, UFS, and LFS positions (Govers et al., 1994; Poesen et al., 1997). Also, repeated moldboard plowing on the edge of an interfluve and shoulder can cause the furrow slice to be cast downslope resulting in the downward movement of the surface layer. Fly-ash deposition patterns at the cultivated site (Table 4) revealed that the highest concentrations (in 0- to 20-cm layer) occur on the UBS, LBS, UFS, and LFS positions, suggesting recent erosion on the IF and SH than on UBS, LBS, UFS, and LFS positions.
The SH and LBS at the cultivated site (Table 4) had <5 mg of fly ash per kilogram of soil below the 20-cm depth. The UBS, UFS, and LFS positions had fly ash above 30 mg kg-1 of soil between the 10- and 30-cm layer. In contrast, the LFS of Transect 7 had above 10 g kg-1 in the 50-cm layer. Fly-ash concentrations of >10 mg kg-1 of soil in the 30- to 50-cm layer suggests the deposition of sediments with fly-ash from higher landscape positions although faunal mixing and flow bypass through macropores could have contributed to some of the fly-ash content of 30 to 50 cm. Sediment accumulation is indicated by the occurrence of fly ash below 20 cm at the UBS, UFS, and LFS positions (Table 4).
The values in Tables 3 and 4 were expressed on a per kilogram basis. We decided to express the data on a volume basis since the bulk density of reforested and cultivated sites often vary (Olson et al., 1994b). For the purposes of expressing our variables on a volume basis, we decided to determine the weight in kilogram of soil in 0.2 m3 (1 by 1 m soil pit with a 0.2-m depth). For organic C, magnetic susceptibility, magnetic minerals, and fly ash, the value of each fraction (Tables 3 and 4) were averaged by relative thickness (either 5- or 10-cm interval) to a depth of 20 cm. These values (Table 5) were expressed on a per kilogram basis for the upper 20 cm of the profile. The mean soil bulk densities for both reforested and cultivated soils were 1.3 Mg m-3. Therefore the weight of soil in 0.2-m3 volume was 0.26 Mg or 260 kg. The average organic C, magnetic susceptibility, magnetic minerals, and fly-ash values for the upper 20 cm of each profile were obtained by multiplying respective average values for the 0- to 20-cm layer by kg of soil in 0.2 m3. Values in Table 5, representing the mean and standard deviation of two transects (Fig. 1), were calculated for the reforested (Transects 5 and 6) and the cultivated (Transects 7 and 8) areas.
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The presence of forages for much of the last 70 yr appears to have helped to retain or increase the organic C content of the entire cultivated site (Table 5). The organic C content levels at the cultivated site would have been reduced more if only used for row crops and subjected to more tillage and erosion. Organic C content of the cultivated site (3.51 ± 0.34 kg per 0.2 m3) is not statistically different from the organic C level of the entire reforested site (4.26 ± 0.85 kg per 0.2 m3). Comparing all landscape positions at the cultivated site with the reforested site, there was an average net loss of 0.75 kg of organic C per 0.2 m3 of soil (Table 5). This difference suggests a 23% soil loss from the 0- to 20-cm soil layer at the cultivated site. The data for the 0- to 20-cm profile suggest lower organic C for the cultivated (Table 5) versus the reforested site on all landscape positions except the IF. No loss of organic C at the IF of the cultivated site compared with the reforested site (Table 5) is probably a result of different rates of organic C accumulation at the cultivated site with forage and cultivated crops in the rotation. Organic C at the reforested site increased for lower landscape segments possibly as a result of more plant available water on the lower landscape segments. The local variability of the organic C content was expressed by the standard deviations presented in Tables 3, 4, and 5.
The authors assumed that the rate of new soil formation over the last 70 ± 10 yr is the same for paired landscape positions (such as a UBS) would be similar and that the rate of soil formation would vary between landscape position (IF, SH, UBS, LBS, UFS, LFS). Consequently, we only compared respective segments with the same landscape position, for example the SH of the cultivated site with the SH of the reforested site. We assumed that the slow soil formation rates at both sites would result in a similar amount of soil development over the last 70 yr. However, the rapid soil erosion rates would result in different amounts of soil loss from the cultivated and reforested sites since 1930 and are the focus of this study.
In the 0- to 20-cm layer, the average magnetic susceptibility and the magnetic minerals (Table 5) were higher on the reforested site than the cultivated site in all landscape positions. The primary reasons for higher magnetic susceptibility in the reforested site might be the presence of more organic C and magnetic minerals. The presence of redox depletions higher in the soil profile (Table 1) suggested more reducing conditions as compared with the cultivated site. It is hypothesized that higher organic C and absence of cultivation provided a substrate for heterotrophic microorganisms that gave rise to the reducing conditions necessary to solubilize Fe-bearing primary minerals. Subsequent oxidation resulted in formation of the high magnetic susceptibility mineral maghemite (Mullins, 1977). In the cultivated site, lower magnetic susceptibility on backslope and footslope positions was because of the surface erosion from the Ap horizon and mixing in of lower magnetic susceptibility subsoil material. Magnetic susceptibility and magnetic minerals decreased at the cultivated site with increased soil loss from erosion.
Comparing the cultivated site with the reforested site (Table 5) on a whole transect basis, the cultivated site had 12 and 4% less magnetic susceptibility and magnetic minerals, respectively, than the reforested site in 0- to 20-cm depth. This loss of both magnetic susceptibility and magnetic minerals suggested the removal of topsoil accounts for 12 and 4%. In this study, these magnetic susceptibility-based estimates calculated into a loss of 2.4 cm of soil (Table 5) from a 20-cm thick soil layer because of accelerated erosion at the cultivated site. Magnetic susceptibility accounts for not only magnetic minerals but also other paramagnetic (clay minerals) and diamagnetic mineral and organic C substances (McBride, 1986) and the differences could be partially because of variability in organic C and inherent soil variability.
Fly-ash comparison on a whole transect basis showed 12% missing fly ash in the 20-cm layer in the cultivated site (Table 5). Erosion on landscapes is a complex phenomenon because of slope gradient, slope length, and slope shape (Moore et al., 1986). Apparently, the LBS gained fly ash because of accelerated erosion. The concave across-slope shape (Lobb and Kachanoski, 1999) at the cultivated site might have trapped sediments from the IF and SH segments (Table 5). Loss of 31% of the fly ash on the upper footslope might be because of accelerated erosion on the footslope (Table 5). The presence of fly ash in the 20- to 30-cm layer (Table 4) and more uniform distribution throughout the 0- to 20-cm layer on lower footslope suggested deposition of sediments rich in fly ash. It is quite possible that the UFS was exposed to erosion during early period (1851–1930) of fly-ash deposition and more fly ash was transported to the adjacent waterway and off this landscape. It is suggested that 2.4 cm (Table 6) of the surface soil layer (12%) has eroded during past 60 to 80 yr from the entire cultivated site because of accelerated erosion which indicates a 4.7 Mg ha-1 yr-1 soil loss. The erosion rate was determined by calculating the weight of 1 ha of soil to a 2.4-cm depth and dividing by the number of years (70) of cultivation. Loss of fly ash from the cultivated site was because of both tillage and the process of accelerated erosion which resulted in the movement sediment that contained fly ash.
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SUMMARY |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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2.4 cm of the original surface layer since 1930 (70 yr) which is attributed to accelerated erosion associated with cultivation. We believe this 12% soil loss estimate, based on fly-ash removal and magnetic susceptibility reduction (Table 6), represents a better estimate of soil loss than obtained using organic C and magnetic mineral content. These fly-ash and magnetic susceptibility values are less subject to changes under soil conditions than are organic C (23% lower) and magnetic mineral content (4% lower). Other evidence that soil erosion had taken place (deposition of fly-ash rich sediment on the UFS and LFS) in the past 60 to 80 yr confirms that accelerated erosion has occurred at the cultivated site. For the entire cultivated landscape, the presence of 88% of the fly-ash content and 88% retention of the magnetic susceptibility suggests the presence of 17.6 cm of the original A horizon, placing the soil in the slightly eroded phase. The organic C and magnetic minerals (Table 6) supports this placement. The estimated annual soil loss amounts to an average of 4.7 Mg ha-1 yr-1 for the past 60 to 80 yr based on loss of fly-ash and reduction in magnetic susceptibility.
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ACKNOWLEDGMENTS |
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Received for publication October 5, 2000.
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This article has been cited by other articles:
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  K. R. Olson, R. L. Jones, A. N. Gennadiyev, S. Chernyanskii, W. I. Woods, and J. M. Lang Accelerated Soil Erosion of a Mississippian Mound at Cahokia Site in Illinois Soil Sci. Soc. Am. J., November 1, 2002; 66(6): 1911 - 1921. [Abstract] [Full Text] [PDF]
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at cultivated site as compared to reference reforested site is from predicted soil loss.