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Pedosphere 20(6): 736746, 2010
ISSN 1002-0160/CN 32-1315/P
c 2010 Soil Science Society of China
Published by Elsevier Limited and Science Press
Effect of Soil Erosion on Soil Properties and Crop Yields
on Slopes in the Sichuan Basin, China1
SU Zheng-An1,2, ZHANG Jian-Hui1,2 and NIE Xiao-Jun1,2
1Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Conservancy,Chengdu 610041 (China)2Graduate University of the Chinese Academy of Sciences, Beijing 100049 (China)
(Received January 7, 2010; revised June 2, 2010)
ABSTRACT
Roles of tillage erosion and water erosion in the development of within-field spatial variation of surface soil properties
and soil degradation and their contributions to the reduction of crop yields were studied on three linear slopes in the
Sichuan Basin, southwestern China. Tillage erosion was found to be the dominant erosion process at upper slope positions
of each linear slope and on the whole short slope (20 m). On the long slope (110 m) and medium slope (40 m), water
erosion was the dominant erosion process. Soil organic matter and soil nutrients in the tillage layer were significantly
related to slope length and 137Cs inventories on the long slope; however, there was no significant correlation among them
on the short slope, suggesting that water erosion lowered soil quality by transporting SOM and surface soil nutrients
selectively from the upper to lower slope positions, while tillage erosion transported soil materials unselectively. On the
medium slope, SOM, total N, and available N in the tillage layer were correlated with slope length and the other properties
were distributed evenly on the slope, indicating that water erosion on this slope was still the dominant soil redistribution
process. Similar patterns were found for the responses of grain yield, aboveground biomass, and harvest index for slopes.
These results indicated that tillage erosion was a major cause for soil degradation and grain yield reduction on the linear
slopes because it resulted in displacement of the tillage layer soil required for maintaining soil quality and plant growth.
Key Words: 137Cs inventory, linear slope, soil degradation, tillage erosion, water erosion
Citation: Su, Z. A., Zhang, J. H. and Nie, X. J. 2010. Effect of soil erosion on soil properties and crop yields on slopes
in the Sichuan Basin, China. Pedosphere. 20(6): 736746.
INTRODUCTION
There is a growing recognition that tillage erosion plays an important role in the redistribution of
soil within a slope landscape (Quine et al., 1999; Van Muysen et al., 1999; Quine and Zhang, 2002;
Kosmas et al., 2001; Nassen et al., 2002; Dercon et al., 2003; Van Oost et al., 2003, 2004; De Alba et
al., 2004; Li et al., 2004; Zhang et al., 2004a, b, 2006; Malo et al., 2005; Papiernik et al., 2005; Liet al.,
2007; Zhenget al., 2007). Goverset al. (1999) reported that the observed patterns of soil redistributionin fields did not agree with the predictions of standard water erosion models. Furthermore, the spatial
signatures of water erosion and tillage erosion are fundamentally different: soil loss by tillage will be
most intense on landscape positions where water erosion is minimal (i.e., convexities and near upslope
field borders), while soil accumulation by tillage often occurs in a place where water erosion is maximal
(e.g., hollows where overland flow concentrates).
As early as 1937, a study by Puhr and Olson (1937) reported the impacts of 50 years of cultivation
on selected soil chemical and physical properties in a few selected South Dakota soils. However, there
had been virtually no systematic study of tillage translocation and erosion until the first attempts were
1Supported by the National Natural Science Foundation of China (No. 40771027), the National Key Technology
R&D Program of China (No. 2008BAD98B04) and the National Basic Research Program (973 Program) of China
(No. 2007CB407206).2Corresponding author. E-mail: [email protected].
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EROSION EFFECT ON SOIL PROPERTIES AND CROP YIELDS 737
made in the 1980s and early 1990s (Goverset al., 1999). Since then, the importance of tillage erosion and
effect of tillage erosion on soil properties and grain yields have been demonstrated all over the world. Da
Silva et al. (1999) reported that tillage erosion, in the long term, altered soil profile and consequently
affected soil water-holding capacity, nutrient availability, and organic matter (SOM) reserve, and grainyields. Several studies prove that tillage erosion increases soil variability and degradation of surface soil
quality on convex slope positions, as well as spatial variability of crop production (Schumacher et al.,
1999; Kosmas et al., 2001; Papiernik et al., 2005). Kosmas et al. (2001) reported that tillage erosion
affected chemical properties of soils mainly in three important ways: loss in soil organic matter, loss in
soil nutrients, and exposure of subsoil with low fertility or high acidity. According to Quine and Zhang
(2002), tillage erosion was the dominant soil redistribution process and correlated closely with the soil
properties and grain yields, suggesting that soil redistribution by tillage was a major contributor to the
spatial variation in soil properties and crop production. In linear slope landscapes (Asia and Africa),
animal-drawn plough and manual hoeing was considered the primary tillage tool and also created tillage
erosion on hillslopes, with significant effect on soil degradation and grain yields (Thapa et al., 1999;
Turkelboom et al., 1999; Nyssen et al., 2002; Li et al., 2004; Zhang et al., 2004a, b, 2006).Although studies by Zhanget al.(2004a, b, 2006) and Ni and Zhang (2007) have clearly demonstrated
that runoff water is not the only erosive factor and tillage erosion has also greatly contributed to total
soil erosion and would be more severe than water erosion in some landscape positions in the Sichuan
Basin, China, these studies focused on the quantities of tillage erosion and impacts of tillage erosion
on soil quality, without addressing tillage erosion effects on grain yields. It is, therefore, urgent to
evaluate soil erosion and erosion-induced depletion of soil nutrients and reduction of grain yield, so as
to understand the process of soil degradation due to erosion and to establish sustainable ecosystems
and soil management practices to prevent the soils from further degradation or to facilitate restoration.
This study initiated to relate caesium-137 derived soil erosion rates and patterns to observed spatial
variations in surface soil properties and crop productivity, with specific objectives of: 1) to apply the137Cs technique to assess contemporary soil erosion and deposition patterns under a long-term cultivation
in the Sichuan Basin; 2) to investigate the spatial variation in surface soil properties and crop yields; and
3) to analyze the relationships between the contemporary (137Cs-derived) soil redistribution patterns
with surface soil properties and grain yields.
MATERIALS AND METHODS
The study area is located in Jianyang County (30 04 2830 39 00 N, 103 11 34104 53
36 E, 400587 m above sea level), Sichuan Province, southwestern China. This area is typical of hilly
areas of Sichuan and it has a humid sub-tropical climate, with a mean annual temperature of 17 C
and a mean annual precipitation of 960 mm. The erosive rainfalls between May and September account
for 62.3% of annual precipitation (Zhang et al., 2008). The land use in the area is characterized by dry
land cultivation where wheat (Triticum aestivumL.) makes up a large part of the crop rotation with
corn (Zea mays L.), sweet potato (Ipomoea batatas (L.) Lam.), groundnut (Arachis hypogaeaL.), and
rape (Brassica napus L.). After harvesting corn and sweet potato or groundnut, the fields are tilled
with a hoe during the late summer or early fall. Local farmers till their land one time at this stage in
order to improve seed bed conditions for seeding of winter wheat or rape as the next crop. Information
provided by farmers in the study area indicates that the fields have been cultivated for at least 100
years. The soil in the study area derived from purple mudstone and sandstone of Jurassic Age enriched
in CaCO3 (Tang et al., 1984), with a pH of 8 and a clay content of 250400 g kg1, is classified as a
Regosol according to the FAO soil taxonomy (FAO, 1988) and as a clay loam according to the USDA
soil taxonomy (Soil Survey Staff, 1996). When cultivated, the soil is highly susceptible to erosion and
the area represents one of the most severely eroded regions in China.
Most of the hillslopes in the study area have been dissected into slope segments and terraces; there-
fore, they are linear slopes and different slope gradients have evolved during the long-term agricultural
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practice. In rare cases, relatively leveled surfaces on terraces have been created by large amounts of
engineering work. In the context of this study, however, attention was given to different lengths of linear
slopes with slope gradients between 9.1% and 25.1%. In the study area, hoeing is the dominant tillage
practice due to small patches of fields and steep hillslopes. Farmers are accustomed to till from thebottom of the field and gradually moving up the slope. The upper parts of the slope segments in the
study area are characterized by a thin soil layer, underlain by rock stratum, whereas thick soil layers
are found in the lower parts of the slope.
Soil sampling was carried out in March of 2006 (dry season) on three slopes, which are located
approximately 500 m apart each other. No systematic variations in soil texture of the Ap or the B
horizons were observed among the three slopes. The first slope is a long hillslope with a gradient
of 10.1% and a slope length of 110 m (Ge et al., 2007). The slope is steep in the upper and middle
parts and gentle in the lower part. The second is a medium slope with a slope gradient of 9.1% and a
slope length of 40 m. The third is a short slope with a slope gradient of 25.1% and a slope length of
20 m. The coordinates and elevation of each sampling point were measured using a differential global
positioning system (DGPS) with horizontal accuracy of a few centimeters. Soil samples were collectedat 5 m intervals along the slope transect using a 6.8 cm diameter hand-operated core sampler. For each
sampling point, two cores were collected along the soil profile to a depth depending on soil thickness ( i.e.,
down to bedrock) and then bulked to make a composite sample. In cases where a subsurface horizon B
was distinguishable below the plow layer (Ap), the two horizons were sampled separately. Soil samples
were air-dried, crushed, and passed through a 2 mm-mesh sieve. Coarse materials such as gravels and
roots were removed. Samples of the < 2 mm fraction were weighted and used for all subsequent analyses.
A subsample of the total core was then prepared for 137Cs analysis by carefully mixing a/(a+ b) kg
sample of the < 2 mm particle-size fraction from the plow layer (Ap) with b/(a+b) kg sample of the
< 2 mm particle-size fraction of the sub-plow layer (B), where a and b are the weights of the samples
from Ap and B horizons, respectively. This subsample was then packed into a plastic beaker, and 137Cs
activity was measured using a hyperpure lithium-drifted germanium detector (ORTEC, USA) coupled
with a Nuclear Data 6700 multichannel gamma-ray spectrophotometer with counting time from 40 000
to 60 000 s. The relative errors of test results were lower than 5%. The inventories of137Cs were originally
expressed on a unit mass basis (Bq kg1) and were then converted to an area basis (Bq m2) using the
total mass of the bulked core sample.
Soil physical and chemical properties were determined according to the regular analysis methods.
Soil particle-size fractions were determined by the pipette method following H2O2 treatment to de-
stroy organic matter and subsequent dispersion of soil suspensions by Na hexametaphosphate (ISSCAS,
1978). Soil bulk densities were determined using oven-dried weight and sample volume. Soil organic
matter was determined using wet oxidation with K2Cr2O7. Total N was measured by the classical Kjel-
dahl digestion method. The determination of total P and K was carried out using the NaOH digestion
method. Available N was measured using NaOH extraction and distilling. Extractable P and K were
determined using NaOH digestion and ammonia acetic extraction, respectively (Liu, 1996).Wheat grain yield and aboveground biomass were measured at the end of the growth period at 5 m
intervals along the transects of the slopes, using a reaping hook from two plots 1 m 2 each. The grains
and total biomass were weighed after wheat was dried in the laboratory. Harvest index was calculated
as the grain yield as a percentage of the aboveground biomass.
The efficient 137Cs reference inventory is used to eliminate impacts of 137Cs surface enrichment
during nuclear explosion period on the assessment of soil losses on cultivated sloping land (Zhang et al.,
2000). The efficient 137Cs reference inventory (Ae, Bq m2) is expressed as:
Ae= Ao(1 RI) (1)
where Ao is the 137Cs reference inventory (Bq m2) and RI is the runoff index. Using a runoff index
of 0.25 as measured by Lei et al. (2003) and a local 137Cs reference inventory of 1 927 Bq m2 from
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EROSION EFFECT ON SOIL PROPERTIES AND CROP YIELDS 739
Zhang et al. (2004c), the efficient 137Cs reference inventory was calculated to be 1 445 Bq m2 (Zhang
et al., 2006). For an eroded site where a total 137Cs inventory (A, Bq m2) is less than the reference
inventory at year t (years), soil erosion rates can be expressed as (Zhang et al., 1990):
= 10dB
P
1
1
X
100
1/(t1963) (2)
where is the mean annual soil erosion rate (t ha1 year1), d the sampling depth (m), B the bulk
density of soil (kg m3),Xthe percentage reduction in total 137Cs inventory (X= (AeA)/Ae 100),
andP the particle size correction factor. In accordance with earlier study results in this area (Zhanget
al., 2004a, b), tillage erosion rates increase with increasing slope gradient and are inversely proportional
to the slope length:
R= (k3+k4S) 10/Ld (3)
whereR is the tillage erosion rate (t ha1 per tillage pass),k3 andk4 are the soil transport coefficients
(kg m1 per tillage pass), Sis the slope gradient (m m1), and Ld is the downslope parcel length (m).Simple linear regression analysis (Pearson product-moment correlation) was carried out to determine
the relationships between SOM, total N, total K, total P, available N, extractable K, extractable P, grain
yield, aboveground biomass, and 137Cs inventory. Analysis of variance was used to test the significance
of differences found between different positions of a slope. All statistical analyses were carried out using
SPSS software (version 11.5).
RESULTS AND DISCUSSION
Soil redistribution
For the long slope (Fig. 1a), 137Cs inventories were found to increase from the upper to lower slope
positions, being the lowest at the top and the highest at the foot position (100110 m), with the latterbeing 2 times higher than that of the former. On the medium slope (Fig. 1b), 137Cs inventories ranged
from a low value of 823.6 Bq m2 at the upper slope positions (05 m) to a high value of 1 386.8 Bq m2
at the foot slope positions (3540 m). Along the short slope transect, 137Cs inventories also increased
from the top to foot slope positions (Fig. 1c), with a 57.1% increase at the foot slope position compared
to that at the top of the slope. These suggest that soil loss occurred at the upper slope positions and soil
accumulation at the foot slope positions. This pattern of soil redistribution, characterized by soil loss
from the upper slope positions and deposition at the lower slope positions and the resultant increase in
soil depth from the upper to lower slope positions, is typical of tillage erosion in the study area (Zhang
et al., 2004a, b, 2006; Ge et al., 2007; Ni and Zhang, 2007). This result is also consistent with those of
the studies by Lobb and Kachanoski (1999), Van Muysenet al.(1999), Kosmasset al.(2001), Quine and
Fig. 1 Distribution of137Cs inventory a long the transects of a long slope (a), a medium slope (b), and a short slope (c).
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740 Z. A. SUet al.
Zhang (2002), Van Oostet al.(2003), Liet al.(2004), Maloet al.(2005), and Papierniket al.(2005). Fur-
thermore, the standard water erosion models show that water erosion is minimal near upslope field
borders (Govers et al., 1999). Therefore, the most serious erosion occurred at the upper slope position,
suggesting that tillage erosion redistributed soil in amounts, which decreased the effect of water erosionat that position on linear slopes.
The tillage erosion rates were much higher on the short slope than on the two longer slopes (Table
I). Tillage transport coefficients, k3 and k4, in the study area reach 30.7 and 141.3 kg m1 per tillage
pass, respectively (Zhang et al., 2004a). Based on one major tillage operation per year in the study
area, tillage erosion rates were estimated at 4.1, 10.9, and 33.1 t ha1 year1 for the long slope, medium
slope, and short slope, respectively (Table I). Tillage erosion rates being 7 times higher on the short
slope than on the long slope and 2 times higher on the short slope than on the medium slope suggested
that tillage erosion was more pronounced on short linear slopes. Compared with the contribution of
tillage erosion to the total erosion on the long and medium slopes (11.8% and 27.3%, respectively), the
contribution of tillage erosion was great (52.1%) on the short slope, showing that tillage erosion was the
dominant erosion process on the short slope, whereas water erosion was the dominant erosion processon the long slope and medium slope.
TABLE I
Lengths, gradients, 137Cs inventories, and erosion rates of the different slopes studied
Item Long slope Medium slope Short slope
Slope length (m) 110 40 20
Number of samples 12 9 5
Slope gradient (%) 10.1 9.1 25.1
Mean 137Cs inventory (Bq m2) 946.5289.54 1071.9773.27 758.30312.70
Tillage erosion rate (t ha1 year1) 4.1 10.9 33.1
Total erosion rate (t ha1 year1) 34.8 40.0 63.6
Contribution of tillage erosion to total erosion (%) 11.8 27.3 52.1
The< 0.02 mm particle-size fraction distribution of the tillage layer also demonstrated that tillage
erosion played a key role on the short slope, whereas water erosion was the dominant erosion process
on the long and medium slopes (Fig. 2). The< 0.02 mm particle size fraction in the tillage layer gra-
dually increased from top to bottom of the long slope and was negatively correlated with the elevation
(r = 0.80, P = 0.02, Fig. 2c). In contrast, the
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TABLE II
Descriptive statistics of tillage layer soil properties and crop properties on different slops studied
Slope Soil or crop Propertya) n Minimum Maximum Mean SDb) C.V.c)
%
Long Soil
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slope, the tillage erosion accounted for more than 50% of the total erosion, indicating that tillage erosion
was a dominant erosion process within the short linear slope. Tillage erosion unselectively transported
soil materials downslope and thus induced insignificant spatial variations within the short slope, whereas
water erosion selectively transported soil fine particles, which had relatively more SOM, total N, andavailable N, and consequently caused notable spatial variations in these elements. Therefore, within
different landscapes, i.e., short, medium, and long linear slopes, different dominant processes of soil
redistribution caused different spatial variations in surface soil properties. Zhanget al. (2006), Ni and
Zhang (2007), and Geet al. (2007) reported similar results, but Quine and Zhang (2002) advocated that
soil redistribution by tillage was the most important process contributing to within-field soil variability
and that higher soil variability could be expected to develop in complex landscapes due to continuous
tillage operation. These differences could be explained by different landscapes (linear slope and complex
landscapes), and different types of tillage operations (downslope vs. upslope and downslope) and tillage
tools (hoe vs. tractor).
Fig. 3 Distribution of soil organic matter (SOM) and total N (TN) in the tillage layer on the long slope (SOMvs. slopelength: r = 0.71, P = 0.009; TN vs. slope length: r = 0.85, P = 0.001).
Fig. 4 Distribution of soil available N (AN), extractable P (EP), and extractable K (EK) in the tillage layer on the longslope.
Fig. 5 Distribution of soil organic matter (SOM), total N, and available N in the tillage layer on the medium slope.
Relationships between soil properties and 137Cs inventories
For the long slope, positive correlations between 137Cs inventories and surface soil properties were
found (Fig. 6), being significant at P < 0.01 with r values of 0.79, 0.87, 0.96, 0.73, and 0.90. On the
medium slope, available N and extractable K in the tillage layer were found to be systematically related
to 137Cs inventories (Fig. 7). However, no significant correlations between 137Cs inventories and surface
soil properties were found for the short slope, implying that SOM and soil nutrients in the tillage layer
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EROSION EFFECT ON SOIL PROPERTIES AND CROP YIELDS 743
did not change significantly. According to Zhang et al. (2006) and Ge et al. (2007), the < 0.02 and
< 0.002 mm particle-size fractions were significantly correlated with total N, available N, extractable
P, extractable K, and SOM, and the fine particles were selectively transported by runoff from upslope
to downslope on the long slope. In contrast, tillage erosion transported soil materials from upslope todownslope unselectively. Therefore, the differences in the distribution of surface soil properties between
different landscapes were attributed to the differences in soil erosion process; tillage erosion was mainly
responsible for symmetrical distribution of surface soil properties at each position on the short slope,
whereas water erosion played a primary role in higher soil SOM and soil nutrients in the tillage layer at
the downslope positions on the long slope.
Fig. 6 Distribution of surface soil propertiesvs. caesium-137 (137Cs) inventories on the long slope. SOM = soil organicmatter.
Fig. 7 Distribution of available N (AN) and extractable K (EK) in the tillage layervs. caesium-137 (137Cs) inventories on
the medium slope.
Relationships between crop properties and 137Cs inventories
On the long slope, the mean grain yield distribution (Table III) was similar to that of 137Cs inven-
tories and soil redistribution by tillage erosion and water erosion (Fig. 1a). Mean grain yield and mean
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744 Z. A. SUet al.
aboveground biomass were found to be the lowest at the upper slope positions on the long slope where
the tillage erosion rate was the highest, whereas at the lower slope positions where soil aggradations were
identified, the grain yield and aboveground biomass were higher. On the medium slope, the grain yield
and aboveground biomass had high values at the foot slope positions where soil aggradations occurred,with no significant differences observed at the other slope positions. On the short slope, the grain yield
and aboveground biomass showed a gradual downslope increase.
The 137Cs inventory ratio of the upper slope position to the foot slope position showed a similar trend
to that for the grain yield and aboveground biomass. However, the value of the 137Cs inventory ratio was
obviously lower than those of the grain yield and aboveground biomass. On the long slope, the ratio was
0.41 for 137Cs inventory and 0.69 and 0.76 for the grain yield and aboveground biomass, respectively. On
the medium and short slopes, the 137Cs inventory ratios were 0.59 and 0.48, respectively, as compared
to the grain yield ratios of 0.89 (medium slope) and 0.82 (short slope) and the aboveground biomass
ratios of 0.88 (medium slope) and 0.93 (short slope). These indicated that the relationship between soil
redistribution and crop production was not a simple one in this field and therefore other factors must
be taken into account, integrating soil, water, plant, and air factors (Quine and Zhang, 2002).
TABLE III
Mean grain yield, mean aboveground biomass, and harvest index of wheat on the different slopes studied
Slope Position n Mean grain yield Mean aboveground biomass Harvest index
kg m2
Long slope Upper 6 0.400.01 1.040.11 0.38
Middle 8 0.430.04 1.240.13 0.35
Lower 6 0.540.02 1.210.06 0.45
Foot 4 0.580.01 1.360.01 0.42
Medium slope Upper 4 0.520.01 1.350.03 0.39
Middle 6 0.500.07 1.200.09 0.42
Lower 4 0.510.01 1.270.01 0.40
Foot 4 0.580.04 1.540.13 0.38Short slope Upper 2 0.480.01 1.260.03 0.38
Middle 2 0.560.11 1.380.27 0.41
Lower 2 0.610.11 1.450.34 0.42
Foot 2 0.580.07 1.350.21 0.43
On the long slope, the harvest index of wheat was found to be lower than 0.40 at the upper and
middle slope positions, greater than 0.40 at the other slope positions in each of the slopes. On the
medium slope, the harvest index was also lower than 0.40 at the upper slope positions, but was greater
than 0.40 at the middle and lower positions. It is interesting to note that the harvest index was lower
than 0.40 at the foot slope position of the medium slope, suggesting that the grain yield did not increase
as much as the aboveground biomass at that slope position. However, on the short slope, the harvest
index of wheat was lower than 0.40 only at the upper slope positions, and greater than 0.4 at allother slope positions. According to Liu et al. (1991), the harvest index of wheat averages 0.35 with a
maximum of 0.40 and a minimum of 0.30. This indicated that soil redistribution due to tillage erosion
and water erosion played an important role in redistributing nutrients and altering grain yields through
changing the depth of soil layer and surface soil properties. Therefore, tillage erosion could be a major
contributor to variation in crop properties in some landscapes, for instance the short linear slope, and
some landscape positions, i.e., upper slope positions of the long slope and medium slope.
CONCLUSIONS
Caesium-137 data indicated that soil erosion occurred at upper slope positions and deposition oc-
curred at the lower slope positions on the long slope, medium slope, and short slope. Further analysis of
soil particle-size fractions suggested that tillage erosion played an important role in the transfer of soil
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EROSION EFFECT ON SOIL PROPERTIES AND CROP YIELDS 745
materials from the upper to lower slope positions. On the long slope and medium slope, water erosion
was suggested to be the dominant erosion process, while tillage erosion was the dominant erosion process
on the short slope. Surface soil properties were strongly related to 137Cs inventories and slope length
on the long slope, showing that soil redistribution determined the variation of surface soil properties onthis slope. On the short slope, 137Cs inventories were found to be correlated with slope length; however,
surface soil properties (particle-size fraction) were distributed evenly on the slope, suggesting that soil
redistribution by tillage was the most important process contributing to within-field soil variability.
On the medium slope, some surface soil properties, i.e., < 0.02 mm particle-size fraction, available
N, and extractable K, were found to be related to 137Cs inventories and other surface soil properties
were stochastically distributed, suggesting that water erosion was still the dominant soil redistribution
process. These results indicated that attention must be given to impacts of tillage erosion on soil degra-
dation within short slope landscapes, particularly at the upper slope positions. Water erosion selectively
transported soil materials, which could result in higher SOM, clay, and nutrients in the tillage layer at
the lower slope positions, while soil materials were relocated unselectively by tillage erosion, resulting
in inappreciable changes in soil chemical and physical composition. Therefore, water erosion loweredsoil quality by transporting surface soil nutrients and SOM selectively from the upper to lower slope
positions. Grain yields and aboveground biomass were found to increase from the upper to lower slope
positions in each slope. However, since the relationship between crop properties and erosion is complex,
more data (i.e., rainfall and soil moisture) for years with different weather and landform conditions are
required to explore these trends fully. It could be concluded that tillage erosion was a major cause for
soil degradation and grain yield reduction in the linear slopes because it resulted in displacement of
tillage layer soil required for maintaining soil quality and plant growth.
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