effects of soil erosion and deposition on soil organic carbon dynamics at a sloping field in black...

ORIGINAL ARTICLE

Effects of soil erosion and deposition on soil organic carbondynamics at a sloping field in Black Soil region, Northeast China

Shulan CHENG1, Huajun FANG2, Tianhong ZHU1, Jiaojiao ZHENG1,Xueming YANG3, Xiaoping ZHANG4 and Guirui YU2

1Graduate University of Chinese Academy of Sciences, Beijing 100049, 2Institute of Geographic Sciences and Natural Resources

Research, Chinese Academy of Sciences, Beijing 100101, China, 3Greenhouse & Processing Crops Research Centre, Agriculture &

Agri-Food Canada, Harrow, Ontario, N0R 1G0, Canada and 4Northeast Institute of Geography and Agricultural Ecology,

Chinese Academy of Sciences, Changchun 130012, China

Abstract

Soil erosion transports light density and fine particle soil material from hills down to low-lying land areas, which

can lead to carbon loss and subsequent sequestration. In the present paper, the profile distribution of soil organic

carbon (SOC) and soil 13C natural abundance (d13C) were analyzed across five geomorphic positions, distrib-

uted along a typical rolling farmland in the Black Soil region of Northeast China. The contents of particulate

organic carbon (POC) and mineral-associated organic carbon (MOC) at each geomorphic position were mea-

sured with physical fraction method. The results showed that soil erosion decreased 5.3–22.4% of SOC and

increased 4.0–6.1% of d13C of surface soils at the eroding sites. At the typical depositional sites, SOC content

and d13C value in the buried surface layer were 1.5 times and 1.1 times as much as those of the current plough

layer, respectively. Soil erosion did not change the POC content, but MOC content decreased by 9.3–35.2%. At

the eroding sites, the coefficient of determination between soil d13C and MOC (R2 = 0.52) was higher than that

between soil d13C and POC (R2 = 0.37). Our study indicated that soil erosion decreased SOC content and

increased d13CSOC in surface layer mainly through transferring fine sized and 13C-depleted SOC fraction. Deep

burial and re-aggregation of eroded materials at depositional sites were in favor of stabilization and sequestra-

tion of SOC.

Key words: 13C natural abundance, black soils, physical separation, soil organic carbon, soil redistribution.

INTRODUCTION

Soil erosion slacks and breaks down water stable aggre-

gates, thus leading to encapsulated C prone to mineraliza-

tion (Lal 2003; Six et al. 1999, 2000). Being a selective

process, soil erosion also preferentially transfers materials

of fine sized particles and light density (Wairiu and Lal

2003). As a consequence, eroded soil materials that are

enriched in plant nutrients are deposited at low-lying

areas (Boix-Fayos et al. 2009). Researchers note that the

above processes are beneficial to promote net primary

production (NPP) of plant, to increase soil organic carbon

(SOC) storage by reducing C mineralization, and to

sequester organic carbon at low-lying areas (Fang et al.

2006a; Jacinthe et al. 2001; McCarty and Ritchie 2002;

Stallard 1998; Van Oost et al. 2007). However, the inher-

ent mechanism, such as the dynamics of new and old soil

carbon in eroding and depositional positions as well as

the fate of eroded carbon, is rarely studied in agricultural

ecosystems. More studies are needed to fully understand

the effects of soil erosion and deposition on SOC loss and

sequestration at a catchment scale.

The 13C natural abundance (or d13C) technique pro-

vides a way to characterize the dynamics of SOC with dif-

ferent turnover time (Balesdent et al. 1996). Using this

technique, Cadisch et al. (1996) and Lynch et al. (2006)

found that coarse sized and light density fractions of soil

organic matter (SOM) were originated from recent plant

residues, whereas SOM in fine sized and heavy density

materials were inherited from old plant materials. When

original C3 grassland has been reclaimed and cultivated as

Correspondence: Huajun FANG, Institute of Geographic Sci-ences and Natural Resources Research, Chinese Academy of Sci-ences, Beijing 100101, China. Email: [email protected]

Received 4 September 2009.Accepted for publication 4 May 2010.

� 2010 Japanese Society of Soil Science and Plant Nutrition

Soil Science and Plant Nutrition (2010) 56, 521–529 doi: 10.1111/j.1747-0765.2010.00492.x

C4 crop (maize) for several decades, coarse sized SOC

fractions often represent maize-derived 13C signals,

whereas fine sized SOC fractions associated with clay and

silt keep the 13C signals of original C3 vegetation (Kris-

tiansen et al. 2005). Our null hypotheses are that: (1) soil

erosion decreases SOC content and tends to increase d13C

due to preferential transfer of fine sized 13C-depleted soil

materials, and (2) sediments accumulated at earlier stage

have higher SOC content and are more depleted in 13C

than those at later stage.

Black soils in Northeast China (43–50�N, 124–127�E)

occupy an area of 5.96 million hectares. Although the

black soils are enriched in organic matter, with an undu-

lating landscape, concentrated rainfall in summer and

bare soil surface without residue cover under conven-

tional management can lead to black soils being very

prone to erosion. Approximately 4470 km2 of land in

the Black Soil region has been subject to moderate to

severe erosion, accounting for 38% of the total black soil

acreage (Fang et al. 2006a). At a typical catchment in

the Black Soil region, our previous study on the impact

of soil redistribution on the SOC budget indicates that

over half of the total eroded SOC from the eroding areas

over the past 100 years is deposited in the depositional

areas (Fang et al. 2006a). Further exploring the inherent

mechanism of SOC loss and sequestration is very neces-

sary, including identifying sources of SOC and quantify-

ing dynamics of SOC in specific soil fractions using

techniques for physical fractionation of SOM in conjunc-

tion with isotopic analyses. In the present paper, the spe-

cific objectives were to: (1) compare the difference of

SOC compositions and soil d13C values among geomor-

phic positions based on their profile distribution, and (2)

evaluate the relationship between soil d13C and SOC

fractions at various geomorphic positions and deposition

periods.

MATERIALS AND METHODS

Site descriptions

The study was conducted at a small catchment (N:

44�43¢, E: 125�52¢) in the southern part of the Black Soil

region, which is 5 km southwest of Songhuajiang town in

Dehui city, Jilin province, Northeast China (Fig. 1). The

altitude of the area is 300 m above sea level. The region is

characterized by a mid-temperate sub-humid monsoon cli-

mate, with mean annual precipitation of 520 mm, of

which nearly 70% falls in the rainy season (June–August).

The annual average temperature is 4.4�C, with an average

temperature of the coldest and hottest month being

)17.5�C and 22.8�C, respectively. The soil is classified as

fine loamy Typic Hapludoll (Soil Survey Staff, 1999). The

natural vegetation is dominated by C3 plants such as

Corylus heterophylla, Setaria viridis, Artemisiasibirica,

Stipa grandis. The land was reclaimed to cultivate spring

wheat about 200 years ago. However, vegetation was

dominated by C4 plants (mainly corn) in the last 70 years

due to climate drying, increase in diseases and insect pests

of spring wheat and requirements of higher yield (Fang

et al. 2006a). Autumn ploughing, hand hoeing during the

growing season and machine-driven ridging parallel to the

slope are common in this region.

Experimental design and sampling

A 200-m long transect, perpendicular to the contour, was

set up along the northern slope. Based on soil erosion and

deposition rates, five slope positions referring to summit,

shoulder-, back-, foot- and toe-slope are divided. A very

flat surface and a short slope length at the summit site are

not conducive to pooling water (Table 1). On the other

hand, windbreaks were planted in the surrounding area,

which can also decrease surface water erosion. So summit

can behave as a control site due to very weak soil erosion.

Shoulder- and back-slope act as eroding sites, whereas

foot- and toe-slope act as typical depositional sites (Fig. 2

and Table 1). The lengths of the summit, shoulder-, back-,

foot- and toe-slope are 20.2, 51.5, 73.6, 30.5 and 19.0 m,

respectively (Fig. 2 and Table 1). The rates of soil erosion

and deposition at each slope site were determined using137Cs technique and mass balance model (Table 1, Fang

et al. 2006b). Three soil pits were excavated down to the

Figure 1 Location of study site and distribution of Black soils inNortheast China.


522 S. Cheng et al.

soil parent materials at each site, and 15 soil profiles could

be used in all. Soil samples were collected at a 5-cm inter-

vals for the top 50 cm soil depth, 10-cm intervals for soils

from 50 to 100 cm deep and 20-cm intervals for soils

below 100 cm. Some selected soil properties were listed in

Table 1 (Fang et al. 2007).

13C natural abundance measurement

Mineral soil samples were air dried at room temperature

and then sieved through a 2-mm sieve to remove roots

and stones. All samples were ground into a fine powder in

a planetary mill and oven dried at 70�C for 24 h before

being analyzed. The d13C values and C contents of sam-

ples were determined simultaneously using an automatic,

online elemental analyzer (Flash EA1112; ThermoFinni-

gan) coupled to an isotope ratio mass spectrometer (Finni-

gan MAT-253, ThermoElectron, Bremen, Gemany).

Carbon isotope values (d13C) were reported in per mil

(&) relative to the Pee Dee Belemnite standard (a carbon-

ate formation, whose generally accepted absolute ratio of13C ⁄ 12C is 0.0112372). The standard deviation of 10

repeated samples was <0.3&.

Prediction of litter d13C in C3 and C4 sites

When the C3 grassland changes into maize fields, the pro-

portion of residual carbon derived from C4 crop (X) and

C3 vegetation (1 ) X) can be calculated by equation (1):

X ¼ ðd� d3Þ=ðd4 � d3Þ ð1Þ

where d is the d13C value of maize soil, d3 is the d13C

value of SOM derived from C3 vegetation, and d4 is the

d13C value of maize residues.

The equation (1) can be rewritten as follows:

d ¼ Xd4 þ ð1�XÞd3 ð2Þ

Supposing the total SOC content is C, carbon content

derived from C4 crop (C4) is XC. The equation (2) can

also be changed into equation (3) through substituting X

with C4 ⁄ C.

d ¼ ðd4 � d3ÞC4=Cþ d3 ð3Þ

Equation (3) indicates that the d13C of SOM is inversely

correlated with SOC content (Piao et al. 2001). The

Table 1 Characteristics of geomorphic positions and soils in the upper 20 cm (mean, standard error [SE] in parentheses)

Geomorphic positions

Summit Shoulder-slope Back-slope Foot-slope Toe-slope

Slope (�) 1.31 3.73 2.75 2.25 1.27

Slope length (m) 20.2 51.5 73.6 30.5 19.0

Soil erosion (t ha)1 year)1)* 2.40 62.02 26.22 )28.53 )106.06

SOM (g kg)1) 26.20 (1.38) 20.34 (0.86) 24.83 (1.03) 20.86 (1.55) 23.79 (1.72)

Total nitrogen (g kg)1) 1.30 (0.08) 1.23 (0.02) 0.88 (0.04) 1.27 (0.11) 1.06 (0.25)

Soil moisture (%) 18.5 (0.9) 19.0 (0.7) 19.1 (1.2) 20.2 (0.7) 20.2 (1.2)

Sand (2.00–0.02) (%) 39.3 (0.37) 35.0 (1.52) 42.3 (0.05) 42.2 (0.05) 46.6 (0.09)

Silt (0.020–0.002) (%) 33.4 (11.66) 46.7 (6.18) 37.4 (2.19) 30.3 (3.92) 32.9 (1.54)

Clay (<0.002) (%) 27.3 (8.81) 18.3 (5.86) 20.4 (3.10) 27.6 (2.96) 20.5 (2.30)

pH 5.91 (0.31) 5.92 (0.26) 6.12 (0.39) 5.87 (0.27) 5.66 (0.16)

*Data are from Fang et al. (2006a). Positive values mean erosion and negative ones deposition

Figure 2 Description of soil profiles at the five geomorphic posi-tions.


Soil redistribution and carbon dynamics 523

intercepts of regression equations between soil d13C and

reciprocal of SOC (1 ⁄ SOC) in the soil section, which was

affected by C4 and C3 plants, represent the d13C values of

C4 crop residue and original C3 vegetation litter, respec-

tively.

SOC separation and measurement

Particulate organic carbon (POC) and mineral-associated

organic carbon (MOC) were determined using a proce-

dure adapted from Cambardella and Elliott (1992).

Approximately10 g of soil samples were weighed and put

into plastic bottles, then 30 ml Hexametaphsphate liquor

was added at 5 g L)1. Soil suspensions were shaken for

15 h on a reciprocal shaker. POC (>53 lm) was recovered

by back-washing the sieve followed by filtration (What-

man filter paper #541). POC consisted of free organic

debris and some larger fragments of organic matter

released by the dispersion of soil aggregates. The MOC

fraction (<53 lm) was recovered by evaporation. MOC

consisted of soil C associated with silt and clay size parti-

cles and some smaller fragments of organic matter

released by the dispersion of soil aggregates. The contents

in POC and MOC were calculated considering the masses

of separated soil fractions and the concentrations of car-

bon. All samples were free of carbonates.

Statistical analysis

The difference of carbon contents and d13C values of sam-

ples among geomorphic positions was tested with one-

way analysis of variance (ANOVA). Means comparison was

conducted using Tukey’s honestly significant differences

(HSD) test. We used SAS version 8.01software (SAS Insti-

tute, Carey, NC, USA) for ANOVA. Regression analysis was

also conducted to test the relationships between soil d13C

and SOC fractions using Sigma Plot version 10.0 software

(Systat Software, Inc., Richmond, CA, USA). Statistically

significant difference was set as P < 0.05 and is stated if

otherwise.

RESULTS

Depth distribution of SOC and d13C

Vertical distribution of SOC concentrations and soil d13C

values at various landscape positions is shown in Figure 3.

At the eroding sites including summit, shoulder- and

back-slope sites, SOC concentrations varied by site and

decreased with increasing soil depth over a depth of

50 cm. Soil d13C values varied by sites too; however, after

decreasing with depth, the soil d13C increased below

35 cm for the summit and back-slope sites and from 25 to

30 cm for the shoulder-slope site (Fig. 3a,c). The average

SOC content in the surface 20 cm layer was

11.8 ± 0.5 g kg)1 at the shoulder-slope, which was signif-

icantly lower than that at the summit (15.2 ± 0.8 g kg)1)

(Table 2). However, soil d13C values ()18.6 ± 0.2& and

)19.0 ± 0.2& at the shoulder- and back-slope, respec-

tively) were significantly higher than that at the summit

()19.8 ± 0.3&) (Table 2).

Vertical distribution of SOC concentration clearly

showed buried soil at a depth around 70 cm at the foot-

and toe-slope sites, where soil d13C decreased with soil

depth from )19.23 ± 0.25& to )23.71 ± 0.17& at the

foot-slope site and from )19.39 ± 0.33& to )23.62 ±

0.16& at the toe-slope site (Fig. 3d,e). In the upper 20 cm

soils, the SOC contents were 12.1 ± 0.9 and 13.8 ±

1.0 g kg)1 at the foot- and toe-slope sites, which were

also significantly lower than that at the summit site

(15.2 ± 0.8 g kg)1) (Table 2). Although there was no sig-

nificant difference in soil d13C values between foot- or

toe-slope and summit, the d13C values were greater at the

eroding shoulder- and back-slope sites than at the deposi-

tional foot- and toe-slope sites (Table 2).

Depth distribution of POC and MOC

Particulate organic carbon and MOC decreased exponen-

tially with the increasing soil depth at three eroding sites

(Fig. 4a,b). In the plough layer, there was no significant

difference in POC content among three eroding sites,

which ranged from 4.4 ± 0.6 to 4.8 ± 0.3 g kg)1

(Table 2). However, MOC content was significantly

lower at the shoulder-slope than at the summit (Table 2).

Percentages of POC to SOC at the shoulder-slope

(18.6 ± 1.6%) and back-slope (20.0 ± 1.5%) were signifi-

cantly lower than that at the summit (31.5 ± 2.2%)

(Table 2).

According to profile distribution of POC and MOC, the

depths of the surfaces of buried soils, at the foot- and toe-

slope sites, appeared around 70 and 80 cm (Fig. 4a, b). At

the foot-slope site, POC and MOC contents were

4.73 ± 0.55 and 12.85 ± 1.67 g kg)1 in the buried layer,

respectively, which was 1.3 and 1.5 times as great as

those in the current plough layer (Fig. 4d and Table 2).

However, at the toe-slope site, POC content was

1.52 ± 0.36 g kg)1 in the buried soils, which was signifi-

cantly lower than that in the current plough layer

(3.25 ± 0.47 g kg)1) (Fig. 4e and Table 2). Also, there

was no significant difference in MOC content between

the buried and the plough layer (10.66 ± 0.46 versus

10.56 ± 0.32 g kg)1) (Fig. 4e and Table 2). In the plough

soils, both POC and MOC contents were significantly

lower at the depositional foot-slope site than at the sum-

mit site (Table 2).

Correlation between soil d13C and SOC fractions

Soil d13C values were negatively correlated with reciprocal

values of SOC (1 ⁄ SOC) in the soil section where SOC was

mainly from C4 plants (Fig. 5a–c). The intercepts of


524 S. Cheng et al.

regression of foot- and toe-slope sites were )15.87 and

)17.45&, respectively, which were lower than those of

eroding sites ranging from )10.43 to )13.81&. On the

contrary, soil d13C values were positively correlated with

1 ⁄ SOC values in the soil section where original C3 vegeta-

tion was dominating. There were similar intercepts of

regression of summit, back- and foot-slope sites ranging

from )22.42 to )22.83& (Fig. 5a,c,d). Below buried lay-

ers of foot- and toe-slope sites, soil d13C values were nega-

tively correlated with 1 ⁄ SOC values (Fig. 5d,e).

In the upper 20 cm layers at the eroding sites, soil d13C

values were negatively correlated with POC and MOC

contents, respectively (Fig. 6a,b). Moreover, the adjusted

coefficient of determination (R2) of the latter was higher

than that of the former (Fig. 6a,b). In addition, soil d13C

value was negatively correlated with POC ⁄ SOC (Fig. 6c).

DISCUSSION

Loss and sequestration of SOC

Soil erosion decreased MOC rather than POC contents at

the eroding sites. This result showed that the preferential

transfer of SOC fractions associated with fine sized soil

(a) (b)

(c) (d)

(e)

Figure 3 Distribution of soil organic carbon (SOC) and d13C with soil depth at the geomorphic positions. (a) Summit; (b) shoulder-slope; (c) back-slope; (d) foot-slope; (e) toe-slope.



materials decreased the total SOC content at the eroding

sites. Martinez-Mena et al. (2008) also suggested that the

more labile OC fraction (POC) lost in soil in the cultivated

area was mainly due to the effect of cultivation (low over-

all biomass production and residue return together with

high C mineralization) rather than to water erosion, given

that the major part of the SOC lost in sediments was in

the form of MOC. Except MOC, the transfer of micro-

aggregates with high carbon content by surface runoff

also causes SOC decrease (Barthes and Roose 2002; Wair-

iu and Lal 2003; Yu et al. 2006). Although POC is apt to

be decomposed by soil microorganisms and be transferred

by surface runoff due to its light density (<1.6 g cm)3)

(Lal 2003), the annual loss of POC can be made up by

new carbon input from annual crop residue (Jacinthe

et al. 2001; McCarty and Ritchie 2002), which brings no

significant difference in POC content among the eroding

sites. In addition, there is no significant difference of soil

CO2 emission among the geomorphic positions with vari-

ous soil erosion rates (Bajracharya et al. 2000). Therefore,

we thought that the difference of SOC content in plough

layer at the different geomorphic sites was mainly caused

by soil erosion and deposition rather than microbial

decomposition.

The profile pattern of SOC and its fractions reflected

the temporal evolvement of slope shape and dynamics of

eroded materials in the study area. At the same study area,

Fang et al. (2006a) reported that the majority of the

deposited soil materials were accumulated at the foot-

slope site before 1954 and then gradually accumulated at

the toe-slope after 1954 using 137C and fly ash as two

time-makers. Simultaneously, transferred POC and MOC

from the eroding sites had relatively higher concentration

and lost little due to its small slope in the early period of

sediment accumulation (Fang et al. 2006a). Anaerobic

environment resulting from deep burial can protect POC

and MOC from being decomposed by soil microbes

(Chan et al. 2002; Polyakov and Lal 2004). Therefore,

the occurred peaks of POC, MOC and SOC in the buried

surface layer at the foot-slope site were greater than those

at toe-slope site.

Isotopic evidence on SOC dynamics

The predicted d13C values of C4 crop residue at the erod-

ing sites ranged from )10.43 to )13.81&, which was sim-

ilar to the actual measures of crop residues ()12.51 ±

0.11&) (Fang et al. 2005). The d13C values of C4 crop lit-

ter at depositional sites were more depleted in 13C than

Table 2 Comparison of soil organic carbon (SOC) fractions and their d13C of plough layer soils among each geomorphic position

Geomorphic positions

Summit Shoulder-slope Back-slope Foot-slope Toe-slope

SOC (g kg)1) 15.2 (0.8)a 11.8 (0.5)c 14.4 (0.6)ab 12.1 (0.9)c 13.8 (1.0)b

d13C (&) )19.8 (0.3)c )18.6 (0.2)a )19.0 (0.2)b )19.5 (0.2)c )19.6 (0.2)c

POC (g kg)1) 4.4 (0.6)ab 4.8 (0.3)a 4.6 (0.3)a 3.7 (0.3)c 3.8 (0.9)bc

MOC (g kg)1) 10.8 (1.9)a 7.0 (0.6)d 9.8 (0.6)b 8.4 (0.4)c 10.1 (1.3)ab

POC ⁄ SOC (%) 31.5 (2.2)a 18.6 (1.6)b 20.0 (1.5)b 22.1 (1.9)b 23.4 (0.7)b

MOC ⁄ SOC (%) 68.5 (2.2)b 81.4 (1.6)a 80.0 (1.5)a 77.9 (1.9)a 76.6 (0.7)a

Mean, standard error (SE) in parentheses; n = 3; There is significant difference with different letters following the value, and vice versa (Tukey’s HSD test).

(a) (b)

Figure 4 Depth distribution of particulate organic carbon (POC) and mineral-associated organic carbon (MOC) at the geomorphicpositions


526 S. Cheng et al.

those at the eroding sites, which can partly be attributed

to increase of soil moisture and decrease of water use effi-

ciency of crop (Table 1).

Soil erosion tended to increase d13C values in the sur-

face soils at the eroding sites through transporting MOC.

Powers and Schlesinger (2002) also suggested that soil

d13C was positively correlated to slope gradient over land-

scape (R2 = 0.61, P < 0.01), and soil erosion tended to

increase d13C value of topsoil. In croplands with long his-

tory of cultivating C4 crops, coarse sized and light density

soil particles are enriched in 13C much more than those

of fine sized and heavy density ones (Roscoe et al., 2001;

Liu et al. 2002). In our study area, the average d13C of

particulate organic matter in the upper 20 cm soil layer

near the study area is )19.43 ± 0.47&, which is signifi-

cantly higher than that of mineral associated organic mat-

ter ()21.03 ± 0.28&) (Yu et al. 2006). Therefore, soil

erosion increased d13C values in surface soil layers

through mainly decreasing MOC that depleted in 13C.

The higher correlation of surface soil d13C to MOC

content than to POC content in our study also supported

this assumption (Fig. 6)

The profile distribution of soil d13C values at the depo-

sitional sites helps to estimate the cultivation history of C4

crops. Using 137Cs and fly ash as two time marks, Fang

et al. (2006a) deduced that the time in 35 and 70 cm

depth at the foot-slope site was 1954 and 1903, respec-

tively. The d13C of plough layer in black soils where corn

has been cultivated for more than 50 years was approxi-

mately )20.15 ± 0.45& (Liu et al. 2004), which is near

to that in 45 cm depth with the time of about 1932.

Above the original soil layer, eroded material from C3

vegetation is dominant before 1932, and shifts to C4 crop

residue after 1932. Other processes that transport C in the

soil such as bioturbation and dissolved organic carbon

leaching may also influence the vertical patterns of d13C

in soil profiles, but mineralization is expected to be the

dominant mechanism (Powers and Schlesinger 2002).

Therefore, we can deduce that the history of cultivating

corn is about 72 years.

(a) (b)

(c) (d)

(e)

Figure 5 Relationship between d13C and soil organic carbon (SOC) at the geomorphic positions (a) summit; (b) shoulder-slope;(c) back-slope; (d) foot-slope; (e) toe-slope.



In sum, our study suggested that a large amount of

eroded carbon from the upper slope positions was seques-

tered in the depositional areas since the field was culti-

vated. Moreover, the transferred C was mainly MOC

fraction with strong chemical recalcitrance and long mean

turnover time (MRT), which caused the depositional

areas a great C sink. We must consider the temporal and

spatial patterns of eroded materials as calculating soil

carbon budget and evaluating organic matter quality in

agricultural ecosystems.

Conclusions

Compared profile distribution of SOC fractions and soil

d13C values, our study indicates that soil erosion decreases

SOC contents in surface layer at the eroding sites through

transferring fine sized MOC fractions. Accordingly, the

significant accumulation of MOC occurs in topsoil at

the depositional sites. Deep burial and re-aggregation

of eroded materials at the depositional sites are in favor of

stabilization and sequestration of SOC. The intercept of

regression equations between soil d13C and 1 ⁄ SOC in the

special soil section can nicely deduce the d13C value of

vegetation litter. Soil erosion tends to increase topsoil

d13C value through decreasing 13C-depleted MOC con-

tent and POC ⁄ SOC. In addition, depth variation of SOC

fractions and soil d13C can help to identify surface layer of

buried soils and to deduce cultivation history of C4 crops

in the study area.

ACKNOWLEDGMENTS

This research was funded by National Natural Science

Foundation of China (30600071, 40601097, 30590381),

National Key Research and Development Program

(2010CB833502), Knowledge Innovation Project of

the Chinese Academy of Sciences (KZCX2-YW-432,

O7V70080SZ, LENOM07LS-01) and the President Fund

of GUCAS (O85101PM03).

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