Soil reinforcement by a root system and its effects on sediment yield in response to concentrated flow in the loess plateau

doi:10.4236/as.2011.22013

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Vol.2, No.2, 86-93 (2011)

doi:10.4236/as.2011.2 2013

Agricultural Sciences

Soil reinforcement by a root system and its effects on

sediment yield in response to concentrated flow in the

loess plateau

Peng Li1*, Zhanbin Li1,2

1School of Water Resources and Hydroelectric Power, Xi’an University of Technology, Xi’an, China;

*Corresponding Aut h o r : [email protected]

2Institute of soil and Water Conservation, Chinese Academy of Sciences, Ministry of Water Resources, Yangling, China.

Received 3 January 2010; revised 1 March 2011; accepted 23 March 2011.

ABSTRACT

The import ance of root s in soil conse rvation has

long been underestimated due to a lack of sys-

tematic studies conducted to evaluate root dis-

tribution patterns and their effects on soil ero-

sion. Current knowledge regarding root morp-

hology and it s impact on soil erosion by water is

limited; therefore, detailed analysis of the role

that root systems play in controlling soil ero-

sion is needed. In this study, stratified runoff sc-

ouring at different soil depths in the field was

conducted in a grassland area. The results indi-

cated that both root biomass and soil water-

stable aggregates decreased as soil depth in-

creased at a ll three sites, while there was almost

no change in soil bulk density at 1.3 g/cm3. Se-

diment yields under different runoff discharge

at different sites showed similar trends, and the

sediment yield increased as the soil depth in-

creased at all three sites. Further analysis re-

vealed that close relationships existed between

root biomass and the amount of water-stable

aggregates and soil organic matter content, and

that these factors greatly influenced soil ero-

sion. Based on the data generated by the exper i-

ment, equations describing the relationship be-

tw een sediment production at different soil depths

and root biomass were determined.

Keyw ords: Root; Soil Properties; Soil Erosion; Se-

diment Yield; Loess Plateau

1. INTRODUCTION

The Loess Plateau is one of the most eroded areas in

the world, and the resistance of loess to erosion forces

has attracted a great deal of attention from researchers. A

study conducted by Zhu [1] revealed that the low resis-

tance of loess was related to its unique deposition pat-

terns during its formation, and that it showed greatly

improved resistance when vegetation was present. Due

to the differences in soil properties across the profile,

soil erosion exhibits various patterns, and these patterns

tend to be more complex when vegetative coverage and

root systems exist [2-4]. Because numerous studies [5-9]

evaluating the impact of vegetation characteristics on

soil erosion have been conducted, it is often assumed

that all aspects of vegetation have been studied. How-

ever, although many studies have investigated the effects

of plant components such as leaves, stems, organic mat-

ter, roots and exudates, on soil erosion, attention has

predominantly been paid to the effects of the above-

ground biomass on runoff hydraulics and soil erosion

[5,10,11]. Conversely, systematic root studies are lacking,

primarily due to difficulties in direct observation of their

effects [12]. Despite this lack of information, a few stu-

dies have verified that roots played a crucial role with

respect to the effects of rainfall and runoff on soil ero-

sion [3,4,9]. The presence of roots in soil provides me-

chanical reinforcement; therefore, their presence is re-

garded as one of the most important contributions of

vegetation to soil stab ility [11,13,14].

The reinforcement of soil resistance to erosion by

plant roots can be attributed to two aspects. First, roots

and root remnants physically bind soil p articles, forming

mechanical barriers to soil and water movement [15].

Major parameters influencing the mechanical influence

of roots include root diameter, degree of bifurcation,

appearance of root hairs, friction between roots and soil,

and root system distribution [16]. Second, roots and root

remnants excrete binding agents and form a food source

for microorganisms that, in turn, produce other organic

bindings [17,18]. These bindings increase the amount of

stable soil aggregates over the long term, thereby reduc-

P. Li et al. / Agricultur al Sciences 2 (2011) 86-93

ing soil erodibility [19]. Of these two asp ects, the first is

essential with respect to soil erosion by concentrated

flow.

Several publications [3,4] describing the influence of

roots on soil erosion by runoff have emphasized the need

for further research in this area. To determine the effects

of root systems on soil erosion quantitatively and to re-

veal the mechanism of rill and gully development as

well as their relationship to root biomass distribution,

soil properties, and sediment yield, it is necessary to

study these relationships across soil profiles under field

conditions. In this study, ru noff scouring at different soil

depths was conducted to investigate the vertical changes

in soil resistance to runoff erosion forces. In addition,

both vertical root distribution and related soil properties

were analyzed to demonstrate the relationships among

those parameters.

2. MATERIALS AND METHODS

2.1. Site Conditions

The experimental sites are located in Wangdong Wa-

tershed of the Changwu Field Experimental Station of

the Institute of Soil and Water Conservation (ISWC),

Chinese Academy of Sciences (CAS). The elevation of

this area ranges from 950 m to 1225 m. The area is sub-

jected to a warm temperate c ontinental seasonal climate,

with an average annual temperature of 9.1˚C. The annual

rainfall in the area is 584.1 mm, most of which is con-

centrated between July and September. The main soil

type on most sites is loess, with a clay (<0.01 mm) con-

tent of 25%.

The dominant species on most slopes are perennial

herbacious grass species of Stipa bungeana and Bothri-

ochlon ischaemum, with similar coverage and biomass.

Runoff plots on different sites were not differentiated

and were considered to be in th e same condition for run-

off scouring (Table 1).

2.2. Setting-Up Runoff-Scouring Plots at

Different Soil Depths

Zhou Peihu [1] and Jiang Dingsheng [20] developed

runoff scouring in fields in loess regions for evaluation of

soil anti-sco urability. This tec hnique is considered to be a

good technology because there is no disturbance of local

soil, vegetation, or topography. This technology was em-

ployed in our study during the growing season of 2002

with a runoff scouring plot size of 1 × 4 m. Twenty-four

hours after scouring of the soil surface, sub-surface soil of

0~5 cm was removed carefully by hand, and runoff with

the same former runoff discharge was conducted again.

After 24 hours, the next sub-surface layer of soil of 5~10

cm was removed and subjected to similar runoff scouring.

This process was repeated until scouring was conducted at

soil depths of 5, 10, 15, 20, 25, 30, 40, and 50 cm (Figure

1). The runoff volume and sediment yield were measured

each minute during the application of runoff.

2.3. Root Biomass Investigation

The soil drilling method was applied for the root in-

vestigation. For each runoff plot, eight points were dis-

tributed evenly on both sides (Figure 2) of the selected

plot. Root samples from each 10 cm layer were collected

and brought back to the laboratory and dried in an oven

at 85˚C for 24 h to measure the biomass. The biomass

density was calculated using the following equation:

Root density (RD) of certain layer (g/m3) was calcu-

lated as follows:

RD iRh







 (1)

where R = the radius of the soil auger (3.4 cm); h = the

Table 1. General conditions of runoff scouring plots by layers.

Plot

No Runoff discharge

(L/min) Gradient

(º) Cover Upper part bio-

mass (g/m2) Bulk density

(g/cm3) Exposition Position on

slope Location

1 6.5 20 0.86

216.21 1.28 Southern Upper

2 8.5 20 0.81

200.77 1.25 Southern Upper

3 10.5 20 0.82

206.59 1.23 Southern Upper

4 12.5 20 0.81

208.55 1.24 Southern Upper

5 14.5 20 0.85

213.74 1.27 Southern Upper

HBY

6 10.5 25 0.82

212.67 1.27 Southern Middle

7 12.5 25 0.8

209.55 1.21 Southern Middle

8 14.5 25 0.81

209.53 1.29 Southern Middle

TSW

9 10.5 8 0.82 211.69 1.28 Southern Bottom

10 12.5 8 0.82 210.70 1.24 Southern Bottom

YJS

Note: HBY, TSW, YJS indicate the Huangbaiwa, Tongshuwa, and Yuejiashan sites, respec tively.

P. Li et al. / Agricultur al Sciences 2 (2011) 86-93

Figure 1. Cutaway view of runoff

scouring plots on the soil profile.

f r

oot

lin

400cm

Figure 2. Sketch map of root sampl-

ing on runoff scouring plot.

thickness of the soil layer (10 cm); m = the root weight;

and i = the number of sampling points

Soil properties related to soil erosion such as the soil

organic matter content, water-stable aggregate content

and soil bulk density were also measured.

3. RESULTS AND DIS CUS SIONS

3.1. Vertical Root Biomass and Soil

Properties

The vertical root biomass distributions on the three

sites showed a similar decrease as soil depth increased

(Figure 3). Root biomass was concentrated in the sur-

face soil, after which it decreased to less than 0.2 kg/m3

in soil below a depth of 40 cm. There was no significant

difference in the root biomass at the same soil depth at

different sites, indicating that root distribution was uni-

form at the same depth among sampling points. There-

fore, the experimental conditions can be considered to be

the same for all sites. The distribution patterns of th e soil

organic matter content were also similar at the three sites

(Figure 4). The soil organic matter content decreased as

the soil depth increased, and there was no obvious dif-

ference in soil organic matter content in deep soil when

compared at the same soil depth among sites.

As shown in Figure 5, the results of soil water-stable

aggregates indicate that the aggregate content decreased

as the soil depth increased. In addition, the aggregate

content was almost the same at all three sites. Analysis

0.2

0.4

0.6

0.8

1.2

soil de pt h ( c m)

root biomass (kg/m3)

HBY

YJS

TSW

Figure 3. Root biomass distribution in natural

grassland and abandoned lands of different years.

soil depth (cm)

SOM (g/kg)

HBY

TSW

YJS

Figure 4. Vertical soil organic matter distribution

patterns.

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

020406080100120

soil de pth ( cm)

aggregate content (%)

HBY

TSW

YJS

Figure 5. Vertical distribution patterns of soil aggregate

content.

of the aggregate class distribution (Figure 6) revealed

that there was little difference in content among small

diameter classes of aggregates, but that large differences

existed among aggregates with large diameter classes

(2～5 mm and 1~2 mm). Soil bulk density, which is an

important factor influencing soil erosion, was measured,

and the results showed that soil bulk density varied

slightly as the soil depth increased (Figure 7). However,

the soil bulk density in the surface soil was almost the

same (1.3 g/cm3) at all three sites.

20 cm 30 cm

50 cm

Point of root sampling

root biomass (kg/m3)

soil depth (cm)

aggregate content(%)

400 cm

P. Li et al. / Agricultur al Sciences 2 (2011) 86-93

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

020406080100 120

100

200

300

400

500

600

700

800

900

0 24 6 810121416

5c m

15c m

40c m

50c m

sediment yi eld (g/min)

Openly accessible at

soil de pth (cm)

aggregate co ntent (%)

>5mm

5-2mm

2-1mm

1-0.5mm

0.5- 0.25mm

Figure 6. Vertical distribution patterns of soil aggregate

content in different classes on HBY.

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

0 20406080100120

soil depth (cm)

bulk density (g/cm

)

HBY

TSW

YJS

Figure 7. Vertical distribution patterns of soil b ulk density.

3.2. Sediment Yield Processes at Different

Soil Depths under Different Runoff

Discharges and Slope Gradients

To avoid confusion, only sediment yield processes at

soil depths of 5 cm, 15 cm, 40 cm, and 50 cm are illus-

trated in the figures. As shown in Figure 8, the sediment

yield decreased with time at all sites and tended to be

stabilized two minutes after beginning the experiment.

The high sediment yield in the beginning was the result

of soil disturbance caused by soil division in the experi-

ment.

Sediment yield processes under different runoff dis-

charges at different sites demonstrated similar trends

(Figure 8 and Table 2), and the sediment yield increased

as soil depth increased at all three sites. Specifically,

sediment yield on surface soil was similar at these sites.

However, the yield increased sharply at soil depths over

30 cm, and this change was apparently closely related to

runoff discharge and slope gradient. Considering the root

distribution pattern, it can be concluded that, on surface

soil layer, the effect of the root system on sediment yield

was dominant, but that this effect decreased as the soil

depth increased. When the soil depth was greater than 40

time (min)

60c m

HBY—(10.5 L/min)

100

150

200

250

300

350

400

450

500

024681

time (min)0121416

sediment yield (g/min)

5c m

15c m

40c m

50c m

YJS—(10.5 L/min)

100

200

300

400

500

600

700

800

0246810121416

ti me (min)

sediment yield (g/min)

5cm

15cm

40cm

50cm

TSW(25˚)—(10.5 L/min)

Figure 8. Sediment yield process at different depth and sites.

cm, the effect of the root system on soil properties, and

consequently on sediment yield, was especially limited

due to its reduced distribution in deeper soil. As a resu lt,

sediment yield tended to increase as the runoff discharge

and slope gradient increased. Fluctuations in sediment

yield can be interpreted by the existence of animal holes

and biopores of roots and their r emnants.

During the runoff scouring, changes in th e so il ero sion

forms were focused on. Sheet erosion occurred on the

surface soil layer; however, as the soil depth increased,

soil erosion holes appeared on the slope, and their size

and number increased with depth. At a depth of 40 ~ 50

cm, these erosion holes tended to be connected, and

sheet erosion tended to transform into rill erosion, indi-

＞5 mm

5 - 2 mm

2 - 1 mm

1 - 0.5 mm

0.5 - 0.25 mm

soil depth (cm)

5 cm

15 cm

40 cm

50 cm

sediment yield (g/min)

60 cm

5 cm

15 cm

40 cm

50 cm

time (min)

sediment yield (g/min)

time (min)

5 cm

15 cm

40 cm

50 cm

sediment yield (g/min)

time (min)

P. Li et al. / Agricultur al Sciences 2 (2011) 86-93

Table 2. Vertical sediment production on different slopes and in response to different runoff discharge.

Soil depth (cm)

Location Runoff

discharge

(L/min) 0-5 5-10 10-15 15-20 20-25 25-30 30-40 40-50 50-60

12.5 13.14 25.06 31.68 - 37.73 37.84 46.00 59.16 -

YJS 8º 10.5 11.27 20.81 23.87 - 34.73 34.05 38.90 50.84 -

6.5 19.48 18.23 24.81 25.27 25.73 28.14 33.48 62.90 -

8.5 14.24 20.67 12.46 18.13 23.81 16.80 65.71 111.53 -

10.5 16.47 25.17 21.00 36.86 23.24 65.06 84.91 129.91 156.19

12.5 22.47 27.25 31.94 69.32 53.99 80.94 161.42 255.79 -

HBY 20º

14.5 19.20 24.09 25.28 26.46 40.08 114.61177.14 271.77 -

10.5 15.91 19.57 22.50 39.67 111.83 118.42134.43 143.03 -

12.5 22.30 29.67 39.86 71.33 98.93 130.44135.39 149.51 -

TSW 25º

14.5 18.03 25.18 45.39 80.33 107.62 133.19151.42 179.71 -

Note: There was no runoff scouri ng on the soil layers with “-“due to poor control when separating layers.

cating a decrease in the resistance of soil to runoff

scouring. Notably, there were some very fine roots

present in these erosion holes, suggesting that not all

roots improve soil resistance to runoff scouring.

3.3. Relationship between Root Biomass

and Soil Properties

As main bridges for communication between mate-

rial and energy, ecological and physiological features

of the root system had a deep impact on amelioration

and improvement of soil properties, especially those

properties related to soil erosion. According to some

former studies, the main soil properties related to ero-

sion, including soil organic matter and water stable

aggregate content, were closely related to the distribu-

tion of root systems. In this study, the relationships

among root systems, soil organic matter and aggregate

content were established based on experimental data.

As shown in Figure 9, as the root biomass in-

creased, the soil organic matter content or content of

soil aggregates (1 ~ 2 mm) increased logarithmically.

The large data error that was observed may indicate

that these properties in surface soil were greatly in-

fluenced by human activities, such as fire, grazing,

trampling, etc., as well as by natural factors such as

rainfall, freezing and thawing. The relationship be-

tween root biomass and soil properties in deep soil

layers tended to be more significant, especially in

areas in which the root density was lower than 0.2

g/cm3. Furthermore, the relationship between root and

soil properties was not linear, indicating that the exis-

tence of root biomass could only improve soil to a

certain level.

3.4. Relationship between Sediment Yield,

Root Biomass and Soil Properties

The improvement of soil properties and soil resis-

tance to runoff can be attributed to a well-developed

root system. As shown in Figures 10-12, there were

close relationships between sediment yield and root

biomass, soil organic matter content or content of

water-stable aggregates. Thus, further consideration

must be given to selecting indexes to reflect the ef-

fects of vegetation on soil erosion.

As the main source of energy and material in soil,

the root system radically reflects the improvement of

soil properties by vegetation, especially which of

herbaceous vegetation with root systems primarily

composed of fine roots. Those roots are more easily

transformed into soil organic matter due to their

shorter fibers; therefore, it is rational to select root

biomass as the main index to reflect the effects of

vegetation on sediment yield. As shown in Figure 11,

the sediment yield per unit of runoff discharge de-

creases as the root biomass density increases, indicat-

ing that root biomass is effective at improving soil

resistance to soil erosion forces. Following data anal-

ysis, the following equation describing the relation-

ship between sediment production at different soil

depths and root biomass was determined:

Yab x



where Y is the sediment yield at different soil depths,

x is the root biomass, a and b are constants. The si-

mulated results (Table 3) indicated that this fitting

equation reflected the relationship between the root

system and sediment yield well under different runoff

discharges. As shown in Table 3, it is clear that the

value of a and b decreased as the runoff discharge

increased, implying that their reciprocal values may

be related to soil erodibility.

When the root biomass density was greater than 0.2

kg/m3, the effects of root biomass on sediment yield

reduction were remarkable. In addition, when the root

P. Li et al. / Agricultur al Sciences 2 (2011) 86-93

y = 1.8489Ln(x) + 11.905

2 = 0.6451

00.20.4 0.6 0.8

oot biomass(kg/m3

SOM (%)

y = 0.9362Ln(x) + 4.775

R2 = 0.7011

00.20.4 0.6 0.8

root biomass(kg/m3)

soil aggregate 1<D<2mm (%)

Figure 9. Relationship between root biomass and

soil properties.

0.005

0.01

0.015

0.02

0.025

0.03

00.2 0.4 0.6 0.8

root biomass (kg/m3)

sediment yild (kg)

6.5l/min

8.5l/min

10.5l/min

12.5l/min

14.5l/min

Figure 10. Relationship between sediment yield and root

biomass.

0. 00 5

0.01

0. 01 5

0.02

0. 02 5

0.03

024

water s t abl e ag gregate (%)

sediment yield (kg)

6. 5l/mi

8. 5l/min

10. 5 l/min

12. 5 l/min

14. 5 l/min

Figure 11. Relationship between sediment yield and water

stable aggregate content.

0.005

0.01

0.015

0.02

0.025

0.03

46810121

SOM (%)

sediment yield (kg)

6.5l/min

8.5l/min

10.5l/min

12.5l/min

14.5l/min

Figure 12. Relationship between sediment yield and

soil organic matter conte n t.

0.0005

0.001

0.0015

0.002

0.0025

00.2 0.4 0.6 0.8

ro ot bio ma ss density (kg/m3)

sedim ent yield per r uno ff dis charge

Figure 13. Relationship between root biomass and

sediment yield per unit of runoff discharge.

Table3. Determination of the constants of a and b in the

equation.

6.5 L/min8.5 L/m in 10.5 L/m i n 12.5 L/min14.5 L/min

a 547.8126411.9401 338.4305 226.8418200.5907

b 125.8726 92.8135 78.6954 53.084849.8790

Standard Error0.0005 0.0005 0.0006 0.0008 0.0009

Coefficient 0.9817 0.9893 0.9956 0.9933 0.9958

biomass distribution was below 0.2 kg/m3, there was a

linear relationship between root biomass and sediment

yield.

Selection of appropriate root indexes has long been

the subject of debate. In studies related to root physi-

ology and ecology, root length and surface area have

been chosen as indexes because they are directly re-

lated to the area in which the root and soil environ-

ment touch and they reflect the physiological func-

tions of the root in the processes of vegetation im-

provement. The results of the present stud y imply that

there is an intrinsic relationship among the root, soil

organic matter and water-stable aggregates, and that

root biomass turnover is the main reason for the im-

provement of soil organic and water-stable aggregates.

Therefore, these findings indicate that root biomass is

a better index than other root indexes in studies re-

lated to soil erosion.

y = 1.8489 Ln( x) + 11.905

R2 = 0.6451

root biomass (kg /m3)

y = 0.9362 Ln( x ) + 4.7754

R2 = 0.7011

soil agg

egate 1 ＜ D ＜ 2 mm (%)

root biomass density(kg/m3)

root biomass (kg /m3)

P. Li et al. / Agricultur al Sciences 2 (2011) 86-93

In addition, although there were herbaceous grass

roots distributed in deep soil, there were also small,

fine roots in the erosion holes. These findings indicate

that not all the roots effectively improve soil resis-

tance to erosion. Using root biomass as an index for

describing the relationship between vegetation and

erosion not only includes non-significant factors such

as fine roots, but, more importantly, may undermine

the significance of root length and surface area.

4. CONCLUSIONS

Many publications have discussed the impact of

vegetation on soil erosion and reported that the influ-

ence of plants is mainly attributed to the aboveground

biomass. Indeed, the importance of belowground bio-

mass with respect to soil erosion by water can easily

be neglected. In this study, systematic investigations

of the root distribution, soil properties, and sediment

yield were conducted to reveal the relationships

among these factors. Based on the experiments con-

ducted in the field, the following conclusions can be

reached.

Runoff scouring of different soil depth indicated

that, because of the difference in root biomass distri-

bution in the soil profile, there was a remarkable dif-

ference in sediment yield, which emphasized the im-

portance of reinforcement of roots in soil.

There was also a close relationship between root

biomass and soil properties, including soil organic

matter content and content of water stable aggregates.

As the main source of material and energy, root bio-

mass was selected as the main index to determine the

effects of vegetation on sediment yield. The sediment

yield by unit runoff discharge decreased as the root

biomass increased. A root density of 0.2 g/m3 was

found to be the critical value for the sediment yield

patterns.

5. ACKNOWLEDGEMENTS

This article was financially supported by the national natural

scientific foundation (No: 41071182). In addition, the authors

would like to express their appreciation to the staff of Mr. Shen

Mingyun and Mr. Zheng Liangyong for their useful suggestions and

technical assistance during the study.

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