The effects of narrow grass hedges on soil and water loss on sloping lands with alfalfa (Medicago sativa L.) in Northern China

Geoderma 167-168 (2011) 91–102

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The effects of narrow grass hedges on soil and water loss on sloping lands with alfalfa (Medicago sativa L.) in Northern China Bo Xiao a, b,⁎, Qing-hai Wang a, Hui-fang Wang b, Quan-hou Dai c, Ju-ying Wu a a b c

Research & Development Center for Grasses and Environment, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, PR China State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, Yangling, Shaanxi 712100, PR China College of Forest, Guizhou University, Guiyang, Guizhou 550025, PR China

a r t i c l e

i n f o

Article history: Received 14 September 2010 Received in revised form 7 September 2011 Accepted 16 September 2011 Available online 2 November 2011 Keywords: Arundinella hedges Pennisetum hedges Grass strips Soil and water conservation Loess Plateau of China

a b s t r a c t Grass hedges provide an efficient way to prevent soil and water loss on sloping croplands in numerous countries throughout tropical and subtropical regions. However, the effects of grass hedges on sloping land are still not well understood in temperate regions where there is severe soil and water loss. Therefore, the soil and water conservation benefits of two native grass hedges, Pennisetum alopecuroide (Pennisetum alopecuroides (Linn.) Spreng.) and Arundinella hirta (Arundinella hirta (Thunb.) C. Tanaka), were investigated using simulated rainfall with different slope gradients (5, 10, 15 and 20%), rainfall intensities (14, 22, 36 and 63 mm h− 1), and coverage of alfalfa (Medicago sativa L.) on sloping lands of northern China. The results showed that Pennisetum hedges were more efficient in controlling soil and water loss compared with Arundinella hedges, possibly due to the more numerous, finer, and deeper roots in Pennisetum hedges. Overland flow and soil loss in sloping cropland were decreased by 52% and 75%, respectively, using Pennisetum hedges and 29% and 52%, respectively, using Arundinella hedges. The contribution of independent variables to overland flow followed the order rainfall intensityN coverage of alfalfa N type of grass hedges N slope gradient. The contribution to soil loss followed the order slope gradientN types of grass hedgesN coverage of alfalfa N overland flow N rainfall intensity. Tillering ability and root characteristics should be considered in the selection of grass species when designing grass hedges. These findings may be helpful in the remediation of serious soil and water losses on sloping croplands in northern China and similar regions. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Soil and water losses are a major environmental problem that is attracting widespread attention in China because almost one-third of the country's land is affected by excessive soil erosion and runoff. The most serious problems occur not only in the southern region, which has abundant precipitation, but also in the northern region, mainly due to drought followed by concentrated rainfalls, sparse vegetation, loose soils, complex landforms and long-term improper land use (Cha and Tang, 2000; Shi and Shao, 2000). In northern China, the Loess Plateau is well known worldwide for its incredible erosion, which amounts to 15 000–20 000 t km − 2 a − 1 (Cha and Tang, 2000). The large amount of sediment leads to very serious damage to the middle and lower reaches of the Yellow River (Milliman et al., 1987; Saito et al., 2001). According to Xu et al. (2004) and Bennett (2008), 28% of the soil loss came from sloping croplands that accounted for only 7% of the area. In these sloping croplands, 30% had a slope ⁎ Corresponding author at: Research & Development Center for Grasses and Environment, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, PR China. Tel.: +86 010 51503436; fax: +86 010 51503297. E-mail address: [email protected] (B. Xiao). 0016-7061/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2011.09.010

ranging from 8% to 25%, and nearly 5% of them exceeded 25% (Sun et al., 2006). Among the various remedies tested so far, natural engineering solutions including filtering strips are preferred because of their low cost, eco-friendliness and efficiency. Narrow grass hedges, a special type of vegetative filter strips defined as dense and erect vegetation barriers made of stiff-stemmed grass that slows down runoff and reduces erosion (Dabney et al., 1999; Kemper et al., 1992), are commonly used in preventing soil and water loss on sloping croplands all over the world (Cullum et al., 2007; Dalton et al., 1996; Ghadiri et al., 2001; Gilley et al., 2000; Golabi et al., 2005; Kemper et al., 1992; Lin et al., 2009; McGregor et al., 1999; Raffaelle et al., 1997). It has been reported that grass hedges have the potential to reduce runoff by up to 60% and soil loss by up to 80% through filtration, deposition and infiltration (Bhattarai et al., 2009; Gilley et al., 2000; Koelsch et al., 2006). However, the efficiency of grass hedges is site specific and depends mostly on the slope gradient; runoff volume, flow rate, size and density of sediment particles; grass species; density, interval and width of the grass strips; the properties of the underlying soil (mainly soil infiltrability); and even rainfall characteristics (intensity and duration) (Deletic, 2001; Gilley et al., 2000; Robinson et al., 1996). Some studies have indicated that slope gradient and rainfall intensity are the most important factors

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Table 1 Planting details and cultural practices in the experimental plots. Item

Content

Crop Seeding date Seeding rate (kg ha− 1) Seeding method Row spacing (cm) Tillage

Alfalfa (Medicago sativa L.) June 10, 2007 37.5 Strip seeding 40.0 30-cm plowing before seeding with conventional tillage method Hand weeding

Weeding

that significantly affect the effectiveness of grass hedges on a given site (Huang et al., 2010; Sun et al., 2008; Xiao et al., 2010; Yuan et al., 2009). For example, McGregor et al. (1999) found that stiff grass (Miscanthus sinensis) hedges decreased runoff by 5% and soil loss by 75% on plots with 5% slope gradients under natural rainfall; Kim et al. (2008) recorded that switchgrass (Panicum virgatum L.) hedges decreased runoff by 62% and soil loss by 76% on plots with 6.5% slope gradients under natural rainfall; Gilley et al. (2000) reported that switchgrass hedges decreased runoff by 41% and soil loss by 63% on plots with 12% slope gradients and 64 mm h − 1 rainfall

intensities; Raffaelle et al. (1997) documented that volunteer grass (predominantly bermuda grass) hedges decreased soil loss by 63% on plots with 10% slope gradient and 64 mm h − 1 rainfall intensity. Although these studies evaluated the effectiveness of grass hedges at different slope gradients and/or rainfall intensities, systematic research is still lacking, and the influence of slope gradient and rainfall intensity remains unclear. In previous research, many grasses including vetiver grass (Vetiveria zizanioides (L.) Nash), Napier grass (Pennisetum alopecuroides (Linn.) Spreng.), switch grass, tall fescue (Festuca arundinacea Schreb.), and eastern gamagrass (Tripsacum dactyloides (L.) L.) were demonstrated to be very effective in reducing soil and water loss (Angima et al., 2002; Babalola et al., 2007; Dabney et al., 1995; Dercon et al., 2006; Magette et al., 1989; Pansak et al., 2008). Presently, vetiver grass is recommended in numerous countries throughout tropical and subtropical regions due to its unique characteristics such as fast growth, deep and penetrating root system, and high tolerance to adverse conditions (World Bank, 1993). However, vetiver grass should not be used in temperate regions, e.g., in northern of China, because it cannot withstand the low temperatures in winter (the temperature in such regions can be as low as −30 °C, whereas the lowest temperature at which vetiver grass can survive is −9 °C (World Bank, 1993; Wu et al., 2008)). In

(a) 20% slope gradient

15% slope gradient

10% slope gradient

5% slope gradient

5.5 m

T2 T1 CK T1 CK T2 T1 T2 CK T1 T2 CK T2 T1 CK T2 CK T1 CK T2 T1 T2 CK T1 CK T2 T1 T2 CK T1 CK T2 T1 T2 T1 CK

12 m Grass hedges 0.5 m wide

1.5 m

PROS 17

Water pipe

PROS 15

Pump 1# Valve

Water container

Runoff and sediment collector Pump 2# Valve

(b)

Fig. 1. Rainfall simulator and experimental plots: (a) experimental design and rainfall simulator, T1 = Arundinella hedges, T2 = Pennisetum hedges, and CK = control; (b) overview of experimental plots.

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Table 2 Overland flow (mm) in plots with Arundinella hedges, Pennisetum hedges and the control in trial a (21% coverage of alfalfa). Slope gradient 5%

10%

15%

20%

Treatment

22 mm h− 1 Dry run

Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum

a⁎

0.80 0.39b 0.23b 0.92a 0.60b 0.23c 1.44a 1.36a 0.29b 3.56a 2.40b 0.35c

36 mm h− 1 Wet run a

1.53 0.57b 0.30b 1.38a 1.25a 0.36b 2.77a 1.51b 0.35c 5.66a 3.73b 0.92c

Very wet run a

1.69 1.26a 0.53b 2.41a 2.26a 1.28a 4.90a 4.61a 0.50b 6.35a 5.37ab 2.25b

Dry run a

63 mm h− 1 Wet run a

6.38 2.97b 1.05b 6.43a 5.43a 2.91a 8.21a 8.15a 2.07a 13.64a 10.81a 5.08b

10.04 5.19a 3.69a 9.62a 9.56a 4.55a 12.75a 12.10a 4.84b 17.00a 15.29a 12.28a

Very wet run a

11.10 10.72a 5.82a 13.99a 12.40a 7.20a 17.41a 14.88a 7.26b 20.55a 19.84a 12.93b

Dry run a

20.01 12.90ab 8.49b 21.08a 13.22b 8.09b 24.61a 17.30ab 10.80b 23.20a 19.13ab 14.76b

Wet run a

20.55 15.44a 16.92a 22.14a 16.77ab 10.51b 26.18a 18.60ab 13.58b 27.94a 21.07ab 17.71b

Very wet run 26.74a 18.74b 15.44b 32.41a 24.24b 19.84c 35.07a 24.53b 21.61b 35.42a 28.34a 25.33a

⁎In tables 2 to 7, for each slope gradient and within the same columns, the means followed by a different letter are significantly different at the 5% probability level.

temperate regions, some native grass species should be used in grass hedges, and comparative studies should be conducted to evaluate their effectiveness in controlling soil and water loss. According to Wu et al. (2008), Pennisetum (P. alopecuroides (Linn.) Spreng.) and Arundinella (Arundinella hirta (Thunb.) C. Tanaka) are probably the most promising candidates given that there are native perennials, tolerant to the local climate extremes (winter droughts, freezing temperatures), and possess sufficient stem strength to remain erect against flowing water. However, the influences of such native species on soil and water loss on sloping cropland have yet to be determined and should be well understood in order to correctly assess the ecological and environmental effects of such grasses. The objectives of this study were to (1) evaluate the efficiency of Pennisetum and Arundinella grass hedges in reducing overland flow and soil loss; (2) quantify the correlations among dependent variables (overland flow, soil loss) and independent variables including type of grass hedges, slope gradient, rainfall intensity, and extent of soil covers; and (3) analyze the preliminary mechanism of grass hedge root systems in reducing soil and water loss. 2. Materials and methods 2.1. Study area The experiments were conducted at the National Experimental Station for Precision Agriculture in Xiaotangshan, north of Beijing (116°26′ E, 40°10′ N), China. The study area was located in the North China Plain, which is characterized by a continental, semi-humid, monsoon climate in the temperate zone. The average frost-free growing season extends for approximately 190 days from mid-April to midOctober. The dominant crops are maize (Zea mays L.), soybeans (Glycine max (L.) Merr.), and winter wheat (Triticum aestivum L.). The mean

annual precipitation is 640 mm (80% occurring between June and August), and the mean annual temperature is 11.5 °C. The soil is loamy clay in texture with 41.4% sand, 24.2% silt and 34.4% clay. The mean constant infiltration rate in the study area was approximately 13.0 mm h− 1, and the bulk density was 1.37 g cm− 3. The organic matter, total nitrogen and phosphorus for the top 10 cm soils were 14.03 g kg− 1, 2.46 g kg− 1 and 0.63 g kg− 1, respectively. 2.2. Experimental design and measurement The experiment was based on three independent variables: grass hedges (Arundinella hedges, Pennisetum hedges, and without grass hedges as control); slope gradients (5, 10, 15, and 20%); and rainfall intensities (22, 36, and 63 mm h− 1). Thirty-six hydrologically isolated plots (1.6 m by 11 m) were established in June 2005 in an experimental design with three grass hedge levels and four slope gradients in three replications The plots were equipped with a rainfall simulator capable of rainfall intensities from 10 to 100 mm h− 1 (Xianfei Agricultural Engineering High-Tech Co., Beijing, China). Two types of nozzles (PROS-17 and PROS-15, Hunter Company, USA) were used in the simulator, the working pressures were 0.16–0.30 MPa and the drop size of the rainfall was 0.7–5.0 mm. According to the Christiansen method (Mateos et al., 1997), the rainfall coefficient of uniformity of the rainfall simulator was about 86%. Grasses were planted in double rows (0.5-m row spacing and 0.1-m plant spacing) using multiple shoots (2–3 shoots); the hedges were located at the middle and at the lower edge of the plots. The grass hedges were oriented along contour lines, which are perpendicular to the slope direction. Apart from these grasses, alfalfa (Medicago sativa L.) was planted in 2007. Details on the cultural practices of alfalfa are shown in Table 1. The experimental design, setup of the rainfall simulator, and overview of the experimental plots are shown in Fig. 1.

Table 3 Overland flow (mm) in plots with Arundinella hedges, Pennisetum hedges and the control in trial b (38% coverage of alfalfa). Slope gradient

Treatment

5%

Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum

10%

15%

20%

22 mm h− 1

36 mm h− 1

63 mm h− 1

Dry run

Wet run

Very wet run

Dry run

Wet run

Very wet run

Dry run

Wet run

Very wet run

0.41a 0.23b 0.10b 0.61a 0.26b 0.13b 1.47a 1.89ab 0.25b 2.84a 1.54b 0.28c

0.83a 0.28b 0.17b 0.94a 0.50b 0.34c 1.34a 1.34a 0.43b 5.31a 3.42ab 1.92b

1.34a 0.67b 0.36b 1.79a 1.37a 0.64b 3.31a 2.62a 0.43b 6.29a 4.18b 1.81c

1.61a 1.05b 0.12c 2.25a 2.09a 0.18b 5.37a 3.94a 0.77b 7.69a 8.15a 2.90b

5.88a 3.77a 0.64b 6.45a 3.26b 1.12c 11.75a 12.46a 4.07b 14.52a 12.40a 4.84b

12.40a 7.20b 2.21c 11.86a 10.45a 5.49a 18.95a 17.24a 7.38b 19.48a 18.24a 10.27b

11.63a 5.24a 3.68a 10.92a 5.31a 3.21a 13.70a 11.57a 6.43a 22.32a 17.54a 15.25a

15.65a 16.26a 3.07b 22.50a 13.28b 10.16b 21.43a 17.95a 15.11a 28.87a 20.72a 19.84a

20.37a 17.00ab 9.12b 26.21a 20.55ab 14.35b 29.22a 22.87ab 18.24b 33.12a 26.04a 23.55a

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Table 4 Overland flow (mm) in plots with Arundinella hedges, Pennisetum hedges and the control in trial c (71% coverage of alfalfa). Slope gradient 5%

10%

15%

20%

Treatment

22 mm h− 1 Dry run

Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum

a

0.26 0.00a 0.00a 0.25a 0.00a 0.00a 0.33a 0.04b 0.01b 2.72a 0.20b 0.00b

36 mm h− 1 Wet run a

0.18 0.00a 0.00a 0.40a 0.00a 0.00a 0.76a 0.19b 0.02b 2.97a 0.25b 0.00b

Very wet run a

0.44 0.00a 0.00a 0.97a 0.35a 0.03b 1.10a 0.29ab 0.05b 3.34a 0.42b 0.08b

Dry run a⁎

0.74 0.00a 0.00a 1.60a 0.00b 0.00b 2.69a 0.56b 0.00b 6.97a 0.74b 0.18b

The simulated rainfall experiments were conducted from 2007 to 2008. In this period, three experiments (including three rainfall intensities) were implemented at different growth times of the alfalfa: August 28, 2007 labeled as trial a; October 17, 2007 labeled as trial b; and July 8, 2008 labeled as trial c. The coverage of alfalfa measured by the First Growth Cover Measurements (Decagon Devices, Inc., USA) method were 20.8±2.2%, 37.6±3.1%, and 71.2±2.2% (n =36) for trials a, b, and c, respectively. During the experiments, the stem densities of the

63 mm h− 1 Wet run a

1.33 0.00a 0.00a 1.56a 0.00b 0.00b 2.92a 0.56b 0.12b 7.23a 0.65b 0.18b

Very wet run a

2.33 0.00a 0.00a 2.57a 1.54a 0.24a 4.49a 1.89ab 0.15b 9.12a 1.77b 0.32b

Dry run a

4.72 1.03a 0.71a 6.88a 1.59a 0.71b 7.47a 2.83a 1.62a 9.09a 2.69b 2.25b

Wet run a

13.58 4.58ab 2.60b 13.87a 6.26b 4.96b 15.70a 8.56a 7.56a 27.81a 13.22ab 8.68b

Very wet run 15.00a 5.96a 4.28a 15.76a 7.82a 5.43a 16.44a 10.80a 11.22a 29.58a 17.36ab 12.16b

Pennisetum and Arundinella grass hedges were 1275 ±64 stems m− 2 (n =12) and 1185±59 stems m− 2 (n= 12), respectively. For each rainfall intensity experiment, the simulations were repeated under three antecedent soil moisture conditions according to the standard procedure: an initial one-hour rainfall simulation at the low soil moisture condition (“dry run”) was followed by a second one-hour rainfall approximately 24 h later (“wet run”) and a third one-hour rainfall 1 h later (“very wet run”). The rainfall intensity

Fig. 2. Overland flow increase with slope gradient in plots with Arundinella hedges, Pennisetum hedges and control: (a) trial a with 21% coverage of alfalfa; (b) trial b with 38% coverage of alfalfa; (c) trial c with 71% coverage of alfalfa. The errors bars are added at the 5% probability level.

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Fig. 3. Overland flow increase with rainfall intensity in plots with Arundinella hedges, Pennisetum hedges and control: (a) trial a with 21% coverage of alfalfa; (b) trial b with 38% coverage of alfalfa; (c) trial c with 71% coverage of alfalfa. The errors bars are added at the 5% probability level.

system, WinRHIZO (Regent Instruments Inc., Canada), to describe the spatial distribution and characteristics of the grass roots.

and uniformity were monitored through rain gages, and overland flow was collected in containers located at the bottom of each plot. Approximately 1000 mL of runoff with sediment was sampled from each plot after thoroughly stirring, and the sediment was separated and dried at 105 °C. The total soil loss was calculated by multiplying the sediment content in the sample by the collected overland flow. When the simulated rainfall experiments were completed, the roots in a 10 cm × 10 cm section around the grass hedges were dug to a 10-cm depth, washed out, and then analyzed using a root analysis

2.3. Data analysis During the dry runs, the runoff was minimal or even nonexistent. For this reason, the statistical analysis was based only on data obtained during the wet and very wet runs according to the standard procedures. However, the experimental data collected in the dry runs

Table 5 Soil loss (kg ha− 1) in plots with Arundinella hedges, Pennisetum hedges and the control in trial a (21% coverage of alfalfa). Slope gradient 5%

10%

15%

20%

Treatment

22 mm h− 1 Dry run

Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum

a

6.86 2.20b 1.18b 15.23a 4.93b 1.78b 68.00a 51.60ab 10.95b 431.98a 216.61ab 29.41b

36 mm h− 1 Wet run a

14.15 8.53b 2.36c 73.81a 33.92b 4.10c 120.70a 72.87b 25.36c 504.87a 369.95a 41.18a

Very wet run a

17.32 10.64b 3.87c 123.40a 61.88b 17.30c 180.22a 156.65a 49.77b 601.94a 389.13a 98.80a

Dry run

63 mm h− 1 Wet run

a

27.96 6.45ab 1.88b 33.35a 20.88b 4.35c 591.26a 264.32ab 14.44b 1292.17a 1198.09a 138.41b

a

77.46 23.94ab 8.43b 373.55a 56.41a 10.52a 817.69a 539.97b 138.27c 1596.97a 1079.64b 584.95b

Very wet run a

179.34 58.64ab 18.62b 995.12a 674.51a 152.14a 1347.63a 985.80b 390.07c 2434.01a 1580.89a 1290.80a

Dry run

Wet run a

305.64 70.36b 47.07b 788.36a 276.64ab 73.25b 1969.25a 526.72b 138.26b 7224.96a 2084.91b 271.53b

a

1263.76 113.07a 52.92a 1281.36a 615.30a 138.08a 2106.50a 641.32b 154.72b 6649.75a 3780.70b 2370.73b

Very wet run 1458.15a 136.48a 61.38a 1682.27a 598.90a 240.08a 3495.24a 913.62b 248.34b 7436.15a 4604.32ab 2664.05b

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Table 6 Soil loss (kg ha− 1) in plots with Arundinella hedges, Pennisetum hedges and the control in trial b (38% coverage of alfalfa). Slope gradient 5%

10%

15%

20%

Treatment

22 mm h− 1 Dry run

Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum

a

1.57 0.72ab 0.29b 2.12a 0.54b 0.47b 3.42a 2.87a 2.12a 14.70a 10.62a 1.96a

36 mm h− 1 Wet run a

2.18 2.03a 1.01a 2.61a 2.00a 0.63b 5.40a 4.05a 2.02a 22.50a 15.24ab 3.43b

Very wet run a

3.88 3.66ab 2.32b 5.09a 3.69ab 1.99b 7.29a 5.52a 3.34a 48.07a 17.53a 4.20a

Dry run

Wet run

a

a

14.59 4.82ab 1.82b 24.24a 13.14a 2.06a 36.99a 17.62b 7.49b 183.56a 73.40ab 19.66b

were also presented because they may provide additional useful information. The experimental data were analyzed using analysis of variance (ANOVA) and multiple comparisons (Duncan test) with SAS 8.1, and the results were expressed as mean ± standard error. Correlation analysis and optimal scaling in regression analysis were conducted in SPSS 15.0 to determine the correlations between overland flow, soil loss and related factors including grass hedges (defined as an unordered categorical variable), slope gradient, rainfall intensity and coverage of alfalfa. The representation and graphical fits of the experimental data were obtained using OriginPro 8.0. 3. Results 3.1. The reducing effects of grass hedges on water loss The overland flows in the plots with the Arundinella hedges, Pennisetum hedges, or without hedges during trials a, b, and c are presented in Tables 2, 3 and 4, respectively. The control plots without grass hedges had the highest overland flow, followed by the plots protected with Arundinella and Pennisetum hedges for all rainfall intensities (22–63 mm h − 1) and slope gradients (5–20%), suggesting that grass hedges have remarkable reducing effects on overland flow. The reductions in overland flow due to the grass hedges ranged between 7% and 100% depending on rainfall intensity, slope gradient, grass hedges and alfalfa coverage. The results of the statistical analysis (ANOVA) showed that in most cases the differences between the overland flow in Pennisetum protected plots and the control were significant (P b 0.05). However, the differences between the overland flow in the Arundinella-protected plots and the control were not significant (P N 0.05) in most cases. To evaluate changes in the efficiency of the grass hedges on reducing overland flow with slope gradient, the overland flow values under

63 mm h− 1

33.95 13.97a 6.45a 32.29a 15.51b 3.98c 163.85a 27.98ab 15.48b 389.60a 108.59b 84.17b

Very wet run a

43.58 24.62a 11.94a 130.87a 71.79ab 14.88b 258.65a 99.15ab 22.85b 846.99a 342.58b 125.26b

Dry run a

52.86 23.82a 9.82a 71.63a 32.64ab 9.89b 170.05a 58.94ab 29.48b 735.69a 401.41a 232.65a

Wet run a

44.23 35.57a 12.00b 134.08a 62.09b 30.81b 291.03a 89.68b 31.37b 1226.74a 546.87a 512.75a

Very wet run 124.43a 79.26ab 18.17b 197.80a 86.10b 44.74b 661.97a 182.44b 80.71b 1615.68a 764.67ab 351.30b

two of the three rainfall intensities (the dry run was excluded) were summed for each treatment and each slope gradient (Fig. 2). The overland flow in protected and unprotected plots increased almost linearly with slope gradients. Compared with the control, the decreases in overland flow with Arundinella grass hedges for the 5, 10, 15 and 20% slope gradients were respectively 28, 19, 23, and 17% in trial a; 20, 29, 13, and 21% in trial b; and 68, 55, 46, and 58% in trial c. Correspondingly, the decreases in overland flow with Pennisetum grass hedges for the 5, 10, 15, and 20% slope gradients were respectively 40, 47, 51, and 37% in trial a; 72, 54, 47, and 42% in trial b; and 79, 70, 54, and 73% in trial c. We observed statistical significantly differences in the overland flow between the Pennisetum-protected plots and the control plots but insignificant differences between the Arundinella-protected plots and the control plots. On the whole, in the four slope gradients and three trials, the Arundinella hedges reduced overland flow by an average of 33% and the Pennisetum hedges by 56%, indicating that the Pennisetum hedges had a greater effect (approximately 1.7 times) on preventing water loss compared with the Arundinella hedges. Rainfall intensity is another important factor in controlling soil and water loss. In this study, the overland flow positively correlated with rain intensity (Fig. 3). The reductions in overland flow by the Arundinella hedges under the various intensities were respectively 23, 11, and 26% in trial a; 32, 16, and 22% in trial b; and 85, 80, and 50% in trial c. The decreases in overland flow by the Pennisetum hedges were respectively 76, 48, and 38% in trial a; 71, 64, and 43% in trial b; and 98, 97, and 61% in trial c. The variance analysis showed that the differences in the overland flow between the protected plots and the control as well as between the Pennisetum- and Arundinellaprotected plots were both statistically significant. The effectiveness of grass hedges in reducing water loss positively correlated with the coverage of alfalfa (Tables 2, 3, and 4). The

Table 7 Soil loss (kg ha− 1) in plots with Arundinella hedges, Pennisetum hedges and the control in trial c (71% coverage of alfalfa). Slope gradient

Treatment

5%

Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum Control Arundinella Pennisetum

10%

15%

20%

22 mm h− 1

36 mm h− 1

63 mm h− 1

Dry run

Wet run

Very wet run

Dry run

Wet run

Very wet run

Dry run

Wet run

Very wet run

0.16a 0.00a 0.00a 0.44a 0.00a 0.00a 0.28a 0.13a 0.00a 21.04a 0.22a 0.00a

0.39a 0.00a 0.00a 0.56a 0.00a 0.00a 0.31a 0.14a 0.01a 25.00a 0.35a 0.00a

0.45a 0.00a 0.00a 2.27a 0.08a 0.04a 0.92a 0.13a 0.02a 28.49a 0.36b 0.34b

3.95a 0.00a 0.00a 7.67a 0.00a 0.00a 7.36a 0.83b 0.00b 13.63a 0.72a 0.00a

4.51a 0.00a 0.00a 8.90a 0.00a 0.00a 12.81a 0.86a 0.14a 34.36a 1.22a 0.00a

8.12a 0.00a 0.00a 9.18a 3.41a 0.15a 19.38a 2.11a 0.10a 38.89a 4.47b 2.96b

21.98a 8.36a 3.82a 43.32a 13.92a 9.14a 53.53a 21.25a 13.37a 133.02a 32.10a 16.55a

56.59a 21.33a 15.26b 57.47a 27.91a 25.65b 109.26a 55.58b 28.81b 185.05a 59.66a 33.27b

98.16a 41.53a 30.30a 106.72a 46.22a 36.32a 183.98a 69.01a 38.88b 326.79a 81.89a 55.42a

B. Xiao et al. / Geoderma 167-168 (2011) 91–102

decrease in water loss by the Pennisetum hedges increased from 44% to 54% with increasing alfalfa coverage from 21% to 38% and from 54% to 69% with increasing alfalfa coverage from 38% to 71%. Similarly, the decrease in water loss by the Arundinella hedges changed from 22% to 21% and then increased to 57% with increasing alfalfa coverage from 21% to 38% and then to 71%, respectively. When the overland flows under the three rainfall intensities for the three soil covers were summed together for no hedges, the Arundinella hedges and the Pennisetum hedges, the total runoffs for the three levels of grass hedges were 874.9 mm, 624.8 mm and 419.6 mm, respectively. For the total runoffs of the three treatments, the overland flow in the control was significantly higher than that with the Arundinella hedges and significantly higher than that with the Pennisetum hedges (n = 3, F = 28.90, P b 0.001). Our results showed that the two grass hedges significantly decreased overland flow on sloping croplands and that the controlling effect of the Pennisetum hedges was significantly higher than that of Arundinella. The Arundinella grass hedges decreased overland flow by an average of 29%, and the Pennisetum grass hedges decreased overland flow by 52%. 3.2. The reducing effects of grass hedges on soil loss The measured soil losses in the plots with the Arundinella hedges, Pennisetum hedges, or without hedges during trials a, b, and c are

97

presented in Tables 5, 6 and 7, respectively. Similar to the results for overland flow, the plots without grass hedges yielded the highest amounts of sediment, followed by those protected by the Arundinella and Pennisetum hedges. The soil loss rate decreased between 6% and 100% due to the protective hedges and depended on rainfall intensity and slope gradient. We observed the statistically significant differences in soil loss between the Pennisetum hedges and the control and insignificant differences between the Arundinella hedges and the control (Tables 5 to 7). To evaluate the influence of slope gradient on the effects of grass hedges, the total soil losses under two of the three rainfall intensities (the dry run was excluded) were summed for each rainfall intensity and each slope gradient; the results are presented in Fig. 4. The soil losses in the grass hedge-protected and unprotected plots increased almost curvilinearly with the slope gradient. The decreases in soil loss by the Arundinella hedges with the 5, 10, 15 and 20% slope gradients were respectively 88, 55, 59, and 39% in trial a; 37, 52, 71, and 57% in trial b; and 63, 58, 61, and 77% in trial c. The decreases in soil loss by the Pennisetum hedges with the 5, 10, 15 and 20% slope gradients were respectively 95, 88, 88, and 63% in trial a; 79, 81, 89, and 74% in trial b; and 73, 66, 79, and 86% in trial c. The difference in soil loss between plots with and without the Pennisetum hedges was statistically significant, while that between plots with and without the Arundinella hedges was statistically significant only

Fig. 4. Soil loss increase with slope gradient in plots with Arundinella hedges, Pennisetum hedges and control: (a) trial a with 21% coverage of alfalfa; (b) trial b with 38% coverage of alfalfa; (c) trial c with 71% coverage of alfalfa. The errors bars are added at the 5% probability level.

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B. Xiao et al. / Geoderma 167-168 (2011) 91–102

Fig. 5. Soil loss increase with rainfall intensity in plots with Arundinella hedges, Pennisetum hedges and control: (a) trial a with 21% coverage of alfalfa; (b) trial b with 38% coverage of alfalfa; (c) trial c with 71% coverage of alfalfa. The errors bars are added at the 5% probability level.

in some cases. Similarly, only in some cases were the soil losses in plots with the Pennisetum hedges significantly different from those with the Arundinella hedges. On the whole, in the four slope gradients and the

three soil covers, the Arundinella hedges reduced soil loss by an average of 60% and Pennisetum by 80%. In other words, Pennisetum hedges had a greater effect (about 1.3 times) on preventing soil loss compared

Table 8 The results of the correlation analysis (Pearson correlation) among overland flow, soil loss and independent variables. Treatments

Factors

Rainfall intensity

Control (n = 108)

Rainfall intensity Slope gradient Coverage of alfalfa Overland flow Soil loss Rainfall intensity Slope gradient Coverage of alfalfa Overland flow Soil loss Rainfall intensity Slope gradient Coverage of alfalfa Overland flow Soil loss

1.00 0.00 0.00 0.84a 0.40a 1.00 0.00 0.00 0.72a 0.28a 1.00 0.00 0.00 0.76a 0.26a

Arundinella hedges (n = 108)

Pennisetum hedges (n = 108)

a b

Correlation is significant at the 0.01 level (two-tailed). Correlation is significant at the 0.05 level (two-tailed).

Slope gradient

Coverage of alfalfa

Overland flow

Soil loss

1.00 0.00 0.26a 0.33a

1.00 − 0.29a − 0.36a

1.00 0.59a

1.00

1.00 0.00 0.26a 0.33a

1.00 − 0.45a − 0.33a

1.00 0.53a

1.00

1.00 0.00 0.27a 0.33b

1.00 − 0.37a − 0.27a

1.00 0.53a

1.00

B. Xiao et al. / Geoderma 167-168 (2011) 91–102

with Arundinella hedges. These results implied that both Pennisetum and Arundinella grass hedges could significantly decrease soil loss on sloping lands, and the decreases varied with plant species, slope, precipitation and land cover conditions. The total soil loss on the four slope gradients was summed for each treatment under the different rainfall intensities, which showed that soil erosion positively correlated with rainfall rate (Fig. 5). The two grass hedges were significantly different in protecting soil loss. For instance, the reductions in soil loss in plots with the Arundinella hedges under rain intensities of 22, 36 and 63 mm h− 1 were respectively 33, 36, and 55% in trial a; 45, 63, and 57% in trial b; and 98, 91, and 64% in trial c. The corresponding reductions by the Pennisetum hedges were respectively 85, 67, and 77% in trial a; 85, 80, and 75% in trial b; and 99, 98, and 77% in trial c. The effectiveness of grass hedges in preventing soil loss negatively correlated to alfalfa coverage (Tables 5, 6, 7). The decreases in soil loss by the Pennisetum hedges were 83% to 81% and then 76% with increasing alfalfa coverage from 21% to 38% and then to 71%, respectively. Similarly, the decreases in soil loss by the Arundinella hedges were 60% to 54% and then 65% with increasing alfalfa coverage from 21% to 38% and then to 71%. The combined total soil losses under the three rainfall intensities for the four soil covers were 42 kg ha− 1, 20 528 kg ha− 1 and 10420 kg ha− 1, respectively, for the unprotected plots and those protected with Arundinella and Pennisetum. The presence of Arundinella hedges decreased soil loss by 52% and Pennisetum hedges by 75%.

3.3. Correlation and regression analysis The correlations among overland flow, soil loss and the independent variables were analyzed and the results summarized in Table 8. Overland flow and soil loss both significantly correlated to rainfall intensity, slope gradient and alfalfa coverage. However, another factor, the type of grass hedge, was not considered in this correlation analysis because it was an unordered categorical variable. Therefore, to evaluate the contribution of related factors including type of grass hedge, slope gradient, rainfall intensity and coverage of alfalfa to the effectiveness of grass hedges, optimal scaling regression was performed from the experimental data. The regression results (overland flow: n = 324, R2 = 0.830, F = 310.804, P b 0.001; soil loss: n = 432, R2 = 0.602, F = 79.873, P b 0.001) indicated that the regression was very robust. The contribution of each factor according to the optimal scaling regression is listed in Table 9. From the F and P values in Table 9, we assumed that all of the considered factors in the regression significantly influenced overland flow and soil loss. Additionally, the ranking of the factors that influenced the dependent variables revealed that the contribution to overland flow was in the order rainfall intensity N coverage of alfalfa N type of grass hedgesN slope gradient. The contribution to soil loss was slope gradient N types of grass hedgesN coverage of alfalfa N overland flow N rainfall intensity. We therefore confirmed the role of grass hedges in preventing soil and water loss on sloping croplands.

99

3.4. Mechanisms of grass hedges in reducing soil and water loss It has been reported that the spatial distribution and characteristics of grass roots are important in controlling soil and water loss. Some studies have confirmed that grass roots may increase soil porosity (forming water channels for infiltration) and catch surface soil particles, thus decreasing overland flow and soil erosion (Deletic, 2001; Dosskey et al., 2007; Ritchie et al., 1997). In addition to the functions mentioned above, the roots are also very important for holding the grasses erect. Additionally, it is speculated that the stiffness of the grass stems, their shape and orientation with regard to the flow direction are critical to the effectiveness of grass hedges. For this reason, the grass roots were sampled and some of the characteristics including total length, average diameter, volume, surface area and dry biomass weight were measured. The root length, volume, surface area and dry biomass of Pennisetum were consistently higher than those of Arundinella, whereas the root diameter of Pennisetum was slightly smaller (Table 10). These results suggest that Pennisetum not only has more roots in total but also has more fine roots than Arundinella. In addition, the vertical distribution of grass roots as reported in Table 10 showed that the length and biomass of Arundinella roots dramatically decreased from the top 0–20 cm to 20–30 cm, whereas this was not the case for Pennisetum. From this result, it is possible to conclude that Pennisetum has more numerous, finer and deeper roots than Arundinella, which is in agreement with the comparative results of their functions in controlling soil and water loss. Some researchers have documented that one of the most important mechanisms of grass hedges in decreasing soil loss is filtration through their dense stems (Bhattarai et al., 2009), which was supported by the present research. Our results showed that the soil sediment depths on the upper side of the Arundinella and Pennisetum grass hedges were 1.5 cm and 2.5 cm, respectively, in December 2007; 2.5 cm and 4.0 cm, respectively, in December 2008. This finding demonstrated that a significant part of the sediment was filtered and deposited upstream of the grass hedges. These soils in front of the grass hedges formed a small backwater zone; in this zone, the runoff velocity was reduced, and sediment deposition was promoted. Subsequently, the total runoff and soil loss was greatly decreased. Similar results also have been recorded by other researchers (Blanco-Canqui et al., 2004; Ghadiri et al., 2001; Tadesse and Morgan, 1996): Ghadiri et al. (2001) reported that runoff ponding caused sediment deposition upslope of barriers, thereby reducing sediment loss; Blanco-Canqui et al. (2004) observed that the maximum ponding depth was 0.03 ± 0.01 m and extended 0.70 ± 0.01 m above the barriers and attributed the reduction in transported sediment to the ponding effect. Finally, as compared to Arundinella, Pennisetum was more effective at filtering and preventing sediment flow through the hedges, possibly due to its denser stems. This result is in agreement with the above observations on soil and water conservation. 4. Discussion The 36 simulated rainfall events under different slope gradients, rainfall rates and soil coverage indicated that Arundinella grass hedges

Table 9 The results of optimal scaling in regression among overland flow, soil loss and independent variables. Factors Overland flow

Soil loss

Rainfall Slope Type of grass Coverage Overland intensity gradient hedges of alfalfa flow F value 932.42 P value b0.001 Zero order 0.73 Importance 0.65 F value 1.10 P value 0.296 Zero order 0.37 Importance 0.04

98.49 b 0.001 0.24 0.07 50.97 b 0.001 0.38 0.77

139.69 b 0.001 − 0.28 0.10 18.10 b 0.001 − 0.28 0.73

254.16 b 0.001 − 0.38 0.18 41.35 b 0.001 − 0.46 0.57

– – – – 24.94 b 0.001 0.69 0.19

Table 10 The spatial distribution and characteristics of grass roots in the 10 cm × 10 cm surrounding area. Soil depth (cm)

Treatments

Length (cm)

Diameter (mm)

Volume (cm3)

Surface area (cm2)

Dry biomass (g)

0–10

Arundinella Pennisetum Arundinella Pennisetum Arundinella Pennisetum

6759 17 714 5092 16 070 1088 11 427

0.38 0.35 0.47 0.39 0.81 0.38

8.11 18.99 8.34 19.38 7.62 13.08

802 2035 661 1971 326 1364

1.40 2.61 1.07 3.61 0.31 2.36

10–20 20–30

B. Xiao et al. / Geoderma 167-168 (2011) 91–102

Decrease in overland flow (%)

decreased overland flow by 29% and soil loss by 52%, whereas Pennisetum grass hedges decreased overland flow by 52% and soil loss by 75%. The high efficiency of Pennisetum hedges in reducing soil and water loss is possibly related to its denser stems and richer and deeper roots compared with Arundinella hedges, which was also reported by Tadesse and Morgan (1996). In other words, the tillering ability and the root characteristics, which determine to a large extent the efficiency of grass hedges, should be considered in the selection of grass species for grass hedges. The results also suggested that grass hedges are more efficient at preventing soil loss than at preventing runoff. This conclusion agrees well with the results reported by Barfield et al. (1979) and Hussein et al. (2007), who found that grass hedges were more efficient at removing particle pollutants than soluble pollutants including runoff. This result is reasonable because the effect of grass hedges on decreasing overland flow is mainly due to the increased infiltration on the hedges' fields, which may be caused by increased soil porosity (the results of grass roots) and increased infiltration time (the results of grass stems in blocking runoff) (Dosskey et al., 2007).

100

Arundinella hedges Pennisetum hedges

(a) 80

60

40

20

0

Dry run

Wet run

However, the effect of grass hedges on soil loss is mainly due to the increased soil structure stability by grass roots, filtration by grass stems, and decreased carrying capacity of runoff (caused by decreased volume and velocity of runoff) (Blanco-Canqui et al., 2004; Van Dijk et al., 1996). Therefore, it is reasonable that grass hedges are much more efficient in reducing soil loss and other particle pollutants compared with water loss and soluble pollutants. According to the experimental procedure, an initial one-hour rainfall simulation at the low soil moisture condition (“dry run”) was followed by a second one-hour rainfall approximately 24 h later (“wet run”) and a third one-hour rainfall 1 h later (“very wet run”). Thus, the initial soil water content before the simulated rainfalls was very different among dry runs, wet runs and very wet runs. As discussed, the infiltration rate mostly depended on the soil moisture and decreased with increasing soil moisture. Therefore, we can conclude that the order of infiltration rate and the total infiltration water volume during the experiments should be dry run N wet run N very wet run, which also can be inferred from the volume of runoff in the tables

Decrease in overland flow (%)

100

60

40

20

60

40

20

Dry run

Wet run

Very wet run

Dry run

Wet run

Very wet run

Dry run

Wet run

Very wet run

80

60

40

20

0

Very wet run

100

100

(f) Decrease in soil loss (%)

(e) 80

60

40

20

0

Wet run

(d)

80

0

Dry run

100

(c) Decrease in soil loss (%)

Decrease in overland flow (%)

(b) 80

0

Very wet run

100

Decrease in soil loss (%)

100

Dry run

Wet run

Very wet run

80

60

40

20

0

Fig. 6. Performance of grass hedges (calculated as a reduction compared to the control) in reducing soil and water loss in dry runs, wet runs and very wet runs: (a) overland flow decrease in trial a; (b) overland flow decrease in trial b; (c) overland flow decrease in trial c; (d) soil loss decrease in trial a; (e) soil loss decrease in trial b; (f) soil loss decrease in trial c. The errors bars are added at the 5% probability level.

B. Xiao et al. / Geoderma 167-168 (2011) 91–102

Decrease in overland flow (%)

100

Arundinella hedges Pennisetum hedges Linear fit of Arundinella hedges Linear fit of Pennisetum hedges

(a) 80

2

y=66.99-0.92x, R =0.88

60

40

20 2

y=39.58-0.52x, R =0.55

0

0

5

10

15

20

used in association with grass hedges to prevent soil and water loss more efficiently (Rodriguez, 1997). This study investigated the effectiveness of two grass hedges and the influences of slope gradient and rainfall intensity on a micro scale (plots); however, the effectiveness of grass hedges in midlevel (watershed) and large scales (national and landscape) should be evaluated in future studies. To date, almost all of the studies on grass hedges have been conducted on the plot scale, and only Wang et al. (2005) and Rachman et al. (2008) have assessed the effectiveness of some grass hedges, including switchgrass, eastern gamagrass, big bluestem grass (Andropogon gerardii Vitman) and bromegrass (Miscanthus sinenses purpurascens) on the watershed scale (6.6 ha). Although no obvious decrease in the effectiveness of grass hedges was observed when the scale was increased from plot to watershed, it is still necessary to consider scale problems in further research related to grass hedges. In addition, the effectiveness of grass hedges was evaluated in this research but there were several anomalies in overland flow versus soil loss in terms of slope and rainfall intensity. The overland flow rate or depth changes that influence deposition or entrainment may be very helpful for the explanation of these anomalies, but this kind of data collection was lacked in our research. In order to avoid such shortcomings of our study, the overland flow rate or depth should be measured chiefly in further research. 5. Conclusions The soil and water conservation function of two native grass hedges, Pennisetum and Arundinella, were studied by simulated rainfall under different slope gradients, rainfall intensities and extent of soil covers on sloping lands. The overland flow and soil loss in sloping cropland decreased by 52% and 75% by Pennisetum hedges and 29% and 52% by Arundinella hedges, respectively. The effects of grass hedges 100

Decrease in soil loss (%)

and figures. This result indicated that the grass hedges reduced sediment loss in the dry run by enhanced infiltration, reduced runoff and filtration of the grass stems while in the very wet run, the filtering action of the grass stems was more dominant because in this stage, the soil was almost saturated. Thus, we can separate the aspects of filtration and enhanced infiltration in the functional mechanism of the grass hedges and then quantify their respective effects. For this reason, we summed the total runoff and soil loss for dry runs, wet runs and very wet runs for each trial, and then presented the performance of grass hedges (calculated as a reduction compared to the control) in reducing soil and water loss in Fig. 6. This figure shows that the effectiveness of the grass hedges consistently decreased from the dry runs to the wetter runs, but the decreases were not very great (averages: 12% for runoff and 14% for soil loss from dry run to very wet run); however, the reduction due to the grass hedges in the very wet runs was still 43% for runoff and 64% for soil loss. This result indicated that the functional aspect of the grass stems that blocked overland flow and filtered sediments was much greater than the aspects that enhanced infiltration. Our results further indicated that the effects of grass hedges decreased in percentage terms with increasing slope gradient and rainfall intensity (Fig. 7). For example, the reduction by the Pennisetum hedges decreased from 64% to 51% for overland flow and from 82% to 74% for soil loss when the slope gradient increased from 5% to 20%; similarly, the reduction by the Pennisetum hedges decreased from 82% to 47% for overland flow and 88% to 76% for soil loss when the rainfall intensity increased from 22 mm h− 1 to 63 mm h− 1. Therefore, the adoption of grass hedges does not provide a total protection guarantee, especially under some extreme conditions such as steep sloping croplands and frequent heavy rain regions (Blanco-Canqui et al., 2004; Sun et al., 2008). In such conditions, other engineering techniques such as micro-basins, fish scale pits, mulch and even land shaping should be

(b) 80

60 2

y=61.44-0.15x, R =0.05

40

20

0

25

2

y=84.46-0.35x, R =0.23

0

5

Slope gradient (%)

15

20

25

100

(c) 80

2

y=100.11-0.84x, R =1.00

60

40

20

2

y=50.98-0.30x, R =0.78

20

30

40

50

Rainfall intensity (mm h-1)

60

70

Decrease in soil loss (%)

Decrease in overland flow (%)

10

Slope gradient (%)

100

0 10

101

(d)

2

y=94.40-0.30x, R =0.99

80

60 2

y=60.90-0.02x, R =0.02

40

20

0 10

20

30

40

50

60

70

Rainfall intensity (mm h-1)

Fig. 7. The effectiveness of grass hedges (calculated as a reduction compared to the control) in reducing soil and water loss decreased with increasing slope gradients and rainfall intensities: (a) overland flow decrease with slope gradient; (b) soil loss decrease with slope gradient; (c) overland flow decrease with rainfall intensity; (d) soil loss decrease with rainfall intensity. The errors bars are added at the 5% probability level.

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B. Xiao et al. / Geoderma 167-168 (2011) 91–102

decreased in percentage terms with increasing slope gradients and rainfall intensities. The contribution of independent variables to overland flow was in the order rainfall intensity N coverage of alfalfa N type of grass hedges N slope gradient. The contribution to soil loss was slope gradientN types of grass hedgesN coverage of alfalfa N overland flow N rainfall intensity. The high efficiency of the Pennisetum hedges in reducing soil and water loss was possibly related to its denser stems, richer and deeper roots compared with Arundinella hedges. The tillering ability and root characteristics, which determine to a large extent the efficiency of grass hedges, should be considered in the selection of grass species when designing grass hedges. The Pennisetum hedges were more efficient at controlling soil and water loss and should be preferentially used on sloping croplands compared with Arundinella hedges. Acknowledgments This study was funded by the Beijing Novel Program (No. 2009B25), Beijing Municipal Natural Science Foundation (No. 8102015), National Natural Science Foundation of China (No. 41001156), Young Scientist Award of Beijing Academy of Agriculture and Forestry Sciences (2009), and Open Fund from State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau (No. 10501-295). We give our thanks to Dr. Xiaorong Wei (Institute of Soil and Water Conservation, Chinese Academy of Sciences) and Prof. Dr. Vito Sardo (University of Catania, Italy) for their invaluable comments on improving the manuscript. References Angima, S.D., Stott, D.E., O'Neill, M.K., Ong, C.K., Weesies, G.A., 2002. Use of calliandra–Napier grass contour hedges to control erosion in central Kenya. Agriculture, Ecosystems & Environment 91, 15–23. Babalola, O., Oshunsanya, S.O., Are, K., 2007. Effects of vetiver grass (Vetiveria nigritana) strips, vetiver grass mulch and an organomineral fertilizer on soil, water and nutrient losses and maize (Zea mays, L) yields. Soil and Tillage Research 96, 6–18. Barfield, B.J., Tollner, E.W., Hayes, J.C., 1979. Filtration of sediment by simulated vegetation: I. Steady-state flow with homogeneous sediment. Transactions of the American Society of Agricultural Engineers 22, 540–545. Bennett, M.T., 2008. China's sloping land conversion program: institutional innovation or business as usual? Ecological Economics 65, 699–711. Bhattarai, R., Kalita, P.K., Patel, M.K., 2009. Nutrient transport through a vegetative filter strip with subsurface drainage. Journal of Environmental Management 90, 1868–1876. Blanco-Canqui, H., Gantzer, C.J., Anderson, S.H., Alberts, E.E., 2004. Grass barriers for reduced concentrated flow induced soil and nutrient loss. Soil Science Society of America Journal 68, 1963–1972. Cha, X., Tang, K.L., 2000. Study on comprehensive control model of small watershed eco-environment in water and wind crisscrossed erosion zone. Journal of Natural Resources 15, 97–100 (in Chinese). Cullum, R.F., Wilson, G.V., McGregor, K.C., Johnson, J.R., 2007. Runoff and soil loss from ultra-narrow row cotton plots with and without stiff-grass hedges. Soil and Tillage Research 93, 56–63. Dabney, S.M., Meyer, L.D., Harmon, W.C., Alonso, C.V., Foster, G.R., 1995. Depositional patterns of sediment trapped by grass hedges. Transactions of the American Society of Agricultural Engineers 38, 1719–1729. Dabney, S.M., Liu, Z., Lane, M., Douglas, J., Zhu, J., Flanagan, D.C., 1999. Landscape benching from tillage erosion between grass hedges. Soil and Tillage Research 51, 219–231. Dalton, P.A., Smith, R.J., Truong, P.N.V., 1996. Vetiver grass hedges for erosion control on a cropped flood plain: hedge hydraulics. Agricultural Water Management 31, 91–104. Deletic, A., 2001. Modelling of water and sediment transport over grassed areas. Journal of Hydrology 248, 168–182. Dercon, G., Deckers, J., Poesen, J., Govers, G., Sanchez, H., Ramirez, M., Vanegas, R., Tacuri, E., Loaiza, G., 2006. Spatial variability in crop response under contour hedgerow systems in the Andes region of Ecuador. Soil and Tillage Research 86, 15–26. Dosskey, M.G., Hoagland, K.D., Brandle, J.R., 2007. Change in filter strip performance over ten years. Journal of Soil and Water Conservation 62, 21–32. Ghadiri, H., Rose, C.W., Hogarth, W.L., 2001. The influence of grass and porous barrier strips on runoff hydrology and sediment transport. Transactions of the American Society of Agricultural Engineers 44, 259–268. Gilley, J.E., Eghball, B., Kramer, L.A., Mooreman, T.B., 2000. Narrow grass hedge effects on runoff and soil loss. Journal of Soil and Water Conservation 55, 190–196.

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