Effects of grass contour hedgerow systems on controlling soil erosion in red soil hilly areas, Southeast China

International Journal of Sediment Research 30 (2015) 107–116

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International Journal of Sediment Research journal homepage: www.elsevier.com/locate/ijsrc

Original Research Paper

Effects of grass contour hedgerow systems on controlling soil erosion in red soil hilly areas, Southeast China Ji Fan a, Lijiao Yan a,n, Pei Zhang a, Ge Zhang b a b

College of Life Sciences, Zhejiang University, Hangzhou 310058, China Center for Geographic Information Systems, Georgia Institute of Technology, Atlanta, GA 30308, USA

art ic l e i nf o

a b s t r a c t

Article history: Received 6 August 2013 Received in revised form 15 October 2014 Accepted 12 March 2015 Available online 4 May 2015

Soil erosion by water is a well-recognized serious environmental problem in the world. While contour hedgerow systems are an effective method for soil water conservation, there are a few studies on its effect in the red soil hilly areas in Southeast China. With a fixed field experiment, we constructed a runoff plot at hilly area in Zhuji County, Zhejiang province, to evaluate the effect of the grass hedgerows in soil water conservation, and to determine the optimized hedgerow patterns. Hemerocallis citrine (HC) and Ophiopogon japonicas (OJ) were selected to build the hedgerows in patterns of one row and two rows. The REE method was used to trace the source of the sediment for a better understanding of the characteristic and mechanism of erosion with hedgerows control. Our results showed that (1) hedgerows reduced erosion and surface runoff by 31.99–67.22% and 15.44–45.11%, respectively; (2) hedgerows delayed the development of rills; (3) hedgerows reduced the soil nutrients loss; (4) hedgerows reshaped the soil physical properties, especially in increasing 40.25 mm water-stable aggregates. Taken together, our results suggest that two-row OJ is the optimized contour hedgerow pattern in the experiment condition, and downward sloping land should have the highest priority to take measures for soil water conservation. This research comprehensively studied the effects and mechanism of contour hedgerows in controlling soil and water loss in red soil hilly areas, Southeast China, so that the practice of soil and water conservation can be implemented more effectively in these areas. & 2015 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

Keywords: Soil erosion Contour hedgerow Runoff plot REE method

1. Introduction Soil erosion by water is a well-recognized serious environmental problem in the world (Beskow et al., 2009, Blavet et al., 2009, Chen et al., 2012). It leads to a series of negative effects, such as soil deterioration, declining land productivity, contamination of local hydrology systems and reduction of water storage capacity with transported sediments and pollutants (Cerdan et al., 2010). Moreover, since soil is the largest terrestrial C pool, soil erosion even increases the threat of global warming via loss of soil organic carbon (Lenka et al., 2013). In hilly areas, soil erosion is often a more serious problem than that in plain areas (Lin et al., 2009). Conserving soil from being eroded by water is very important to retain the tillage and maintain sustainability in hilly areas. In the coastal areas of China, like Zhejiang Province, with fast economic

n

Corresponding author. E-mail addresses: [email protected] (J. Fan), [email protected] (L. Yan).

development, there is more pressure on soil erosion from intensive cultivation. Planting hedgerows along the contour on steep lands is an effective soil conservation technology in hilly areas. Research has pointed out that contour hedgerows could reduce runoffs, soil erosion (Cullum et al., 2007; Salvador-Blanes et al., 2006) and control non-point source pollution by reducing soil nutrients' loss (Agus et al., 1997; Agus et al., 1999; Chaubey et al., 1995; Lin et al., 2009; Wu et al., 2011). It has also been reported that contour hedgerows can reduce investment on slope farmland (Lin et al., 2007; Wu et al., 2011). Under certain conditions, contour hedgerows could reduce runoffs and sediments by 60–80% and 80–95%, respectively (Lal, 1989; Lin et al., 2009). In the recent two decades, several studies also particularly focused on the hedgerows' influence on reshaping the micro-topographic features of the slope (Dabney et al., 1999; Lin et al., 2009; Zheng, 2006) and its impact on redistributing soil nutrients on the slope (Lin et al., 2009). While several studies have been done on the controlling effect of contour hedgerow on soil erosion in other areas of China

http://dx.doi.org/10.1016/j.ijsrc.2015.03.001 1001-6279/& 2015 International Research and Training Centre on Erosion and Sedimentation/the World Association for Sedimentation and Erosion Research. Published by Elsevier B.V. All rights reserved.

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(Bu et al., 2008; Lin et al., 2009, Tang et al., 2003), there are still no studies in red soil hilly areas of Southeast China so far. It is important to understand contour hedgerow's soil erosion controlling effect specifically in red soil hilly area since depending on hedgerow species, management methods, weather conditions and soil properties, the controlling effects vary. For instance, Bu Chongfeng et al. studied effects of contour hedgerows and found that it increased soil organic matter, total nitrogen, and total phosphorus 5–9 times higher in the soil in Three Gorges Dam area, China (Bu et al., 2008). But results of Agus et al. showed that the concentration of P in the soil reduced from 20 mg/kg to about 14 mg/kg, exchangeable K increased from 0.24 cmol( þ)/kg to 0.29 cmol( þ)/kg, in the Philippines (Agus et al., 1999). The strong climate fluctuation, especially in rainfall quantity, affects the vulnerability of a region to erosion (Imeson & Lavee, 1998). Since Southeast China is covered by subtropical ridges in summer, it often experiences dry weather during that period, and storm events brought by typhoon cause high risk of erosion in this area. Thus, it is important to understand the effects of soil conservation in hilly areas of Southeast China, and find an effective mitigation method. The objectives of this study are: (1) investigating the effects of grass hedgerows on controlling soil erosion in red soil hilly areas of Southeast China; (2) investigating the effects of grass hedgerows on reducing the soil nutrients loss; (3) evaluating the impacts of grass hedgerows on redistributing sediment on the slope and reshaping in the same area; (4) selecting the optimized hedgerow pattern for soil conservation in the area.

2. Materials and methods 2.1. Background of the study area Zhejiang Province is one of the provinces which are most lack of arable lands in China (Ding Xiaodong, 2008). According to the soil monitoring report of Zhejiang Province (2004), there is 13,654.13 km2 land impacted by soil erosion, which equals to 12.95% of total province area. About 89.6% of the erosion area in Zhejiang province is in the terrain with slope more than 81, especially concentrated in slope between 151 and 251. Therefore, studying the erosion of hilly area is important for soil and water conservation in Zhejiang Province. The experiment was conducted in the runoff plots located at Shifei village, Zhuji County, Zhejiang Province, which belongs to the Cao’ejiang River watershed in southeast China, around 29 1440 N, 120 1230 E (Fig. 1(a)). The topography of this region is mainly hilly area, and the soil is dominated by red soil, classified as oxisols according to USDA (Staff., 1999). The climate of this area belongs to subtropical monsoon climate. The average precipitation is 1373.6 mm per year, and the average annual temperature is 16.3 1C. 2.2. Runoff plot establishment Runoff plot observation is a classical method in studying soil erosion. The method was invented in the 1960s by USDA for monitoring the erosion amounts all over USA, and it soon prevailed all over the world as a standard method for soil erosion research. Our experiment was started in October 2008. Lin, C. et al., 2009 pointed out that in some situation, it is better to use grass species than wood species in contour hedgerow system for soil and water conservation (Lin et al., 2009). In this experiment, we selected two species to establish the grass hedgerows. The selecting principles are as follows: (1) the species can grow fast enough under the local

condition to establish the hedgerows in a short time; (2) the species should not spread on the slope, or severely compete with crops on nutrients and water potentially. Finally, we chose Hemerocallis citrine and Ophiopogon japonicas, both of which were commonly used in building grass hedgerows in China (Chen et al., 2006; Li et al., 2003; Shen, 1998). This experiment included 4 runoff plots. Each plot was equally divided into 6 small plots (1.5 m  10 m) by concrete plates as treatments (Fig. 1(b)). In total, there were 6 treatments and each treatment had 4 repetitions. The 6 treatments including the control with no hedgerows (Control), single strip with Hemerocallis citrine (SC), and single strip with Ophiopogon japonicas (SJ), double strips with Hemerocallis citrine (DC), double strips with Ophiopogon japonicas (DJ), and two strips consisted of one strip Hemerocalliscitrina and one strip Ophiopogon japonicas (CJ). There were 3 grass strips in each treatment (Fig. 1(b)). 2.3. Simulating rainfall Simulated rainfall was a supplement to natural precipitation. Simulated rainfall was implemented by sprinkler at one side of the plot. The sprinkler at 0.1 kPa water pressure can cover 4 treatments (6 m long) at one time. To ensure the uniformity of rainfall, the data of the middle 2 treatments were measured each time. Each rainfall was at 0.1 kPa, lasting for 30 min. The rainfall intensity was maintained in the range from 40 mm/h to 45 mm/h. 2.4. Measurement of erosion and runoff After each precipitation event, either natural precipitation event or simulated precipitation event, soil erosion and runoff samples were collected by tanks at the bottom of plots. Befor the sample was collected from each tank, we stirred the liquid in the tank for 10–15 min and made sure the eroded sediment distributed evenly in the liquid. Then the erosion and runoff samples were taken from each tank's outlet. Each sample had 500 mL suspension liquid of sediment. The amount of runoff in each tank was measured in the depth, and multiplied by the area of each tank to get the volume. The formula is as follows: V ¼ ðD  P ÞA

ð1Þ 3

where V is the volume of runoff (mm ), D is the depth in the tank (mm), P is the rainfall depth (mm), and A is the area of the tank (mm2). Because the tanks were not sealed above, the precipitation fell into it. To get the absolute value of the runoff volume, we subtracted the rainfall depth from the measured depth in the tank. The amount of erosion was measured by weighing the soil in the samples and calculating the soil concentration, where the soil was filtered from the suspension liquid and dried in oven. The soil amount in each sample was then multiplied by the total volume of runoff to get the total erosion amount of each plot. The formula is as follows:
E ¼ m=500 V ð2Þ where E stands for the erosion amount from each plot (g), m for the soil weight in each sample (g/500 mL), and V for the volume of runoff (mL). 2.5. Rare earth element tracing method Rare earth elements (REEs) were laid downstream every hedgerow strip to trace the erosion source from three different parts of the slope (Fig. 1(b)). The background REEs' concentrations were measured before the experiment to decide the applied concentration. Eu, Er and Dy were chosen as tracers because of the low background concentrations and the high sensitivity in

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Fig. 1. The location and runoff plot: (a) The location of the runoff plot and (b) top view of the runoff plot (dm).

neutron activation analysis (NAA). The applied concentrations in different segments of the slope are shown in Table 1. The amount of erosion from different slope parts was calculated using Eq. (3) as follows: Ei ¼ m c=coi




ð3Þ

where Ei (g) is the mass of erosion from slope part i, m (g) is the mass of the sediment sample, Ci (ppb) is the concentration of tracer element i in the sediment sample, and coi (ppb) is the applied concentration of tracer element i. The background concentration of tracer elements was too small to be taken into account.

2.6. Measurement of nutrient contents in soil Soil nutrient contents were measured every 3 months during the experiment. Top 15 cm soil was taken as the sample for each treatment. Concentration of organic matters, total nitrogen, alkalihydrolyzable nitrogen, ammonium nitrogen, nitrate nitrogen, total phosphorus, available phosphorus, total potassium and available potassium in each sample was measured. These test methods were following “Handbook of the Soil Analysis” (Pansu, 2006).

2.7. Measurement of soil physical property Physical property was measured after the hedgerow system built up for two years. Samples were taken at up, middle and lower part of each slope. And each sample included two parts: top soil (0–20 cm) and subsoil (20–40 cm). The following indicators of samples were measured: specific weight (g/cm3), soil bulk density (g/cm3), organic matters (%), porosity (%), aggregation degree (%), water stable aggregates (%), sand (%), silt (%) and clay (%). These test methods were following “Handbook of the Soil Analysis” (Salvador-Blanes et al., 2006).

Table 1 Concentration of initial REEs in different places along the slope. Slope

Eu

Er

Dy

Up Middle Lower

1659.98 109.8 57.09

111.44 7691.04 388.23

157.87 463.61 5933.06

Unit: part per billion (ppb).

2.8. Measurement and simulation of the slope micro-topography Slope topography was measured every six months. Nine strings were setting above the slope in each plot, paralleling with the initial slope line marked on the plates which were used to separate the plots. By measuring the vertical distance from slope to strings (15 samples with a same distance from top to bottom of the slope were taken along each string), we simulated the change in the slope surface and the slope micro-topography with interpolation in Matlab.

3. Results 3.1. Erosion and runoff This study was based on the data collected from precipitation events. There is a threshold of rainfall amount in single precipitation event for sediment yield. According to the research of Guo et al. (2001), it is 12.7 mm in the experiment area (Guo et al., 2001). We selected 21 natural precipitation events where the rainfall amounts were larger than the threshold and 5 simulated precipitation events after the establishment of grass hedgerow systems during 2009–2011 to study the effect in controlling erosion and runoff. The runoff and the erosion data of natural precipitation events are listed in Table 2. It is shown that the hedgerows effectively reduced erosion and runoff. As shown in Fig. 2, compared to the control treatment (the

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Table 2 Erosion and runoff data of 21 natural precipitation events during 2009–2011. Amount of runoff (m3)

Amount of erosion (kg) Year N P (mm) SC SJ DC DJ CJ Control

2009 8 295.20 8.77 6.98 5.66 4.46 5.95 13.24

2010 9 405.25 2.69 2.26 1.42 1.22 1.64 3.80

Table 3 Concentration of REE in sediment.

2011 4 276.45 1.32 0.87 1.02 0.47 0.81 1.75

2009 8 295.20 1.70 1.69 1.39 1.12 1.54 2.11

2010 9 405.25 1.95 1.63 1.52 1.41 1.64 2.35

2011 4 276.45 1.28 0.92 0.92 0.67 0.86 1.37

SJ SC DJ DC CJ Control

Eu

Er

Dy

153.76 150.96 203.17 199.43 199.36 106.63

1224.15 1098.01 1223.87 1036.32 1342.77 902.25

1274.13 1401.72 1541.64 1137.46 1679.24 1093.32

Row N is number of natural precipitation events; Row P is total precipitation (mm); SC for single row of Hemerocallis citrine, SJ for single row of Ophiopogon japonicas, DC for double rows of Hemerocallis citrine, DJ for double rows of Ophiopogon japonicas, and CJ for two-row-hedgerows consisted by one row Hemerocallis citrine and one row Ophiopogon japonicas. Control refers to the control treatment which had no hedgerow on the slope.

Fig. 3. Erosion from different slope parts.

3.3. The micro-topography of the slope

data of the control was set as 100%), treatments SC, SJ, DC, DJ, and CJ reduced erosion by 31.99%, 46.19%, 56.89%, 67.22% and 55.30%, respectively, and reduced surface runoff by 15.44%, 27.27%, 34.31%, 45.11% and 30.70%, respectively. Obviously, two-row-hedgerow systems were better in controlling water and soil loss.

Treatment DJ and SJ were selected to see how hedgerows changed the slope surface. As shown in Fig. 5(a) and (b), we can find that erosion was deposited in front of the hedgerow belts, and the slope immediately downstream the hedgerows was obviously eroded. It led the slope to be less steep. Comparing Fig. 5(a), (b) and (c), we can see that the Treatment SJ displayed a smoother surface than DJ, while the control was at the similar status with SJ, which means hedgerow pattern DJ was more effective in shaping the slope. Moreover, hedgerows also controlled the development of rills. Comparing the result of slope surface simulations (Fig. 5(d)–(f)), it showed that rills reduced in order of the control, SJ and DJ.

3.2. Erosion from each slope part

3.4. Concentration of soil nutrient contents

Concentration of REE in sediment was measured after each single precipitation. To eliminate the error induced by sediment deposited on the slope, each REE concentration was summed up (Table 3). Ratios of sediment from up, middle and down sections of the slope were roughly in a range from 18:32:50 to 27:30:43. As is shown in Fig. 3, the percentage of the sediment from slope sections was different among treatments. Compared to the control, the hedgerow systems changed the percentages, and the effect was different depending on the hedgerow patterns. Treatment SC barely changed the percentage, while treatment DC changed the percentage significantly. The order by percentages of sediment from upslope, from less to more, is control, treatment SC, SJ, CJ, DJ and DC. And in the same order, percentages of sediment from downslope decreased. We also studied the sediment from the runoff plot along the local river to understand the pattern of deposition. As is shown in Fig. 4(a), the concentrations of REEs along the river declined with distance. Compared to the concentration of REEs in the tank, the concentration of REEs in the river was smaller. Erosion from different sections of the slope can also be calculated by Eq. (3), and the result is shown in Fig. 4(b). There is no significant difference among the samples from different sample spots in the river.

Concentration of soil nutrient contents was measured every 3 months after the hedgerow system built up for 2 years. Nine indicators were measured in this experiment including organic matters, total nitrogen, alkali-hydrolyzable nitrogen, ammonium nitrogen, etc. Here we selected 4 indicators which were organic matters, total nitrogen, total phosphorus and total potassium to show the soil nutrient, and used the data of 2010 to analyze the effect of hedgerow systems in controlling soil nutrient loss. The result showed that there was more organic matter in the slopes with hedgerows than in the control, as well as the increased levels of total nitrogen and total phosphorus. At the end of 2010, the concentration of organic matters in treatments with hedgerows was 7–24% higher than that in the control, and the concentration of total nitrogen was 8–17% higher. The concentrations of total phosphorus in the treatments with hedgerows were generally 12–18% higher than that in the control, except the concentration of treatment DC. However, the concentrations of total potassium kept the same among the treatments.

Fig. 2. The reduced percentage of cumulative erosion and surface runoff of each treatment compared to the control.

3.5. Soil physical properties Soil erodibility is a measure of soil susceptibility to be eroded, which is determined by soil physical properties (Tejada & Gonzalez,

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Fig. 4. Erosion from the plots along the local river: (a) concentrations of REEs and (b) percentage of erosion from different slope parts.

Fig. 5. The effect in changing the micro-topography of the slope. (a) Profile of treatment DJ, (b) profile of treatment SJ, (c) profile of the control, (d) slope surface simulation of treatment DJ, (e) slope surface simulation of treatment SJ, (f) slope surface simulation of control. In (a), (b) and (c), the right side is the upstream, and the hedgerows were roughly at sample sequences 5, 9 and 13. In (d)–(f), the left side is the upstream, the x-axis is the slope length, and the z-axis is the surface relative height.

2006). Seven indicators were selected and measured, including specific weight, soil bulk density, organic matters, porosity, waterstable aggregates, and proportion of sand, silt and clay. The result showed that these indicators of treatments with hedgerows were higher than that of the control, except the proportion of Sand (%). Specifically, compared with the control,

the treatments with hedgerows were 7–17% lower in soil bulk density, 15–30% higher in organic matters, 8–13% higher in porosity, 3–5% higher in 45 mm water-stable aggregates, 11–19% higher in clay, 14–21% less in sand, and 2–4% less in silt. Moreover, the difference was much more significant between the treatments with double-row hedgerows and the control.

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4. Discussion 4.1. Effects in controlling the erosion and runoff As shown in results, hedgerows reduced erosion and slope surface runoff. It means that all the hedgerow systems had a good controlling effect on erosion and runoff. Among the treatments, the controlling effects were different. We examined the difference using paired-samples T-test. As shown in Tables 4 and 5, we found that compared to the control, treatments with hedgerows had significant difference (P o0.05 or P o0.01) both in erosion and runoff. Among the hedgerow patterns, there was significant difference between single row and double rows, including SC and DC, SJ and DJ. It indicates that the single-row hedgerow pattern could not replace double-row, though single-row hedgerow would be cheaper and would compete less fiercely with crop for water and nutrients. However, there was no significant difference among double-row hedgerows (DC, DJ and CJ). It indicates that the more important factor is hedgerow pattern and that in double rows, the effect is species agnostic. However, integrating all the results shown in Fig. 2, we concluded that DJ was still the best hedgerow pattern to control the erosion and surface runoff. There are two possible reasons. Table 4 Significance of erosion amounts of different treatments.

SC SJ DC DJ CJ B n

SC

SJ

DC

DJ

CJ

Control

– 0.025* 0.006** 0.005** 0.002** 0.001**

– 0.067 0.003** 0.084 0.002**

– 0.068 0.671 0.001**

– 0.072 0.001**

0.001**

–

Significant, two-tailed Po 0.05. Significant, two-tailed Po 0.01.

nn

4.2. Effects in shaping the slope

Table 5 Significance of average surface runoff amounts of different treatments.

SC SJ DC DJ CJ Control n

SC

SJ

DC

DJ

CJ

Control

– 0.037* 0.011* 0.003** 0.99 0.001**

– 0.199 0.012* 0.608 0.000**

– 0.036* 0.417 0.000**

– 0.010** 0.000**

– 0.001**

–

Significant, two-tailed Po 0.05. Significant, two-tailed Po 0.01.

nn

Generally speaking, there are two factors that cause soil erosion by water: one is splash erosion by rain drop, and the other is erosion by slope shallow surface runoff (Ellison, 1947). Hedgerow plants can mitigate these two factors to reduce the soil erosion. To reduce the splash erosion, plants should cover the soil well enough from the rain drop. In morphology, Hemerocalliscitrina (HC) was taller than Ophiopogon japonica (OJ), and lower in LAI than OJ. But LAI of both plants were more than 1, which means rain drops were always blocked by plants. However, rain drop would get more kinetic energy after being blocked by HC because it was higher than OJ, so that soil would be easier to be eroded by rain splashing. The other factor is runoff. To eliminate the hedgerows' impact, the relationship between runoff and erosion was measured here by 9 simulating precipitation events on the control plot. The result is shown in Fig. 6. The denser the plants' stalks are, the more effective the plants are in reducing runoff (Shen et al., 2010). Stalks of OJ were much denser, which made it the better species to be a hedgerow plant. Soil water content is also believed to be an important factor to control the erosion, because it determines how much precipitation could permeate into the soil and lead to a less runoff (Foster et al., 1984; Nearing et al., 1997). We also tested the soil water content before rains in different treatments by simulating precipitation. As shown in Fig. 6(b), at the same plot, runoff volume was less when the soil water content was low, and vice versa. But when it came to the different plots with different hedgerow patterns, the result was complicated. Since the runoff volume was not the only factor, sometimes, there would be more soil water content and runoff but less erosion. Table 6 shows that hedgerow reduced the soil water content by a certain degree, and it ascribed transpiration and water utilization to the plants. However, this data was obtained in autumn when soil water content remained at a high level. When weather was too drought, instead of consuming water, plants could keep the water in the soil by increasing the organic matters (Täumer et al., 2005; Vogelmann et al., 2013), and could diminish soil loss when the next storm came by increasing the concentration of water stable aggregates (Le Bissonnais, 1996).

Fig. 3 shows that the sediment was mainly from the middle and lower parts of the slope, and the slope surface simulation (Fig. 5) shows the same results. Since the surface runoff started to accumulate from the top of the slope, and the volume gradually increased along the slope, the upslope was less eroded by runoff. The velocity of surface runoff increased from upslope to downslope, which led to more erosion at the middle and lower parts. Not only the surface runoff but also the interflow increased from the upslope to downslope, and it made the soil water content increase in the same direction. Compared to the top, the rainfall in

Fig. 6. (a) The relationship between erosion and runoff volume; (b) the relationship between soil water content before the precipitation and runoff volume. The data used in this figure were gained under simulating precipitations at the control plot.

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the middle and lower parts became harder to permeate into the soil, which also increased the surface runoff in the middle and lower parts. As the results shown in Fig. 5, the hedgerows changed the slope shape. Only the control and the two treatments with best control ability were simulated here. Hedgerows shaped the slope in two directions, along the slope and across the slope. The sediment eroded from the upslope tended to deposit in front of the hedgerow belt, while the soil below the belt was washed away more severely. That is because the runoff was slowed down at the hedgerow belt and sediment was blocked by the hedgerows. And downstream the belt, the runoff speeded up again. The process made the slope become a terrace gradually. The result was similar to the research of Lin et al. (2009). And across the slope, the hedgerows slowed down the development of rills (Fig. 5d–f), which made the slope smoother than the bare slope. Rills developed were mainly driven by the surface runoff and caused more severe erosion than interrill erosion (Li, Li, Ding, Liu, & Yao, 2006). According to Fig. 2, we could see there was the least runoff in Treatment DJ. Since the hedgerows reduced the runoff significantly, hedgerows were effective in controlling the rill development. However, because there were no other plants between Table 6 Runoff and erosion under one simulating precipitation.

DC SC CJ SJ DJ Control

Runoff (L)

Erosion (g)

Soil water content

228.66 189.46 78.4 117.59 104.53 267.85

113.41 75.78 13.07 27.44 31.36 125

25.23% 24.18% 21.01% 22.12% 22.95% 27.00%

113

hedgerow strips, rills still developed and there was no big difference among treatments with different hedgerow patterns in rill development.

4.3. Effects in reducing the nutrient loss Soil erosion is always accompanied by the loss of soil nutrient contents. There are two ways for the loss of soil nutrients during the soil erosion by water: (1) soluble matter washed away by dissolving in runoff, (2) eroded with soil particles. Fig. 7 shows that the concentration of nutrients in the control was the lowest among all the treatments. It means hedgerows could reduce the nutrients' loss. Total nitrogen showed an obvious decreasing trend, while the other indicators were fluctuating during the experiment. In the study area, precipitation was abundant, hence there would be some weeds on the slope, which led to the fluctuation. Most of the nitrogen in the soil was washed away by resolving in runoff in the form of ammoniacal nitrogen ́ and nitrate nitrogen (Han et al., 2010; Ramos & MartınezCasasnovas, 2004; Udawatta et al., 2006). Lin Chaowen et al. (2007) (Chaowen, Shihua et al., 2007) also pointed out that soil nutrients and organic matters except potassium would be accumulated upstream the hedgerows belt accompanied with clay particles. Soil properties were often used to explain the erodibility. Our results indicated that the hedgerows could increase the porosity of the soil and soil bulk density, which could increase the water permeability, and delay the surface runoff production. It also indicated that the hedgerows could increase the big water-stable aggregates in the soil. By increasing the content of organic matters in the soil, the big water-stable aggregates were increasing, and

Fig. 7. Change of soil nutrients in one year. (a) is the organic matters, (b) is the total nitrogen, (c) is the total phosphorus, and (d) is the total potassium.

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Fig. 8. (a) 40.25 soil aggregates in different sections of slopes; (b) destruction rate of soil aggregates and (c) soil constitution.

the sand proportion was decreasing. These two effects led to less erosion by water. Soil aggregates are known as an important factor in measuring the soil erodibility (Barthès & Roose, 2002; Bryan, 2000; Le Bissonnais, 1996; Le Bissonnais & Arrouays, 1997). There were already lots of research in studying the effect of the soil aggregates on soil erosion (Chen et al., 2011; García-Orenes et al., 2009; Shi et al., 2012). Additionally, we conducted more studies about aggregates in this research, both in dry sieve and wet sieve analyses. Soil samples were taken at a depth of 0–5 cm. Soil aggregates were divided into 7 tiers by diameter, 45 mm, 5–3 mm, 3–2 mm, 2– 1 mm, 1–0.5 mm, 0.5–0.25 mm, and o0.25 mm. Water-stable aggregates 40.25 mm indicated the soil dispersion rate. The smaller it is, the easier that soil is to be eroded (Chen et al., 2011). Bernard (Barthès & Roose, 2002) pointed out that the amount of erosion would be inversely proportional with concentration of 40.25 mm water-table aggregates at the shallow surface. As shown in Fig. 8(a), from upslope to down slope, the content of 40.25 mm water stable aggregates was decreasing in all of the treatments. Specially, the content was the lowest in the control, regardless the slope sections. Compared among the treatments, treatment DJ was the highest in this indicator at each slope section. The content of 40.25 mm water stable aggregates was significantly less in up slope and middle slope at treatment with hedgerows than that in the control, which means that the erodibility became stronger at these two parts because of the hedgerows, and the finding was consistent with the result of REE method.

Comparing the results by two sieving methods, we also calculated the destruction rates in every tier. Destruction rates of 45 mm aggregates and 40.25 mm aggregates are shown in Fig. 8 (b). Destruction rates indicate the ability of the aggregates to keep integrity from rain drop splashing and surface runoff eroding. As shown in Fig. 8(b), the destruction rate of treatment DJ was the lowest, and that of the control was the highest. Fig. 8(c) shows the constitution of the soil at different slope parts. Treatments with hedgerows were apparently higher in clay and lower in sand than the control. It indicated that hedgerows could prevent the soil from desertification. Because clay particles were easier to accumulate upstream the hedgerow belts (Chaowen et al., 2007), there would be less erosion if clay ratio was higher. And the proportion of sand was the lowest in Treatment DJ. In this respect, DJ was also the best hedgerow pattern to improve the soil erodibility. Moreover, according to the research of Chaowen et al. (2007) in purple soil area, because of the hedgerows, the eroded soil particles and the nutrients would be re-distributed along the slope section between two hedgerow strips, and the trend kept the same among slope sections. In this study, to illustrate the trend of the hedgerow's effect on the redistribution of eroded soil on the whole slope, and considering the distribution of the sediment deposit on the slope sections were the same, we only used one sample point of each section of the slope. However, it needs further study for the redistribution of the eroded soil on the slope section between hedgerow strips in the red soil area.

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5. Conclusion This study based on runoff plot observation, from several aspects, analyzed the effect of different-pattern contour hedgerows in controlling erosion by water at slope land. The results could be summarized as: (1) contour hedgerows could significantly reduce the soil loss and surface runoff; (2) contour hedgerows could change the ratio of the sediment source and shape the micro-topography of the slope; (3) contour hedgerows could reduce the soil nutrients' loss and improve the soil physical composition. Integrating all the results, double row Ophiopogon japonicas had the best ability in controlling the soil and nutrient loss in the condition of the experiment area. The species selected in this study were all grass. Though grass may be weaker in reducing erosion than woody plants, it could be used to build up contour hedgerows in a much shorter time. In this study, the hedgerows were mature enough 2 years after being planted to control the soil loss, while the hedgerows built by woody plants need at least 5 years. Moreover, different parts of this research all indicated the middle and lower parts of the slope would be more easily to be eroded. In further research and cultivated practice, water and soil conservation in these two parts of the slope should be considered in priority, especially the lower part. Several studies had pointed out that hedgerows would compete water, sunlight, nutrients and space with the crop (Agus et al., 1999; Dercon et al., 2006; Oshunsanya, 2013). Considering that the effect depends on the weather and environment, how to keep a balance between controlling soil loss and avoiding competing with crop would be the direction of further research in this area.

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