Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=sagb20 Acta Agriculturae Scandinavica, Section B — Soil & Plant Science ISSN: 0906-4710 (Print) 1651-1913 (Online) Journal homepage: http://www.tandfonline.com/loi/sagb20 Effects of crop-slope interaction on slope runoff and erosion in the Loess Plateau Bo Ma, Gang Liu, Fan Ma, Zhanbin Li & Faqi Wu To cite this article: Bo Ma, Gang Liu, Fan Ma, Zhanbin Li & Faqi Wu (2019) Effects of crop-slope interaction on slope runoff and erosion in the Loess Plateau, Acta Agriculturae Scandinavica, Section B — Soil & Plant Science, 69:1, 12-25, DOI: 10.1080/09064710.2018.1488988 To link to this article: https://doi.org/10.1080/09064710.2018.1488988 Published online: 16 Jul 2018. Submit your article to this journal Article views: 18 View Crossmark data
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Full Terms & Conditions of access and use can be found athttp://www.tandfonline.com/action/journalInformation?journalCode=sagb20
Acta Agriculturae Scandinavica, Section B — Soil & PlantScience
Effects of crop-slope interaction on slope runoffand erosion in the Loess Plateau
Bo Ma, Gang Liu, Fan Ma, Zhanbin Li & Faqi Wu
To cite this article: Bo Ma, Gang Liu, Fan Ma, Zhanbin Li & Faqi Wu (2019) Effects of crop-slopeinteraction on slope runoff and erosion in the Loess Plateau, Acta Agriculturae Scandinavica,Section B — Soil & Plant Science, 69:1, 12-25, DOI: 10.1080/09064710.2018.1488988
To link to this article: https://doi.org/10.1080/09064710.2018.1488988
Effects of crop-slope interaction on slope runoff and erosion in the Loess PlateauBo Maa, Gang Liub, Fan Mac, Zhanbin Lia and Faqi Wud
aState Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University,Yangling, People’s Republic of China; bInstitute of water Resources and hydro-electric Engineering, Xi’an University of Technology, Xi’an,People’s Republic of China; cInstitute of Desertification Control, Ningxia Academy of Agriculture and Forestry Science, Yinchuan, People’sRepublic of China; dCollege of Resource and Environment, Northwest A&F University, Yangling, People’s Republic of China
ABSTRACTCrops are the most important ground cover on slope farmland and have a significant impact on thesoil erosion. But soil erosion on slope farmland is also affected by many other factors, such astopography and rainfall. In order to explore the effect of crop growth on soil erosion on differentslope gradient of slope farmland, and analyze the interaction of crop growth and slope gradienton soil erosion, this study used artificial simulated rainfall to observe the runoff rates and soilloss amounts under different slope gradients for maize, soybeans, and winter wheat in differentgrowth stages. Results showed that crops and slope gradient both significantly affectedproduction and development of slope runoff. Compared with bare land, mean runoff rate onslopes was reduced by 24%, 32%, and 94% respectively, and sediment yield was decreased by44%, 55%, and 99% respectively on maize, soybean, and winter wheat fields. Inhibitory effects ofcrops on slope runoff rate and sediment yield were enhanced with crop growth and decreasedwith increasing slope gradient. Crop growth and coverage could offset the impact of increasingslope gradient on runoff and sediment to some extent and reduced water and soil loss onslopes. Sediment yield was produced largely when the slope gradient was greater than 10degrees on maize and soybean fields, but soil erosion was effectively inhibited when the slopegradient was less than 15 degrees on winter wheat fields. Crop planting can effectively reducethe impact of slope gradient on soil erosion, especially during the flourishing period of crop growth.
ARTICLE HISTORYReceived 16 November 2017Accepted 7 June 2018
Water and soil loss occurs mainly on sloping farmlandand, especially in China, accounts for 50%–60% of totalyield loss (Tang 1999). Therefore, preventing water andsoil loss on sloping farmland is an outstanding problemin modern soil erosion research (Tang 1999). Cropsprovide the main coverage on sloping farmland andhave a different effect on slope runoff and sedimentyield than forest and grass coverage due to their shortgrowing season and extensive influence by humanity.Studies have shown that the runoff rate in farmlandswas far greater than in forest and grassland, the sedi-ment yield was 2–4 times that of forest land and 4–7times that of fallow land, and the range of variationwas large, suggesting that the erosion resistance of farm-land was poor (Li et al. 2006). For example, a runoff plotstudy in China demonstrated that the planting of milleton sloping farmland increased sediment yield by10.37% compared with grassland, while the sedimentyield produced by sloping farmland where sweet pota-toes were planted increased by 6.08 times comparedwith wasteland (Xiao et al. 2015, 2017). Compared with
bare land, farmland with a certain degree of coveragecould lower runoff rate and postpone runoff production.Crops had positive effects on reducing slope runoff rateand sediment yield (Song et al. 1998; Sun et al. 2005;Singh et al. 2011; Wang et al. 2011). For example, inthe artificial rainfall condition, the runoff coefficient ofthe bare slope reached 90%, while the slopes of theplanted peanut and sweet potato were 36% and 39%respectively; Compared with the bare slope, the runoffrate of the sweet potato and peanut cultivation plotsdecreased by 133% and 148% respectively, and the sedi-ment yield decreased by 10.0 and 2.2 times respectively(Zhu et al. 2016). Slope gradient is a driving factoraffecting slope runoff and erosion, and the runoff vel-ocity is determined by the slope gradient (Yao andTang 2001). Studies of the effects of slope gradient onsoil erosion have mainly focused on gentle slopes(<10°) (Fox and Bryan 2000; Valmis et al. 2005; Assoulineand Ben-Hur 2006). Within a certain range, the larger theslope gradient, the greater will be the slope runoff rateand the amount of soil loss (Foster and Martin 1969;Wei and Zhu 2002; Li et al. 2008; Geng et al. 2010;
CONTACT Bo Ma [email protected] State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation,Northwest A&F University, No. 26 Xinong Road, Yangling, China 712100
García-Ruiz et al. 2015; Zhao et al. 2015). However, soilerosion on slopes does not increase continually withincreases in slope gradient, and there is a critical gradewhere runoff and erosion behaviour changes. Accordingto simulated rainfall experiments and field observations,when the slope gradient reaches a certain limit, soilerosion decreased as slope gradient increased,meaning that a critical erosion gradient exists (Yair andKlein 1973; McCool et al. 1987; Liu et al. 1994; Jin 1995;Kapolka and Dollhopf 2001). Most researchers attributedit to an increase in slope, the increase in the area of rain-fall collection, the precipitation, runoff rate and sedimentyield per unit area would be reduced accordingly (Jin1995; Wang et al. 2004; Li et al. 2016). Because of differ-ences in research methods, soil properties, prevailinghydrometeorological conditions and farming practicesetc., the critical gradient in different studies variesgreatly. The critical gradient obtained by theoreticalinference and field observations is generally greaterthan 40 degrees (Renner 1936; Horton 1945; Cao 1993;Liu et al. 2001), but values of less than 30 degrees areobtained by runoff plot testing and indoor simulatedrainfall (Jin 1995; Wang, et al. 2004; Zhang et al. 2015;Li, et al. 2016). Under conditions of crop and other veg-etation cover on sloping farmland, there is a similarrelation between slope gradient and soil erosion, manystudies have shown that soil erosion on farmlandincreased with the increased of slope gradient(Woodruff 1947; Song et al. 2000; Huang et al. 2005;Wang, et al. 2011; Kateb et al. 2013; Dragomir et al.2016; Ma et al. 2016; Anache et al. 2017; Bagio et al.2017; Vaezi et al. 2017). A filed experiment in Chinausing 33 small erosion plots were carried out to deter-mine the soil erosion from sloping farmland, grasslandand forestland, the results showed that plots growthwith maize (Zea mays) and tea (Camellia sinensis (L.)O. Ktze.) at slope gradient ≤30° had significantly highersoil erosion than grassland and forestland. For themore, tea plantation, maize cultivation and grasslandproduced higher soil erosion on moderate slop thanthat on slight slope (Kateb et al. 2013). According tothe research on the erosion process of vineyards, theslope is one of the important causes of soil erosion,and high soil erosion occurs on steep slopes with lowcoverage (Comino et al. 2016). A meta-analysis ofrunoff and soil erosion plot-scale studies under naturalrainfall in Brazilian could be known that more soilerosion produced in cropland and pastures than that inforest, however croplands and pastures on averagehad lower slopes than forest (Anache et al. 2017).Some degree of crop coverage can greatly reduce soilerosion when the slope gradient is less than 5 degreesand soil erosion is produced mainly by rainfall.
However, when the slope gradient is greater than 9degrees, soil erosion increases due to runoff scouring(Woodruff 1947). As for sloping farmland with crops,under the same coverage and rainfall, the larger theslope gradient, the higher will be the slope runoff rate,but the law producing sediment yields is not obvious(Song et al. 2000). Turunen et al. (2017) indicated that,more erosion occurred in the steep slope than in theflat slope, some studies on Chinese loess and purplesoil regions have also obtained similar results (Huanget al. 2005; Lin 2011; Wang et al. 2011, 2008; Zhongand Zhang 2011). Compared with bare land, crop cover-age can effectively reduce runoff rates and sedimentyields. However, such reducing effects differ amongcrop types and vary with slope gradients. For instance,Wang, et al. (2011) reported that on slopes of 5° and15°, maize reduced the runoff rates by 29.2% and12.2% compared to bare land, whereas the decrementof runoff rates under alfalfa (Medicago sativa L.) cultiva-tion changed from 78.5% to 75.0% when slope gradientincreased from 5° to 15°. Similar but even more pro-nounced reducing patterns was observed on sedimentyields, highlighting the varying effects of vegetation cov-erage to soil erosion control on slopes of different gradi-ents. However, related studies were all based onobservations during the crop exuberant period andmature stage, and studies of the effects of slope gradienton soil erosion during the whole crop growing seasonhave been neglected. On sloping farmland with crops,changes in soil erosion with slope gradient have beenaffected by other factors, such as crop species and culti-vation practices, making the pattern change morecomplex. Maize, soybeans (Glycine max), and winterwheat (Triticum L.) are important food and commercialcrops on the Loess Plateau. The purpose of this researchis to evaluate leaf area index, slope runoff rate and sedi-ment yields in runoff plots with different slope gradient,and the interaction of crops and slope gradient on slopeerosion was studied. This study will provide a theoreticalreference and a basis for water and soil loss preventionon sloping farmland as well as water and soil resourceutilisation in farmlands, thus making a contribution tofurther understanding of fundamental erosionmechanism.
Materials and methods
Study area
This study was carried out from 2009 to 2010 at the Soiland Water Conservation Engineering Laboratory, North-west A&F University (Shaanxi Province, P.R. China), situ-ated on the southern fringe of the Chinese Loess
Plateau. The climate is continental monsoon with semi-humid, and the average annual temperature is 12.9°C.The warmest month is July, with an average temperatureof 26.1°C; and the coldest month is January, with anaverage temperature of −1.2°C. The mean annual pre-cipitation is 635.1 mm, mostly concentrated from Julyto September with larger interannual variations; Soils inthe study area are Eum-Orthic Anthrosols, which is akind of cinnamon soil in World Reference Base for SoilResource (WRB). The size distribution and componentsof soil particles were 0.40% of 1–0.25 mm, 8.60% of0.25–0.05 mm, 44% of 0.05–0.01 mm, 13% of 0.01–0.005 mm, 22% of 0.005–0.001 mm, and 12% of<0.001 mm (Lei et al. 2005). Vegetation in the studyarea is mainly forest steppe zone, but the natural veg-etation is almost replaced by artificial vegetation. Atpresent, the study area is mainly dominated by cultivatedland.
Research methods
The study was carried out in the runoff plots of the Soiland Water Conservation Engineering Laboratory atNorthwest A&F University, and observations of thedifferent growth stages of crops were conducted bythe simulated rainfall method (Cheng et al. 2007; Ma2009). The length of the runoff plot was 4 m, and thewidth was 1 m, with a height of 0.4 m from the outletto the surface; because a gently rolling topographywith fewer steep slopes land in the study area, thedesign slope gradient was 3°, 5°, 10°, and 15°, and each
slope gradient had four plots. Set the plot outlet to a con-tracting rectangle to collect runoff and sediment(Figure 1). Under each slope gradient, two plots were cul-tivated with crops according to local customs, and theother two plots were left uncovered as a control (bareland). At every stage of observation, soil water contentin surface soil in the runoff plots was measured by thesoil auger (diameter of the soil auger was 5 cm). Thesoil moisture content was measured by the dry weighingmethod (drying temperature ≥105°C). The determi-nation of soil moisture content is mainly used to deter-mine whether there is a large difference in soilmoisture content before each simulated rainfall. Theresults showed that the soil moisture content beforethe simulated rainfall with little difference. A rainfallsimulator was used to simulate rainfall in the four plotsfor each slope gradient. After the start of rainfall, whenrunoff was generated, runoff samples were collectedfor 1 min using polyvinyl chloride kegs. Collection of 1-min samples was performed every 2 min until rainfallended. The collected runoff samples were taken to thelaboratory for measurement of total volume. Thesamples were then allowed to settle for 24 h to removethe supernatant. Most of the supernatant in the barrelwas directly drained, and then a small portion of theremaining liquid was sucked out with syringes for injec-tions. The precipitated sediment was then dried at 105°Cand weighed to obtain the sediment weight. The rainfallintensity in the study was 80 mm/h, and the duration ofeach rainfall event was 60 min according to the localstorm characteristics occurring in summer and autumn.
Figure 1. Schematic of rainfall simulator (a) and runoff plot (b).
14 B. MA ET AL.
The test crops weremaize, soybeans, and winter wheat,which are the main food crops in the Loess Plateau inChina and were planted in runoff plots in 2009 and2010 and also in fields near the runoff plots to observethe leaf area index (LAI). Maize and soybean wereplanted use hole sowing methods. In the runoff plotsand fields, the planting spacing arrangement of maizewas 60×25 cm2 (row spacing × plant spacing), the plant-ing spacing arrangement of soybean was 40×20 cm2
(row spacing × plant spacing). The winter wheat wassown in drill with seeding quantity of 130 kg/ha, andthe row spacing was 20 cm. The planting densities ofmaize, soybean and winter wheat are all typical andcommon planting densities in the study area. Sincedifferent tillage methods will have a greater impact onthe soil erosion on sloping farmland, straight slopeswere used in this study, and the land was only levelledbefore sowing. After the crop has germinated, no moretillage is done and only manual weeding is performedto avoid the impact of the tillage on the results of thisstudy. The runoff rate and sediment yield on the slopeof maize were observed for six times according to maizegrowth stages, and observed for five time on the slopeof soybean and winter wheat respectively according tosoybean and winter wheat growth stages. The vegetativegrowth stages and leaf area index for each sampling dateare shown in Table 1. Due to the large differences in plantmorphology, canopy structure, and leaf area indexbetween maize, soybean, and winter wheat, the averageobservations during the whole growth period of cropswere used to compare and analyze the differences insoil erosion among the three crops.
At the end of each stage, the leaf areas of maize, andwinter wheat were determined by means of the length-to-width ratio, and total leaf area was determinedaccording to the following formula:
AL =∑n
i=1
(K × Li ×Wi), (1)
where AL is the total area of each plant (cm2), K is amodification coefficient (maize 0.75, millet and winterwheat 0.85) (Cai and He 1994; Feng and Shi 2005), Li isthe length of the i-th leaf (cm), Wi is the width at the
widest point of the i-th leaf (cm), and n is the numberof leaves on the plant.
Soybean leaves were scanned by a scanner at 600 dpi,and then leaf area was measured using Image J.
Ten maize, soybean and winter wheat plants wereused to measure the total leaf area of a single plant ineach growth stage and to calculated the average value.With the measured leaf area, the average total leaf areaon the land could be divided by the total land area toobtain the leaf area index (LAI) (Myneni et al. 2002).
The side-sprinkling rainfall simulation system usedwas designed and constructed by the Institute of Soiland Water Conservation, Yangling, China. Rainfalldevices included the rainfall system and the watersupply system. The rainfall system consisted of twosingle vertical rainfall brackets. A vertical rainfallbracket included the side sprinkler nozzle, nozzlestents, and a pressure-control section. The side-sprinklernozzle was made up of a nozzle body, steam breaker,and outflow orifice. The nozzle was installed on the ver-tical rainfall bracket and fixed by a tripod. Each nozzlewas 6 m above the ground, and the raindrop sprayheight was 1.5 m as it sprayed out of the outfloworifice. Therefore, the height from which raindropsreached the ground was 7.5 m, and the effective rainfallarea was 5×7 m2. The simulated rainfall pattern wascreated by opposing sprays from these two single verti-cal rainfall brackets, forming a superimposed rainfallarea. Supply pressure was controlled by a pressuregauge, and rainfall intensity was controlled mainly byadjusting the supply pressure and bore diameter of theoutflow orifice. Rainfall intensity could be controlledover a range of 30–140 mm/h. The kinetic energy ofthe side-sprinkling rainfall simulator was similar to thatof natural rainfall, and rainfall uniformity was >80% ascalculated by the following formula using rain gauge.
RN = 1−∑n
i=1|Pi − �P|n�P
(2)
where, RN is rainfall uniformity (%), Pi is the rain capacityin the ith rain gauge (mm), �Pis average rain capacity(mm), and n is the number of rain gauges.
Table 1. Vegetative stage and average leaf area at each sampling date.Maize Soybean Winter wheat
Vegetative Stage Symbol Leaf area index Vegetative Stage Symbol Leaf area index Vegetative Stage Symbol Leaf area index
4th leaf V4 0.41 2nd trifoliate V2 0.61 5th node detactable Feekes 7.6 3.356th leaf V6 1.35 Beginning flowering R1 1.99 Boot just swollen Feekes 10.0 4.428th leaf V8 2.32 Full flowering R2 3.82 1/2 of head emerged Feekes 10.3 5.2712th leaf V12 3.52 Full pod R4 4.83 Flowering half complete Feekes 10.51 5.8Tasseling VT 4.45 Full seed R6 6.59 Stubble field SFStubble field SF
Note: The stubble height of maize is about 20 cm, and the stubble height of winter wheat is about 10 cm. Surface litter keeps the original state.
The extent of the impacts of leaf area index and slopegradient on soil erosion could be differentiated use themethod proposed by Cheng et al. (2007). Becauseslope lengths are short in the Loess Plateau and thelowest slope gradient is only 3 degrees in the presentstudy, 3 degrees was taken as the standard slope gradi-ent. The slope runoff rate increment on any slope with aslope gradient of X° (5°, 10°, 15°) was calculated accord-ing to the following formula:
variation of runoff rate caused by slope gradient(VQslope)
= runoff rate on bare land (X◦)–runoff rate on bare land (3◦);
variation of runoff rate caused by crops (VQcrop)
= runoff rate in the crop field (X◦)–runoff rate on bare land (X◦);
total runoff rate increment (VQT )
= runoff rate in the crop field (X◦)–runoff rate on bare land (3◦);
contribution index of slope to runoff rate(CIslope)
= VQslope/VQT (using absolute values); and
contribution index of crops to runoff rate(CIcrop)
= VQcrop/VQT (using absolute value).
The contribution indices of slope gradient and cropsto slope sediment yield were calculated using theabove equations.
Statistical analysis
The IBM SPSS statistics 20.0 (IBM Inc.) software was usedin statistical analysis of the data. To compare the signifi-cance of differences between treatments, a Duncan test(α = 0.05) was used to explore the significance of theeffect of slope gradient on the runoff rate and sediment
yield. Univariate analysis of variance was used to explorethe significance of the effect of LAI and slope gradient onthe runoff rate, sediment yield and sedimentconcentration.
Results
Slope runoff rate and sediment yields underdifferent slope gradients during crop growth
Crops had significant effects on slope runoff and sedi-ment interception under the same slope gradient(Table 2, Figures 2 and 3). Univariate analysis of varianceshowed a significant difference (p < 0.0001) in the effectsof different crops (maize, soybeans, and winter wheat) onslope runoff rate and sediment yields and a significantdifference (p < 0.0001) in the effects of different leafarea indices and slope gradients on slope runoff rateand sediment yields for the same crop. The mean sloperunoff rates were 37.65, 34.26, and 3.14 L/m2 respectively
Figure 2. Slope runoff rates under different slope gradients and leaf area indices for maize (a), soybeans (b), and winter wheat (c). LAI isthe leaf area index. The various letters mean a significant difference between different slope gradients (α = 0.05). BS is bare land. SF isStubble field.
Table 2. Slope runoff rate and sediment yield under differentslope gradient
Crop type ** **Slope gradient ** **Crop type×Slopegradient
** **
Note: Runoff rate and sediment yield under different slope gradient wereaverage value of the whole growth stages. **at 0.01 significant levelfrom the variance analysis.
16 B. MA ET AL.
during the whole growth stages of maize, soybeans, andwinter wheat, representing decreases of 23.08%, 31.88%,and 93.28% respectively compared with bare land(Figure 2). The mean slope sediment yields were202.64, 165.34, and 2.15 g/m2·h respectively during thewhole growth stages of maize, soybeans, and winterwheat, representing decrease of 44.25%, 54.95%, and99.41% respectively compared with bare land (Figure 3).Soil erosion varied significantly in different cropgrowing stages, and the runoff rate and sediment yielddecreased as the leaf area index increased. The runoffrate and sediment yield were the greatest in maizeplots and second-greatest in soybean plots; in runoffplots where winter wheat was planted, runoff rate andsediment yield were the lowest. There was hardly anysoil loss on the sloping surface cultivated with winterwheat under the experimental conditions used here. Itis noteworthy that after harvesting of maize and winterwheat, due to some remaining surface litter, sloperunoff and sediment increased at the stubble stage,but remained less than that of bare land. In summary,the inhibitory effect of crops on soil erosion graduallyincreased as crops grew, and therefore slope runoffrate and sediment yields both decreased. However,there are many factors influencing soil erosion onsloping farmland. For instance, due to crop transpirationand soil evapotranspiration, the soil moisture content ofplots before rainfall may be different, and may also affectthe results of this study. Our study cannot completelyrule out such effects.
There was a significant difference in runoff rate andsediment yields under different slope gradients in thesame growth period in crop fields (P < 0.0001, Table 2).The effects of crop leaf area index and slope gradienton slope runoff rate and sediment yields representedthe interaction of time and space on slope soil erosion,and this interaction caused marked changes in sloperunoff rate and sediment yield. On each slope gradientof plots, runoff rate and sediment yields under cropcover were less than bare land, and the erosion gradually
increased as slope gradient increased. This suggestedthat crops could effectively reduce slope runoff and sedi-ment yield under various slope gradients, but that therewas nonetheless a significant difference due to slope gra-dient (P < 0.0001, Table 2). Take a plot of maize as anexample; during the whole growth period of maize, themean slope runoff rate increased by 52.21% as theslope gradient increased from 3 to 15 degrees, but thisincrease was 1.9% less than for bare land under thesame slope change. Maize could reduce the runoff rateby 24.68% compared to bare land on a sloping surfacewith a gradient of 3 degrees. This value increased to25.16% at 15 degrees and was less than 1% away fromdecreasing the amplitude on the slope with a slope gra-dient of 15 degrees. In the plots where maize was grown,the sediment yield decreased by 53.43% compared withbare land at a slope of 3°, and decreased by 36.94% at15°.Plots planted with soybeans and winter wheat hadsimilar behaviour and characteristics (Figure 2). Whenthe slope gradient increased from 3°to 15°, in plots culti-vated with soybeans and winter wheat, the runoff rateincreased by 69.56% and 76.79% respectively, and thesediment yield increased by 306.88% and 96.58%respectively compared with bare land. This suggestedthat soil erosion on slopes increased with slope gradient,but erosion inhibition decreased as slope gradientincreased.
Runoff and sediment yield processes duringdifferent crop growth stages
Due to soil infiltration and rainfall interception by maizecanopies, the precipitation for runoff formation was5 mm in each test period, and the rainfall amount atwhich slope runoff started gradually increased as themaize grew (Figure 4(a)). However, at the stubble stageof maize, the mean precipitation was 2.24 mm whenslope runoff started because coverage was low andtherefore runoff started earlier. In plots planted in soy-beans, the precipitation values when slope runoff
Figure 3. Slope sediment yield under different slope gradients and leaf area indices for maize (a), soybeans (b), and winter wheat (c).LAI is the leaf area index. The various letters means a significant difference between different slope gradients (α = 0.05). BS is bare land.SF is Stubble field.
started were all within 3 mm at each growth stage, andthe runoff start times was all less than 2 min (Figure 4(b)). The precipitation value at which slope runoffstarted was less than 5 mm at the Feekes 7.6 andFeekes 10.0 stages on winter wheat slope, similar tobare land, but was greater than 29 mm at the Feekes10.3 and Feekes 10.51 stages and was 13 mm at thestubble stage of winter wheat (Figure 4(c)). Thisshowed that winter wheat had a strong influence onslope runoff.
During rainfall, the runoff rate in cropland was alwaysless than on bare land, and the higher the leaf area index,the lower were the process curves for runoff (Figure 4). Inplots cultivated with maize and soybeans, runoff for-mation trends in the V4 stage of maize and the V2stage of soybeans approached that of bare land andvaried greatly as rainfall continued. However, runoff for-mation curves in the VT stage of maize and the R6 stageof soybeans was much lower than that of bare land inFigure 4(a) and 4(b), and the fluctuations were small.The process curves for runoff in different growth stagesall emerged as curves showing the trend of increasefirst and then stable fluctuation and varied greatly atdifferent crop growth stages. The common runoff for-mation process curve indicates that the initial stage ofrunoff production increases rapidly, then more slowly,and gradually approaches a stable state. Compared
with bare slope, on the slope with stubble, the firstrunoff peak was reached in 25 min after runoff formation.However, due to the lack of surface protection, runoff stillshowed large fluctuation after reaching the peak. Unlikemaize and soybeans, the peak value of the runoff for-mation process curve was less than 0.09 mm/min inthe winter wheat field, but 1.06 mm/min on bare land(Figure 4(c)). The process curve for runoff varied greatlyon bare land, but changed little and remained smoothin the wheat field. Although the process curve forrunoff was volatile in the stubble period comparedwith other growth periods of winter wheat, the variationwas less than on bare land.
In plots cultivated with crops, the sediment yield for-mation process curves for different growth periods wereall lower than on bare land (Figure 5), but the fluctuationswere small after the corn V8 stage, the soybean R1 stage,and the winter wheat Feekes 10.3 stage. The sedimentyield formation process was consistent with runofftrends. Because of interception storage by crop canopiesand soil infiltration, runoff on slopes significantlydecreased as crops grew. Because runoff formation pro-cesses changed smoothly, under their influence, sedi-ment yield formation processes also changed gentlyand smoothly. Slope sediment yields and their fluctu-ations decreased with vigorous crop growth, thus stabi-lising the sediment generation process.
Figure 4. Process curves for runoff rate at different growth periods of maize (a), soybeans (b), and winter wheat (c). The plot at 10degrees is given as an example.Note: V4 4th leaf, V6 6th leaf, V8 8th leaf, V12 12th leaf, VT Tasseling, BS bare land, SF Stubble field.
Figure 5. Process curves for sediment yields at different growth periods of maize (a), soybeans (b), and winter wheat (c). The plot at 10degrees is given as an example.Note: V2 2nd trifoliate, V4 4th leaf, V6 6th leaf, V8 8th leaf, V12 12th leaf, R1 Beginning flowering, F7.6 5th node detactable, F10.0 Boot just swollen, F10.3 1/2 ofhead emerged, F10.51 Flowering half complete, VT Tasseling, BS bare land, SF Stubble field.
18 B. MA ET AL.
Runoff and sediment yield formation processesunder different slope gradients
The strength and volatility of the runoff and sedimentyield formation processes changed greatly with slopegradient (Figures 6 and 7). In the same crop growthstage, increasing the slope gradient could reduce thetime required to generate runoff. Under planting ofmaize and soybeans, the runoff formation processesunder different slope gradients took the form of amulti-peak curve, and this trend curve gradually rosewith increasing slope gradient. The pattern was thatslope runoff quickly increased in the first 20–30 minfrom the start of runoff, then increased more slowly,and finally tended to a stable state with somefluctuations.
In plots cultivated with maize and soybeans, thecurves of sediment yield on the 3-degree, 5-degreeand 10-degree slopes are similar, and are relativelystable (Figure 7). However, the sediment trend curveswere higher and more volatile on slopes with gradientsof 15 degrees (Figure 7). On 3-degree and 5-degreeslopes, the terrain slopes gently, which can enhance
rainfall infiltration on the slope surface, and surfaceflow velocity is relatively slow, leading to a significantdecrease in surface runoff sand-carrying capacity andcausing lower sediment yields and a smooth sedimentyield process on 3-degree and 5-degree slopes. Asslope gradient increases, the flow velocity of surfacerunoff is accelerated by the steep inclined plane; runofftends to generated more easily in the steep slopes ascompared with the gentler slopes. The increase in flowvelocity and the inclined plane can easily make runoffconverge on the slope surface and form rills, whichincreased turbulence in runoff and enhanced its sedi-ment-carrying capacity, thus greatly increasing sedimentyield on the slope surface. According to visual obser-vations on site, rills gradually formed on sloping surfaceswith gradients of 10 and 15 degrees as rainfall continuedin maize and soybean fields, and the form of the erosionchanged from sheet and splash erosion to rill and interrillerosion. Obvious and complete eroding rills could beobserved under crop canopies at the end of rainfall.Slope runoff moved in the form of a stream due to therills, accelerating runoff velocity and making slope
Figure 7. Process curves for sediment yields for different slope gradients under maize (a), soybean (b), and winter wheat (c) cultivation.For example, the maize VT stage had an LAI of 4.45, soybeans in the R6 stage an LAI of 6.59, and winter wheat at the Feekes 10.3 stagean LAI of 5.80.Note: VT Tasseling, LAI leaf area index.
Figure 6. Process curves for runoff rate for different slope gradients under maize (a), soybean (b), and winter wheat (c) cultivation. Forexample, the maize VT stage had an LAI of 4.45, soybeans in the R6 stage an LAI of 6.59, and winter wheat at the Feekes 10.3 stage anLAI of 5.80.Note: LAI leaf area index, VT Tasseling.
runoff and sediment flows unstable. At slope gradients of3 and 5 degrees, the terrain sloped gently, so that rillswere not obvious during rainfall, and only some fineerosion cracks appeared on the slope with the 5-degree gradient. Erosion on the 3- and 5-degree slopeswas mainly splash and sheet erosion. The lower flow vel-ocity on gentler slopes greatly reduced runoff volumeand energy and lowered slope runoff rate and sedimentyield.
The formation curves of runoff and sediment yield onwinter wheat plots and their variation with slope gradi-ent differed widely from the maize and soybean plots(Figures 6(c) and 7(c)). Slope runoff and sediment yieldvaried to a lesser extent under different slope gradients,and the highest runoff rate was 0.097 mm/min and thehighest sediment yield 0.08 g/m2·min, values whichwere significantly less than for the other two crops. Thetrend curves of runoff and sediment yield were closelyintertwined under different slope gradients. Increasingthe slope gradient advanced the runoff start time, andthe precipitation before runoff on the slope surfacedecreased from 43.28 mm at 3 degrees to 13.17 mm at15 degrees.
Relationship between the effects of crops andslope gradients on slope runoff and sedimentyields
Univariate analysis of variance showed that the inter-action of leaf area index and slope gradient had signifi-cant effects (p < 0.0001) on runoff rate and sedimentyield. Interactions among leaf area index and slope gra-dient had no obvious influence (p > 0.05) on sloperunoff rate, but significant effects (p < 0.0001) on sedi-ment yields in maize and soybean fields. From the analy-sis of variance, it can be seen that the interactionbetween crop type and slope gradient and the inter-action between crop growth stage and slope have sig-nificant effects on the runoff rate and sediment yield
(p < 0.0001, Table 3). Under the maize and soybeanplanting conditions, the interaction of crop growthstage and slope gradient had no significant effect onthe runoff rate (p > 0.05), but the impact on the runoffrate under planting winter wheat reached a significantlevel (p < 0.0001). However, the interaction of cropgrowth stage and slope has a significant effect on thesediment yield of these three crops (p < 0.0001). Onwinter wheat fields, interactions among leaf area indexand slope gradient had significant effects (p < 0.0001)on runoff rate and sediment yields. The extent of theimpacts of leaf area index and slope gradient on soilerosion could be differentiated. In this study, the contri-bution indices of leaf area index and slope gradient onrunoff and sediment yield were obtained by quotingand improving the contribution rates of leaf area indexon sloping surfaces proposed by Cheng et al. (2007).
According to the calculations, the contribution indicesof the two factors described in the chart trended asshown in Figures 8 and 9. For the runoff rate, the slopegradient increased from 3 degrees to 15 degrees, thecontribution indices of maize to slope runoff ratedecreased from the 1 (baseline value) to 0.5, and
Figure 8. Contribution indices of slope gradient and crops on runoff rate under different slope gradients. Data in this figure are themean values of runoff and sediment yield in every observation stage of crops under different slope gradients.
Table 3. Interaction analysis between crop growth stage andslope gradient.
Note: ** at 0.01 significant level from the variance analysis. ns was notsignificant.
20 B. MA ET AL.
contribution indices of slope gradient from 0 (baselinevalue) increased to 0.69. In the soybean-growing plot,the contribution indices of soybean to runoff ratedecreased from 1 (baseline value) to 0.51, and the contri-bution indices of slope gradient to runoff rate increasedfrom 0 (baseline value) to 0.63. For winter wheat, the con-tribution indices of winter wheat to runoff rate changeslittle, from 1 (baseline value) to 0.97, and the contributionindices of the slope gradient increases from 0 (baselinevalue) to 0.36 (Figure 8). The change of the contributionindices of crops to the sediment yield is similar to that ofrunoff rate (Figure 9). The contribution indices of crops toslope runoff rate and sediment yield all decreased asslope gradient increased, and the contribution index ofslope gradient gradually increased also. From Figures 8and 9, it can be seen that there is an intersectionbetween the polyline of contribution indices of maizeand soybean and polyline of contribution indices ofslope gradient. This is the balance between crop andslope effects on the runoff and sediment production.That is to say, the reduction effect of crops on runoffand sediment is equal to the increase effect of slope gra-dient on runoff and sediment. This intersection (inflec-tion point) occurs at a slope of about 10 degrees. Thevalue of 10 degrees was an important turning point atwhich runoff and sediment behaviour changed onsloping surfaces with maize and soybeans. When theslope gradient was less than 10 degrees, maize and soy-beans could offset the contributions of increasing slopegradient to slope runoff and sediment yield. Comparedwith maize and soybeans, the contribution indices ofwinter wheat to slope runoff and sediment decreasedslightly with slope gradient, but always remained high.The effects of water storage and silt retention bywinter wheat greatly counteracted the negative effectsof slope gradient, leading to very low levels of sloperunoff and sediment yield and only slight changes withincreasing slope gradient. From the Feekes 7.6 stage tothe SF stage, winter wheat plants played a dominantrole in the variation of slope runoff and sedimentyields, but slope gradient had little effect on this
variation. The turning point at which winter wheatoffsets the impact of slope gradient could be seen onlyon slopes with a gradient of more than 15 degrees.Therefore, further research must be carried out onwinter wheat.
The above analysis suggested that runoff rate andsediment yield increased with slope gradient whiledecreasing with the presence of crops, a phenomenonwhich played an important role in soil and water losson sloping farmland. The interaction of slope gradientand crops made the changes in runoff rate and sedimentyield more complex, and the effects of slope gradient onslope runoff and sediment yield decreased to someextent as crops grew. However, the effects of crops onrunoff and sediment yields increased greatly with cropgrowth and gradually became dominant in the variationof slope runoff and sediment yields. According toregression analysis, there was a linear relationshipbetween slope gradient, leaf area index, and runoff andsediment yields under different crops, and the corre-lation was significant (Table 4). According to significancetests, each regression equation was significant (p <0.0001).
Discussion
Many studies have shown that planting crops can reducethe amount of soil erosion on sloping farmland (Song
Figure 9. Contribution indices of slope gradient and crops on sediment yields under different slope gradients. Data as for Figure 8.
Table 4. Regression between runoff rate, sediment yield, andleaf area index and the slope gradient of maize land.Crops Regression relation R2 F value
et al. 1998; Sun et al. 2005; Singh et al. 2011; Wang et al.2011; Zhu et al. 2016). In this study, maize, soybean andwinter wheat were planted on runoff plots, and theresults and laws obtained by artificial simulation of rain-fall experiments were similar to those of previous studies.Wang et al. (2011) conducted a simulated rainfall exper-iment and the results showed that, in the south of theLoess Plateau, maize planting reduced the runoff by28.6% averagely, and the sediment yield was reducedby 30.3% averagely. The results of this study show thatmaize can reduce the runoff by 24% and reduce the sedi-ment yield by 44% averagely (Figures 2 and 3). Althoughthe soil texture of the two research areas is the same, inthe former experiment, the planting density of the maizeand the size of the runoff plot are different from those ofthis study, which may be the reason for the differences inthe experimental results. Both crops and slopes have asignificant impact on soil erosion on sloping farmland(Table 3 and 4), our data support the conclusion bySong et al. (Song et al. 1998; Kaspar et al. 2001; Huanget al. 2005; Sun et al. 2005; Singh et al. 2011; Wanget al. 2011; Kateb et al. 2013; Dragomir et al. 2016; Maet al. 2016; Anache et al. 2017; Bagio et al. 2017; Xiaoet al. 2017).The process of runoff rate and sedimentyield under different crops cover (Figures 4 and 5) anddifferent slope gradient (Figures 6 and 7), the law ofchange and the results of this study support thefindings by Ma (2009). Many studies have shown soilerosion on farmland increased with the increased ofslope gradient, and the results of this study are similar,as shown in Table 2 (Woodruff 1947; Song et al. 2000;Huang et al. 2005; Wang et al. 2011; Kateb et al. 2013;Dragomir et al. 2016; Ma et al. 2016; Anache et al.2017; Bagio et al. 2017). As shown in Figures 8 and 9,the results of this study suggest that when the slope isbelow 10 degrees, planting maize and soybeans canplay a good role in controlling soil erosion, but whenthe slope is greater than 10 degrees, it will still producehigher soil erosion, which is in accordance with thework of Woodruff, who found that 9 degrees was thecritical slope gradient (Woodruff 1947).
In the observations, maize and soybeans were plantedby the sow method. Compared with maize, soybeanshave a higher leaf area index, but have greater runoffrate and sediment yield. One of the possible reasons isthat soybeans have a higher planting density thanmaize and therefore have a higher leaf area index.However, maize is a gramineous plant. Its special leafshape is not prone to bending in the course of rainfall,but it is also better able to withstand the impact of rain-drops, so it has better role in intercepting rainfall (Maet al. 2015). In the experiment, due to the softer leavesof soybeans, we found that during the rainfall process,
bending deformation was more likely to occur due toraindrops (Ma et al. 2015). This results in the exposureof the crown surface, increasing the risk of erosion.Causes the land below the soybean canopy to beexposed, increasing the risk of erosion. Winter wheat isalso a gramineous plant but sowing in drill. Meanwhile,tillering is a specific branching characteristic duringwinter wheat growth and development, which promoteshigh plant density. A row of winter wheat plantsresembles a ‘wall’, slowing down the flow velocity thusreducing the runoff rate and sediment yield. Althoughin the runoff plot where winter wheat is planted, whenthe slope exceeds 15 degrees, soil erosion on the plotcould still be significantly reduced. However, winterwheat is the autumn sowing crop, which mainly growsin winter and spring, and the rainfall during this periodis relatively small. The stubble period of winter wheat isjust in the late spring and early summer, and theamount of rainfall gradually increases. At this time, thesummer sowing crops such as maize and soybean havejust started sowing or have not yet formed effective cov-erage on the soil surface, so it is easier to produce soilerosion under the influence of rainfall. The winterwheat stubble can effectively control the generation ofsoil erosion. In this study, the winter wheat stubblefields in plots of 3 degrees to 15 degrees reducedmore than 92% of the runoff averagely, and theaverage sediment yield was reduced by more than99%, indicating a good soil erosion control.
In the study of the effects of vegetation on soilerosion, canopy interception and energy dissipation ofrainfall cannot be ignored. Because of the large differ-ences in leaf shape and canopy structure betweenmaize and soybean, their energy-dissipating effects onrainfall will also vary greatly, leading to great differencesin splash erosion and surface erosion (Ma et al. 2013,2015). Since only the change of leaf area index atdifferent growth stages of crops was considered in thisstudy (Table 1), the influence of canopy interception onthe slope runoff rate and sediment yield was not takeninto account. Future studies also need to analyze cropdensity, canopy structure, and leaf morphology toclarify the impact of rainfall kinetic energy reduction onthe slope runoff and sediment yield. There are manyuncertainties in the experiment that can affect the obser-vations, such as the previous soil moisture, soil texture,etc. These factors need to be considered in futurestudies.
In this study, an artificial simulated rainfall methodwas used to simulate the soil erosion process undersingle rainfall conditions (Figure 1). This short-term plotexperiment was used to observe the effects of crop soilerosion prevention at different growth stages of crops.
22 B. MA ET AL.
The use of artificial rainfall simulation methods to controlrainfall without observation under natural rainfall con-ditions can avoid the impact of extreme rainfall eventson the research results. Under natural rainfall conditions,the probability of encountering extreme events duringshort-term filed experiments is lower than that of long-term field experiments, which will greatly increase theestimation of the soil erosion, and observations inlong-term field experiments tend to be closer to thelong-term average or natural reality (García-Ruiz et al.2015). The short-term experiments that producedifferent results depend on the choice of study period(Hjelmfelt et al. 1986). The purpose of our research is toobserve the effects of relatively fast-growing crops onsoil erosion, so it is more appropriate to use short-termobservational experiments. However, in order to furtherstudy the impact of crop cultivation on soil erosion, itis necessary to conduct long-term field observations inrunoff plots to make up for short-term experiments.
Water and soil loss on sloping farmland is always acore ecological and environmental problem, especiallyin the Chinese Loess Plateau. In China, sloping farmlandwith a gradient of more than 25 degrees should bereturned to forest or grassland according to theChinese Green for Grain Project. In the Loess Plateau,the policy is, on the one hand, to plant trees to restoreecological systems, and on the other hand, to build ter-races to mitigate the adverse impacts of slopes onrational use of water and land resources, and also to alle-viate the problem of arable land reduction caused byecological restoration. Maize, soybeans, and winterwheat (referred in this paper) are all grain crops grownon the Loess Plateau. The planting area of maize and soy-beans is largest in summer and autumn with favourablehydrothermal conditions and plays an important role inthe local environment and the agricultural economy.This research has found that soil erosion was still sub-stantial when crops were cultivated on slopes with a gra-dient of more than 10 degrees. Taking account of thelower limit of 25 degrees for returning farmland toforest or grassland, sloping farmland with a slope gradi-ent of more than 10 degrees and less than 25 degreesshould be managed to prevent soil erosion, a strategywhich is related to the rationality of the lower slope gra-dient limit designated in the Green for Grain Project inChina. Considering that sloping farmland accounts for alarge proportion of arable land, reducing the lowerlimit of slope gradient for returning farmland to forestand grassland could increase the ecological restorationarea, but would significantly decrease farmland areaand is not realistic. Therefore, depending on the localsituation, terraces can be built and contour tillage per-formed on slopes with gradients between 10 and 25
degrees to mitigate water and soil losses in these areasand improve the local ecological environment.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This research for this thesis has been supported by the NationalNatural Science Foundation of China [grant number 41771311]and [grant number 41561144011], the Western light project ofCAS No. XAB2016B08 and the National Basic Research Programof China (973 Program, 2007CB407201-5).
Notes on contributors
Dr. Ma Bo is mainly engaged in the research on soil erosionprocess and its mechanism in sloping farmland.
Dr. Liu Gang’s research direction is in the impact of basin eco-logical and hydrological processes and land-use change on thesoil erosion process in watershed.
Dr. Fan Ma’s research focuses on soil erodibility and the mainfactors affecting soil erodibility.
Dr. Li Zhanbin is expert in soil erosion and soil and water con-servation engineering. He is mainly engaged in the dynamicprocess of soil erosion and prediction model, soil and waterconservation technology and its environmental effects.
Dr. Faqi Wu, expert in soil erosion and soil and water conserva-tion engineering. The main research areas are soil erosion andsoil and water conservation, land resources and regional eco-logical environment restoration.
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