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Acta Agriculturae Scandinavica, Section B — Soil & PlantScience

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Yield and water use efficiency of dryland potatoin response to plastic film mulching on the LoessPlateau

Qiang Li, Hongbing Li & Suiqi Zhang

To cite this article: Qiang Li, Hongbing Li & Suiqi Zhang (2017): Yield and water use efficiencyof dryland potato in response to plastic film mulching on the Loess Plateau, Acta AgriculturaeScandinavica, Section B — Soil & Plant Science, DOI: 10.1080/09064710.2017.1377761

To link to this article: http://dx.doi.org/10.1080/09064710.2017.1377761

Published online: 18 Sep 2017.

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Yield and water use efficiency of dryland potato in response to plastic filmmulching on the Loess PlateauQiang Lia,b*, Hongbing Lia,b* and Suiqi Zhanga,b

aState Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy ofSciences and Ministry of Water Resources, Yangling, People’s Republic of China; bUniversity of Chinese Academy of Sciences, Beijing, People’sRepublic of China

ABSTRACTWater shortage is a key factor limiting potato yield in dryland farming regions, and the practice ofsoil mulching with plastic film has proven to be effective for increasing crop yield and improving soiland water conservation. This study investigated the effects of plastic mulching on potato yield andwater use efficiency (WUE) on the Loess Plateau of China based on a synthesis of 44 publications(1987–2016) from 17 sites. Our results indicated that relative to no mulching, plastic mulchingsignificantly increased the yield and WUE by 29.2% and 31.7%, respectively. The effects on yieldunder different mulching methods were ranked as follows: ridge-furrow mulching (RFM) > flatmulching (FM), and the effects on yield with different mulching extents were ranked full-areamulching (FAM) > partial-area mulching (PAM). The effects of plastic mulching on the yield usingdifferent colors of film were ranked as white plastic mulching (WPM) > black plastic mulching(BPM) and under different rainfall levels as (precipitation < 400 mm) > (precipitation > 400 mm).The rankings of the effects of plastic mulching on WUE were the same as for potato yield. Theyield-evapotranspiration (ET) relationship could be described using a quadratic polynomial, andthe yield response curve to ET under plastic mulching fell above the no-mulching curve,indicating that plastic mulching can result in higher yields than no mulching when ET is equal.The yield-ET relationship indicated that potatoes required an ET of 334 and 360 mm to achievethe highest yield under plastic mulching and no mulching, respectively, and potato WUEincreased linearly with yield under both plastic mulching and no mulching. This study indicatedthat RFM, FAM and WPM are better potato management measures and that there is potential toimprove the yield and WUE on the Loess Plateau.

ARTICLE HISTORYReceived 21 June 2017Accepted 5 September 2017

KEYWORDSPlastic film mulching; LoessPlateau; yield; water useefficiency; potato; economicbenefits

Introduction

Drylands account for approximately 45% of the Earth’sland area (Schimel 2010). In China, rain-fed agriculturalarea covers approximately one-third of the arable land,approximately 40% of which is located on the LoessPlateau (Li 2004), and water stress is the main factorlimiting agricultural production in this region (Zhanget al. 2009). Due to global climate change, the inci-dence of drought on the Loess Plateau has dramati-cally intensified in the past 50 years; the averageannual temperature has climbed sharply, and theaverage annual precipitation has tended to decrease(Xiao et al. 2013). These changes can cause fluctuationsin crop yield in arid and semi-arid environments (Lobellet al. 2008), increasing crop water-use efficiency (WUE)in these regions plays an important role in increasing

crop yields and promoting the sustainable develop-ment of rain-fed agriculture.

On the Loess Plateau, the average annual precipitationranges from 300 to 600 mm, and more than 90% of farm-land cannot be irrigated (Fan, Stewart, Yong, et al. 2005;Wang et al. 2016). In addition, the precipitation is unevenlydistributed with more than 60% of the annual rainfalloccurring in the summer months (July to September) (Liet al. 2000). Furthermore, most rainfall events occur inthe form of low-intensity precipitation that cannot be effi-ciently utilized by crops, while a few rainfall events arethunderstorms that generally cause tremendous soilerosion (Wang et al. 2008). As one of the main staplefood crops in this region, potato has traditionally beenplanted using a flat or row-drilling method, but such con-ventional practices have led to considerable loss of soil

© 2017 Informa UK Limited, trading as Taylor & Francis Group

*These authors contributed equally to this work.Supplemental data for this article can be accessed https://doi.org/10.1080/09064710.2017.1377761.

CONTACT Suiqi Zhang sqzhang@ms.iswc.ac.cn State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and WaterConservation, Chinese Academy of Sciences and Ministry of Water Resources, Yangling, Shaanxi 712100, People’s Republic of China University of ChineseAcademy of Sciences, Beijing, 100049, People’s Republic of China

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moisture and poor soil physical conditions (Huang et al.2003). Due to the scarce and erratic rainfall combinedwith improper field management in the region, thepotato yield is relatively low, and the WUE and economicbenefits of potato cultivation are also low (Hou et al.2010). Thus, enhancing theuseof rainwater and improvingfield management to increase potato yield are necessary.

Plastic film mulching first came to China in 1978 andhas since been widely used, especially in the northwes-tern rain-fed regions (Zhao et al. 2010), because this prac-tice can increase crop yield (Zhang et al. 2011). Plastic filmcan retain soil water content, allow full utilization oflimited rainfall (Zhou et al. 2009), increase the tempera-ture of the topsoil (Wanget al. 2005), reduce the loss of fer-tilizer through eluviation, increase nutrient availability(Haraguchi et al. 2004) and decrease infestation bymany annual and perennial weeds (Cohen et al. 2008);therefore, its use may lead to increases in yield and WUE(Li et al. 2013). However, due to prolonged higher soiltemperatures (Wang et al. 2009) or higher CO2 concen-trations in the micro-environment that may result fromits use (Li, Song, et al. 2004), performing plastic mulchingduring an entire growing season could greatly reduce thecrop yield. In addition, large amounts of plastic filmresidue may have a detrimental effect on the environ-ment, soil structure, water and nutrient transport andcrop growth (Liu et al. 2014).

The relationship between crop yield and water use is amajor issue in agricultural research in the arid and semi-arid areas of the world (Musick et al. 1994; Zhang et al.1999; Fan, Stewart, Payne, et al. 2005). The yield-wateruse relationship takes different forms and varies withsoil texture, climate, crop type and variety, and irrigation,fertilization, and tillage practices. Kang et al. (Kang et al.2002) reported a curvilinear yield-evapotranspiration(ET) relationship for winter wheat when limited irrigationwas applied on the Loess Plateau of China, and similarrelationships have been found for wheat under long-term nitrogen fertilization regimes (Zhong and Shang-guan 2014) and under five mulching and three tillagepractices in northwest China (Wang and Shangguan2015). Other researchers have showed that grain yield islinearly related to ET (Huang et al. 2004; Fan, Stewart,Payne, et al. 2005). Crop WUE values are primarilyrelated to yield, and some studies have indicated thatWUE increases linearlywithgrain yield (Zhong andShang-guan2014;WangandShangguan2015)while others havefound a quadratic WUE-yield relationship (Musick et al.1994; Zhang, Wang, You and Liu 1999; Huang et al.2004). In cereals, such as wheat, the yield-water userelationship has been widely investigated, but thepotato yield-water use relationship under plastic mulch-ing remains unclear. Therefore, investigating the potato

yield-water use relationship is of great importance to sus-taining rainfed agriculture on the Loess Plateau.

Although many field experiments have been con-ducted to assess the impacts of plastic film mulching onpotato yields and WUE, no comprehensive, quantitativeanalyses of the available published data have been per-formed thus far. The present study investigated theeffects of plastic mulching on the yield, WUE and ET ofpotatoes through a synthesis of published results. Theobjectives of this study were (1) to quantify the responsesof potato yield,WUE and ET to plasticmulching and estab-lish the potato yield-water use relationship on the LoessPlateau and (2) to compare the effects of differentplastic mulching methods, mulching areas, colors ofplastic film, and levels of rainfall on yield and WUE.

Materials and methods

Data compilation

A comprehensive literature search of electronic databases,including the Web of Science (http://www.isiknowledge.com/) and the China National Knowledge Infrastructure(http://www.cnki.net/), was conducted. The literaturesearch terms included ‘mulch’ or ‘mulching’, ‘potato’ or‘Solanum tuberosum’, ‘yield’, ‘water-use efficiency’ or ‘eva-potranspiration’ in the article title, abstract, and keywords.A flow diagram of the data search and collection processis presented in Figure 1. Two investigators independentlyscreened the titles, abstracts and full texts of the studies inthe search based on the following criteria: (1) fieldexperimental studies must have compared a plastic filmmulching treatmentagainst ano-mulchingcontrol (conven-tional tillage), (2) potatoes must not have been irrigatedduring the entire potato-growing season, and (3) for multi-factorial experimental studies, only observations of controland plastic mulching were included (i.e. interactionsbetween treatments were excluded). In addition, themean values, standard deviations and sample sizes for theyield,WUE, and ETdata had tobedirectly available or calcul-able from the data obtained during the study. After thesearch results were carefully checked, 44 studies (6 inEnglish and 38 in Chinese) conducted at 17 sites met theselectioncriteria (Figure2). Two investigators independentlyextracted the relevant data from each study, and if necess-ary, the third investigator was consulted to resolve any dis-agreements. If the data used for analysis were in the form offigures or charts, they were transformed from digital ver-sions into numerical values using GetData Graph Digitizerversion 2.25 (http://getdata-graph-digitizer.com/).

Regarding the effects of plastic mulching on potatoyield, WUE, and ET, we considered four groups of control-ling factors: the mulching method, mulching extent,

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color of plastic film, and precipitation (see Table 1 fordetails). The plastic mulching methods were divided intotwo subgroups, ridge-furrow mulching (RFM) and flatmulching (FM), and the mulching extent was categorizedas either full-area mulching (FAM) or partial-areamulching(PAM). There were two types of plastic film color, blackplastic mulching (BPM) and white plastic mulching(WPM). Relevant studies were divided into two subgroupsaccording to annual precipitation levels (< 400 mm and >400 mm), as described in previous reports (Li and Xiao1992; Huang et al. 2015).

Data processing

In this study, WUE (kg ha−1 mm−1) was defined asfollows:

WUE = YET

(1)

where Y is the potato yield (kg ha−1), and ET is the eva-potranspiration (mm) reported in the study.

The data were analyzed using the meta-analysismethod described by Hedges et al. (Hedges and Olkin1985). The effect size of plastic mulching for each obser-vation of yield, WUE, and ET was estimated according tothe SMD (standardized mean difference, g):

g = (�XE − �XC)Swithin

J (2)

Swithin =�����������������������������NE − 1(SE)

2 + NC − 1(SC)2

NE + NC − 2

√(3)

J = 1− 34(NC + NE − 2)− 1

(4)

Figure 1. Flowchart of the process of obtaining data from the literature to build a database for this study.

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Vg = NC + NE

NCNE+ d2

2(NC + NE)

( )J2 (5)

where �XE and �XC are the means for the treatment andcontrol groups, respectively; Swithin is the comprehen-sive within-group standard deviation for every study;NE and NC are the sample sizes for the treatment andcontrol groups, respectively; SE and SC are the standarddeviations for all comparisons in the treatment andcontrol groups, respectively; and Vg is the variance ofindependent research.

The meta-analysis was performed using RevMan 5.1software (developed by the Cochrane Collaboration,Oxford, UK), and in the present study, all of the variablesof interest were continuous (Liu et al. 2011; Wang andShangguan 2015; Xu, Liu, et al. 2015; Xu et al. 2016). Het-erogeneity was assessed by both a chi-square test and anI2 test (Wang and Shangguan 2015; Xu, Liu et al. 2015). Theinverse-variance statistical approach was adopted for the

Figure 2.Map of the Loess Plateau. The locations of the potato field experiments reported in the peerreviewed literature are shown onthe map.

Table 1. Brief descriptions of the materials and methodsassociated with the different mulching methods, mulchingareas and colors of plastic film used in potato fieldexperiments on the Loess Plateau.Item Mulching practice Brief description

Mulchingmethod

Conventional tillage(CT)

The soil was plowed withoutmulching until the next sowing (nomulching)

Flat mulching (FM) Flat planting and mulching withplastic film 7 or 8 μm thick

Ridge-furrowmulching (RFM)

Ridge-furrow planting with plasticfilm mulching; the ridge heightwas 10–30 cm, and the ridgebreadth was 40–80 cm

Mulchingarea

Full area mulching(FAM)

The entire planting area (flat orridge-furrow) was mulched withplastic film

Partial areamulching (PAM)

Alternating strips (flat or ridge)mulched with plastic film and bareland

Color ofplastic film

Black plasticmulching (BPM)

Mulching with black plastic film 7 or8 μm thick

White plasticmulching (WPM)

Mulching with white plastic film 7 or8 μm thick

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meta-analysis. The differences in the means between theplastic-mulching and no-mulching groupswereweightedbased on their sample sizes and standard deviations, asdetermined by the RevMan program, and the confidenceintervals (CI) were calculated according to their weightedeffects. If the 95% CI values of the effect size for a variabledid not overlap with zero, the effect size was consideredsignificantly different. Statistical analyses were performedusing SPSS statistical software (Version 20.0 for Windows,SPSS, Chicago, USA), and differences were evaluated atthe 0.05 level of significance.

Results

Overview of the dataset

The potato yield and WUE were in the range of 9975–47,588 kg ha−1 and 32.9–138.3 kg ha−1 mm−1, respect-ively, under plastic mulching. Seasonal ET was in therange of 142.8–442.6 mm under plastic mulching, butthe mean values of 308.4–388.3 mm showed a relativelynarrow range (Table 2).

Among the 648 observations analyzed in the meta-analysis, 340 involved potato yield; 154 involvedpotato WUE; and 154 involved seasonal ET (Figure 3).In Figures 3(a) and (b), most of the points fall abovethe 1:1 line, indicating that the potato yield and WUEmarkedly increased under plastic mulching comparedwith those values for no mulching. For seasonal ET(Figure 3(c)), the points are distributed almost equallyon both sides of the 1:1 line, demonstrating that thepositive and negative effects were nearly equal andindicating that there was no significant difference inthe effects of plastic mulching and conventional prac-tices on seasonal ET.

Overall effects of plastic mulching on potatoyield, WUE and ET

The overall effect sizes for potato yield and WUE were1.57 (1.32–1.82) and 1.18 (0.79–1.58), respectively, andthe 95% CIs for these parameters did not includezero, indicating a significant positive effect of plasticmulching on yield and WUE (Figure 4(a) and (b),Table 3). However, the overall effect size for ET (0.23,−0.05 to 0.51) indicated no notable differencebetween plastic mulching and conventional tillage(Figure 4(c), Table 3).

On average, the potato yield under plastic mulching(27614 kg ha−1) increased by 29.2% compared with thatof conventional tillage (21,375 kg ha−1) (Figure 5(a)).Similarly, the potato WUE under plastic mulching(84.5 kg ha−1 mm−1) increased by 31.7% comparedwith that of conventional tillage (64.2 kg ha−1 mm−1)(Figure 5(b)). However, there was almost nodifference in the average ET under plastic mulching(361.3 mm) and conventional tillage (357.8 mm)(Figure 5(c)).

Effects of plastic mulching under differentmulching methods

The effects of plastic mulching varied with the mulchingmethod. The mean effect size of plastic mulching for thepotato yield was 1.64 (1.37–1.91) under RFM and 1.18(0.56–1.8) under FM (Figure 4(a)), so the RFM methodcaused a greater improvement in yield (30.0%) than FM(24.8%) (Figure 6(a)). The effect size for the potato WUEunder RFM (1.33, 0.9–1.76) was significant, but theeffect size for potato WUE under FM (0.17, −0.74 to1.08), whose 95% CI included zero, was not significant(Figure 4(b)). The WUE increased under RFM by 32.5%,representing better performance than under FM, 26.4%(Figure 7(a)). The mean effect sizes for potato ET underRFM (0.3, −0.01 to 0.60) and FM (−0.2, −0.98 to 0.58)

Table 2. Actual values of potato yield, water use efficiency(WUE), and evapotranspiration (ET) under plastic mulching onthe Loess Plateau.

ItemCategoricalvariable

Yield(kg ha−1)range

WUE (kg ha−1

mm−1) rangeET (mm)range

Mulchingmethod

FM 11300–43845

74.3–111.4 275.1–432.9

CTFM 9600–30248

41.5–90.7 285.1–439.9

RFM 9975–47588

32.9–138.3 142.8–442.6

CTRFM 9600–34991

24.3–103.2 216.7–439.9

Mulchingarea

FAM 10575–47588

36.5–138.3 235.3–435.4

CTFAM 9600–34991

24.3–103.2 253.8–439.9

PAM 9975–45060

32.8–130 142.8–442.6

CTPAM 9600–34991

24.3–103.2 216.7–439.9

Color ofplastic film

BPM 14371–47588

36.5–138.3 202–442.6

CTBPM 10583–34991

24.3–103.2 219–439.9

WPM 9975–45060

32.9–116.4 142.8–436.1

CTWPM 9600–33199

24.3–90.7 216.7–436.1

Precipitation < 400 mm 9975–47422

59.3–138.3 142.8–413.7

CT< 400 mm 9600–34991

31.2–103.2 216.7–385.6

> 400 mm 13394–47588

32.9–122.2 235.3–442.6

CT> 400 mm 10583–33199

24.3–90.7 306.5–439.9

Notes: FM: flat mulching, RFM: ridge-furrow mulching, FAM: full-area mulch-ing, PAM: partial-area mulching, BPM: black plastic mulching, and WPM:white plastic mulching.

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were not significantly different, and the average seasonalET under RFM (364.1 mm) was higher than that under FM(340.1 mm) (Figure 8(a)).

Effects of plastic mulching with different mulchingextents

The dataset was separated into two sub-datasets accord-ing to different mulching extents. The mean effect sizefor the potato yield under FAM (1.73, 1.34–2.12) waslarger than that under PAM (1.45, 1.12–1.77) (Figure 4(a)), and the increase in the yield under FAM (32.9%)was greater than that under PAM (25.7%) (Figure 6(b)).The mean effect size for potato WUE under FAM (1.28,0.64–1.92) was slightly larger than that under PAM(1.11, 0.62–1.6) (Figure 4(b)), and potato WUE benefitedmore from FAM (35.7%) than from PAM (26.2%)(Figure 7(b)). Neither the effect size for potato ET underFAM (0.06, −0.29 to 0.41) nor that under PAM (0.38,−0.1 to 0.86) were significant (Figure 4(c)), and theaverage ETs under FAM (360.8 mm) and PAM(361.8 mm) were approximately the same (Figure 8(b)).

Effects of plastic mulching with different colors ofplastic film

The effects of plastic mulching were also affected by thecolor of the plastic film. The mean effect size for thepotato yield under BPM (1.34, 1.01–1.68) was smallerthan that under WPM (1.74, 1.39–2.08) (Figure 4(a)).The mean effect size for potato WUE under BPM (0.95,0.54–1.37) was also smaller than that under WPM (1.67,0.78–2.55) (Figure 4(b)), and the increase in potatoWUE under BPM (30.4%) was smaller than that underWPM (33.8%) (Figure 7(c)). The effect on potatoET under BPM (−0.04, −0.26 to 0.33) was not significant,but the corresponding value under WPM (0.72, 0.07–1.38) indicated a significant positive effect (Figure 4(c)).

Effects of plastic mulching under different levelsof rainfall

The effect of rainfall < 400 mm (1.84, 1.21–2.47) on thepotato yield was larger than the effect of rainfall >400 mm (1.46, 1.21–1.71) (Figure 4(a)), and similarly, the

Figure 3. Results of the comparison of plastic mulching and conventional tillage. (a) Potato yield, (b) water use efficiency (WUE), and(c) evapotranspiration (ET). In the Figure, each black point represents the result of one individual comparison between plastic mulchingand the control. The points lying on the 1:1 line indicate that the corresponding values responded similarly to plastic mulching and thecontrol, whereas the points distributed above or below the line indicate positive or negative effects, respectively, under plasticmulching.

Figure 4. The relative effect sizes for potato yield, water-use efficiency (WUE), and evapotranspiration (ET) under different plasticmulching methods, mulching areas, colors of plastic mulch and annual precipitation on the Loess Plateau. Error bars represent the95% CI, and values close to the bars represent the corresponding numbers of observations. FM: flat mulching, RFM: ridge-furrow mulch-ing, FAM: full-area mulching, PAM: partial-area mulching, BPM: black plastic mulching, and WPM: white plastic mulching.

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increase in yield under rainfall < 400 mm (31.3%) wasgreater than the increase in yield under rainfall >400 mm (28.4%) (Figure 6(d)). The effect on potato WUEunder rainfall < 400 mm (2.68, 0.97–4.4) was much largerthan the effect under rainfall > 400 mm (0.95, 0.6–1.3)(Figure 4(b)). There was greater improvement (47.4%) inpotato WUE under rainfall < 400 mm than under rainfall> 400 mm (23.7%) (Figure 7(d)). Neither the effect onpotato ET under rainfall < 400 mm (0.05, −0.67 to 0.77)nor the effect under rainfall > 400 mm (0.24, −0.04 to0.53) were significant (Figure 4(c)). The average seasonalET under rainfall >400 mm (388.3 mm) was much greaterthan that under rainfall <400 mm(308.4 mm) (Figure 8(d)).

Input, output and net revenue of potato

Among 44 publications included in this synthesis, 10 pub-lications reported the economic benefits of potato underplastic mulching. The economic benefit was much higherin plastic film mulching than no mulching despite itshigher cost in mulching material and labor (Table 4). Theoutput/input and net revenue were increased by 10.86%and34.43% forplasticmulchingcompared tonomulching.

Water consumption characteristics

On the Loess Plateau, the relationship between thepotato yield and seasonal ET under plastic film mulchingcould be described by a single quadratic function

obtained via regression analysis after pooling all of thedatasets (y =−0.9344x2 + 624.0280x – 71469.2079, P <0.01, Figure 9(a)). A curvilinear ET-yield relationship wasalso found under no mulching (y =−0.5374x2 +386.5037x – 43957.3058, P < 0.05, Figure 9(a)). Thepotato yield decreased when ET exceeded a certain criti-cal value, which was approximately 334 mm underplastic film mulching and 360 mm under no mulching(Figure 9(a)). Regression analysis also produced parabolicWUE-ET relationships under plastic mulching (y =−0.0017x2 + 1.0013x – 44.6867, P < 0.01) and no mulch-ing (y =−0.0016x2 + 0.9472x – 61.8693, P < 0.05) (Figure9(b)). The relationship between WUE and yield isshown in Figure 9(c); potato WUE was observed toincrease linearly with increasing yield under bothplastic film mulching and no mulching. On the LoessPlateau, an increase in yield of 1 kg ha−1 led to anincrease in WUE of 0.0021 kg ha−1 mm−1 under plasticmulching and an increase of 0.0019 kg ha−1 mm−1

under no mulching (y = 0.0021x + 26.0003, R2 = 0.7348,P < 0.01 and y = 0.0019x + 25.326, R2 = 0.555, P < 0.01).

Discussion

Responses of yield and water-use efficiency toplastic mulching

Our dataset consisted of 648 observations from 44studies (Figure 1) conducted at 17 sites on the Loess

Table 3. Results of the meta-analysis (heterogeneity analysis) of the effects of plastic mulching on the yield, water-use efficiency (WUE),and evapotranspiration (ET) of potato on the Loess Plateau using random-effects models.

ItemCategoricalvariable

Number ofobservations

P-value of effectsize

Heterogeneity analysis

chi-square df

P-value of chi-squaretest

Heterogeneity (I2)(%)

Yield Mulching method FM 46 0.0002 26.52 13 0.23 17RFM 294 < 0.0001 175.75 40 0.05 17

Mulching area FAM 164 < 0.0001 103.81 32 0.04 22PAM 176 < 0.0001 103.14 39 0.11 16

Color of plasticfilm

BPM 150 < 0.0001 82.65 21 0.23 10WPM 190 < 0.0001 115.91 31 0.06 19

Precipitation < 400 mm 104 < 0.0001 78.17 17 0.009 35> 400 mm 236 < 0.0001 121.99 26 0.36 4

WUE Mulching method FM 18 0.72 8.35 3 0.4 4RFM 136 < 0.0001 93.27 19 0.02 28

Mulching area FAM 82 < 0.0001 51.37 14 0.11 22PAM 72 < 0.0001 52.25 15 0.03 33

Color of plasticfilm

BPM 92 < 0.0001 51.47 11 0.24 13WPM 62 0.0002 52.92 11 0.006 43

Precipitation < 400 mm 52 0.002 50.45 10 0.002 50> 400 mm 102 < 0.0001 53.51 9 0.34 7

ET Mulching method FM 18 0.62 6.52 3 0.59 0RFM 136 0.06 86.64 19 0.05 23

Mulching area FAM 82 0.74 45.72 14 0.25 13PAM 72 0.12 49.12 15 0.06 29

Color of plasticfilm

BPM 92 0.8 34.45 11 0.87 0WPM 62 0.03 56.56 11 0.002 47

Precipitation < 400 mm 52 0.9 41.46 10 0.02 40> 400 mm 102 0.1 52.12 9 0.39 4

Notes: FM: flat mulching, RFM: ridge-furrow mulching, FAM: full-area mulching, PAM: partial-area mulching, BPM: black plastic mulching, and WPM: white plasticmulching.

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Plateau (Table 1, Figure 2). Plastic mulching showed pro-moting effects for yield and WUE of potato comparedwith no mulching (Figure 3). In an analysis of wheatand maize yields under soil mulching, Qin et al. (Qinet al. 2015) found that plastic mulching increased boththe yield and WUE of wheat by 20% and increasedboth the yield and WUE of maize by 60%. The meta-analysis conducted in the present study demonstratedthat plastic mulching had significant positive effects onthe potato yield and WUE on the Loess Plateau (Tables2 and 3, Figure 4); our results based on data collectedfrom the peer-reviewed literature showed that plastic

mulching increased potato yield and WUE by 29.2%and 31.7%, respectively (Figure 5). Plastic film mulchingcan decrease soil bulk density and increase both soil por-osity and available nutrient contents (Hou et al. 2015),decrease soil evaporation, enhance soil water infiltrationand redistribute soil moisture, thereby relieving waterstress to some degree (Li, Wang, et al. 2004; Gan et al.2013). In addition to modifying the soil micro-environ-ment, plastic film mulching can increase the photosyn-thetic capacity and the endogenous hormone levels ofpotato plants (Hou et al. 2015). Due to these manyadvantageous effects, plastic film mulching increases

Figure 5. Potato yields (a), WUE (b), and ET (c) under plastic mulching and conventional tillage.

Figure 6. Yields under plastic mulching and conventional tillage. (a) Mulching method, (b) mulching area, (c) color of plastic film, and(d) precipitation. FM: flat mulching, RFM: ridge-furrow mulching, FAM: full-area mulching, PAM: partial-area mulching, BPM: blackplastic mulching, and WPM: white plastic mulching.

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potato yields both quantitatively and qualitatively andenhances WUE (Qin et al. 2014; Zhao et al. 2014). Ourstudy also showed that there was no significant differ-ence in ET under plastic mulching and no mulching(Figures 4, 5 and 8). The results are consistent with pre-vious study of wheat on the Loess Plateau (Wang andShangguan 2015).

Wang and Shangguan (Wang and Shangguan 2015)studied the effects of different mulching and tillage prac-tices on wheat yields on the Loess Plateau and foundthat, among the five mulching practices examined,RFM was the most effective practice for improvingwheat yields. In the present study, similar results wereobtained for potatoes, another crop that is widelyplanted in the same region (Figure 4). The increases inyield and WUE under RFM were higher than thoseunder FM (Figures 6 and 7). RFM can enhance thetopsoil temperature early in the growing season(spring) when temperatures are low, and this highersoil temperature is essential for seed germination (Ganet al. 2013; Hu et al. 2014). Tian et al. (Tian et al. 2003)reported that the average runoff efficiencies (expressedas the ratio of runoff to rainfall) under three mulched

ridge treatments (mulched ridges with widths of 0.3,0.45 and 0.6 m) were 53.1%, 63.3% and 69.7%, respect-ively, when the average precipitation was 223 mm.Zhao et al. (Zhao et al. 2012) showed that RFM wasassociated with a high rainfall harvesting efficiency ofmore than 85% when the average precipitation was382 mm. Due to this efficiency, especially in a regionwith little rainfall, RFM retains more stored water andincreases the amount of water available to plants thanFM (Tian et al. 2003), and the higher temperatures andhumidity in the topsoil create favorable conditions forseed germination and seedling growth. Increased emer-gence rates and strong seedling establishment result invigorous growth and high potato yields. In addition,RFM can increase the receiving surface area for solar radi-ation, raise the temperature on a ridge during the dayand increase the cooling area at night, thereby increas-ing the difference in soil temperature between day andnight, which is conducive to crop growth.

Our results suggest that FAM leads to better perform-ance than PAM on the Loess Plateau (Figure 4). Fullmulching with plastic film can greatly improve tuberyields and WUE in comparison with no mulching

Figure 7.Water-use efficiency (WUE) under plastic mulching and conventional tillage. (a) mulching method, (b) mulching area, (c) colorof plastic film, and (d): precipitation. FM: flat mulching, RFM: ridge-furrow mulching, FAM: full-area mulching, PAM: partial-area mulch-ing, BPM: black plastic mulching, and WPM: white plastic mulching.

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because the increase in yield and WUE decreases as themulching area decreases (Figures 6 and 7). For example,in Dingxi (Gansu, China) in 2009, FAM increased thepotato yield and WUE by 57.4% and 70.3%, respectively,compared to the yield and WUE of the control group,while half mulching increased the potato yield andWUE by only 33.1% and 31.9%, respectively; in earlygrowing stages, FAM raised the soil temperature to amuch greater extent than half mulching (Hu et al.2014). Full mulching has also been shown to decreasesoil water evaporation, increase the penetration of rain-water in the soil, and contribute to the accumulation ofsoil moisture (Ramakrishna et al. 2006). Additionally,the combined benefits of higher topsoil moisture andtemperatures under FAM not only improve the potatoemergence rate but accelerate the speed of germination(Zhao et al. 2012; Hu et al. 2014). Plant height, the leafarea index, the relative growth rate and the dry matter

content have also been shown to be higher under FAMthan under PAM (Hu et al. 2014; Qin et al. 2014), andFAM was found to prolong the growth period from seed-ling to maturation, thus contributing to better tuberdevelopment. Therefore, FAM leads to higher potatoyields and WUE than PAM.

Studies on colored plastic film mulch have provideddifferent or even contradictory results. Chang et al.(Chang et al. 2016) reported that potato grown fromthe clear plastic mulch produced more total and >80 gyields than with black plastic mulch. Ibarra et al.(Ibarra-Jimenez et al. 2011) found that the use of blackplastic mulch resulted in a lower total yield than lightermulch colors (aluminum, white or silver) in Mexico.Zhang et al. (Zhang, Wang, et al. 2017) reported thatpotato grown with white plastic mulch had higherWUE than potato grown with black plastic mulch underdrip irrigation in Gansu Province of Northwest China.

Figure 8. Evapotranspiration (ET) under plastic mulching and conventional tillage. (a) Mulching method, (b) mulching area, (c) color ofplastic film, and (d) precipitation. FM: flat mulching, RFM: ridge-furrow mulching, FAM: full-area mulching, PAM: partial-area mulching,BPM: black plastic mulching, and WPM: white plastic mulching.

Table 4. Analyses on economic benefits of potato.Item Output (RMB/ha) Input (RMB/ha) Net revenue (RMB/ha) Output/Input (O/I) Increase in Net revenue (%) Increase in O/I (%)

No mulching 26296 7515 18781 3.50Plastic mulching 34005 8758 25247 3.88 34.43 10.86

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Similar result was also found in cucumber cultivation(Yaghi et al. 2013). In contrast, Singh and Ahmed(Singh and Ahmed 2008) reported that the tuber yieldunder black polythene mulching was higher thanunder white polythene mulching; emergence, plantheight, and the number of stems were also improvedwith black polythene mulching. Anikwe et al. (Anikweet al. 2007) found that black plastic mulched plotsprovide superior edaphic environment and higher yieldfor cocoyam when compared to other treatments used.

For the 44 publications included in this synthesis, mulch-ing practices in 22 publications were white plastic mulch,12 publications were black plastic mulch and 10 publi-cations (one publication reported that potato yield ofwhite plastic mulch was significantly higher than thatof black plastic mulch, three publications showed thecontrary results and other six found that there were nosignificant difference between the two colors) containedboth colors. Because we could not obtain a comprehen-sive result directly from the original literature, thereforewe used meta-analysis and calculated the effect sizesto conduct quantitative comparison of the two colors.WPM was found to perform better than BPM on theLoess Plateau (Figure 4), which may be because themean annual temperature at most sites in this region islower than 10°C, which constrains crop growth. Addition-ally, the light transmissivity of white plastic film is greaterthan that of black plastic film; thus, most of the solar radi-ation can pass through white plastic film, be directlyabsorbed by the soil and raise its temperature, therebyaffecting crop growth. It is possible that the effect ofWPM on increasing temperature is better than that ofBPM, thus WPM results in a higher potato yield than BPM.

Highly effective collection and utilization of rainfall iscrucial to increasing agricultural productivity in semi-arid regions, where the crop yield generally depends onrainfall. Qin et al. (Qin et al. 2015) synthesized data regard-ing water input levels and found that the mean effect ofplastic mulching on maize yield was 60% with low waterinput (< 370 mm) and 40% with high water input(> 370 mm), while the mean effect of plastic on themaize WUE was 70% and 40%, respectively. Our resultsshowed that the increases in the potato yield and WUEassociated with plastic mulching under rainfall< 400 mm were greater than those under rainfall> 400 mm compared with those with no mulching(Figures 6 and 7), which is consistent with the results ofZhou (Zhou et al. 2009) showing that plastic mulchingexhibited great potential to increase crop productionunder limited rainfall. Similarly, plastic mulching signifi-cantly increased the maize yield by 15–26% in dry years,but no significant increase yield was observed in rainyyears in northeast China (Xu, Li, et al. 2015). Such yieldincreases in dry years or dry regions resulted from agreater rate of dry matter accumulation due to thehigher topsoil temperature and water content underplastic mulching.

Economic benefit of plastic mulching

In this study, the output value of plastic film mulchingbased on tuber yield was considerably higher than thatof no mulching. Compared to no mulching, plastic

Figure 9. Relationships among the yield, water-use efficiency(WUE), and evapotranspiration (ET) of potatoes under plasticmulching on the Loess Plateau. (a) Relationship between ETand yield, (b) relationship between ET and WUE, and (c) relation-ship between yield and WUE.

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mulching required additional plastic film and labor.However, no mulching also required more labor forweeding. The input value of plastic mulching washigher than that of no mulching. The resulting increasein output value offset the increase in preharvest costsand assured a positive net revenue. Consequently, thenet revenue was increased by 34.43% for plastic mulch-ing in comparison to no mulching (Table 4). Similarresults were also found in other crops such as maize,cabbage and cotton (Dong et al. 2009; Paranhos et al.2016; Zhang, Wei, et al. 2017).

Relationships among potato yield, WUE, and ETunder plastic mulching

Wang and Shangguan (Wang and Shangguan 2015)summarized data for dryland wheat from 1995 to2014 and found a quadratic relationship betweenwheat yields and seasonal ET. Our study indicated aquadratic yield-ET relationship for potato under plasticmulching and no mulching (Figure 9(a)), and similarrelationships have been found for winter wheat underlimited irrigation and long-term nitrogen fertilizationregimes in northwest China (Kang et al. 2002; Zhongand Shangguan 2014). The potato yield-ET relationshipunder plastic mulching described in this study has notbeen previously reported, and the equations shown inFigure 9(a) demonstrated that potatoes required anET of 334 and 360 mm to achieve the highest yieldunder plastic mulching and no mulching, respectively.Previous studies have indicated that curvilinearrelationships exist between wheat WUE and seasonalET on the Loess Plateau (Kang et al. 2002; Zhong andShangguan 2014; Wang and Shangguan 2015), butthere have been no relevant studies of potato. In thepresent study, significant parabolic relationshipsbetween potato WUE and ET were observed underplastic mulching and no mulching (Figure 9(b)). WhenET was relatively low, water availability was the limitingfactor for yield, and an increase in ET resulted in a sig-nificant increase in both yield and WUE. However, theaugmentation of yield and WUE began to decrease asET further increased. For the yield-ET relationship, thecurve for plastic mulching fell above that for no mulch-ing, demonstrating that with the same ET, a higheryield could be obtained under plastic mulching thanunder no mulching. Plastic mulching was associatedwith a higher attainable yield than no mulching, anda similar WUE-ET relationship existed under plasticmulching and no mulching. The analysis of the yield-ET and WUE-ET relationships provides a possiblemeans of exploring the potential potato yield and theyield gap between conventional tillage and plastic

mulching on the Loess Plateau. Consistent with findingsfor dryland wheat in the same region, potato WUEincreased linearly as yield increased under bothplastic mulching and no mulching (Figure 9(c)). Inaddition, the slopes (ratios of WUE and yield) for pota-toes under plastic mulching and no mulching were0.0021 and 0.0019, respectively, which are similar tothe corresponding value (0.002) for wheat (Wang andShangguan 2015). The yield-WUE line for plastic mulch-ing fell above that for no mulching, which indicatedthat a higher WUE could be achieved under plasticmulching than under no mulching when the yieldwas equal. Our results demonstrated that a higherWUE could generally be obtained with a higher yield.A higher WUE means that the crop can obtain ahigher yield with less water, so increasing WUE isimportant for balancing the conflict between a higheryield demand and a lower water supply in rain-fedareas.

Limitations of the meta-analysis and futureexpectations

The data presented in this synthesis were derived froma screening of the literature regarding field exper-iments on the Loess Plateau involving differentplastic mulching methods, mulching extents, colorsof plastic film and levels of precipitation, and hetero-geneities may have derived from the followingfactors: (1) the researchers had different personalexperiences and preferences; (2) many of the fieldexperiments did not include any long-term obser-vations; (3) the experimental fields exhibited differentnutrient statuses and fertilizer application rates beforesowing; and (4) climate conditions and field manage-ment practices differed during the potato-growingseason. Although the effect of plastic mulching canbe evaluated via the meta-analytical method, theunderlying sources of meta-analytical uncertaintiesrequire further research.

Some effective planting patterns associated withplastic film mulching have been implemented in recentyears to increase potato productivity and WUE on theLoess Plateau (Zhao et al. 2012; Hu et al. 2014; Qinet al. 2014), but the development of more appropriatefield management strategies remains difficult in thisregion. Using a combination of different field manage-ment practices, such as tillage, mulching, fertilization,and intercropping and crop rotation, crop yields andWUE may be further improved in the future. Such inte-grated measures will not only decrease fluctuations incrop productivity but will provide diverse environmentalbenefits.

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Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by National Science and TechnologySupporting Programs [grant number 2015BAD22B01]; 111project of the Chinese Education Ministry [grant numberB12007]; and the Special Funds for Scientific Research Pro-grams of the State Key Laboratory of Soil Erosion and DrylandFarming on the Loess Plateau [grant number A314021403-C5].

Notes on contributors

Qiang Li is from the Institute of Soil and Water Conservation,Yangling, China. His major research directions include yieldpotential of dryland potato.

Hongbing Li is from the Institute of Soil and Water Conserva-tion, Yangling, China. The major research directions includeecological plant physiology.

Suiqi Zhang is from the Institute of Soil and Water Conserva-tion, Yangling, China. The major research directions includecrop eco-physiological adaptation to arid environment andfield crops management.

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