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Soil water dynamics and water use efciency in spring maize ( Zea mays L.) eldssubjected to different water management practices on the Loess Plateau, China

Yi Liu a ,b ,c ,1 , Shiqing Li a ,b ,*, Fang Chen c , Shenjiao Yang a ,b , Xinping Chen a ,ba State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest Sci-Tech University of Agriculture and Forestry, Yangling 712100, Chinab Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resource, Yangling 712100, Chinac Wuhan Botanical Garden, Chinese Academy of Sciences, Wuhan 430074, China

1. Introduction

Spring maize is one of the main crops on the Loess Plateau inChina, and its high yield, averaging about 12 t hm 2 , is assumed tobenet fromprolonged sunshine,providing adequate light and heatduring its growing season ( Xue et al., 2008 ). However, drought haslong been the primary limiting factor for production of this cropbecause of the shortage and uneven distribution of water resourcesin the area( Kang et al., 2002; Wang et al., 2009; Zhang et al., 2009 ).Toimprove theefciencywith which thelimited water resources areused, it is essential tohave detailed knowledge of thecropeldwaterbalance and evapotranspiration (ET) in the region.

Water and its movement through the soilplantatmospherecontinuum is considered to be one of the most important factors

affecting crop productivity ( Boyer, 1982 ). Water loss throughrunoff, soil surface evaporation, plant transpiration, and soilwaterstorage changes have been studied ( Jin et al., 1999; Liu andZhang, 2007; Liu et al., 2002 ). Frequently, ET, consisting of soilsurface evaporation and plant transpiration, is a major componentof water balance in ecosystems ( Gentine et al., 2007; Parasuramanetal.,2007 ). Several studies use estimates forET to construct waterbudgets for various ecosystems ( Watanabe et al., 2004; Suyker andVerma, 2008 ). Gain yields(GY) canbe described as a linearfunctionof total evapotranspiration (ET) for most crops ( Vaux and Pruitt,1983 ). However, the relationships between GY and ET appeared tobe curvilinear under certain circumstances such as over-irrigation(Sandhu et al., 2002; Liu and Zhang, 2007 ), e.g., excessive irrigationcould lead to an increase in ET without a corresponding increase incrop yield ( Liu et al., 2002 ).

WUE is a comprehensive index that represents the overallefciency of plant water use ( Turner, 1987 ). Thus, it is commonlyused to develop and evaluate optimum water managementstrategies to ensure the most efcient use of water resources.Several soil and crop management practices can increase the cropGYand WUE ( Huang etal.,2005; Fang etal.,in press ). Mulching has

Agricultural Water Management 97 (2010) 769775

A R T I C L E I N F O

Article history:Received 29 September 2009Accepted 12 January 2010Available online 1 February 2010

Keywords:Loess plateauWater management practiceSpring maizeSoil water storageEvapotranspirationWater use efciency

A B S T R A C T

Soil water supply is the main limiting factor to crop production across the Loess Plateau, China. A 2-yeareld experiment was conducted at the Changwu agro-ecosystem research station to evaluate variouswater management practices for achieving favorable grain yield (GY) with high water use efciency(WUE) of springmaize ( Zea mays L.). Four practiceswereexamined: a rain-fed (RF) systemas thecontrol;supplementary irrigation (SI); lm mulching (FM); and straw mulching (SM) (in 2008 only). The soilprole water storage ( W ) and the crop evapotranspiration (ET) levels were studied during the maizegrowing season, and the GY as well as the WUE were also compared. The results showed that mean soilwater storagein the top 200 cm of the prole wassignicantly ( P < 0.05) increasedin the SI (380 mm in2007, 411 mm in 2008) and SM (414 mm in 2008) compared to the FM (361 mm in 2007, 381 mm in2008) andRF (360 mmin 2007, 384 mm in 2008) treatments.The soil water content waslower at theendof growing season than before planting in the 60140 cm part of the prole in both the RF and FMtreatments. Cumulative ET and average crop coefciency ( K c) throughout the whole maize growingseason were signicantly ( P < 0.05) higher in the SI (ET, 501 mm in 2007, 431 mm in 2008; K c , 1.0 in2007, 0.9in 2008) treatment than in theother treatments. Both FM and SI signicantlyimprovedthe GY.The WUE were increasedsignicantly (2325%; P < 0.05) under theFM treatment. It wasconcluded that

both SI and FMare benecialfor improvingthe yield of springmaize onthe Loess Plateau. However, FMispreferable because of the shortage of available water in the area. 2010 Elsevier B.V. All rights reserved.

* Corresponding author at: State Key Laboratory of Soil Erosion and DrylandFarming on the Loess Plateau, Northwest Sci-Tech University of Agriculture andForestry, Yangling 712100, China. Tel.: +86 29 87016171; fax: +86 29 87016171.

E-mail addresses: [email protected] (Y. Liu), [email protected] (S. Li).1 Tel.: +86 27 87510433.

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0378-3774/$ see front matter 2010 Elsevier B.V. All rights reserved.

doi: 10.1016/j.agwat.2010.01.010
mailto:[email protected]:[email protected]://www.sciencedirect.com/science/journal/03783774http://dx.doi.org/10.1016/j.agwat.2010.01.010http://dx.doi.org/10.1016/j.agwat.2010.01.010http://www.sciencedirect.com/science/journal/03783774mailto:[email protected]:[email protected]


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long been widely used in crop eld management in many parts of theworld. Thesurface mulch favorablyinuences thesoil moistureregime by controlling evaporation from the soil surface ( Raeini-Sarjaz and Barthakur, 1997; Wang et al., 2009 ), improvinginltration and soil water retention, decreasing bulk density andfacilitating condensation of soil water at night due to temperaturereversals ( Acharya et al., 2005 ). Soil microclimate under mulchingalso favors seedling emergence ( Albright et al., 1989 ) and rootproliferations ( Osuji, 1990 ) and suppress weed population ( Lalithaet al., 2001 ). Thus, it has been widely reportedthatboth the GY andWUE are increased under mulches ( Li et al., 2001b; Li and Gong,2002 ). However, on occasion, the grain yield of crop can decreaseconsiderably with lmmulching forthe whole growth period( Lietal., 2001a ). Furthermore, the widespread use of non-degradableplastic lm mulch over many years may damage the sustainabilityof rain-fed agro-ecosystems by accelerating the decomposition of soil organic matter, changing the soil structure, and inuencingroot development ( Acharya et al., 2005 ). Straw mulching effectsdepend on the climatic condition and soil type ( Acharya et al.,2005 ). The application of straw mulch is restricted in some place,since it is liable to lower the soil surface temperature, leading toreduction in the yield ( Gao and Li, 2005; Edwards et al., 2000 ).

Irrigation can also have benecial effects on plant waterrelations and yields. For instance, scheduled irrigation at differentgrowth stages can improve WUE according to several studies(Wang et al., 2002; Fang et al., in press ). However, Jin et al. (1999)reported that excessive irrigation can reduce crop WUE, whileeffective decit irrigation may result in higher production andWUE. While Olesen et al. (2000) augued irrigation had little or noeffect on WUE or harvest indices, and that its effects were almostentirely due to increased transpiration. Hence, the reported effectsof irrigation are variable, and the responses of grain yield (GY) andWUE to irrigation were strongly inuenced by soil water contentsand irrigation schedules ( Kang et al., 2002 ).

Most studies have concentrated on examining the soil waterbalance in farmland exposed to only one water managementpractice; few studies have made comparisons among a variety of

water management practices. Our objectives were to: (i) quantifythe soil water storage ( W ) and ET during the maize growingseason; and (ii) determine the effects of eld water managementpractices on soil water balance and WUE.

2. Materials and methods

2.1. Site description

The present study was conducted from 2007 to 2008 at theChangwu Agri-ecological Station on the Loess Plateau (35.2 8 N and107.8 8 E) in Shaanxi Province of China. The experimental site islocated about 1206.5 m above sea level. The loess is more than100-m thick. The soils are Cumuli-Ustic Isohumosols, according tothe Chinese Soil Taxonomy ( Gong et al., 2007 ), and contain 37%clay, 59% silt, and 4%sandand have a pHof 8.4 and a bulk density of 1.3 g cm 3 . The amounts of organic matter, total nitrogen,available phosphorus, available potassium and inorganic nitrogenpresent are 11.8 g kg 1 , 0.87 g kg 1 , 14.4 mg kg 1 , 144.6 mg kg 1

and 3.15 mg kg 1 , respectively. The average annual precipitation is578 mm, with 55%falling between July and September. The annualaverage temperature is 9.2 8 C. The common regional croppingsystem is one crop a year (wheat or maize). Rain-fed agriculture isthe dominant production system.

2.2. Experimental design and treatments

Four water management practices a rain-fed (RF) system(Fig. 1 a), supplementary irrigation (SI) ( Fig.1 b),lm mulching (FM)(Fig. 1c), and straw mulching (SM) ( Fig. 1d) (in 2008 only) wereusedin spring maize elds.The soil water supplyfor the RF, FMandSM treatments relied on natural rainfall, while forthe SI treatment,sufcient moisture in the soil (7085% of the eld water capacity)was maintained using tap water delivered by furrow irrigation. Inthe SI treatment, the crop was irrigated ve times in 2007 (May 8,May 20, June 14, July 14 and August 15) and four times in 2008(May 22, June 5, July 7 and August 4), and the irrigation quota on

Fig. 1. Photographs showing (a) rain-fed (RF), (b) supplementary irrigation (SI), (c) lm mulching (FM) and (d) straw mulching (SM) treatments.

Y. Liu et al. / Agricultural Water Management 97 (2010) 769775770


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each occasion amounted to 33.7 mm and 25.6 mm in 2007 and2008, respectively. On May 8 in 2008, around the time of seedlingemergence, corn straw cut into 0.1 m long segments, was appliedat a rate of 6000 kg hm 2 uniformly on both ridges and furrows inthe SM plots. The treatments were arranged in a completelyrandomized block design, with four replicates for each treatment.The size of each experimental plot was 50.7 m 2 (7.8 m 6.5 m).

Ridge cultivation, a common maize cultivationpracticeacrosstheLoess Plateau, was adopted in all of the four treatments. First,110 kg N hm 2 inthe formofurea (N46%), 50kg P hm 2 in the formof Calcium superphosphate (P 2 O5 12%), and 100 kg K hm 2 in theform of Potassium Sulfate (K 2 O 45%) were broadcast over the soilsurface as a base fertilizer; the elds were then plowed, thus turningthesoiland relocatingthe nutrients tobelowthe surface. Ridgeswereconstructed by banking up soil from both sides to a height of 0.1 mfrom the base, constructing a ridge 0.45 m wide at the top andfurrows 0.15 m wide at the base. In the plots mulched with plastic, alm (0.7 m wide and 0.005 mm thick) was used to cover the soilsurface of the ridges but not the furrows; the edges were securedunder the soil in the bottomof the furrows ( Fig.1 ). Spring maize ( Zeamays L.pioneer 335)was sownin 5 cmdeepholesspaced0.2 m apartalong the midline on the top of the ridge, using a human-poweredhole-drillingmachine,on April20 in2007 andApril 18in 2008. Beforebacklling, water was added as required, to encourage seedlingemergence. Additional nitrogen, in the form of urea, was applied atthe jointing and tasseling stages, at rates of 80 kg N hm 2 and90kgNhm 2 , respectively, following a nutrient management planaimed at achieving a nal yield of 14 t hm 2 . Maize cobs wereharvestedgradually,accordingto theirripeness,from 28Augustto 13September 2007 and from 8 to 20 September2008. Manual weedingwas undertaken as required during the crops growing season.

2.3. Sampling measurements and data calculation

Sampling and measurement procedures were the same in bothcrop growing seasons. Soil water content was determined gravi-metrically by oven-drying (105 8 C for 24 h) the core samples that

weretaken at depthintervals of20 cm down the 0200 cm prole ineachplotat planting time(PT), sixth leafstage(V6),twelfth leafstage(V12), silking stage(R1), milk stage (R3) and physiological maturity(R6). The soil water storage ( W ) in the prole was considered to bethe total storage in all of the sampled layers in the plot, as wascalculated using the formula: W = h r u 1000, where h is soildepth(cm), r is soil bulk density (g cm 3 ), u is soilgravimetricwatercontent (g g 1 ). Change in soil water storage ( D W ) in the 0200 cmprole was calculated using the formula: D W = W t 2 W t 1 , whereW t 1 and W t 2 arethe soil water storage in the0200 cm soil prole attimes t 1 and t 2, respectively.

Evapotranspiration (ET) was determined using the formula:ET = P + I D W , where P is the precipitation (mm) during the crop

growth season and I is the total irrigation amount (mm) ( Zhang etal., 1999, 2005 ). Reference evapotranspiration (ET 0 ) was estimatedeach day using the FAO PenmanMonteith equation according toAllen et al. (1998) , written as

ET0 0:408 D Rn G 900 g u2 es ea =T 273

D g 1 0:34 u2

where ET 0 is reference crop ET (mm d 1 ); D is slope of thesaturated vapor pressuretemperature curve (kPa 8 C 1 ); Rn is netirradiance (MJ m 2 d 1 ); G is soil heat ux (MJ m 2 d 1 ); es and eaare respectively saturated and actual vapor pressure (kPa); g ispsychrometric constant (kPa 8 C 1 ); T is mean air temperature ( 8 C),u2 is wind speed at the height of 2 m (m s 1 ).

The crop coefcient ( K c) was calculated as the ratio of ET toreference ET (ET 0 ) (i.e., K c = ET/ET0 ). WUE was calculated as GY inkg hm 2 divided by total water use in mm (evaluated as ET in thepresent study), i.e., WUE = GY/ET.

2.4. Statistical analyses

The effects of the treatments on the measured parameters wereevaluated by one-way ANOVA. When F -values were signicant,Duncans new multiple range test was used to calculate the leastsignicant difference (LSD) between means. In all cases differenceswere deemed to be signicant if P < 0.05.

3. Results

3.1. Rainfall and reference evapotranspiration (ET 0 ) during theexperimental period

Quite good coincidence was exhibited between the spring maizegrowing season andthe main rainy period in both years ( Fig. 2). Thespring maize growing season rainfall was 302 mm in the 2007experiment and 340 mm in the 2008 experiment, occupying 58.2%and 65.2% of the annual precipitations, respectively.

Diurnal reference evapotranspiration (ET 0 ) ranged from0.5 mm d 1 to 8.1 mmd 1 in 2007, and from 0.6 mm d 1 to7.4 mm d 1 in 2008 ( Fig. 3). The amplitude of seasonal uctuationsin daily ET 0 was quite large during the study period. The ET 0 wasgenerally higher early in the growing season (i.e., May and June)when it was dry and there were high wind speeds and airtemperatures, than later in the growing season (from the July tothe September) when it was rather rainy. The cumulative ET 0 overthewholegrowingseasonwas522 mmin2007and491 mmin2008.

3.2. Growing season evapotranspiration (ET)

The ET in the reproductive stages (R1R6) contributed to thegreatest proportion of the total ET over the whole growing season

Fig. 2. Distribution of monthly rainfall at the experimental site during 2007 and 2008; the maize and wheat growing seasons at the site are also shown.

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(Table 1 ); the shooting stage (V6R1) accounted for the secondhighest level of ET, and the seedling stage (PTV6) for the least.Total ET over the whole growing season was signicantly(P < 0.05) higher in SI (501 mm in 2007, 431 mm in 2008)treatment than the FM (357 mm in 2007, 372 mm in 2008) andRF (372 mm in 2007, 368 mm in 2008) treatments, while there wasno signicant difference in the ET between the two lattertreatments; this indicates that the irrigated plants consumedmuch more water.TotalET wassignicantly( P < 0.05) lower in theSM treatment than the FM and RF treatments in 2008.

The ET/ET0 ratio is also commonly referred to as the cropcoefcient ( K c). The seasonal dynamic of K c in the spring maizeelds appears to have been much the same in both years. K cincreased from the seedling stage until mid-season (stages R1R3)when it reached its maximum value, varying from 0.8 to 2.5; the K cdeclined gradually after that time ( Table 2 ). The average K c of maize over the whole season under the RF, SI, and FM treatmentswas, respectively, 0.7, 1.0 and 0.7 in 2007, and 0.8, 0.9 and 0.8 in2008. The average K c of maize was 0.7 over the whole season forthe SM treatment. The average K c throughout the whole growing

season was signicantly ( P < 0.05) higher in the SI treatment thanthe FM and RF treatments, and the prolonged period of plant

physiological functioning, from growth stage R1 to R3, when the K cwas maintained at a high level, would benet assimilation andtransportation, and hence would benet GY.

3.3. Soil water storage

A large difference in water storage in the soil prole (0200 cm) was recorded in the different years, i.e., it was higher in2008 than in 2007 ( Fig. 4), this was linked to the higherprecipitation in 2008. In 2007, despite there being rather lessprecipitation during the crop seedling stage than the later periodof the growing season, water storage did not decline, and evenslightlyincreased because maize consumesonly a limited amountofwater atthisstage. Duringtherainyseasonon theLoess Plateau,i.e., from July to September, thespring maizetakes up a great dealof water to maintain its luxuriant growth; the variation inseasonal soil water storage was, therefore, mainly affected by theamount of precipitation and maize growth. As a result, waterstorage decreased in 2007 but increased in 2008 during cropgrowth stages V12 to R3 (from June to July) ( Fig. 4); during this

period the precipitation was 200 mm in 2008, compared to132 mm in 2007.

Fig. 3. Diurnal reference evapotranspiration (ET 0 ) over all growth stages.

Table 1Distribution of the evapotranspiration (ET-mm) in different growth stages underrain-fed(RF), supplementaryirrigation (SI), lmmulching(FM) andstraw mulching(SM) treatments in 2007 and 2008.

Year Growthstage

RF SI FM SM

2007 PTV6 7 11 b 61 14 a 3 9 bV6V12 74 16 b 109 20 a 73 12 bV12R1 62 15 ab 69 13 a 52 10 bR1R3 100 25 b 133 22 a 89 16 cR3R6 130 20 a 132 16 a 140 19 aWhole

growthseason

372 32 b 501 28 a 357 24 b

2008 PTV6 28 12 a 33 15 a 30 8 a 22 9 aV6V12 55 17 b 91 34 a 62 23 b 33 17 cV12R1 99 15 b 115 21 a 99 17 b 103 23 abR1R3 103 18 a 84 23 b 86 15 b 52 16 cR3R6 84 10 b 109 22 a 97 19 ab 109 17 aWholegrowthseason

368 26 b 431 33 a 372 27 b 319 31 c

Values are given as means standard error of means ( n = 4). Values followed bydifferent letters within a row are signicantly different ( P < 0.05).PTV6: from planting time to 6th leaf stage.V6V12: from 6th leaf stage to 12th leaf stage.V12R1: from 12th leaf stage to silking stage.R1R3: from silking stage to milk stage.

R3R6: from milk stage to physiological maturity stage.

Table 2Seasonal variation in the K c (ET/ET0 ) in different growth stages under rain-fed (RF),supplementary irrigation (SI), lm mulching (FM) and straw mulching (SM)treatments in 2007 and 2008.

Year Growthstage

RF SI FM SM

2007 PTV6 0.0 0.1 b 0.3 0.1 a 0.0 0.1 bV6V12 0.8 0.2 b 1.1 0.2 a 0.6 0.1 c

V12R1 0.9 0.2 ab 1.0 0.2 a 0.8 0.1 bR1R3 1.9 0.5 ab 2.5 0.4 a 1.5 0.3 bR3R6 1.6 0.2 a 1.6 0.2 a 1.3 0.2 bWholegrowthseason

0.7 0.1 b 1.0 0.1 a 0.7 0.1 b

2008 PTV6 0.2 0.1 a 0.2 0.1 a 0.2 0.0 a 0.1 0.1 aV6V12 0.5 0.2 ab 0.8 0.3 a 0.6 0.2 a 0.3 0.2 bV12R1 1.6 0.2 a 1.9 0.3 a 1.6 0.3 a 1.7 0.4 aR1R3 1.6 0.3 a 1.3 0.4 ab 1.3 0.2 ab 0.8 0.3 bR3R6 1.0 0.1 a 1.3 0.3 a 1.2 0.2 a 1.3 0.2 aWholegrowthseason

0.8 0.1 b 0.9 0.1 a 0.8 0.1 b 0.7 0.1 b

Values are given as means standard error of means ( n = 4). Values followed bydifferent letters within a row are signicantly different ( P < 0.05).PTV6: from planting time to 6th leaf stage.V6V12: from 6th leaf stage to 12th leaf stage.V12R1: from 12th leaf stage to silking stage.R1R3: from silking stage to milk stage.

R3R6: from milk stage to physiological maturity stage.



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Mean soil water storage, calculated by averaging the readingstaken at the sampled crop growth stages over the growing season,weremuchhigherin the SI (380 mmin 2007, 411 mm in 2008) and

SM (414 mm in 2008) treatments than the RF (360 mm in 2007,384 mm in 2008) and FM (361 mm in 2007, 381 mm in 2008)treatments ( Fig. 4 ). SI and SMsignicantly ( P < 0.05) enhanced soilwater storage in the 0200 cm prole compared to the FM and RFtreatments, while there was no signicant ( P < 0.05) difference insoil water storage between the FM and RF treatments.

The soil water content was signicantly higher at the end of thecropping season (stage R6) than at the beginning (stage PT) in the040 cm soil layer under each treatment in 2007, indicating waterrecharge in the soil over the crops growing season. This was notthe casein 2008( Fig. 5). It is likely that heavy Septemberrainfall in2007 improved the moisture conditions near the surface of the soillate in the growing season ( Fig. 2). In the 60140 cm soil prole,there was a clear decrease in soil water content after the maize

growing season in both the RF and the FM treatments in 2007 and2008, indicating a soil water decit in the prole, but the supply of additional water (SI) seemed to compensate for the sub-surfacedepletion during the maize growing season; the SM treatment hadthe same effect. However, the soil water deeper down the prole(160200 cm) rarely changed under any of the treatments over themaize growing season ( Fig. 5).

D W PT-R6 (the change in soil water storage over all the growthstages, i.e., D W PT-R6 = W R6 W PT ) represents the integratedcontribution of ET (soil water depletion) or precipitation andirrigation (soil water recharge) to soil water storage throughoutthis period; this is an important indicator of the sustainability of farmland water. As shown in Fig. 6, the D W PT-R6 was 11 mm and

24 mm for RF, 30 mm and 17 mm for SI, and 4 mm and 28 mm

for FM in 2007 and 2008, respectively. TheD

W PT-R6 was 26 mm for

SM in 2008. The value of D W PT-R6 for both years was negative forRF, and positive for SI. That is, SI not only promoted ETbut also wasassociated with a small amount of water being retained in the soil

prole. However, Rainfall could not compensate for the ETassociated with plant growth in the RF treatment, stored soilwater made up for this decit. For FM, the value of D W PT-R6 waspositive in 2007 (4 mm), and negative in 2008 ( 28 mm), thedifference between the 2 years was mainly related to the differentrainfall amounts and distribution.

Fig. 4. Soil water storage (0200 cm) under rain-fed (RF), supplementary irrigation (SI), lm mulching (FM) and straw mulching (SM) treatments during the experimentalseasons in 2007 and 2008. Error bars are twice the standard error of the mean ( n = 4).

Fig. 5. Soil water content down the prole at the beginning (stage PT) and the end of the maize growing season (stage R6) under rain-fed (RF), supplementary irrigation (SI),lm mulching (FM) and straw mulching (SM) treatments in 2007 and 2008. Error bars are twice the standard error of the mean ( n = 4).

Fig. 6. Change in soil waterstorage (0200 cm) overthe allgrowth stages ( D W PT-R6 )under rain-fed (RF), supplementary irrigation (SI), lm mulching (FM) and strawmulching(SM) treatments in 2007and 2008. Error barsare twice thestandard errorof the mean ( n = 4). Letters on the graph show the result of DMRT ( P > 0.05);

different notations refer to signicant differences between mean values.



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3.4. Grain yield (GY) and water use efciency (WUE)

Both theSI andFM treatments signicantlyincreased thespringmaize GY compared to the RF treatment ( P < 0.05); the enhance-ment was, respectively. 24.6% and 19.7% in 2007, and 36.8% and24.6% in 2008 ( Table 3 ). However, there was no signicantdifference in GY between the SI and FM treatments. Despite soilwater storage (0200 cm) being maintained at a high level in theSM treatment, the GY was still rather low; it is likely that strawmulching decreased the soil temperature and may thus havedecreased root and microbial activities ( Gao and Li, 2005 ).Immobilization of N by corn straw in the SM treatment may beanother reason for low yield in this treatment ( Acharya et al.,2005 ).

FM signicantly ( P < 0.05) increased the grain yield WUEcompared to the RF treatment, with an increase of 2325% in bothyears ( Table 3 ). This was probably the result of restricted waterloss by evaporation, which has no plant physiological signicance,and also the increase in transpiration. However, there was nodenite effect of irrigation on WUE, which varied according to thesoil water conditions. SI wasassociated with an increase in thecropWUE by 12% in 2008, but a signicant ( P < 0.05) decrease in 2007(Table 3 ).

4. Discussion

Maize is characterized by high ET. In the present study, thetotal ET over the whole maize growing season varied in the range319501 mm; these growing season ET values are generallycomparable to results from other studies conducted in the region.For example, Kang et al. (2003) reported a 10-year averageseasonal ET based on lysimeter measurements of 424 mm in asemi-humid region of northwest China. The total ET of springmaizein Inner Mongoliawas 572.5 mm,with a daily average ETof 4.09 mm ( Li et al., 2003 ). Li et al. (2008) found a total ET of 476 mm, with a daily average value of 2.96 mm, and a high K c inthe mid- and late-season.

It has long been known that climate has a major inuence

on K c values, and that it varies as a result of cloudiness,radiation, wind speed, temperature, and so on ( Wang et al.,2007; Drexler et al., 2008 ). In order to obtain good estimatesusing the FAO 56 approach, many researchers have calculatedmonthly K c values or values for different plant growth stages.Allen et al. (1998) calculated maize K c values in the middle andat the end of the growing season of 1.20 and 0.600.35,respectively. A 10-year average maize K c was 0.45 in June(initial stage), 1.43 in August (middle stage), and 0.93 in October(end stage), with an average of 1.04 over the whole season innorthwest China ( Kang et al., 2003 ). Li et al. (2008) calculated anaverage K c of 1.04 over the whole season. Compared to previousstudies, our maize K c (in the range 0.71.0) had a lower thanaverage value. This may be due to the semiarid climate on the

Loess Plateau of China.

Even though the effect of eld water management practices onwater storage was much less than the variations in precipitation,small effects on water conservation during the crops growingseason could greatly affect spring maize yield and ET as well asWUE. SI improved both soil water storage and ET and, hence,signicantly increased the GY ( Table 3 ). It is likely that irrigationafter a soil drying cycle stimulated the physiological processes andcaused compensation or over-compensation in plant growth andGY (Deng et al., 2006 ). However, the WUE did not exhibitconsistent performance in both years under the SI treatment; itwas lower in 2007 than in 2008 when the amount of supplemen-tary water applied was reduced. It appears that additionalirrigation supplementary when the soil water condition is optimalwould have little effect on yield and may even be detrimental ( Jinet al., 1999 ); in addition, excessive irrigation would enhance soilsurface evaporation ( Olesen et al., 2000 ). This could have causedthe reduction in WUE in 2007.

The GY in the FM treatment plots was higher than that in the RFcontrol plots ( Table 3 ). This was probably because mulching withplastic lm reduced soil evaporation, augmented the inltration of rainwater into the soil ( Ramakrishna et al., 2006 ), and enhancedsoil water retention ( Ghosh et al., 2006 ). The FM treatment alsoincreased the WUE ( Raeini-Sarjaz and Barthakur, 1997; Wang etal., 2009 ). However, the whole season average soil water storageand cumulative ET were nearly the same in the two treatments (RFand FM). It is likely that FM increased the physiologicallysignicant canopy transpiration ( Raeini-Sarjaz and Barthakur,1997; Wang et al., 2009 ); plant physiological processes were thusenhanced to ensure plant productivity and GY formation ( Li et al.,2001b; Li and Gong, 2002 ) compared to the RF treatment. Incontrast, there was signicant soil water depletion in the RFtreatment. This is a result of soil surface evaporation, especially inthe early growth stages. At this stage, most of the soil surface wasexposed to the direct irradiation and a dry atmosphere; therefore,plant growth activities were notably restricted by water decit,leading to reduction in WUE.

Straw mulching is regarded as one of the best ways of

improving water retentionin thesoil andreducingsoil evaporation(Baumhardt and Jones, 2002; Zhang et al., 2009 ). Effects of strawmulch on crop yield and WUE, however, have been variable, andthis can be mainly attributed to differences in climatic conditions.The differences of yield and WUE between SM and RF treatmentwere not signicant in this experiment ( Table 3 ). These results arein agreement with recent investigations on straw mulch effectsfrom temperate climates ( Edwards et al., 2000 ). As pointed out byDoring et al. (2005) , mulching affects crop yields in many andcomplex ways. Higher yields under mulch have mostly beenattributed to increased soil water under arid and semiaridconditions ( Huang et al., 2005; Zhang et al., 2009 ); reduced yieldsunder straw mulch have also been reported and have beenattributed to below-optimum soil temperature, reduced soil

nitrate levels, and mulching too early ( Gao and Li, 2005 ).We conclude that the crop GY wassensitiveto altered soil waterconditions under different water management practices in thespring maize elds on the Loess Plateau, China. Correspondingly,ET and WUE were also affected, resulting in differences in plantproductivities. In the low precipitation area, the runoff is usuallylittle, and most of the harvested rainwater gathering at the surfaceis lost through evaporation. While the plastic lm covered ridgeswould improve rainwater harvesting and subsequently increasecrop yield. The ndings suggest that farmers can adopt the rationalplastic lm mulching technologies to obtain the optimal effect inincreasing crop yield and improving water use efciency.Supplementary irrigation can have a substantial effect to increasecrop yield, but farmers must consider its cost before using on a

commercial scale.

Table 3Grain yield (GY) and Water use efciency (WUE) under rain-fed (RF), supplemen-taryirrigation (SI), lmmulching(FM) andstraw mulching(SM) treatments in 2007and 2008.

Treatments GY (thm 2 ) WUE (kg hm 2 mm 1 )

2007 2008 2007 2008

RF 12.2 1.1 b 11.4 1.4 b 28.5 1.5 b 27.0 2.7 bSI 15.2 0.8 a 15.6 1.8 a 26.4 2.0 c 31.4 4.5 abFM 14.6 0.6 a 14.2 1.0 a 35.6 1.2 a 33.1 2.6 aSM 10.1 1.0 b 27.5 2.2 b

Values are given as means standard error of means ( n = 4). Values followed bydifferent letters within a column are signicantly different ( P < 0.05).



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Acknowledgements

This work was supported by the National Basic ResearchProgram of China (2009CB118604) and the Natural ScienceFoundation of the State Key Laboratory of Soil Erosion and DrylandFarming on the Loess Plateau (10502-Z04; 10501-247).

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