REGULAR ARTICLE

Effects of drip irrigation on deep root distribution, rootingdepth, and soil water profile of jujube in a semiarid region

Li-hui Ma & Xiao-li Liu & You-ke Wang & Pu-te Wu

Received: 17 April 2013 /Accepted: 12 August 2013 /Published online: 29 August 2013# Springer Science+Business Media Dordrecht 2013

AbstractAims Aimed to understand how soil water was depletedby deep roots, the effects of drip irrigation and stand ageon the deep root distribution, rooting depth, and soilwater profile dynamics were investigated in a jujube(Ziziphus jujube Mill. CV. Lizao) plantation.Methods A soil coring method with a LuoYang shovelwas used for sampling until no more roots were found.Results It showed that the maximum fine rooting depth(<2 mm in diameter) increased with stand age and itextended deep into the soil rapidly during the first 4 years,but more slowly in the subsequent 4 years. Themaximumrooting depth reached 5 m in a 9-year-old jujube planta-tion, but it stabilized and did not increase thereafter.However, it was 10 m in a 12-year-old jujube plantationthat lacked irrigation.Conclusions We found that the application of 33.3 mmof irrigation water (equivalent to 7 % of the local annualprecipitation) could halve the maximum rooting depth,thereby reducing deep soil water depletion. Our resultsshowed that a low-volume water supply reduced the

maximum rooting depth in jujube and prevented thedepletion of the deep soil water. Appropriate drip irriga-tion is an effective water management strategy for sus-tainable artificial forest development in semiarid regions.

Keywords Artificial economic forest . Fine root dryweight density . Jujube plantation .Maximum rootingdepth . Soil water depletion . Stand age

Introduction

The deep soils explored by deep-rooted forests are nowrecognized to be a major ecosystem component withimportant ecological implications (Nepstad et al. 1994;Jackson et al. 2000). The rooting depth and root distri-bution define the soil volume that plants can potentiallyexplore to extract water and nutrients (Schulze et al.1996). These features are important in seasonally dryenvironments because they allow forests to access waterstored in the deep soil during dry periods, therebymaintaining transpiration (Sarmiento et al. 1985;Meinzer et al. 1999; Oliveira et al. 2005). Schenk andJackson (2002) hypothesized that deep rooting depthswould mainly be found in water-limited ecosystems.However, it remains unclear how and to what extent soilthe water in the accessible deep soil is utilized by roots.Answering this question may help us to understand thefunctional significance of deep roots and their contribu-tion to plant water consumption in semiarid regions.

The distribution of roots in deep soil layers in poorlydescribed compared to shallow layers (Stephen et al.2001). Most reported soil profiles were not sampled tothe maximum rooting depth because of sampling diffi-culties (Schenk and Jackson 2002), so our understanding

Plant Soil (2013) 373:995–1006DOI 10.1007/s11104-013-1880-0

Responsible Editor: Peter J. Gregory.

L.<h. Ma :Y.<k. Wang : P.<t. WuInstitute of Soil and Water Conservation, Northwest A&FUniversity, Yangling, Shaanxi 712100, China

L.<h. Ma :Y.<k. Wang : P.<t. WuInstitute of Soil and Water Conservation, Chinese Academyof Sciences and Ministry of Water Resources,Yangling, Shaanxi 712100, China

X.<l. Liu (*)College of Water Resources and ArchitecturalEngineering, Northwest A&F University, Yangling,Shaanxi 712100, Chinae-mail: 404270234@qq.com

of the deep root distribution and the rooting depth, andtheir general relationships with the soil water profileremain inadequate. The most widely used technique forroot biomass assessment is the coring or excavation ofsoil and the subsequent separation of roots. Severaltechniques have been used to estimate the amount ofroots present at greater depths (Axel and Martin 1998;Schenk and Jackson 2002), but the results were notbased on actual samples. It is not possible to evaluatethe difference between the estimated and actual values,so it is necessary to collect samples from the maximumdepth possible.

Water is the main factor that limits vegetative growth,restoration, and improved productivity in semiarid eco-systems (Tallon and Si 2004). The excessive depletionof deep soil water (defined as a dry soil layer) was firstrecorded in the 1960s in semiarid regions (Li 2001) andit received greater attention after the 1980s in China(Wang et al. 2001). However, deep soil water depletionoccurs frequently in artificial forestlands and grasslandsin other arid and semiarid regions throughout the world(Yang and Han 1985; Jipp et al. 1998; Robinson et al.2006).

In the semiarid loess hilly region of China, theconcentrated rainfall and steep topography make thesoil vulnerable to surface runoff, so there is a high soilerosion modulus of ca 2,000–7,000 t km−2 year−1 (Zhu1989). Thus, great effort has been invested in plantingtrees on slopes, especially in artificial economic for-ests. This region contains a deep loess soil to about 80–120 m with an almost uniform soil texture (Liu et al.1978). Thus, the vegetation present in this region maybe ideal for exploring the deep root distribution,rooting depth, and soil water profile. The groundwatertable is deep and it is considered to have a negligibleeffect on the soil water in the root distribution zone (Li1983). Therefore, deep soil water consumption (belowthe maximum rainfall infiltration depth) depends solelyon root water uptake.

Jujube (Ziziphus jujube Mill. CV. Lizao) trees aredrought tolerant and the jujube fruit has great economicvalue because of its abundant nutrients. Jujube hasbecome the main tree species used in China duringprojects that aim to transform farmland to forestland.Drip irrigation was introduced in 2006 to improve thejujube fruit yield and more fine roots were subsequentlyfound at a 0–1 m depth with shallower taproots (Maet al. 2012). However, few field experiments have ex-amined the effects of stand age and water management

on the rooting depth, deep soil water depletion, and soildrying in forest land.

Therefore, the objectives of this study were to: (1)determine the vertical root distribution and the maximumrooting depth with stand age; (2) determine the soil waterprofile content with stand age; (3) compare the rootingdepth and soil water profiles in irrigated and nonirrigatedjujube plantations.

Material and methods

Study area and stand characteristics

The study was conducted in Mizhi County, ShaanxiProvince (37° 5′ N, 119° 49′ E). For details of the sitedescription, meteorological conditions, and topographyplease refer to Ma et al. (2013). The soil texture was asandy loam with an average soil bulk density of1.30 g cm−3 in the 0–1 m depth and 1.31 g cm−3 in the1–10 m depth (Wang et al. 2008), with a field watercapacity of 23.4 % and a saturated water capacity of39.8 %. The soil bulk density was smaller in the 0–1 mwhich changed obviously than it in the lower-horizonsoil below 1 m which remained relatively stable. Thewilting point was approximately 5.5 % (gravimetric)according to the method proposed by Wei (Wei et al.2004) who defined the wilting point as the soil watercontent when the soil water suction is equal to1.47 MPa. The composition of the soil was as follows:clay, silt, and sand particles accounted for 5.5 %,27.4 %, and 67.1 % respectively. The rainfall infiltrationdepth was less than 2 m (Zhao et al. 2009).

Treatments and experimental design

The jujube plantation was planted in 1999 and most ofit had been drip irrigated since 2006. There were fivestands including four dense dwarfed jujube plantationareas (2.4 m between trees and 2.4 m between treerows, 1,736 trees ha−1, 2 m tree height) with irrigation(from F1 to F4) and one widely spaced jujube planta-tion area (4 m between trees and 5 m between treerows, 500 trees ha−1, 6 m tree height) without irrigation(F5). The stands were F1=2 years, F2=4 years,F3=9 years, F4=12 years, and F5=12 years at the timeof sampling in 2011. Trees were drip irrigated usingtwo emitters placed 60 cm from the trunk on oppositesides. The drip emitter had a flow rate of 4 L h−1.

996 Plant Soil (2013) 373:995–1006

Irrigation water was applied at a rate of 32 L perdripper three times per year during the bloom growthstage, fruit swelling period, and fruit harvesting period.Thus, 64 L of water was released by the two emittersduring each irrigation period so 192 L water was ap-plied to each tree per year over a 5.76 m2 surface area,which was equivalent to 33.3 mm of natural precipita-tion. The applied fertilizer was the same from F1 to F4sites. For details of tree pruning, fertilization, and thesoil chemical characteristics please refer to Ma et al.(2013). F5 was not pruned, fertilized, or irrigated. Thedetailed stand characteristics are provided in Table 1.

Field sampling and measurement

Four neighboring trees at similar growth stages on asampling grid (the distance between the tree rows wasthe width and that between trees was the length) wereselected in the F1–F4 stands and the five samplingpoints in each grid are shown in Fig. 1. Four samplingpoints located 0.6 m from the tree trunk (located in thenorth, south, east, and west directions, respectively) onthe sampling grid were chosen in F5. The trees were alllocated on south-facing upper slopes with a similarslope gradient to minimize any location-related differ-ences as much as possible and there were three repli-cates per stand.

Soil samples were collected to estimate the fine rootand soil water content during a 10-day dry periodwithout rain for F1, F2, and F5 sites, while 3-dayperiod after rain (20.1 mm) for F3 and F4 sites duringSeptember 2011 (the jujube fruit-ripening period). Allweeds were removed within the grid before sampling

to minimize any interference from other plant roots.Fine roots (diameter <2 mm) were defined as white-colored living roots, whereas weed roots were grey andsmelled strongly. Dead weed roots were wrinkled,broke easily, and were dark in color. The root colorand morphology were discriminated based on a visualinspection. At each sampling point, a soil core methodusing a LuoYang shovel was employed, which wassupported by iron scaffolding to access the deep soilat 0.2 m increments in a vertical direction. LuoYangshovel was used widely in archeological excavationbecause they can drill into the deep soil for over 10 mwith internal diameter of 0.16 m.

A small part of the soil from each soil layer thatlacked roots was transferred to an aluminum specimenbox while the rest of the soil sample was washed in situusing a 1 mm sieve, primarily to eliminate dead rootsand impurities so as to separate the roots from the soil,which were stored in polythene plastic bags. We sam-pled the deep soil until no more roots were obtained intwo consecutive soil intervals. This process required2 h to sample a depth of 5.4 m in stands F1–F4 and 5 hto sample of depth of 10.4 m in stand F5.

The root samples were washed thoroughly andsorted again using a fine spray of water on a 1 mmsieve. The roots retained on the sieve were pickedusing tweezers. The oven-drying method was used tomeasure the fine root dry weight and soil water content(gravimetric). The fine root biomass was converted tothe root dry weight density per 1 m3 of soil.

We applied the concept of Schenk and Jackson tocontrast and analyze the vertical distribution of the rootdry weight density in different stands (2002), i.e., the

Table 1 Site characteristics of the jujube stands (records from 2011)

Characteristic F1 F2 F3 F4 F5

Stand age (year) 2 4 9 12 12

Trunk diameter (mm) 37.6e±6.3 58.1d±3.4 70.8c±5.5 85.3a±2.5 77.5b±12

Tree height (m) 1.26d±0.47 1.94c±0.14 2.33b±0.27 2.31b±0.22 6.15 a±0.74

Crown diameter (m)f 1.35d±0.29 1.75c±0.18 1.90c±0.25 2.23b±0.15 3.58a±0.37

Fruit yield (kg ha−1 year−1)g 2,250 15,000 19,800 19,800 3,000

Stand area (ha) 6 6 20 25 15

Values are means ± SE where SE indicates the standard error for all trees in each plot area per stand agea,b,c,d,e Different letters denote significant differences among stands (P<0.05, Duncan’s multiple range test)f Average crown length and widthg Average jujube fruit yield from 2007 to 2011

Plant Soil (2013) 373:995–1006 997

depth above which 95 % of the roots were located inthe soil profile (D95). Similarly, D50 was the depthreached by 50 % of the roots. We used these two valuesto describe the depth profiles. We also calculated thefine root dry weight density in each soil layer relativeto the overall soil depth and the cumulative fine rootdry weight density in each specific soil layer. The driedsoil layer has been defined as having a soil watercontent between the wilting point and 60 % of the fieldcapacity (Wang et al. 2010).

Statistical analysis

The data were analyzed using SPSS v.17.0 (IBMCompany). We tested the assumption of data variancenormality. A one-way ANOVA was performed to testthe effects of stand age and soil depth on the fine rootdry weight density in stands F1–F4. The post hoc sta-tistical analysis applied Duncan’s new multiple rangetest with P<0.05.

Results

Root distribution with stand age

Figure 2 shows the vertical root distribution in standsF1–F4. The fine root dry weight density increased withthe stand age and declined sharply with the soil depth.The overall fine root dry weight densities were 619.1,1109.84, 1360.14, and 2319.41 g/m3 in F1, F2, F3, and

F4, respectively. Thus, the stand age had a significanteffect on the total fine root dry weight density. Most ofthe fine roots were present in the top 1-m soil fraction(89.54 %, 70.38 %, 72.62 %, 57.58 % of the total rootsin F1, F2, F3, and F4, respectively), the maximum rootdry weight densities all occurred in the 0–0.2 m soillayers of all stands (39.4 %, 23.71 %, 32.8 %, and16.6 % of the total roots in F1, F2, F3, and F4, respec-tively) with no significant differences. There were sig-nificant differences in the 0.2–3.8 m soil layers but nosignificant difference in the 3.8–5.0 m soil layers.

Rooting depth with stand age

The maximum rooting depth is shown clearly in Fig. 2.We found that the fine roots tended to extend deeperwith stand age, but they did not extend further after thejujube tree reached 9 years of age in the drip-irrigatedplantation areas. Figure 3 shows the cumulative rootdry weight density. The cumulative root dry weightdensity was higher in the upper soil horizon than thelower soil, i.e., D50 was located in the 0–0.8 m soildepth in all cases while D95 tended to move down withincreasing stand age in the four dwarfed stands. In F1,the maximum rooting depth was 2 m, while D50 andD95 were located in the 0.2–0.4 m and 1.0–1.2 m soillayers, respectively. In F2, the maximum rooting depthextended to 4 m, while D50 and D95 were located in the0.4–0.6 m and 2.2–2.4 m soil layers, respectively. In F3and F4, however, the maximum rooting depths extend-ed to 5 m, while D95 were located in the 3.2–3.4 m soillayer and D50 were located in the 0.2–0.4 m and 0.6–0.8 m soil layers, respectively.

Soil water profile and stand age

In general, the deep soil water content (>2 m) de-creased with increasing stand age. The soil water con-tent increased initially from 0–2.4 m to 0–2.2 m, thendeclined with increasing soil depth to 4.4 m and 3.6 m,before increasing a little in lower depth in F1 and F2respectively (Fig. 4a, b). The soil water content haddifferent trends in F3 and F4 where it decreased from 0to 2 m, remained stable between 1.8 m and 3.6 mwith alow soil water content, before increasing again in thelower soil depths (Fig. 4c, d).

Overall, the soil water varied greatly in the uppersoil horizon whereas it varied little in the lower soil

2.4

mbe

twee

n tr

ee r

ows

1

2 4

3

5

0.6 m0.6 m0.

6 m

0.6

m

2.4 m between trees

Fig. 1 Sampling positions in the densely planted jujube planta-tion area. Empty circles represent the sampling positions

998 Plant Soil (2013) 373:995–1006

horizon. A low soil water zone was present betweenthe rainfall infiltration depth and the maximum rootingdepth, and the average contents in the low soil waterzone were 5.8 % and 5.5 % for F3 and F4, respectively.

Comparison of drip-irrigated and nonirrigated jujubeplantation areas

Figure 5 shows the vertical root distribution in F4 and F5.The maximum rooting depth in F5 reached 10 m whilethe total fine root dry weight density was 1971.37 g/m3.Compared with F4, F5 had a lower fine root dry weightdensity and a deeper rooting depth.

Figure 6 shows the soil water distribution in F4 andF5. The deep soil water content (>2 m) in F5 (average =

6.65 %) was significantly lower than that in F4 stand(average = 8.5 %). The low soil water content zone wasdeeper in F5 than F4 and they extended from 1.8–4.6 mto 1.8–3.6 m, respectively.

Discussion

This study is the first investigation of the effects ofstand age and drip irrigation on the root distribution,rooting depth, and soil water profile of jujube. Wefound a low soil water zone between the maximumrainfall infiltration depth and determined the maximumrooting depth in water-limited ecosystems. The maxi-mum rooting depth was significantly affected by the

Root dry weight density (g m-3)

0 100 200 300 400 500 600

Soi

l lay

er in

terv

al (

m)

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.0

2-yr-old

Root dry weight density (g m-3)

0 100 200 300 400 500 600

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.02.0-2.22.2-2.42.4-2.62.6-2.82.8-3.03.0-3.23.2-3.43.4-3.63.6-3.83.8-4.0 4-yr-old

Soi

l lay

er in

terv

al (

m)

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.02.0-2.22.2-2.42.4-2.62.6-2.82.8-3.03.0-3.23.2-3.43.4-3.63.6-3.83.8-4.04.0-4.24.2-4.44.4-4.64.6-4.84.8-5.0

9-yr-old

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.02.0-2.22.2-2.42.4-2.62.6-2.82.8-3.03.0-3.23.2-3.43.4-3.63.6-3.83.8-4.04.0-4.24.2-4.44.4-4.64.6-4.84.8-5.0

12-yr-old

Fig. 2 Root distributionwithstand age in four denselyplanted jujube plantationsites. Each point representsan average of five samplingposition on the same grid inthe same soil layer (n=3).Error bars represent 95 %confidence intervals. a, b, c

Different letters denote sig-nificant differences amongstands (P<0.05, Duncan’smultiple range test)

Plant Soil (2013) 373:995–1006 999

water regime. We conclude that plant roots can adjusttheir depth in the soil depending on the surface watersupply.

Root distribution pattern

Deep-rooted plants are always present in arid andsemiarid environments. In general, the fine root bio-mass increases with stand age (Bouillet et al. 2002;Makkonen and Helmisaari 2001; Vogt et al. 1983),which was also found in our study. However, Finéret al. (2007) and Lin et al. (2011) reported the oppositeeffect in Fagus sylvatica and Hevea brasiliensis. Börjaand Nilsen (2009) reported that the fine roots increasedwith stand age initially, but they reached their maxi-mum biomass within a specific period then declinedgradually, before stabilizing. In our study, the rootswere still progressed, the oldest stand was only 12 yearsold which was far less than the 100 years of someknown jujube trees (unpublished data).

In our study, the fine roots were most abundant inthe upper soil horizons (>50 % of fine root weredistributed in the 0–1 m soil layer) but this abundancedeclined sharply with soil depth. D95 tended to extenddeeper with increasing stand age and this was a general

pattern (Jackson et al. 1996; Parker and van Lear1996). Püttsepp et al. (2006) and Bakker et al. (2006)reported that the maximum root dry weight density wasconcentrated in the 0–0.2 m soil layer and our studyfound the same distribution. Yu et al. (2007) reportedthat the rooting depth of the highest length-densityroots determined the primary water depletion depth.Kang et al. (2010) showed that the root biomass indifferent soil layers indicated the plant growth abilityand accumulated biomass in specific soil layers wherea higher accumulated biomass indicated a greater abil-ity to absorb soil water and nutrients from the soillayer. Thus, our study showed that jujube tree rootshad a high capacity for absorbing soil water and nutri-ents in the 0–0.2 m soil layer. D50 and D95 might beused to schedule irrigation regimes more effectively.D50 is the main root distribution layer that may be usedto determine soil wetting depth in drip irrigation. Thatis, the maximum depth of vertical wetting front shouldbe less than the main root distribution layer, whichensure the efficient use of water. According to theinformation of D95 combined with rainfall data, wecould adjust irrigation frequency and complement thewater requirement in time to ease and inhibit rootgrowth to a greater depth.

Cumulative root dry weight density (% of total)

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Soi

l dep

th (

m)

0.00.20.40.60.81.01.21.41.61.82.02.22.42.62.83.03.23.43.63.84.04.24.44.64.85.0

2-yr-old4-yr-old9-yr-old12-yr-old

D50

D95

Fig. 3 Cumulative root dryweight density with standage in four densely plantedjujube plantation sites. D50

and D95 represent the soildepth reached by 50 % and95 % of the total root dryweight. Dotted arrows indi-cate the D50 and D95 thresh-olds for the specific propor-tions of roots with differentstand ages. Note that the D95

threshold for 9-year-old treeswas about the same as thatfor 12-year-old trees

1000 Plant Soil (2013) 373:995–1006

Rooting depth

The rooting depth is usually referred to as the maximumrooting depth, which determines the maximum amountof water that can be acquired from the soil duringtranspiration. Zhang et al. (2011) reported that the soilwater depth depleted by roots increased gradually withstand age. Our study showed that the maximum rootingdepth generally increased with stand age, but it extendedrapidly in younger stands (from 2 m in F1 to 4 m in F2)and more slowly in older stands (from 4 m in F2 to 5 min F3). This was probably because the jujube trees hadcompleted their major growth period and their fruit yieldhad reached 15,000 kg ha−1, so large amounts of waterwere required. The roots extended 4 m within the first

4 years. The soil water content below 2mwas low soweconcluded that some of the soil water below 2 m hadbeen utilized. We did not know whether the 4 m rootingdepth was produced to facilitate full water uptake by 4-year-old trees but this deep-rooted feature may be acharacteristic of jujube species, or it may be requiredto support the aboveground tree growth. The 9-year-oldjujube trees yielded 19,800 kg ha−1 and they had a largertree crown and diameter, which demanded high waterconsumption from the soil water supply stored deeper inthe soil. Overall, root growth is a response to the dy-namic water balance. We suggest that the level of pre-cipitation is lower than the water requirements after4 years in the jujube plantation, so the roots may havecontinued to grow to a greater depth to absorb soil water

Soil water content (gravimetric, %)

2 4 6 8 10 12 14

4-yr-old

Soil water content (gravimetric, %)

2 4 6 8 10 12 14

Soi

l lay

er in

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al (

m)

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.02.0-2.22.2-2.42.4-2.62.6-2.82.8-3.03.0-3.23.2-3.43.4-3.63.6-3.83.8-4.04.0-4.24.2-4.44.4-4.64.6-4.84.8-5.0

2-yr-old

12-yr-oldSoi

l lay

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m)

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.02.0-2.22.2-2.42.4-2.62.6-2.82.8-3.03.0-3.23.2-3.43.4-3.63.6-3.83.8-4.04.0-4.24.2-4.44.4-4.64.6-4.84.8-5.0

9-yr-old

The maximum rainfall infiltration depth

Fig. 4 Soil water profile con-tent with stand age in fourdensely planted jujube plan-tation sites. Solid circles rep-resent two-yr-old jujubeplantation sites, empty circlerepresent 4-year-old jujubeplantation sites, solid trian-gles represent nine-yr-old ju-jube plantation sites, andempty triangles represent 12-year-old jujube sites. Dashedlines represent the maximumrainfall infiltration depth.Each plotted point representsthe average of five samplingposition on the same grid inthe same soil layer (n=3). Er-ror bars represent the 95 %confidence intervals

Plant Soil (2013) 373:995–1006 1001

to sustain growth. However, evidence of water deficien-cy was slight in the local environment.

Studies showed that nutrient availability affected rootlength (McCulley et al. 2004), the loess in this regiondeveloped from loess parent material is characterized assurface soil (0–1 m) have the high fertility throughcultivation, while fertility is low below 1 m, and distrib-ute uniformly (She et al. 2010). It illustrate that there arelittle nutrient in deep soil absorbed by root.

No fine roots were found below 5m (F1 to F4) in thedrip-irrigated plantation areas in our study. We cannot

exclude the possibility that fine roots might have beenfound below 5 m depth after further sampling, but theymust have been scarce.

Soil water profile

The rainfall infiltration depth was usually 0–2 m and itwas the main source of available water for jujube inthis region. However, the soil water below 2 m con-tributed to the actual soil water status available to theroots. The soil water in the 0–2 m was mainly affected

Root dry weight density (g m-3)

0 100 200 300 400 500 600

Soi

l lay

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terv

al (

m)

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.02.0-2.22.2-2.42.4-2.62.6-2.82.8-3.03.0-3.23.2-3.43.4-3.63.6-3.83.8-4.04.0-4.24.2-4.44.4-4.64.6-4.84.8-5.05.0-5.25.2-5.45.4-5.65.6-5.85.8-6.06.0-6.26.2-6.46.4-6.66.6-6.86.8-7.07.0-7.27.2-7.47.4-7.67.6-7.87.8-8.08.0-8.28.2-8.48.4-8.68.6-8.88.8-9.09.0-9.29.2-9.49.4-9.69.6-9.8

9.8-10.0

Without irrigatedDrip irrigated

Fig. 5 Root distributions of12-year-old trees with andwithout drip irrigation.Empty triangles representdrip-irrigated jujube planta-tion sites and solid trianglesrepresent nonirrigated jujubeplantation sites. Each pointrepresents an average of fivesampling positions locatedin the same soil layer on thesame grid in the drip-irrigated jujube plantationare and an average of foursampling points in thenonirrigated jujube planta-tion area (n=3). Error barsrepresent the 95 % confi-dence intervals

1002 Plant Soil (2013) 373:995–1006

by rainfall and root uptake but it only reflected the soilwater status when the soil was sampled. Soil samplesallowed us to investigate the soil water regime in F1,F2, and F5 were under dry-soil conditions, while F4and F5 were under wet-soil conditions.

The soil water content decreased gradually below2 m in F1 where the maximum rooting depth was only2 m and there were no roots below 2 m, so the soil watercould not be absorbed by the roots. This low soil watercontent may have been due to the internal dry loess,

which developed from the wind-deposited loess parentmaterial (Zhu et al. 1983). The soil water content de-clined gradually in the 2–3.6 m layer in F2 but the valuewas higher than the wilting point. This indicated that thejujube plantation had depleted the deep soil water slight-ly. There was a low soil water content (approximatelyequal to the wilting point) in the 1.8–3.6 m and 1.8–4.6 m layers in F4 and F5, respectively. This showedthat the roots had completely absorbed the water storedin this soil layer and there was a severe dry soil layer.

Soil water content (%)

4 6 8 10 12

Soi

l lay

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al (

m)

0-0.20.2-0.40.4-0.60.6-0.80.8-1.01.0-1.21.2-1.41.4-1.61.6-1.81.8-2.02.0-2.22.2-2.42.4-2.62.6-2.82.8-3.03.0-3.23.2-3.43.4-3.63.6-3.83.8-4.04.0-4.24.2-4.44.4-4.64.6-4.84.8-5.05.0-5.25.2-5.45.4-5.65.6-5.85.8-6.06.0-6.26.2-6.46.4-6.66.6-6.86.8-7.07.0-7.27.2-7.47.4-7.67.6-7.87.8-8.08.0-8.28.2-8.48.4-8.68.6-8.88.8-9.09.0-9.29.2-9.49.4-9.69.6-9.8

9.8-10.0

Without irrigatedDrip irrigated

Fig. 6 Soil water profile con-tent of 12-year-old trees withand without drip irrigation.Empty triangles representdrip-irrigated jujube planta-tion sites and solid trianglesrepresent nonirrigated jujubeplantation sites. Each pointrepresents an average of fivesampling positions located inthe same soil layer on thesame grid in the drip-irrigatedjujube plantation and an av-erage of four sampling pointsin the nonirrigated jujubeplantation area (n=3). Errorbars represent the 95 % con-fidence intervals

Plant Soil (2013) 373:995–1006 1003

Nepstad et al. (1994) reported the constant depletionrate of deep soil water, which indicated that water up-take via the roots occurred at great depths. Cao et al.(2006) reported that the depth where the minimum soilwater content occurred in the root distribution zone(excluding any rainfall infiltration effect) was the soillayer with the maximum root absorption of water forplant growth and transpiration. Our study also foundthat this low soil water content zone was locatedbetween the rainfall infiltration depth and the maxi-mum rooting depth, which decreased with increasedstand age. We suggest that the jujube plantation waterconsumption increased with the stand age and that theroots were required to reach this layer to absorb soilwater.

Though roots may move water from the surface todeeper levels (inverse hydraulic redistribution) (Burgesset al. 1998), We found that soil water content was closeto the wilting point below 2 m, which showed that thewater delivered by inverse hydraulic redistribution waslower than that absorbed by root from deep soil. Thewater redistributed to the deeper soil layers throughinverse hydraulic remains to be determined. The processof inverse hydraulic redistribution can definitely slowdown the progression of the root downward extension.

Effect of drip irrigation

Studies have shown that the water use efficiency is animportant tool for improving the sustainability of agri-culture (Azevedo et al. 2003). Appropriate irrigationmanagement promotes maximum productivity andcontinuous fruit harvesting. Tanasescu and Paltineanu(2004) found that there were significantly more fineroots under drip irrigation, which was consistent withour study. The F4 stand with irrigation contained morefine roots than the F5 stand without irrigation.

In addition, the maximum rooting depth was 5 m,which extended downward for no longer than 9 yearsunder drip irrigation, whereas the roots extended to 10mwithout irrigation. The similar maximum rooting depthsin F3 and F4 may have been due to their similar pruningconditions, yield and water consumption despite verydifferent canopy size and diameter. We concluded thatthere may be a dynamic balance between the waterrequirement and water supply, the water deficit betweenwater requirement and rainfall was slight, drip irrigationcan complement the water deficit to maintain a stableand optimum rooting depth. It showed that a small

amount of irrigation can reduce the root extension todeeper soil layers.

In F4, there was a much lower maximum rootingdepth, a smaller low soil water content zone, and a higheryield compared with F5. This indicated the major effectsof drip irrigation on jujube tree management. We suggestthat drip irrigation may complement the surface soil waterwhere the fine roots absorb water from the upper soildepth but from lower soil depths when no more water isavailable. Thus, drip irrigationmay inhibit downward rootgrowth, reduce the rate of deep soil water depletion, andeven prevent the formation of a deep soil dry layer. In ourstudy, the addition of the equivalent of 33.3 mm water(7% of the annual rainfall) had large effects on the rootingdepth and deep soil water depletion. This implies that therooting depth is sensitive to the water volume applied, somicro-variations in precipitation could lead to largechanges. If the precipitation level decreases continuously,the deep soil water will be excessively depleted. Only asmall amount of water was supplied via drip irrigation inthis study but it had an important role in combating waterdeficiency in an artificial forest in a semiarid region.

Our study did not obtain lateral spread of roots in thetop 20 cm and the planting density effect on competi-tion for soil water. Lateral soil space obtained by rootwas supposed to be smaller in densely plantation thanwidely plantation, thus root in densely plantation shouldextend deeper to absorb deep soil water (Li et al. 2012).However, we got opposite results in drip irrigated andno drip irrigated plantation. It illustrated again the im-portant role of drip irrigation on the root depth and deepwater depletion.

Bravdo et al. (1992) also showed that root distribu-tions were very flexible and could change throughoutthe life of a tree. Global climate change may lead toincreased precipitation and the anticipated increase isabout 0.5–1 % per decade in this century throughout theworld (IPCC 2007). This increasing precipitation mayproduce more roots in surface soil layers and slow downthe depletion of deep soil water by roots, but artificialforests still need to extend their roots into deep soil toabsorb soil water gradually. Our study showed that theprovision of a small scale water supply could ensure thesustainable development of artificial forests.

Acknowledgments This work was funded by the NationalNatural Science Foundation of China (No. 31200343), the NationalScience & Technology supporting program (No. 2011BAD29B04),the West Light Foundation of the Chinese Academy of Sciences,and Northwest A & F University Science Foundation for Youth.

1004 Plant Soil (2013) 373:995–1006

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