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Soil-Plant Relationships in the HetaoIrrigation Region Drainage Ditch Banks,Northern ChinaYang Zhao a , Xin-rong Li a , Zhi-shan Zhang a , Yi-gang Hu a & Pan Wua

a Shapotou Desert Research and Experiment Station , Cold and AridRegions Environmental and Engineering Research Institute, ChineseAcademy of Sciences , Lanzhou , ChinaPublished online: 28 Oct 2013.

To cite this article: Yang Zhao , Xin-rong Li , Zhi-shan Zhang , Yi-gang Hu & Pan Wu (2014) Soil-PlantRelationships in the Hetao Irrigation Region Drainage Ditch Banks, Northern China, Arid Land Researchand Management, 28:1, 74-86, DOI: 10.1080/15324982.2013.812997

To link to this article: http://dx.doi.org/10.1080/15324982.2013.812997

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Soil-Plant Relationships in the Hetao IrrigationRegion Drainage Ditch Banks, Northern China

Yang Zhao, Xin-rong Li, Zhi-shan Zhang, Yi-gang Hu, andPan Wu

Shapotou Desert Research and Experiment Station, Cold and AridRegions Environmental and Engineering Research Institute, ChineseAcademy of Sciences, Lanzhou, China

Species-environment relationships is a central issue in ecology and important to plantreconstruction and management in degraded ecosystems. We explored how the inter-actions among soil nutrients, salinity, and ion ratios influence vegetation distributionin the Hetao Irrigation Region drainage ditch banks. Twoway indicator speciesanalysis (TWINSPAN) techniques and Canonical Correspondence Analysis(CCA) were used to classify the vegetation and to examine the relationships betweenvegetation and soil chemical properties. The plant communities of Saussurea salsa–Phragmites australis–Sonchus arvensis and Leymus chinensis–Sonchus arvensisoccurred within 161 of a total 245 plots. Edaphic factors exerted the strongestinfluence on vegetation patterns and distributions, with available soil nutrient contentbeing identified as the dominant factor, followed by soil salinity and soil pH.Maintaining soil nutrient and salinity at moderate levels is an efficient approach toprevent species loss in the drainage ditch banks.

Keywords available nutrient, CCA, ion ratios, soil salinity, vegetationdistribution

Species-environment relationships is a central issue in ecology and necessary tovegetative reconstruction and management of degraded ecosystems (Guisan andZimmermann, 2000; Fallu et al., 2002; Li et al., 2009). Water availability was themost important factor in arid and semi-arid regions and high salinity soils, control-ling the plant species distribution (Rogel et al., 2001; Li et al., 2008). In irrigatedfarmland regions, however, water limitation can be mitigated with regular irrigationand can be beneficial to the establishment and survival of plant species. For example,the Hetao Irrigation Region, which uses Yellow River water to supply drainageditches among crops during the growth season, is the oldest and largest artificial

Received 3 April 2013; accepted 5 June 2013.The authors gratefully acknowledge two anonymous reviewers for valuable comments on

the manuscript, as well as Christine Verhille at the University of British Columbia for herassistance with English language and grammatical editing of the manuscript. This work wassponsored by the National Basic Research Program of China (2013CB429901) and theNational Natural Scientific Foundation of China (41271061, 31170385, and 41101081).

Address correspondence to Yang Zhao, Cold and Arid Regions Environmental andEngineering Research Institute, Chinese Academy of Sciences, 320 Donggang West Road,730000, Lanzhou, China. E-mail: zhaoyang66@126.com

Arid Land Research and Management, 28:74–86, 2014Copyright # Taylor & Francis Group, LLCISSN: 1532-4982 print=1532-4990 onlineDOI: 10.1080/15324982.2013.812997

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irrigation region of northern China (Cooper et al., 2003). The main factors limitingindividual plant species and plant community distributions in arid and semi-aridregions when water is unlimited are still unknown.

Soil chemical properties [i.e., total nitrogen (TN), total phosphorus (TP), soilorganic matter (SOM), available nitrogen (AN) and phosphorus (AP), total soilsalinity (TSS), and soil ion concentrations (i.e., Cl-, SO2�

4 , Ca2þ, Mg2þ, Naþ, andKþ)] are usually thought to control plant community patterns and species richnessin high salinity soils, arid and semi-arid regions and drainage ditch banks. A previousstudy showed species number and vegetation distribution to be affected in plots sup-plemented with nitrogen fertilizers (Siddique et al., 2010). Potentially, a reduction innitrogen input could be sufficient to achieve soil AN levels allowing species-rich com-munities (Leng et al., 2010). However, in many cases, the cessation of the nitrogenfertilization is insufficient. Soil NO3-N and NH4-N have other origins (e.g., minera-lization of organic matter) which could be important to vegetation distribution. Soil Pcan have positive or negative impact on species richness, and the impact is dependenton the range (Clark and Tilman, 2008) and soil conditions (Perroni-Ventura et al.,2006). Soil salinity has also been reported as a major factor affecting the distributionof vegetation in high salinity soils (Li et al., 2008). Usually, the TSS significantlyaffects plant survival and growth (Zhang et al., 2010). Additionally, high ion concen-tration (e.g., Cl-, SO2�

4 , Ca2þ, Mg2þ, Naþ, and Kþ) in the plant body leads to toxicity.Rogel et al. (2001) concluded that the relative ratios of ions in saline soil environmentsare fundamental factors impacting plant development. Interactions among the differ-ent soil chemical properties (e.g., nutrient contents, salinity and ion ratios) mayimpact plant distributions; however, few quantitative studies have investigated theseinteractions. A comprehensive investigation of drainage ditch banks in the HetaoIrrigation Region was conducted to answer two questions: 1) do soil chemicalproperties strongly influence plant community distributions in these drainage ditchbanks, and 2) what are the key factors that limit the dominant and nondominantplant distributions in these drainage ditch banks?

Materials and Methods

Study Site

The Hetao Irrigation Region (40�190–41�180 N, 106�200–109�190 E), is located in thewestern arid area of the Inner Mongolia autonomous region of China. The averageelevation is 1027m and there are 12 main drainage ditches and more than 20000minor drainage ditches of various sizes inside the Hetao Irrigation system. Theaverage air temperature is 8.1�C, annual precipitation is 188mm, and potentialevaporation is 2300mm (Feng et al., 2005).

Vegetation Surveys and Sampling

The fieldwork was conducted in Hetao Irrigation Region drainage ditches in Augustand September of 2010. A total of four drainage areas were selected for the survey,comprising 4 main, 12 feeder and 32 lateral drainage ditches. Five, three, and threetransects were lain equidistantly from each other and oriented upstream to down-stream along each main, feeder, and lateral drainage ditch, respectively. At eachtransect, 1 or 2 plots (0.5m� 0.5m), in which herbaceous species were identified,

Soil-Plant Relationships in the Hetao Irrigation Region 75

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were set perpendicular from the bottom to top of the slope, and another plot(5m� 5m) was set up on the slope in which the largest herbaceous and shrub specieswere identified. The hydrographic characteristics and chemical properties of thestudied drainage ditches were measured. Along each transect, one sample of theupper 20 cm of the soil layer was taken from each plot and one water sample wastaken if water was present. In total, 245 soil samples and 50 water samples weretaken in the study area. At the same time, pH and dissolved oxygen (DO) of thewater was determined using an YSI 556 Multi-Probe System (YSI IncorporatedCompany, USA).

The water samples were analyzed within a short time after sampling. TN, TP, andtotal dissolved-solid contents (TDS) in water were measured following the methodsdescribed in the Water and Wastewater Monitoring Method (State EnvironmentalProtection Agency, 2002). The average depth of water, water surface width, ditchslope length, and velocity were 0.46� 0.17m, 6.46� 2.81m, 10.4� 2.96m, and0.50� 0.02m=s, respectively. Average pH, TN, TP, TDS, and DO of the watersamples were 8.21� 0.07, 2.11� 0.91mg=L, 0.16� 0.07mg=L, 1298� 907mg=L,and 6.46� 0.40mg=L. Air-dried, crushed, soil samples were passed through a 2mmsieve. Soil TN, TP, and pH that was measured in soil diluted five-fold with water, totalsalt in soil (TSS) and soil HCO�

3 , Cl-, SO2�

4 , Ca2þ, Mg2þ, Naþ, and Kþ concentrationswere measured following the methods described in the General Analysis Methods ofSoil Agriculture Chemistry (Agriculture Chemistry Specialty Council, 1983).

Data Analysis

To analyze vegetation and related environmental factors, classification andordination techniques were used (He et al., 2007). The importance value index(IV¼ [Relative Density (%)þRelative Height (%)þRelative Cover (%)]=3) of thespecies was used to classify the plot - species data matrix with a two - way indicatorspecies analysis by TWINSPAN (version 2.3) (Hill and Milauer, 2005). We used twosampling plot sizes (0.25m2 and 25m2) in our study. We transformed the data of25m2 plots to 0.25m2, if one transect occurred within two plot sizes, summed andthen calculated IV. All ordinations and plots were performed using the computer pro-gram CANOCO 4.5 and drawn by CANODRAW 4.12. To describe the relationshipsbetween the plant species composition and the measured environmental factors weused Canonical Correspondence Analysis (CCA) (Ter Braak, 1986). The similaritiesand dissimilarities between soil chemistry properties of different plant communitieswere compared by analyzing total and available nutrients, total anion and cationconcentrations as well as the ratios of total nutrients, available nutrients and ions(i.e., the ratio of the same variable between different plant communities).

Results

Species composition and diversity of these drainage ditches in the same plots duringthe same time period were reported in Zhao et al. (2013) using Shannon’s Index, andRichness.

The CCA ordination biplot (Figure 1a) shows the effect of soil chemical proper-ties on plant species distribution in the Hetao Irrigation Region drainage ditch banks,with soil-plant correlations of 0.567 and 0.546 on the first and second axes, respect-ively (Table 1). The first and second axes explained 42.4% and 24.0% of the variance

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in soil-plant relationships, respectively. The dominant environmental factor relatingto the first CCA axis was TP (Table 1) and pH, TSS, TN, and ion concentrations(i.e., HCO�

3 , Cl-, SO2�4 , Ca2þ, Mg2þ, and KþþNaþ) were also significantly

(p< 0.05) related to the first axis. Soil pH, salinity, and soil nutrient were the domi-nant environmental variables correlated with the second CCA axis.

Some dominant plant species (such as, Polygonum aviculare, Peganum nigellas-trum, Sonchus arvensis, Typha angustifolia, Scirpus trigueter, and Swainsonia salsula)

Figure 1. Canonical Correspondence Analysis (CCA) ordination diagram of the first two axesshowing the distribution of the 62 species and the environmental variables (soil organic matter(SOM), total nitrogen (TN), total phosphorus (TP), and available nitrogen (NH4-N and NO3-N) and phosphorus (AP), total soil salinity (TSS), Cl-, SO2�

4 , Ca2þ, Mg2þ, and Naþ þKþ) (a),and the distribution of the stands with their TWINSPAN groups and environmental variables(b). Species are abbreviated as: Eccr: Echinochloa crusgalli (L.) P. Beauv.; Lper: Lolium perenneL.; Cirg: Chloris virgata Swartz; Aspl: Achnatherum splendens (Trin.) Nevski; Cpse: Calama-grostis pseudophragmites (Hall F.) Koeler; Csch: Crypsis schoenides (L.) Lam.; Cait: C. aculeata(L.) Ait.; Pcom: Phragmites australis (Cav.) Trin. ex Steud; Lvel: Leymus chinensis (Trin.)Tzvel.; Scap: Stipa capillata Linn.; Xsib:Xanthium sibiricum Patrin exWidder; Scro: Scorzoneraparviflora Jacq.; Tvul: Tripolium vulgare Nees; Sarv: Sonchus arvensis Linn.; Isal: Inulasalsoloides (Turcz.) Ostenf.; Ibri: I. britannica L.; Ltat: Lactuca tatarica (L.) C. A. Mey.; Tmon:Taraxacum mongolicumHand-Mazz.; Cirs: Cirsium arvense (Linn.) Scop.; Sspr: Saussurea salsa(Pall.) Spreng; Arte: Artemisia argyi H. Lev. & Vaniot.; Halo: Halogeton arachnoideus Moq.;Srut: Salsola kali subsp. ruthenica (Iljin) Soo.; Koch: Kochia scoparia (L.) Schrad. var. sieversi-ana; Sall: Suaeda salsa (Linn.) Pall.; Sgla: S. glauca (Bge.) Bge.; Bdas: Bassia dasyphylla (Fisch.et Mey) O. Kuntze; Cgla: Chenopodium glaucum L.; Cser: C. serotinum L.; Asib: Atriplexsibirica L.; Atri: A. centralasiatica Iljin; Moff: Melilotus officinalis (Linn.) Pall.; Salo: Sophoraalopecuroides L.; Ssal: Swainsonia salsula (Pall.) Taub; Vici: Vicia amoena Fisch; Oxyt:Oxytropis glabra Lam.; Pavi: Polygonum aviculare L.; Plap: P. lapathifolium L.; Psib: P.sibiricum L.; Kobr: Kobresia myosuroides (Villrs) Fiori; Cusc: Cyperus fuscus L.; Stri: Scirpustrigueter L.; Ctri: Carex tristachya Thunb.; Pnig: Peganum nigellastrum Bunge; Nitr: Nitrariasibirica Pall.; Sali: Salix matsudana Koidz; Popu: Populus alba Linn. var. pyramdalis Bunge;Glau: Glaux maritime L.; Plan: Plantago depressa Willd; Tram: Tamarix ramosissima Ledeb.;Elox: Elaeagnus oxycarpa Schlecht.; Hibi: Hibiscus trionum L.; Cchi: Cynanchum chinenseR.Br.; Hale: Halerpestes cymbalaria (Persh) Greene; Lchi: Lycium chinense Miller; Lobt:Lepidium obtusum Basin.; Sper: Spergularia salina J.et C. Presl; Tpal: Triglochin palustre L.;Tang: Typha angustifolia L.; Doda:Dodartia orientalis L.; Conv:Convolvulus arvensis L.; Lapp:Lappula myosotis V. Wolf. (Figure available in color online.)

Soil-Plant Relationships in the Hetao Irrigation Region 77

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were closely related to soil TN, TP, SOM, AN, Ca2þ, and Mg2þ. Other dominantplant species (Cyperus fuscus, Saussurea salsa, Leymus chinensis, S. arvensis, Lappulamyosotis, and Tamarix ramosissima) were associated with AP. Some non-dominantspecies were closely associated with soil salinity content (e.g., Dodartia orientalis,Inula salsoloides, and P. sibiricum).

With TWINSPAN classification, six site groups were identified within the 245sites (Figure 2); each group was named according to its dominant species. The dis-tributions of all groups determined by the biplot were influenced by soil chemicalproperties. Soil chemical properties of each study site are described in Table 2.Group 3 was highly associated with TP, NH4-N, TSS, SO2�

4 , and Cl-. Group 6 in

Table 1. Correlation coefficients between environmental variables (soil organicmatter (SOM), total nitrogen (TN), total phosphorus (TP), and available nitrogen(AN) and phosphorus (AP), total soil salinity (TSS), Cl-, SO2�

4 , Ca2þ, Mg2þ, andNaþþKþ) and the first two axes of the Canonical Correspondence Analysis (CCA)

Variables Axes 1 Axes 2 Variables Axes 1 Axes 2

SOM �0.011 0.286�� TSS �0.235�� �0.171�

TN �0.050� 0.188�� HCO�3 �0.048� �0.159��

NH4-N �0.101� 0.039 Cl- �0.229�� �0.186��

NO3-N �0.128� �0.167� SO2�4 �0.202�� �0.078�

TP �0.319�� 0.233�� Ca2þ �0.090� 0.149��

AP 0.032 0.025 Mg2þ �0.180�� 0.214��

pH �0.097� �0.236�� KþþNaþ �0.219�� �0.225��

�p< 0.05. ��p< 0.01.

Figure 2. Dendrogram of the TWINSPAN for vegetation in the Hetao Irrigation Regiondrainage ditch banks. Indicator species are together with importance values index. Speciesare abbreviated as: Pavi: Polygonum aviculare; Tang: Typha angustifolia; Stri: Scirpus trigueter;Cusc:Cyperus fuscus; Sspr: Saussurea salsa; Pcom: Phragmites australis; Sarv: Sonchus arvensis;Lvel: Leymus chinensis; Sarv: Sonchus arvensis; Pnig: Peganum nigellastrum; Lapp: Lappulamyosotis; Tram: Tamarix ramosissima; Ssal: Swainsonia salsula; E: Eigen; N: Number of plots.

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Table

2.Environmen

talvariables(soilorganic

matter

(SOM),totalnitrogen

(TN),totalphosphorus(TP),available

nitrogen

(NH

4-N

andNO

3-N

)andphosphorus(A

P),andtotalsoilsalinity(TSS)

Group1

n¼29

Group2

n¼21

Group3

n¼92

Group4

n¼69

Group5

n¼13

Group6

n¼21

SOM

mgkg�1

11590�400

11630�1610

10300�470

11890�700

12060�1440

8560�870

TN

mg�kg�1

550�50

550�50

510�20

570�30

680�90

470�50

NH

4-N

mg�kg�1

1.52�0.40

1.44�0.38

1.57�0.13

1.45�0.16

2.05�0.62

1.85�0.32

NO

3-N

mg�kg�1

10.16�2.92

5.66�0.65

14.09�2.23

15.00�3.63

15.12�4.19

39.18�14.42

TPmg�kg�1

500�20

510�20

500�10

520�10

520�20

550�20

APmg�kg�1

4.92�0.86

5.81�1.18

6.08�0.48

5.14�0.44

6.20�0.75

21.11�9.91

pH

7.6�0.12

7.7�0.10

7.6�0.04

7.5�0.04

7.7�0.08

7.5�0.12

TSSmg�kg�1

6850�1090

5240�1200

9880�680

8980�930

8180�1800

12530�2360

HCO

� 3mg�kg�1

2288�56

1811�47

3460�25

3248�28

2922�63

3884�42

Cl-mg�kg�1

1756�471

1155�530

2579�272

2257�424

2093�761

3978�812

SO

2�

4mg�kg�1

425�170

495�193

473�109

416�152

500�127

362�134

Ca2þmg�kg�1

163�24

114�40

196�19

206�25

222�60

252�55

Mg2þmg�kg�1

231�48

133�58

255�28

281�38

212�63

271�57

KþþNaþmg�kg�1

1917�357

1530�351

2961�217

2576�291

2429�550

3769�793

Notes:Cl-,SO

2�4,Ca2þ,Mg2þ,andNaþþK

þatthe0–20cm

soildepth.Mean�SEin

thesitesrepresentingthesixvegetationgroups(G

1:Polygonum

aviculare;G2:Typhaangustifolia-Scirpustrigueter

-Cyperusfuscus;G3:Saussureasalsa-Phragmites

australis-Sonchusarvensis;G4:Leymuschinensis-

Sonchusarvensis;G5:Peganum

nigellastrum;G6:Lappula

myosotis-Tamarixramosissim

a-Swainsonia

salsula)obtained

byTWIN

SPAN.

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Figure 3. The ratios of AN=AP (a); ANþAP=TNþTP (b); Cl�þ SO2�4 þHCO�

3 concen-tration (c); KþþNaþþCa2þþMg2þ concentration (d); TSS (e); Kþ=Naþ (f); Ca2þ=Mg2þ

(g); KþþNaþ=Ca2þþMg2þ (h); Total Nutrient Content=Total Soil Salinity (TSS) (i); TotalNutrient Content=Total Anion Concentration (j); Total Nutrient Content=Total CationConcentration (k); and trends of ANþAP and Total Soil Salinity (TSS) (l); in the soil ofthe Group 1 to Group 6 (G1: Polygonum aviculare; G2: Typha angustifolia - Scirpus trigueter- Cyperus fuscus; G3: Saussurea salsa - Phragmites australis - Sonchus arvensis; G4: Leymuschinensis - Sonchus arvensis; G5: Peganum nigellastrum; G6: Lappula myosotis - Tamarixramosissima - Swainsonia salsula; G: Group.). The value is Meanþ SE. Different lettersrepresent significance differences at the p< 0.05 level.

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the biplot and was associated with AP, KþþNaþ, NO3-N, pH, and HCO�3 . Groups

2, 4, and 5 were closely associated with SOM, TN, and AP. Group 1 was associatedwith many variables such as TP, Mg2þ, and Ca2þ(Figure 1b).

Groups 3, 4, and 5 were associated with higher AN=AP, than Groups 2 and 6.Group 6 was associated with significantly higher (ANþAP)=(TNþTP) than allother groups (Figure 3a, b). Group 6 was also associated with the highest TSS, totalanion and total cation levels. Groups 3, 4, and 5 were associated with moderate levelsof these variables, and Groups 1 and 2 were associated with lowest levels. Groups 1and 2 were associated with the highest Kþ=Naþ ratios, compared to Groups 3, 4,and 5, which were associated with moderate Kþ=Naþ ratios, and Group 6, whichwas associated with lowest Kþ=Naþ ratios. Groups 5 and 6 were associated withhigher Ca2þ=Mg2þ ratios than Groups 1, 2, 3 and 4 were. Groups 2, 3 and 6 wereassociated with higher (KþþNaþ)=(Ca2þþMg2þ) ratios than Groups 1, 4, and 5were (Figure 3c–h). Groups 1 and 2 were associated with the highest Total NutrientContent=Total Soil Salinity (TSS) ratio, Total Nutrient Content=Total Anion Con-centration, and Total Nutrient Content=Total Cation Concentration ratios. Groups3, 4, and 5 were associated with moderate ratios, and Group 1 and 2 were associatedwith lowest ratios. The available nutrient and TSS of the soil also showed the samevariation trends, namely high soil available nutrients corresponded to high TSS in allGroups (Figure 3i–l).

Discussion

Effect of Soil Salinity Content and Ion Concentrations on PlantsSpecies Distribution

Soil salinity content and ion concentrations are the main factors determining thedistribution of plant species in arid and semi-arid regions and high salinity soils (Tothet al., 1995; Li et al., 2008; Naz et al., 2013). TSS is also considered an important vari-able explaining plant distribution (Rogel et al., 2001). High soil salinity content isthought to inhibit plant seed germination and limit plant community establishment(Khan and Gul, 2001), and can cause lethal to sub-lethal damage to plants (Kaushalet al., 2005) due to its effects on plant-water uptake potential (Vance et al., 2008). Thesalinity tolerance of each species plays a vital role in maintaining vegetation patternsin harsh environments (Clarke and Hannon, 1970). Dominant plant species that arehighly adapted to salinity can grow in high TSS conditions (Jafari et al., 2003). In thepresent study, plant species distribution, especially non-dominant plant species, wassignificantly controlled by soil salinity content. Discharge water containing highsoluble salt (averaged 1298mg=L) from adjacent agricultural fields resulted in saltaccumulation in the soil of ditch banks (averaged 9.05mg=g). High soil salinity hada negative influence on plant survival and growth, especially for nondominant plantspecies, however, dominant plant species (such as P. australis) were well adapted toendure high soil salinity. Many species of halophytic plants, such as Suaeda, Salsola,Tamarix, and Atriplex are indicators of saline soils. Furthermore, the appearance ofhalophytic plants, in conjunction with the disappearance of salt-sensitive plants, isone of the earliest and most recognizable signs of soil salinization (Dehaan andTaylor, 2002). In our study, T. ramosissima and group 6 plants occurred along withhigh soil salinity. Therefore, we should pay close attention to the spatial distributionof indicator plant species for identification of soil salinization and therefore better

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targeting of drainage ditch banks requiring management. Soil pH was also found tobe a vital variable influencing plant distribution in our study region, in particular forplant communities composed of L. myosotis–T. ramosissima–S. salsula (Group 6).Soil pH influences both seed establishment and plant survival (Wamelink et al.,2005), and thus has been shown to influence species distributions by many investiga-tors in a number of different regions (Fallu et al., 2002; He et al., 2007).

Survival and growth of aquatic and semi-aquatic plants can be determinedby water retention time, which also influences plant distributions (Lundholm andLarson, 2003). Longer water retention times in drainage ditches can allow aquaticand semi-aquatic plants to colonize and grow, while shorter water retention times pre-vent aquatic plant survival and growth due to drought stress (Bouldin et al., 2004).In our study, Typha angustifolia, Scirpus triqueter, P. australis, and Carex tristachyaare aquatic and semi-aquatic species. Drainage ditch transects with longer waterretention times provided the opportunity for their colonization. Most weedy species(species such as Echinochloa crusgalli, Lolium perenne, Chloris virgata, Sonchus arven-sis, Cirsium arvense, and Convolvulus arvensis in our study region) are well adapted totheir immediate natural environment, with high fecundity and germination andseedling establishment being the most important factors for successful spread(Malıkova et al., 2010). After periods of harsh conditions (such as water scarcity,nutrient-poor soil, and high soil salinity), improvement of environmental conditionscan trigger seed germination and seedling establishment. Once the suitable conditionsoccur, weedy species can easily emerge and establish in this area; therefore, the distri-bution of weedy species in the habitat is more likely a chance.

Effect of Relative Proportions of Ions on Plant Species Distribution

The relative ratios of Ca2þ, Naþ, Mg2þ, and Kþ in soil are known fundamental fac-tors impacting plant development in saline environments (Rogel et al., 2001). Becauseplants can absorb the required ions from the soil only when ion ratio is a constant inthe soil, elevated soil-soluble salt concentrations within plant rooting zones affectplant-water uptake, which negatively impacts plant growth and distribution (Vanceet al., 2008) and is potentially toxic. Naþ is the most poisonous ion in salinized soils,because low Naþ and high Kþ concentration in the cytoplasm are essential to main-tain a number of enzymatic processes (Yang et al., 2009). Rogel et al. (2001) con-sidered that higher Kþ=Naþ and lower Ca2þ=Mg2þ ratios were beneficial for plantgrowth. In our study, the plant communities of S. salsa–P. australis–S. arvensis(Group 3) and L. chinensis–S. arvensis (Group 4) occupied plots with relatively highratios of mean Kþ=Naþ, low ratios of Ca2þ=Mg2þ and even lower (KþþNaþ)=(Ca2þþMg2þ) ratios in soils. These conditions may be beneficial to the growth ofthese plants, which likely explains why communities of S. salsa–P. australis–S.arvensis (Group 3), and L. chinensis–S. arvensis (Group 4) were widely distributedthroughout our study area.

Effect of Soil Nutrition on Plant Species Distribution

Soil nutrient contents can be vital factors controlling the distribution of dominantspecies in arid and semi-arid regions (Venterink, 2011; Zhao et al., 2013). N is anessential plant nutrient and increasing N availability can be beneficial to plants in

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extremely nutrient-poor soils (Bobbink et al., 2010). In our study, the plantcommunity of P. nigellastrum (Group 5) was governed by a unique dominant species.This appeared to be due to high soil TN and NH4-N content negatively influencingspecies number and vegetation distribution, and is consistent with Siddique et al.(2010). NO3-N is known for its negative effect on the diversity of plant communities(Janssens et al., 1998). The highest soil NO3-N content occurred in the L. myosotis–T. ramosissima–S. salsula community (Group 6), which likely explains why thisgroup is composed of only 21 plots. P is another important element for plant growth,and species richness decreases with increasing P. That is to say P limitation can favorhigher species richness (Venterink, 2011). In our study, the plant communities of P.aviculare (Group 1) and S. salsa–P. australis–S. arvensis (Group 3) were significantlyaffected by TP. Previous researchers have shown that low levels of AP can maintainhigher species numbers (Janssens et al., 1998). The AP influenced the plant distri-bution of L. myosotis–T. ramosissima–S. salsula (Group 6) community, which hadthe highest AP content. Our results also suggest that SOM might be a factor in deter-mining species distribution and community composition. SOM plays an importantrole not only in soil fertility, but also in plant water absorption from the upper layerof the soil. Our results were consistent with the findings from the Rubio andEscudero (2000) and He et al. (2007) studies in arid and semi-arid regions and theJi et al. (2009) study in the Liaohe Delta of Northeast China.

The soil available nutrient content was also a major factor determining the veg-etation distribution in high TSS conditions. High TN or AN can lead to low speciesdiversity, however, increasing the N availability in extremely nutrient-poor soils (e.g.,salinized soils) can increase species diversity (Khan and Gul, 2001; Bobbink et al.,2010). Also Perroni-Ventura et al. (2006) found a positive relationship between APand species richness in the Tehuacan-Cuicatlan region. Kitayama and Aiba (2002)suggested that increasing P content in a P - limited ecosystem onMount Kinabalumaybe able to maintain forest structure stability, and that P deficiencies will prevent plantspecies from adjusting to the substrate. In the present study, the irrigation ditch soilscan be defined as nutrient - poor soils (Zhao et al., 2013). TN and TP in dischargedwater accumulated in the bank soil and caused increased TN and TP content in thesoil, thus contributing to high species richness on the ditch banks. As TSS contentof plots increased, the nutrient uptake by the plants increased, suggesting that nutri-ent supply limited nutrient uptake. At high levels of TSS content, the plants appearedto keep their internal nutrient concentration more or less constant. This may suggesta depletion of nutrients at the root surface in soils with low levels of TSS, and anaccumulation in soils with high levels of TSS (Morris and Ganf, 2001). Our fieldworktested the finding described above, and we can clearly see that the soil availablenutrient content increased with TSS content. Insufficient nutrient availability hada major influence on plant community distributions, such as P. aviculare community(Group 1). Also, the highest available nutrient contents and TSS appeared to limit thedistribution of plant species, as seen with the L. myosotis–T. ramosissima–S. salsulacommunity (Group 6). Morris and Ganf (2001) also suggested that in soil - plantsystems, maintaining soil nutrients and salinities at moderate levels can bebeneficial to plant growth. This likely explains why communities of S. salsa–P.australis–S. arvensis (Group 3) and L. chinensis–S. arvensis (Group 4) make up a largeportion of the plants inhabiting the sample plots in our study area.

In a summary, soil available nitrogen was the most vital factor limiting plantdistribution, followed by soil salinity and pH. In drainage ditch banks, therefore,

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maintaining soil nutrient and salinity at moderate levels is efficient approach toprevent species loss.

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