Effects of using wastewater as nutrient sources on soil chemical properties in peri urban agricultural systems

If the content of trace m ctals increascs above a ccrtain critical concentration due to th eừ accum ulation in soil, this can have negative en v iro n m en tal effccts, w hich can includ

V NU ỉo u rn a l of Science, F.arth S ciences 24 (2008) 87-95

Effects of using wastewater as nutrient sources on soil chemical properties in peri-urban agricultural systems

N g u y e n M a n h K h a i1'4, P h a m T h a n h T u a n 2, N g u y e n C o n g V in h 3, I n g r id O b o r n 4

2 Department o f Environmental Impact Asscssmcnt and Appraisal,

M inistry o/N aturaỉ Resources and Environmcnt ( MONRE)

’ Soils and Fertiỉizers ỉnstitutc

4 Department of Soil Sãcnces and Environment,

Sĩocdish University o f Agriadturaỉ Scicnccs (SLU)

Kcccivod 27 M ay 2008; rcccivcd in rcv iscd ío rm 5 July 2008

A b stract R cu sin g d o m e stic w a stc w n te r for irrigation and a p p ly in g b io so lid s as a íertilise r in crop

p ro d u c tio n a rc co m m o n p ra cticcs in p cri-u rb an arcas of V ietnam T his stu d y in v estig atcs th c cffects

o f u sin g d o m c stic w a stc w a te r in ficld cx p c rim cn ts on riu v is o ls soils in p cri-u rb an arc a s of l ỉanoi

a n d N am D inh cities VVe c o m p a rc d long-tcrm (30-50 ycars) w a sto w a te r-irrig a tc d ricc -d o m in ated

ía rm in g sy stem s U sin g w a stc w a tc r for irrigation sig n iíican tly affcctcd pl 1, electrical c o n d u c tiv ity

(KC), ex ch an g ea b lc K a n d N a a n d rc v e rsc aqun rí$/rt-digestiblc (R cv A q R cịị ) c o p p c r (Cu), lcad (Pb)

a n d zinc (Zn) in the in v e stig a tc d a rc a s co m p ared \vith con tro i p lo ts irrig n tcd u sin g riv e r w atcr

ĩh c rc w crc n o sig n iíica n t cffccts o f w a stc w a tc r irrigation o n th c NI I N O vextrnctablc íractio n of

c a d m iu m (C d) an d o th c r tra c c m ctals, but th c r.D T A -extractable írac tio n of C u # Pb a n d Zn w as

sig n iíica n tly in crcascd

Kci/ĩOữrds: SF A sia; hcn v y m ctals; irrig atio n ; p a d d y soils; tracc elem o n ts

1 In tro d u ctio n

U rbanisation and in d u strialisatio n are

leading to p ro d u ctio n of a h u g c volum e of

eíílu en ts in m an y countries Industrial,

agricultural an d d o m estic cffluents such as

biosolids and w astow ater are eith er d u m p c d

on land or used for irrigation a n d íertilisation

purposes, vvhich creates both o p p o rtu n itics

an d problem s [24]

* Corrosponding author Tcl.: 84-4*5583306.

K-mail: khainm ểvnu.vn

The advantagos of reusing w astew ater arc that it p rovidcs a convcnient disposal of

w aste p ro d u cts and has the beneíicial aspects

of a d d in g valuable p lan t nu trien ts and organic m atter to soil Purthcrm ore, the reusc

of w astow atcr for irrigation as a íertiliser source is a com m on an d po p u lar practice, cspocially in pcri-u rb an aroas W astowater is often the only source of w atcr for irrigation Evon in arcas vvhere w astew ater is not the solc vvater source for agricultural irrigation, íarm crs still proíor using sew age for irrigation

by rcason of its n u tritiv c valuc, vvhich ređuccs

ex p cn d itu rc on chcm ical ícrtilisers [10,17]

87

88 Ngm/cn Maiĩh Khni ctnl / VNU Ịountnl o f Science, Eartlt Sciences 24 (2008) 87-95

Hovvever, as vvastes are pro d u cts of

h um an society, onhancod concentrations of

potential toxic substances including traco

m etals are generally ío u n d in waste\vater,

w hich m ay lim it the long-term usc of

effluents for agricultural p u rp o se s d u e to the

likclihood of phytotoxicity, hoalth and

en vừ onm ental effects (1, 14] A nothcr

agricultural lan d is tho potentially phytotoxic

n aturc of organic vvastes, m ainly as a result of

com bination of íactors such as high salinity or

excess of am m onium ions, organic com pounds

or low m olccular w eight íatty acids, vvhich

e.g m ay inhibit soed germ ination | 6 , 10]

If the content of trace m ctals increascs

above a ccrtain critical concentration due to

th eừ accum ulation in soil, this can have

negative en v iro n m en tal effccts, w hich can

includc negative effccts on soil biota and

honce on m icrobial and íaunal activity [7ị

Furtherm oro, tracc m etals can affect crop

grow th and quality, an d th u s pose risks for

hum an health [2, 6, 12] Thereíorc, thc risk of

contam ination by trace m otals m ust bc

considcred vvhon wastcvvater is applicđ and

u n d erstan d in g of thc behaviour of m ctals in

environinental risks of ap p ly in g w astew ater

in agro-ocosystem s

T h e m ain objective (>f this p ap o r w as to

q u a n tiíy thc cffects of reu se of vvastow atcr as

n u tric n t sources by: (i) in v cstig atú ig the effccts of long-term vvastcvvatcr ừ rig atio n on soil p H , EC, organic carbon, total nitrogcn and trace m etals (cad m iu m (Cd), co p p er (Cu)y lcad (Pb) an d zinc (Zn)); (ii) in v cstig atin g th e cííects of application of w astcw atcr,

accu m u latio n an d solubility

2 M a te ria ls and m e th o d s

2.7 Locatiou o f thc rcscarch arcas

Soil sam ples w cro collccted from peri-

u rb a n arcas in tvvo pro v in ces of V ietnam ,

in c lu d in g H anoi, N am D inh (Table 1) Tho sam p le d arcas are locatod in dclta an d

lo w lan d areas vvith a tropical m onsoon clim ate T h e annual rainíall is 1500-2000 m m ,

an d m o rc than 50% of th c rainíall is

co n cen trated d u rin g Ju n c to A ugust T h e

m ean m onthly tem p o ra tu re varies betvveen 17 and 29"c, w ith tho w arm cst pcriod from Ju n c

to A u g u st and the coldest d u rin g Decom bor and Jan u ary

T able 1 D cscrip tio n of w a stc w a tc r (full-scale case s tu d ic s ) in c x p e rim e n ts on P lu v iso ls in

p c ri-u rb a n arcas o f H an o i and N a m D in h citics, V ictnani

No L ocation Soil irrig a tc d by Posỉtion N a m c C ro p A p p lic a tio n S a m p lin g tim c

1 Ila n o i

F lu v iso ls W a stc w a tc r N: 20°57.52'

K: 105°49.68'

T rc a tm e n t Ricc S ince 1960s Ju n c 2004

R iver w n tcr N: 20°58.12'

E: 105°48.15’

C e n tra l Rice Ju n c 2004

2 N am D inh

F luvisols W a stc w a tc r N: 20°44.93’

E: 106°20.98'

T re a tm e n t Rice S incc 1980s Ju n c 2004

R ivcr w a tc r N: 20°43.43'

E: 106o20.68’

N^UI/CIÌ M anh Khai ct nị / VNU Ịourtml o f Sãencc, ĩ.nrth Sciences 24 (2008) 87-95 89

2.2 Wastaoatcr irrigation in pcri-urban agriailtural

production systems

The sevvage ừ rig atio n stu d y arcas are

located ừi u rb a n rcgions dovvnstream from

Hanoi City an d N am Dinh City Tho soil

typcs arc Eutric Fluvisol at tho H anoi site,

and H um i-E ndogleyic Fluvisol (Eutric) at tho

N am Dinh sito according to tho World

Rcíorcnce Baso for Soil Resources T he soils

are íertile an d su itab le for g ro w in g crops

Rice has b een th e m ain crop in th ese areas,

but therc is a ten d en cy of changing from rice

to vegotable p ro d u c tio n d u e to incrcasing

dcm and from th e innor City m arkots of Hanoi

and N am Dinh At tho H anoi site, sevvage

w atcr has been usod for irrigation since thc

1960s Bocause o f vvater scarcity, agricultural

land has boen irrig ated by sevvage from Kim

N guu River, w hich ru n s th rọugh tho urban

arca to rural ag ricu ltu ral land [8, 9] At the

Nam Dinh site, irrig atio n u sin g vvastevvater

started in thc 1980s as a rosult of increasing

urbanisation Tho sow agc m ainly com prises

dom cstic w ater b u t also includes vvastovvater

and dischargcs from in d u strial activities in

the urban areas |4, 20] In N am Dinh, the soil

sam ples vvere takcn in the íields vvhore the

DANIDA-IWMI project on vvastevvater reuse

in agriculturc in V ietnam w as carried o ut [20]

2.3 Soil sampling stratcgy and sainplc prcparation

For assessm ent of the im pact of wastovvater,

soil sam ples w erc takcn from the topsoil (0-20

cm) of all stu d y sitcs in p cri-u rb an arcas of

Hanoi (n=4) and N am D inh (n=8) u sin g a soil

auger At cvery sa m p lin g point, 3 to 5 sub-

samplos w crc takon from approxim ately 250

m 2 and m ixed to o b tain a bulk sam plc Non-

vvastewater irrig atcd soils ("natural" river

irrigation) w ere also sam p lcd for com parison

(n=4 for H anoi, a n d n=8 for N am Dinh)

A íter air d ry m g at room tcm peraturo, thc soil sam plos vvere ground and sievod to rem ovc particlos > 2 m m , and then stored in plastic bags The soil sam plos w crc brought to

Sw edon (SLU) for analysis

2.4 Soil analysis

Total N (Ntot) and total organic carbon (TOC) vvas detorm incd on íinoly ground sam plcs on a LECO CH N analyser (Lcco

C H N *CH N 932 analyser) Prior to the analyses, tho sam plos worc trcatcd by 4M HC1 (1:1 soil:solution ratio) for dissolution of carbonates T he soil EC an d pH vvere

m oasurcd in deionisod H :0 (1:5 soil:solution ratio), and pHcaci: vvas doterm inod aftcr

ad d itio n of 0.5M C aC l; [18] Tho soil sam plcs wcru cxtractod w ith IM N H jN O j for 2 hours (1:2,5 soil:so!ution ratio) to quan tiíy thc exchangoablo an d spociíically adsorbod

íraction of trace m ctals (i.c Cd, Cu, Pb, Zn)

|3| Potentially bioavailable m etals wcro extracted vvith 0.025 M (Na):EDTA (1:10 soilisolution ratio) for 1.5 h |19| The reversc

acịua rcgia (3:1 HNO.;:HCl ratio)-digestible

ừaction (Rcv Aij Rcg) of C d ; Cu, Pb an d Zn

w as extracted using a m ethod dcscribcd by

Stcvens et al [18] A íter contriíugation, íiltration a n d dilu tio n (if neccssary) m etal concentrations vvere d eterm in ed by inductivcly couplcd plasm a-m ass spoctrometry (ICP-MS, Perkin Elm er ELAN 6100)

2.5 VVatcr sampling and ĩoatcr analysis

In N am D inh and H anoi, vvater sam ples

w ere colloctcd in su m m e r 2004 from the Red Rivèr an d w astew ater channcls, w hich w cre the irrigation sourcos at the stu d y sites The

pH an d EC in these vvatcr sam ples vvere

d cterm in ed directly after sam pling Polyethylene bottles that had been

pre-90 N guyên M anh Khni et a i / VNU Ịounm l of Science, Lnrth Sciences 24 (2008) 87-95

w ash ed w ith acid and distilled w ater and

dricd wero u sed , and aíter sam pling, a few

drops of co n cen tratcd HC1 vvere ad d ed prior

to chem ical analysis

VVater sam p les vvcre analysed for thcừ

total concentrations of Ca, Cd, Cu, K, Mg, N,

p, Pb and Zn O n e aliquot of the sam ples vvas

digested w ith boiling concentrated H N O3

beíore determ ination of the total conccntration

of K by ílam e spectrom otry; Ca, Mg and Na,

by atom ic ad so rp tio n spcctrophotom etry

(AAS, Perkin E lm er 300); C d, Cu, Pb and Zn

by ICP-MS, a n d total p by H N O3 digestion

followod by d e tc rm in a tio n of PO4-P w ith the

ascorbic acid m eth o d [5] Total N vvas

quan tiíied as described elsevvhere [9Ị

2.6 Statistical analysis

Data from tho exp erim en ts w ero analyscd

u sin g the G en eral Lincar M odcl (GLM)

pro ced u re of M initab Softw are version 14

T reatm ent m ean s w hich shovvòti signiíicant

differenccs at th e probability level of p<0.05

w ere com parcd u sin g T u k e y 's pairw ise

com parison procedure The sourco of irrigation

w ater w ithin sites (vvastcvvater an d river

vvater) w as usod as a íactor in the m odel Tho

statistical m odel u sed vvas ]/,j = ụ + a, + c ,j,

w h ere ụ is the m e a n value for all treatm ent, a,

the diíícren t betw con m ean valuo-' of

treatm cnt i w ith ovorall m ean, and e,j is the

ran d o m error

3 R esults

3.1 Irrigation watcr quality

Tho results of tho irrigation vvatcr analysis

are presentod ừì Tablc 2 Tho pH and EC vvere

signiíicantly highcr in thc vvastevvater comparcxi

w ith the river vvater Tho \vastow ator also h ad signiíicantly h ig h cr conccntrations of

nu tricn ts an d traco m ctals com parcd vvith the river w ater T his indicatcd that non -treatcd vvastcwater containod both n u trien ts that are

of value for irrigation of crops in agricultural system s, and potontial toxic elem ents that can aííect soil p ro d u ctio n capacity a n d crop quality A com parison botw eon w astew aters

in H anoi and N am Dinh show ed that the conccntrations in H anoi vvastevvater w ere significar>tly h ig h er for m ost elcm ents included in the stu d y (i.c Cd, Cu, K, Na, N,„„ P1o1/ Pb, Zn)

3.2 Effccts o f applying ĩoastaoater on soil pH and elcctrical conductivity

A pplying w astew ater for irrigation signiíicantly increased soil pH (pHn20 a n d

pH can:) at b o th s tu d y sites (H anoi a n d N am Dinh) (Tablo 3), probably d u c to w astcw ator being m ore alkalino than rivcr vvater (Tablc 2) T he sim ilar cííoct w as obscrved for electrical conductivity, w hich \vas highcr in the vvastevvater troatm cnts th an in the control (river)

3.3 Effccts o f applying ĩvastaoatcr on soil organic

carbon and total rtitroỊỊcn contcnts

Rcusc of vvastevvater for irrigation causod

an ừicrease in total organic carbon (TOC) contcnt an d total n itro g en (Nio,) in the soil at

b oth study sites (Fig 1) Tho soils that h ad reccived vvastevvater for irrigation had 1.68%

TO C and 0.19% N|0I at the H anoi site an d 2.67% TOC an d 0.26% N,ci at the Nam D inh site T he co rresp o n d in g valucs for control sam ples w erc 1.29% (TOC), 0.15% (Nuh) an d 1.85%, (TOC), 0.21% (N,ot) for Hanoi and Nam

D inh, respectivcly

Nguì/Cĩí M anh Khai ct aỉ / VNU Ịũuntaỉ c f Science, Earth Sciences 24 (2008) 87-95 91

T ab le 2 VVatcr q u a lity o f th c R ed R ivcr w a tc r an d w a ste w a tc r u sc d for irrig atio n in I Ianoi a n d N a m Dinh

D ifferen t lcttcrs (a, b) d c n o tc sig n iíican t diffcrcnccs b etw een so u rc es of irrig atio n w a tc r w ith in sitc s (P<0.05)

R ed River (n=4)

W a stc w a te r (n=5)

Red R iver (n=4)

YVastexvater (n=6)

4 lo ta l p h o s p h o ru s (IV,) m g i ; 1 0.6a 2.0 0.5“ 4.4b

T able 3 Electric c o n d u c tiv ity (KC, ỊiS cnV1), pl I, cxchangcablc Ca, M g, N a, K (IM NI Ỉ 4 N O 3 ex tractab lc; g k g '1)

in topsoỉl ( 0 - 2 0 cm ) sa m p le s fro m cx p e rim cn ts vvỉth rc u se of w a stc w a tc r I)iffcrcnt lcttcrs d e n o te sig n iíica n t

d iffc ren c cs b etw e en tre a tm e n ts at th c s a m c sitc (P<0.05)

S ite E C 1 pÈ 1*1120 pl ỉ 2CaC12

E xchangeablo

I la n o i

C on troi 62.80“ 6.45* 5.69a 0.06a 0.04a 0.89 0.19 VVastevvntcr 102.75b 6.70b 5.96b 0.16b 0.09b 1 0 0 0 2 2

N a m D inh

C o n tro l 78.25* 5.99* 5.42* 0.06* 0.04* 1.55 0.18 VVastevvator 179.38b 6.36b 5.71b 0 1 2 b 0.170 1.60 0 2 1

1 p l ỉ in H ị O , ratio soiỉ: loatcr - ĩ: 5

2 p H in 0.05 M CnC.Ỉ 2 , ratìosoiỉ: soỉutioìì = 1:5

3 IM N ỉ I 4 N O 3 extractable, ratio soil: solution = 1:2.5

3.0

s 2.5

ễ 2 0

8

1.5

1.0

0 5

0.0

S C o n t r o l I b

a T r e a t m e n t

■

■

r b

3 Ị P I I

T I

1

I

Fig 1 Lffcct of w a s te w a te r irrig a tio n o n soil chem ical p ro p e rtie s, to tal o rg a n ic carbon (T O C %), to tal n itro g en (Nịov %)• D ifferent lc ttc rs d c n o te sig n iíica n t diffcrcnccs b etw ccn tre a tm c n t a n d control at th c sa m e site (P<0.05).

92 Ngui/ai Manh Klmi et al / VNU Ịourììnl oỊScicncc, F.nrtlỉ Sciences 24 (2008) 87-95

3.4 EJfects ofapplying umstcĩvatcr on trace mctal

conccntrations in soil

The concentrations of reverse aqua regia

(Rev Aq ífcg)-digestible Cu, Pb and Z n in soils

receiving vvastevvater vverc signiíicantly

higher than those in soils receiving river

vvater Thcre w as no signiíicant differencc in

Cd concentration (Rev Aq Rcg) bctvveen

vvastcvvatcr ừrigated soils and control soils

(Fig 3) T he potontially bioavailablc

conccntrations of Cu, Pb and Zn

(EDTA-45

40

35

ì 30

D

ố 25

cn

cú

(r 20

cr

< 15

ồ

ír 10

5

0

HTreatment r a

3 m

IIHanoi Nam Dinh

0.20

0.16

0.12

3

O)

Ề 0 08

cr

<

ĩ 0.04

£

0.00

extractable) in vvastevvater-treated soils vvere signiíicantly hig h cr th an in control soils (no difference for Cd) The N H4N03-extractcd ÍTactions of C d, Cu, Pb and Zn constituted only a sm all p ro portion of tho EDTA-

ex tracted íractions Hovvevcr, thcro w as no signiíicant diffcrenco betvvcon treatcd soils

an d control soils in tho exchangeable (NH^NO.ị) íraction of thcso m etals (Table 4)

concentrations in com bination vvith a variation bctvveen thc rcplicates

140 Ị 120

O) 100 j

£

•*»»

c 80 N co

<y 60 • cr

< 40

ồ

cr 20

-0 I

4 5 •

40 •

3 5 • ơ)

E 30

-Q

O. 2 5 I

cn

0 20

òẽ

cr 15

<

>

ó 10 lỉ:

5 0

Fig 2 Effect of rc u sc o f w a ste w a te r on rc v ersc nqua rcgia (R ev A q R eg)-cxtra c ta b le C đ, Cu, Pb a n d /.n (m g k g '1)

conoentrations in soil Diffcrcnt letters dcnotc significant differences bct woen trcatm ent and control at tho sam c sitc (P<0.05).

Tabie 4 Effect of w a ste w ater application o n 0.025 M EDTA (m g kg ' 1 d w ) a n d IM N ỉ Ỉ 4 N O 3 extractablc (m g kg 1 dvv) tracc m ctals Differcnt lottcrs d en o te signiíicant differcnccs bct\v ccn trc a tm c n t a n d contro! at thc sam c sitc (P<0.05)

Si te E D T A -extractable N I l4N O _rCxtractable

H a n o i

C ontrol

T re atm en t

8.21a 9.53b

1.85* 0 1 1 2 a 2.9 7b 0.105a

7.18a 8.89b

0.002 0.150 0.0059 0.004 0.205 0.0076

0.004 0.007

N a m D in h

C o n tro l 10.99a 1.63* 0 1 2 0 b 11.10* 0.009 0 1 2 0 0.0093 0 0 1 1

T re a tm e n t 12.65b 1.75b 0.126b 15.32b 0.006 0.180 0.0125 0.038

N guì/ai M anh Khai et ni / V N U Ịountal o f Science, Larth Sciences 24 (200S) 87-95 93

4 D iscu ssio n

A nalyses of soil sam plcs collected to

assoss the im pacts of sew agc irrigation on the

irrigated ag ricu ltu ral soils of p eri-u rb an areas

of H anoi and N am Dinh cities sh o w ed that

reuso of m u n icip al vvastevvater for irrigation

had signiíicantly in crcased both TO C an d Nioi

in soils This íin d in g is in agreem ont vvith

previous stu d ies w h e re wastevvatcr irrigation

had becn sh o w n to increase soil org an ic c

and N [15] Hovvover, a potential h a /a r d to

peri-urban crop p ro d u ctio n vvas revealcd duc

to the accu m u latio n of trace m etals in

íigricultural soils irrigatod vvith sovvagc

M unicipal vvastevvater contains a variety

of inorganic su b stan ces from dom estic and

industrial sources, including a nu m b o r of

potontially toxic elcm cnts such as arsenic

(As), cadm ium (Cd), ch ro m iu m (Cr), coppcr

(Cu), m ercury (H g), load (Pb), zinc (Zn), ctc

Ị 16) A ccording to tho annual rep o rt on the

en v iro n m en tal statu s of Viotnam m a d e by

VEPA Ị22|, íhesc p otential toxic elem en ts are

com m only presont in dom ostic vvastevvater of

m any cities in V ietnam Even if potontial toxic

clcm ents in w asto w ater are not p rescn t in

conccntrations likoly to dircctly aííect

h u m an s and th u s liinit thcir ag ricu ltu ral use,

thoy socm to be h ig h cr than in n a tu ra l river

vvater, vvhich w o u ld co n tam in ate tho

agricultural soils in the long-term As a rcsult,

tho concentrations of tracc inotals (Cu, Pb and

Zn) in thc vvastevvater-irrigated soils vvere

signiíicantly hig h o r th a n in control soils,

indicating that th e application of vvastevvater

had cnrichcd the soil vvith trace m etals Liu ct

al (2005) studied tho impact of sevvago irrigation

on trace m etal contam ination in Beijing and

reported that thc trace m ctals vvcro enrichcd

in thc soil d u o to sevvage irrig atio n [11] This

w as also íound in earlicr publications about effccts of sowago irrigation on soils [13, 21] The application of w astcw ater m the peri- urban sites of H anoi and N am Dữứi citics incrcased soil pH by approxim ately 0.3 units com parod w ith the non-w astcw ator irrigatcd sites Prcvious researches |8, 23] have indicated that tho vvastevvater applied for irrigation at Hanoi and N am Dinh sitcs is in

m ost cases ncutral to alkaline (Ó.5-8.5) The prcsont stu d y also ío u n d that the pH w as signiíicantly highor for w astew ator com pared

w ith n atu ral river vvator (Table 2) In addition, the highor conccntration of cations such as Na and K in w astcw atcr lod to an incrcaso in EC and cxchangoablo Na and K in soils irrigatcd vvith vvastewatcr The high pH

of soils irrigatod w ith vvastevvater m ight red u ce the m obility of thc trace m etals accum ulatod in these soils

5 C o n clu sio n s

Reuse of w astew atcr as nutricnt sourcos has becom o com m on practico in Vietnam, espccially in p eri-urban areas The rcusc of thesc nutrionts h ad som o bcncíicial eííects on soil íertility, such as increased total organic carbon and nitrogen This study íound that both organic carbon con ten t and total nitrogen vverc im proved (ừicreased) in soils treated vvith wastcwater Hovvever, thesc bcncfits wcre lừnitod by tho presence of som e potential toxic traco m otals in w astew atcr It vvas concluded that tho reuse of w astow atcr for ừrigation incrcascd soil pH , EC, TOC, N loi and total concentration of Cu, Zn and Pb The EDTA- extractable íraction of C d, Cu and Zn was signiíicantly higher for w astcw atcr-irrigated soils

94 Nguì/en M anh Khai ct al / VNU Ịounĩal ọ f Science, Enrth Sciences 24 (2008) 87-95

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