1

0038-075C/01/16612-000–000 December 2001Soil Science Vol. 166, No. 12Copyright © 2001 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A.

DYNAMICS OF PHYSICAL-CHEMICAL PROPERTIES IN SOILS WITHANTHROPIC FLOODING, BUENOS AIRES PROVINCE, ARGENTINA

Perla A. Imbellone1, Beatriz A. Guichon2, and Jorge E. Giménez1

In Argentina, redox processes have been studied in natural ecosystems,but little is known about these processes in managed environments. InBuenos Aires province (southern Pampean region), large-scale experi-ments aimed at commercial rice production have recently taken place inlow-lying areas of the Depressed Pampa . The objective of this study wasto establish the physicochemical modifications that take place in rice fieldsoils as a result of temporary flooding. Soils (fine, illitic, thermic VerticArgiudolls) developed in loessial sediments are widespread in level inter-fluves of the La Plata river basin. We measured Fe2�, Mn2� and NO3

� inthe Ap and BA horizons of an artificially flooded and a nonflooded soilduring two rice cultivation cycles. Periods of preflooding, flooding, andpostflooding were distinguished in each cycle. Only the Ap horizon pre-sented a definite reduced condition during the flooding period, as indi-cated by redox potential and the contents of Fe2�, Mn2� and NO3

�. Af-ter irrigation was discontinued, the soil returned to an oxidizing statesimilar to that of the nonflooded soil. The anaerobic conditions did notpersist long enough to modify the morphology of the flooded soil as nomacroscopic redoximorphic features were observed. (Soil Science 2001;166:000–000)

Key words: Redox potential (Eh), hydromorphism, iron, manganese,paddy soils.

FLOODING for rice culture generates seasonalanaerobic conditions, thereby elevating Fe2�

and Mn2� and increasing denitrification. In Ar-gentina, these processes have been studied in nat-ural environments (Taboada and Panuska, 1985;Guichon et al., 2000). However, little is knownabout these processes in managed environments.No studies have been conducted in Buenos Airesprovince (southern Pampean region), where riceis grown only for experimental purposes. How-ever, large-scale experiments have recently begunin low-lying areas of the Depressed Pampa aimedat the commercial production of rice. The objec-tive of this investigation was to compare thephysicochemical modifications that have taken

place in a Vertic Argiudoll planted to rice andtemporarily flooded to a nonflooded soil underfallow conditions.

MATERIALS AND METHODS

The soils were studied (fine, illitic, thermicVertic Argiudolls) at the Rice Experiment StationIng. Agr. J. Hirschhorn (Faculty of Agrarian andForest Sciences, National University of La Plata).The selected physical and chemical properties areindicated in Table 1. The soils were developed inloessial sediments, which are widespread in levelinterfluves of the La Plata river basin.

Soil morphology was described according tothe Soil Survey Manual (Soil Survey DivisionStaff, 1993). pH was measured in situ with a glasselectrode as described by Vizier (1970). Particle-size analysis was determined by the pipettemethod (National Soil Survey Center, 1996), or-ganic carbon by the Walkley-Black method (Al-lison, 1965), and moisture was determined gravi-metrically.

1Instituto de Geomorfología y Suelos, Universidad Nacional de La Plata, Argentina.Dr. Imbellone is corresponding author. E-mail:imbellon@netverk.com.ar 2CONICET, Calle 3 No. 584, B1902CIX La Plata, Argentina. E-mail: igs@museo.fcnym.unlp.edu.ar

Received Oct. 23, 2000; accepted Aug. 16, 2001.

SS-12/01-99518-Imbellone 11/01/2001 9:52 AM Page 1

2 IMBELLONE, GUICHON, AND GIMÉNEZ SOIL SCIENCE

TABLE 1

Selected physical and chemical properties of the Control soil

Grain-size composition (%)Textural class Organic

Horizon Depth (cm) sand silt clay (USDA) pH (paste) matter (%)

Ap 0–18 7.7 61.2 31.1 Silty clay loam 5.2 4.4BA 18–36 8.1 57.4 34.5 Silty clay loam 5.4 3.6Btss1 36–84 5.6 36.1 58.3 Clay 5.0 0.9Btss2 84–113 6.3 38.0 55.7 Clay 5.4 0.6BC 113–160 8.4 49.5 42.1 Silty clay 5.6 0.4C 160–180� 6.8 46.7 46.5 Silty clay 6.0 0.1

Redox potential (Eh) was measured with amillivolt meter with platinum-tipped (Pt) elec-trodes and a calomel reference electrode. The Ptelectrodes were calibrated with the Zobell solu-tion (Zobell, 1946) using the equation correctedby Lévy and Toutain (1979) adapted for thecalomel electrode (Eh � 182 mV � 2.4/25 t).Ehmeasurements were corrected to pH 7 with thefactor ddE(V)dpH � 0.05974 volts (Bohn,1971). The Pt tips of the electrodes were placedin the Ap and AB horizons of the flooded soilsduring two cultivation cycles (1995–1996 and1996–1997) and in one nonflooded soil whererice was never grown. Readings were made ap-proximately every month during the rice-growingcycle. The Eh values included in Table 3 are themean of three to five readings, following themethod of Vizier (1970).

Fe2� and Mn2� were extracted with a 3%AlCl3 solution (Ignatieff, 1941). Fe2� was deter-mined colorimetrically with o-phenanthrolineand Mn2� by atomic absorption spectrophotom-etry.NO3

� was extracted with a saturated CaSO4solution and measured by phenoldisulfonic acidcolorimetry (Bremner, 1965). Samples were keptin air-tight plastic jars immediately after collec-tion, refrigerated during transport, and stored in arefrigerator at 4 �C until analyses, which weremade within 24 h.

Irrigation water was applied by floodingfrom December 1, 1995, to March 30, 1996, inthe first cycle and from December 1, 1996, toMarch 20, 1997, in the second cycle. Irrigationwater was pumped from an aquifer at about 40 mdepth. Chemical properties of the irrigation wa-ter are given in Table 2. Carbonate and bicar-bonate were determined by acid titration, chlo-ride by Mohr titration, sulfate by gravimetry withbarium chloride precipitation, calcium and mag-nesium by EDTA titration, and sodium by flamephotometry (National Soil Survey Center, 1996).

Rice has been grown in each plot every 3years for a period of 20 years. The plots remainunder fallow condition in the intervening years.Fertilizer (50 kg ha�1 of diammonium phos-phate) is applied at planting, and herbicides usedafter seedlings emerged. An 8-cm thick watersheet is maintained during the growing cycle.The cultivars of rice used in the first and secondcycles were IRGA 409, of Brazilian origin, andLa Plata ITAPE, created at this Experiment Sta-tion, respectively.

The measurements were divided into threeperiods: preflooding, flooding, and postflooding.The first period included preliminary tillage andinitial irrigation with discontinuous accumula-tions of water on the surface. The flooding pe-riod was set between the dates, with Eh below300 mV in the Ap horizon, a limit established for moderately reduced soils by Ponnamperuma(1972). During this period, the soil presented asheet of water at least 3 cm in thickness. Finally,during postflooding, irrigation is interrupted, thesheet of surface water disappears, and the soil be-gins to lose moisture. The three periods cannotbe separated by exact dates. The transitions are

TABLE 2

Properties of the irrigation water

pH 8.3Electrical conductivity (dS m�1) (25�C) 1.01Dissolved solids (mg L�1) 700Ca2� (mmolc L�1 1.3Mg2� (mmolc L�1) 1.0Na� (mmolc L�1) 7.2CO3

2� (mmolc L�1) 0.9CO3H� (mmolc L�1) 6.7Cl� (mmolc L�1) 1.0SO4

2� (mmolc L�1) 1.3Sodium Adsorption Ratio 6.71Residual Sodium Carbonate 5.3

SS-12/01-99518-Imbellone 11/01/2001 9:52 AM Page 2

VOL. 166 ~ NO. 12 PROPERTIES OF SOILS WITH ANTHROPIC FLOODING 3

TA

BLE

3

Evo

lutio

n of

Eh,

Fe2�

,Mn2

�,N

O3�

,pH

and

moi

stur

e

1995

–199

6 cy

cle

1996

–199

7 cy

cle

Para

met

erH

oriz

onPr

efloo

ding

Floo

ding

Post

flood

ing

Prefl

oodi

ngFl

oodi

ngPo

stflo

odin

gan

d so

ilD

ec 2

8Ja

n 11

Feb

12A

pril

9M

ay 1

5Ju

ne 1

1N

ov 1

2D

ec 2

6Ja

n 2

Jan

8Fe

b 11

Mar

11

Apr

il 8

May

12

June

10

Ap

NF

477

543

485

385

430

409

484

429

386

356

417

405

426

427

446

Eh

F45

957

121

180

510

522

480

314

358

297

193

124

315

368

418

mV

BA

NF

502

515

506

446

465

497

473

440

436

459

473

430

445

418

428

F50

743

729

635

333

552

248

444

446

846

639

631

952

235

243

5A

pN

F0.

780.

340.

010.

01�

0.01

0.01

0.63

�0.

01�

0.01

�0.

010.

66�

0.01

�0.

01�

0.01

0.01

Fe2�

F0.

860.

011.

6884

.740

.60.

530.

02�

0.01

�0.

0113

.02

1.96

350.

181.

111.

230.

65m

g kg

�1

BA

NF

0.56

�0.

010.

010.

01�

0.01

�0.

010.

01�

0.01

�0.

01�

0.01

�0.

01�

0.01

�0.

01�

0.01

0.01

F0.

420.

0145

.70.

72�

0.01

�0.

01�

0.01

�0.

01�

0.01

�0.

010.

830.

82�

0.01

0.02

0.76

Ap

NF

2.11

1.77

1.13

0.69

0.41

0.01

1.80

2.40

2.20

4.10

2.00

1.63

1.06

1.46

1.27

Mn2

�F

18.9

43.

9218

9.24

402.

5826

6.22

2.81

1.73

3.18

6.52

24.2

452

.22

367.

6978

.92

46.3

24.

52m

g kg

�1

BA

NF

1.24

0.65

0.48

0.86

0.80

0.01

0.60

0.01

0.70

1.30

0.81

0.77

0.88

0.60

0.57

F0.

84�

0.01

214.

2210

.15

11.3

91.

060.

234.

4012

.47

1.79

44.5

696

.85

6.65

23.7

97.

16A

pN

F0.

660.

460.

360.

011.

690.

950.

013.

102.

201.

100.

820.

632.

232.

190.

01N

O3�

F13

.88

2.13

�0.

01�

0.01

�0.

012.

1928

.59

2.20

0.90

�0.

010.

010.

420.

011.

17�

0.01

mg

kg�

1B

AN

F0.

950.

711.

170.

012.

370.

01�

0.01

1.30

2.20

0.80

0.89

0.95

1.25

1.93

1.64

F19

.68

1.58

�0.

01�

0.01

�0.

010.

9924

.69

14.2

02.

200.

800.

47�

0.01

0.70

1.43

�0.

01A

pN

F5.

56.

65.

75.

25.

65.

56.

76.

15.

35.

36.

16.

26.

66.

56.

5F

8.4

8.2

6.8

6.5

7.6

7.1

5.9

6.4

6.0

6.5

6.8

7.0

7.1

5.6

6.9

pHB

AN

F5.

25.

65.

45.

35.

35.

45.

65.

46.

25.

76.

26.

06.

36.

16.

1F

6.4

6.5

6.7

6.3

6.9

6.9

5.9

6.4

6.1

6.6

6.6

6.8

7.2

6.2

5.9

Ap

NF

16.1

13.3

27.5

26.3

28.6

34.7

32.4

19.2

15.2

36.2

30.4

15.1

17.2

31.1

36.4

Moi

stur

eF

32.2

33.2

42.7

40.4

31.4

31.4

31.9

36.6

42.2

48.6

40.1

42.9

39.5

19.0

38.3

%B

AN

F15

.317

.522

.923

.229

.227

.928

.218

.215

.523

.319

.315

.916

.630

.031

.4F

30.3

32.3

33.6

30.7

27.9

25.0

26.2

29.1

32.5

37.8

38.4

34.6

31.8

28.1

34.1

NF:

Non

-floo

ded

(con

trol

) so

il.F:

Floo

ded

soil.

SS-12/01-99518-Imbellone 11/01/2001 9:52 AM Page 3

4 IMBELLONE, GUICHON, AND GIMÉNEZ SOIL SCIENCE

gradual and occur some time between two read-ings. Consequently, in order to set conventionallimits for each period, the dates of the measure-ments taken under conditions of preflooding orflooding, as described previously, were adopted.

For both the control and the flooded soils,each of the variables was analyzed statistically forthe three periods and the two horizons, utilizinga two-way analysis of variance with replications(Davis, 1986). Similar to the flooded soil, the pe-riods in the control soil were separated by datesto determine if it was possible to detect variationsresulting from natural causes, not from irrigation.

In addition, the measurements of each period(samples) and horizon (treatments) of the two cy-cles for both soils were grouped after testing thenull hypothesis of variance. Thus, the measure-ments of the ‘96–‘97 cycle were considered repli-cates of the ‘95–‘96 cycle. A level of significanceof P � 0.05 was used.

The Tukey test was performed to determinethe differences among means. Paired correlationswere calculated for each variable and horizon bythe Pearson formula, considering the two cyclestogether, with the same significance level usedwith the ANOVA.Fe2�,Mn2�, and NO3

� values

were log transformed so that they approach anormal distribution.

RESULTS

Table 3 shows the evolution of pH, moisture,Eh, Fe2�, Mn2�, and NO3

� in the Ap and BAhorizons of the rice field soil and control duringthe two studied cycles (1995–1996; 1996–1997).Statistical analyses are shown in Tables 4 to 8.

Eh DynamicsFlooded Soil

The ANOVA showed differences by periodand horizon as well as an interaction betweenboth effects (Table 4). The Tukey test indicated asignificant difference between the mean of the Ahorizon during flooding with respect to theother periods and horizons (Table 5)

Ap Horizon: During preflooding, Eh valuesranged in both cultivation cycles from 314 to 571mV. During flooding they dropped to 80 to 297mV but rose again to 315 to 522 mV during post-flooding.

BA Horizon: The trend of the curves is sim-ilar to those of the Ap horizon, but the values did

TABLE 4

Two-way ANOVAs for effects of periods and horizons of the variables: Flooded soil

Variable Effect df SS F F�

Eh period 2 189639.8 14.67 3.4horizon 1 41515.2 6.42 4.26period by horizon 2 46652.6 3.61 3.4error 24 155129.2

Fe2� (log) period 2 32.7353339 19.89 3.4horizon 1 15.1117379 18.36 4.26period by horizon 2 5.98112006 3.63 3.4error 24 19.750

Mn2� (log) period 2 11.8874682 9.95 3.4horizon 1 3.83294976 6.42 4.26period by horizon 2 0.11930448 0.10 3.4error 24 14.33

NO3� (log) period 2 28.1534394 16.74 3.4

horizon 1 0.73535028 0.87 4.26period by horizon 2 0.03153996 0.02 3.4error 24 20.1863339

Moisture period 2 378.192667 8.99 3.4horizon 1 202.8 9.64 4.26period by horizon 2 36.194 0.86 3.4error 24 505.092

pH period 2 0.07466667 0.03 3.4horizon 1 0.972 0.86 4.26period by horizon 2 0.504 0.22 3.4error 24 27.018

Values in boldface indicate significant difference at the p � 0.05 level.

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VOL. 166 ~ NO. 12 PROPERTIES OF SOILS WITH ANTHROPIC FLOODING 5

not remain uniformly low during flooding asthey did in that horizon. During preflooding, thevalues ranged from 437 to 507 mV;during flood-ing from 296 to 466 mV; and during postflood-ing from 352 to 522 mV.

Control

The ANOVA indicates that no differenceswere observed as a result of any of the effects stud-ied in each period (Table 6). During the two stud-ied cycles, Eh ranged from 356 to 543 mV in theAp horizons and from 418 to 515 mV in the BAhorizons. The behavior of the control soil can beascribed to rainfall dynamics. In two cases, Ehdropped below 400 mV in correspondence withrainfall higher than 20 mm in the two precedingdays (21.5 mm:Eh 376 mV;48.3 mm:Eh 356 mV).

Fe2� DynamicsFlooded Soil

Similar to Eh, the ANOVA of this variableshowed differences for period and horizon; therewas also an interaction between both effects(Table 4). However, when the Tukey test wasperformed, it was possible to differentiate the

means of Ap and BA horizons during flooding;this difference was also observed during post-flooding in the Ap horizon (Table 5).

Ap Horizon: During preflooding, Fe2� wasabsent or its concentration was lower than 0.86mg kg�1. It was always present during flooding,although its maximum level was reached after 1month of flooding in the first cycle and 2 monthsin the second cycle, varying between 1.68 and350.18 mg kg�1. During postflooding, Fe2� be-gan to diminish, as expected, although it was de-tected 2 months after the sheet of surface waterhad disappeared.

BA Horizon: During preflooding, Fe2� be-havior was similarl to its behavior in the Ap hori-zon. In the first cycle, however, it disappeared atthe end of flooding, unlike in the Ap horizon.

ControlAlthough the ANOVA apparently showed a

difference between periods (Table 6), the Tukeytest revealed no significant differences among themeans of each period (Table 7). For both cyclesand horizons, Fe2� levels never reached 1 mgkg�1.They varied erratically and showed no cor-relation with heavy rains or low Eh values.

Mn2� DynamicsFlooded Soil

The ANOVA showed differences by periodand horizon, but there was no interaction be-tween effects (Table 4). The Tukey test revealeddifferences between the means of the floodingand preflooding periods but not with the post-flooding period.This behavior is similar to that ofFe2� (Table 5).

Ap Horizon: During preflooding, Mn2�

showed detectable values earlier than Fe2�.Mn2�

levels ranging from 1.73 to 18.94 mg kg�1 werefound before ponding was observed in the field.During flooding, a marked increase of Mn2� (upto 402,58 mg kg�1) was observed in the two cy-cles. During postflooding, the concentration ofMn2� declined, although it was detected 2months after the end of irrigation.

BA Horizon: Mn2� was present in the threeperiods and in both cycles, with one exception.The maximum value was 214.22 mg kg�1. Thevalues detected did not relate to the content ofMn2� in the Ap horizon.

ControlThe ANOVA did not reveal any differences

either by period or by horizon (Table 6) The

TABLE 5

Tukey test for different variables: Flooded soil

EhBA-Preflooding 468 aA-Preflooding 436 aBA-Postflooding 433 aA-Postflooding 426 aBA-Flooding 366 aA-Flooding 181 b

Fe2�

A-Flooding 1.22095986 aA-Postfloding 0.25480948 aBA-Flooding �0.32946947 aA-Preflooding �1.97515198 bBA-Postflooding �2.1870068 bBA-Preflooding �2.24131987 b

Mn2� (log)Flooding 1.77751723 a A 1.40455202 aPostflooding 1.12264634 ab BA 0.68966743 aPreflooding 0.24116561 b

NO3� (log)

Preflooding 0.76037406 aPostflooding �0.97807673 bFlooding �1.50754986 b

Moisture %Flooding 39 a A 36.69 aPreflooding 32.65 ab BA 31.49Postflooding 30.65 b

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6 IMBELLONE, GUICHON, AND GIMÉNEZ SOIL SCIENCE

TABLE 6

Two-way ANOVAs for effects of periods and horizons of the variables: Control soil

Variable Effect df SS F F�

Eh period 2 6386.86667 2.10 3.4horizon 1 6106.13333 4.02 4.26period by horizon 2 2512.86667 0.83 3.4error 24 36422

Fe2� (log) period 2 2.6886268 3.57 3.4horizon 1 0.98456703 2.61 4.26period by horizon 2 0.60417266 0.80 3.4error 24 9.04416816

Mn2� (log) period 2 1.47102473 1.88 3.4horizon 1 1.21544762 3.10 4.26period by horizon 2 0.65156807 0.83 3.4error 24 9.39871053

NO3� (log) period 2 0.35305145 0.20 3.4

horizon 1 0.011554 0.01 4.26period by horizon 2 0.0241552 0.01 3.4error 24 21.7011126

Moisture period 2 425.682667 5.10 3.4horizon 1 68.403 1.64 4.26period by horizon 2 43.944 0.53 3.4error 24 1000.64

pH period 2 0.398 0.86 3.4horizon 1 0.432 1.86 4.26period by horizon 2 0.278 0.60 3.4error 24 5.564

Values in boldface indicate significant differences at the p � 0.05 level.

concentration of Mn2� was erratic in both cyclesand horizons, and it was almost always less than 2mg kg�1. The content was generally higher inthe Ap than in the BA horizon throughout theexperiment. The maximum value was 4.10 (Apin the second cycle).

NO3� Dynamics

Flooded Soil

The ANOVA showed a difference caused bythe flooding effect (Table 4). The Tukey test re-vealed a significant difference between the meansof preflooding and the means of the other peri-ods (Table 5).

Ap Horizon: During preflooding of the firstcycle, nitrates began to diminish as irrigation in-creased. For both cycles, NO3

� concentrationdropped to less than 1 mg kg�1 during flooding,and it increased 2 months after irrigation was in-terrupted in the first cycle. The trend in the de-crease of NO3

� levels for both cycles coincidedalmost exactly with the sharp increase of Fe2�

and Mn2�.BA Horizon: A pattern similar to that de-

scribed for Ap horizons was observed in both cy-

cles. The levels and distribution of NO3� in the

control soil showed no relationship with any ofthe measured parameters.

Control

No difference between periods and horizonswas revealed by the ANOVA (Table 6).

pH DynamicsFlooded Soil

The ANOVA did not show any differencesbetween periods and horizons (Table 4).

Ap Horizon: The initial value for both cycleswas about 6. pH variations were more pro-nounced in the first cycle than in the second cy-cle. During flooding, pH had a decreasing trendin the first cycle and an increasing trend in thesecond.

BA Horizon: Except for the first measure-ment, pH remained more stable than in the Aphorizon, ranging from 5.9 to 7.2.

Control

The ANOVA did not show any differencesbetween periods and horizons (Table 6).For both

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VOL. 166 ~ NO. 12 PROPERTIES OF SOILS WITH ANTHROPIC FLOODING 7

horizons and cycles, lower values were recordedin the control (5.2–6.7) than in the flooded soil.Likewise, the BA horizon remained more stablethan the A horizon.

Moisture DynamicsThe ANOVA showed differences caused by

period and horizons (Table 4). However, theTukey test revealed significant differences onlybetween the means of the flooding and post-flooding periods (Table 5). In the control soil, theANOVA indicated differences during the flood-ing period (Table 6), but the means were similarfor the three periods according to the Tukey test(Table 7).

As expected, moisture in the flooded soil washigher than in the control soil during the flood-ing period in both horizons.The moisture differ-ence was greater than 10% in both horizons andcycles and attained a maximum of nearly 30%.Conversely, during preflooding and postfooding,the difference is variable. In the two cycles, mois-ture in the BA horizon of the flooded soil islower than in the Ap horizon.

DISCUSSION

Behavior of Waterlogged MediaSoil microbes utilize several substances as

electron acceptors during respiration. The mostimportant in aerated conditions is oxygen. Dur-ing flooding, oxygen is quickly depleted throughthe respiration of aerobic microbes and the pop-ulation of facultative and strict anaerobic mi-croorganisms increases.As the period of floodinglengthens, several chemical reactions characteris-tic of anoxic media begin (Barlett and James,1993). Numerous organisms cause the total orpartial oxidation of organic matter of soil andsediments, utilizing iron and manganese as elec-tron acceptors (Starkey and Halverson, 1927;Vizier, 1990). Microorganisms reduce the less

crystalline mineral oxides initially since the verywell crystallized minerals such as hematite andgoethite reduce very slowly (Lowley, 1995).

Sequential Redox ReactionsThe reduction of oxygen and other electron

acceptors is usually a sequential process: O2 is thefirst component to be reduced, followed byNO3

�,Mn4�,Fe3�,SO42�, and CO2 (Turner and

Patrick, 1968;Ponnamperuma,1972).Bohn et al.,(1985) describe a thermodynamic sequence of re-duction of an electrochemical series in which acompound appears only after the previous one inthe series is totally reduced; for example, Mnwould become reduced first, followed by the re-duction of oxidized Fe. However, Barlett (1986)found that when there is a sharp decrease of Eh,both Fe and Mn can reduce simultaneously, andthe reduction rate and quantity of the reducedforms depend on the soil microbiological andchemical systems, which are difficult to deter-mine. Patrick and Jugsujinda (1992) experimen-tally demonstrated that during a slow and con-trolled reduction, NO3

� and Mn4� reactions aresequential without formation of Mn2�, whereasNO3

� was present at approximately 200 mV.In our field experiment the reactions are, in

part, sequential. Mn2� appears earlier than Fe2� inboth cycles. However, the correlation betweenboth elements (r Fe2�/Mn2� � 0.64; P � 0.05)indicates a degree of association between them. Inthe first cycle, both elements in the BA horizonincreased sharply 46 days after the beginning ofthe experiment. In the Ap horizon, the maximumvalues were reached later (after about 70 and 100days for Fe and Mn, respectively). In the secondcycle,Fe and Mn reached the maximum about 120days after the beginning of the experiment, coin-cident with the lowest Eh value. Nitrates are alsoabsent during the maximum segregation of Mn,150–250 days after the beginning of the experi-ment; nevertheless, small quantities of Mn2� weredetected before and after the period in which ni-trates are very scarce (r Fe2�/NO3

� � �0.55; rMn2�/NO3

� � �0.56; P � 0.05) (Table 8).In the first cultivation cycle, some variations

are observed in the occurrence of reduced com-pounds in the transition from the oxidized to thereduced state and vice versa. The rate of the reac-tions in the flooding period is slower than in thepostflooding period.Eh decreased from 571 to 96mV after approximately 2 months of submer-gence.Mn2� increased from 3.92 to 290 mg kg�1during that period. Fe2� increased markedly inthe BA horizon after 45 days of flooding, from

TABLE 7

Tukey test for the variables Fe2� moisture: Control soil

Fe2� (log)Preflooding �1.32576854 aFlooding �1.84975047 aPostflooding �2.03203024 a

MoisturePostflooding 28.31 aFlooding 24.01 aPreflooding 19.09 a

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8 IMBELLONE, GUICHON, AND GIMÉNEZ SOIL SCIENCE

�1 to 46 mg kg�1, when Eh dropped to 211mV. This rise probably occurred earlier, but in-termediate measurements are not available. Thevalues found are similar to those found elsewhere(e.g., Ponnamperuma, 1972). During postflood-ing, Eh rose sharply (80 to 510 mV) in about 35days, whereas the inverse process took approxi-mately 60 days. After irrigation ends, the soilevolves quickly toward aerobic conditions. Dur-ing the second cycle, the periods lasted 27 and 63days, respectively.

Throughout the experiment, Eh showedcloser association with moisture (r Eh/moisture� �0.55, P � 0.05) than with Fe2� and Mn2�

(Table 8).In the control soil, small quantities of nitrates

were detected in both horizons during the cycle,possibly ascribable to residual fertilizer from theprevious cultivation. Throughout the entire cy-cle, the soil was in an oxidized state, with slightmoisture fluctuations related to rainfall contribu-tion. Minimum quantities of Fe and Mn thatcould not be related to heavy rains were detectedin both horizons. A similar situation is found atrelatively low Eh values (350–400 mV).

Reduction DegreesNumerous authors have used redox potential

as a measure of soil reduction intensity. Criticalreduction potentials and subsequent dissolutionfor different systems were established: Patrick(1960) for NO3

� reduction; Gotoh and Patrick(1972) and Patrick and Henderson (1981) forMn4�; Gotoh and Patrick (1974) and Patrick andHenderson (1981) for Fe2�. Patrick and Jugsu-jinda (1992) found that in an oxidizing to reduc-ing system, the critical measured potential atwhich all of the NO3

� is reduced and Mn2� be-gins to appear is about 200 mV. The critical po-tential for Fe2� appearance is 100 mV. In allredox systems, the critical potentials for the tran-

sition from the oxidized to the reduced state wereabout 50 mV lower than for the inverse transi-tion. The redox potential in natural systems is ofmixed potential (Bohn, 1971), with oxidizers andreducers that do not belong to the same redoxcouple. However, Fe and Mn behavior can bestudied through redox potential and pH fluctua-tions.

Plants may not tolerate excess soluble Fe(Tanaka and Navasero, 1966; Ponnamperuma etal., 1955). In paddy soils,Vizier (1978) establishedreduction degrees as a function of Eh and Fe2�

values and their relationship with the criticalconcentration of Fe2� (300 mg kg�1), abovewhich crop yields are affected by the toxic effectof this element. The reduction degree is lowwhen Fe2� is below 0.5% and Eh is above 200mV; it is very high, with toxic effects, when Fe2�

is higher than 7.5% and Eh is lower than �100mV. The soils studied in both cultivation cyclesshowed a low reduction degree (0.09–0.35% ofFe2� and 80–124 mV Eh). Therefore, no Fe2�

toxicity symptoms were observed, and no toxic-ity risk is anticipated under the managementpractices of the plots. Another factor that influ-ences the amount of Fe2� is the mineralogy ofthe parent materials (Motomura and Yokoi,1969). The loessial sediments of the region havea predominantly illitic/montorillonitic composi-tion (Imbellone and Teruggi, 1993) and a rela-tively low amount of soluble Fe.

Mn is translocated readily to plant tops,where toxicity symptoms are easily seen as black-ish spots or stripes on veinal areas.No such symp-toms or yield decline were observed in the riceplants despite the high levels of Mn2� detected inthe flooded soil, which are above the probablelimit of toxicity (�100 mg kg�1 reducible Mn)reported by Coppenet and Juste (1979). Accord-ing to these authors, rice is capable of accumulat-ing high levels of Mn without damage. Ponnam-peruma (1985) reports that Mn toxicity has notbeen observed in wetland rice, though acid up-land soils may have this problem. It should also bepointed that the high levels of Mn occurred onlyduring a part of the growing cycles and affected,in almost all cases, only the upper horizon.

No high reduction degree was ever reached,possibly because a contribution of dissolved O2from the water was pumped into the rice field.Rice roots are able to release O2 into the rhizo-sphere due to anatomical adaptations - well de-veloped aerenchyma - (Kirk et al., 1990), buttheir influence on redox processes has not beenstudied.

TABLE 8

Simple correlations among variables

Fe2� Mn2� NO3�

(log) (log) (log) Eh Moisture

Fe2� (log)Mn2� (log) 0.64NO3

� (log) �0.55 �0.56Eh 0.06 0.07 �0.20Moisture 0.18 0.07 �0.12 �0.55pH �0.21 �0.08 0.25 0.16 0.14

p � 0.05 Critical value � 0.361 (n � 30)

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VOL. 166 ~ NO. 12 PROPERTIES OF SOILS WITH ANTHROPIC FLOODING 9

Presence of Redoximorphic Features in the SoilPermanent or discontinuous anoxic condi-

tions produce features that are characteristic ofthe hydromorphic process and are evidence ofthe reorganization of substances sensitive to re-dox processes. These features were reproducedexperimentally through successive cycles of satu-ration and drainage during 5 consecutive monthsby Vepraskas and Bouma (1976). No hydromor-phic features were observed after each cultivationcycle in the present study. The natural conditionsof the soils are not profoundly affected becausethe plots are waterlogged for 3 months every 3years, and then a predominantly oxic regime isreestablished. Thus, the alternating and successiveredox conditions are not sufficiently long to pro-duce hydromorphic features.

CONCLUSIONS

Eh, Fe2�, Mn2�, and NO3� were influenced

by flooding in the Ap horizon during the grow-ing period. This was revealed by statistical testsfor the mentioned variables between the differentperiods (preflooding, flooding, and postflooding)and the control soil.

Flooding produced deeper and more pro-longed effects in Fe2�, Mn2�, and NO3 than inEh. Changes in those ions were observed duringthe flooding and postflooding periods in bothhorizons, whereas Eh returned rapidly to highvalues when irrigation was interrupted and thevariations affected only the upper horizon.

The statistical analyses of the variables in thecontrol soil did not show significant differencesin the two cultivation cycles.Thus, no exogenousnatural processes are affecting those variables

The BA horizon in the flooded soil was lessaffected by flooding than the other horizons. Sig-nificant differences were found only with Fe2�

and Mn2� between the three periods.After irrigation is discontinued, the soil re-

turned to conditions in equilibrium with the cli-mate of the area under an oxidizing state similarto the control.

Anaerobic conditions did not continue longenough to modify the morphology of the floodedsoil as no macroscopic redoximorphic featureswere observed. A slight surface crusting was ob-served occasionally but disappeared after the es-tablishment of adventitious vegetation.

ACKNOWLEDGMENTS

The authors are grateful to the staff of theIng.Agr. Julio A. Hirschhorn Experiment Station

and Rice Project for kindly permitting the use ofthe experiment plots and for technical discus-sions. We thank Dr. Julio C. Merodio and MabelVázquez of Ing. Agr. for their valuable assistancein statistical analysis. We also thank two anony-mous reviewers for their helpful suggestions.

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