ORIGINAL ARTICLE

Soil iron fractionation and availability at selected landscapepositions in a loessial gully region of northwestern China

Xiaorong WEI1,2, Mingan SHAO1,2, Jie ZHUANG2,3 and Robert HORTON4

1State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest Sci-Tech, University of Agriculture &

Forestry, Yangling, 712100, China, 2Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water

Resources, Yangling, 712100, 3Institute for a Secure and Sustainable Environment, The University of Tennessee, Knoxville, Tennessee,

37996, USA and 4Department of Agronomy, Iowa State University, Ames, Iowa, 50011, USA

Abstract

Soil Fe fractions and availability vary with landscape positions, because landscape position affects soil chemical

properties and water conditions. In the present study, we investigated Fe fractions and availability at selected

landscape positions in the loessial gully region of northwestern China. Four landscape positions, plateau, slope,

terrace, and gully bottom were investigated. For each landscape position, soil samples were collected at 20-cm

increments to a depth of 80 cm. Iron in the soil samples was fractionated by a modified sequential extraction

method. Available Fe was assessed by diethylene thiamine pentacetic acid (DTPA) extraction procedure. The

results showed that soil profile distributions of DTPA-Fe varied greatly with landscape position in the study

area. The largest content of DTPA-Fe content was observed in the plateau soils, while the smallest content was

observed in the gully bottom soils. Iron in soils existed mainly in the mineral bound fraction, which accounted

for about 73 to 96% of the total Fe. The content of Fe in soil fractions varied greatly with landscape position.

Exchangeable Fe and organic matter bound Fe were direct sources of available Fe, but exchangeable Fe contrib-

uted little to the total available Fe due to its low content in the soils. Oxides bound Fe was an indirect source

of available Fe. The results of the present study indicate that landscape position strongly influences soil profile

distribution and capacity of available Fe by influencing soil Fe fractions and organic matter distributions.

Key words: availability, fraction, iron, landscape position, loessial gully watershed, soils.

INTRODUCTION

Iron (Fe) is one of the major constituents of the litho-

sphere and pedosphere. The content of Fe in natural soils

ranges between 7 and 42 g kg)1 (Kabata-Pendias 2000;

Xing and Zhu 2003). However, plant available Fe is low

in most soils. Iron in soils is mainly associated with metal

oxides and hydroxides or contained in primary and sec-

ondary minerals. The percentage of soil Fe unavailable to

plants can be as high as 92% (Kabata-Pendias 2000; Xing

and Zhu 2003). The availability of soil Fe for plant

growth varies greatly with soil conditions and depends

particularly on soil redox and pH conditions. Certain soil

conditions, such as high soil pH, free calcium carbonate,

and low organic matter, often cause Fe deficiency in

plants. Currently, approximately one-third of the soils in

the world suffer Fe deficiency, which limits plant growth

(Abadia 1995; Connolly and Guerinot 2002).

On the Loess Plateau of China, the total content of Fe

in soils is 28 g kg)1 on average, and DTPA extracted Fe

varies between 0.01 and 5.6 mg kg)1 with an average

content of 4.2 mg kg)1 (Yu et al. 1991). The content of

DTPA-Fe in 40% of the soils is lower than 4.5 mg kg)1,

which is the critical level of Fe deficiency for winter wheat

and millet (Yu et al. 1991). On the Loess Plateau, Fe defi-

ciency for plants (e.g. apple, pear, grapevine, maize and

soybean) is frequently observed (Zhang et al. 2002).

Soil iron deficiency is mainly related to soil physico-

chemical properties and Fe fractionation that determine

the availability of Fe to plants (Abadia 1995; Barton and

Correspondence: X. WEI, State Key Laboratory of Soil Erosionand Dryland Farming on the Loess Plateau, Northwest Sci-TechUniversity of Agriculture & Forestry, Yangling, Shaanxi Prov-ince, 712100, China. Email: xrwei78@163.com

Received 20 January 2010.Accepted for publication 26 April 2010.

� 2010 Japanese Society of Soil Science and Plant Nutrition

Soil Science and Plant Nutrition (2010) 56, 617–626 doi: 10.1111/j.1747-0765.2010.00497.x

Abadia 2006; Zhou et al. 2004). As a valence-variable

element, Fe exists in well drained soils in different forms

of Fe oxides. Soil redox conditions can cause changes in

Fe valence, which impacts Fe availability and fractional

distribution (Li et al. 2007; Plekhanova 2007). Landscape

position impacts soil water movement and transport of

solutes and soil particles in the soil profile. As a result,

landscape position impacts soil physicochemical proper-

ties and water conditions, which in turn affect Fe fractions

in soils and Fe availability to plants (Li et al. 2007;

Plekhanova 2007; Zhang and Zheng 2002).

The objective of the present study was to investigate soil

Fe fractionation and Fe availability at selected landscape

positions (e.g. plateau, terrace, slope and gully bottom)

in a loessial gully watershed of northwestern China.

A sequential extraction procedure modified from that

described in Tessier et al. (1979) was used to fractionate

soil Fe into exchangeable, carbonate bound, organic mat-

ter bound, oxide bound, and mineral bound fractions. We

expect to discover the profile distributions of Fe in soils at

the selected landscape positions, which will be of theoreti-

cal and practical importance for improving nutritional

management of soil Fe in the degrading Loess Plateau

environment.

MATERIAL AND METHODS

Study area

The experiment was conducted in Wangdougou

watershed in Changwu County, Shaanxi Province, China

(35�12¢–35�16¢N, 107�40¢–107�42¢E). The watershed is

a part of the Agro-ecological Experiment Station of the

Chinese Ecology Research Network (CERN), and is

located in a typical gully region of the Loess Plateau with

an altitude of 800–1200 m and an area of 8.5 km2. The

study site has a warm-temperate zone subhumid conti-

nental climate. During the period 1984 to 2005, the

average annual temperature was 9.1�C with a frost-free

period of 171 days. The accumulation of temperatures

higher than 0 and 10�C was 3866 and 3029�C, respec-

tively. The average annual precipitation was 584 mm.

Rainfall mainly occurs between June and September with

large variation of intensity within a year and between

years. The soil is a silty loam Yellow Mian soil, a Camb-

isol according to the International Soil Taxonomy

(FAO ⁄ ISRIC ⁄ ISSS 1998), which is developed in eroded

loess and is influenced by humus accumulation, eluvia-

tion and illuviation.

Soil sampling and chemical analysis

Soil samples were collected from the ground surface to a

depth of 80 cm in 20-cm increments using a 5-cm diame-

ter tube auger at 12 locations in the plateau, 12 locations

in the terrace, seven locations in the slope, and seven loca-

tions in the gully bottom in July of 2005. The distribution

of sampling sites is described in Figure 1. The plateau and

terrace were mainly farmland of spring corn or winter

wheat, while the slope and gully bottom were grass and

forest lands, respectively. In each sampling location, three

soil profiles were sampled to make a composite soil sam-

ple. After removal of large pieces of un-decomposed

organic matter (e.g. plant roots), the soil samples were air-

dried in a laboratory and ground to pass through 1.00

and 0.25 mm nylon screens for analysis.

Soil pH, organic matter and cation exchange capacity

(CEC) were measured using the methods described in

Page et al. (1982). Soil pH was determined using an elec-

trode pH-meter in a soil : water ratio of 1:2. Cation

exchange capacity was determined by replacement of

exchangeable cations by ammonium acetate (pH 7).

Organic matter was determined using the Walkley–Black

method.

Since available Fe is determined by a complex combina-

tion of soil, plant, and environmental variables, we

assessed the availability of Fe in the present study by the

DTPA (diethylene thiamine pentacetic acid) procedure,

which was designed for calcareous soils (Lindsay and

Norvell 1978) and has been widely used for several dec-

ades. Twenty mL of 0.005 mol L)1 DTPA + 0.1 mol L)1

TEA (trietanolamine) + 0.01 mol L)1 CaCl2 (pH 7.3)

were added to 10 g air-dry soil (<1.00 mm). Then, the

Plateau Terrace Slope Gully bottom

Figure 1 The distribution of sampling sites across the watershed.

� 2010 Japanese Society of Soil Science and Plant Nutrition

618 X. Wei et al.

suspension was shaken for 2 h at 25�C, filtered through a

filter paper and stored in a polyethylene bottle at 4�C for

analysis. Total Fe content was measured by a tri-acid

digestion method (Shuman 1985). Half a gram of

<0.25 mm soil was placed into a teflon beaker on a hot

plate, and digested with a mixture of HNO3–HClO4–HF.

After completion, the solution and suspended solids was

transferred into a 100-mL flask and stored in a polyethyl-

ene bottle at 4�C for analysis.

The sequential extraction scheme used in the study was

a modification of that of Tessier et al. (1979). During the

fractionation, 5.0 g of sieved soil were used in 80-mL

polypropylene centrifuge tubes. Each of the chemical frac-

tions was extracted as follows:

1 Exchangeable Fe (Ex-Fe): Soil samples were shaken at

25�C for 2 h with 25 mL of 1 mol L)1 NH4NO3 at

pH 7.0 and centrifuged at 1776 g for 10 min. The

supernatant was filtered with Whatman No. 5 filter

paper.

2 Carbonate bound Fe (Carb-Fe): The residue from (1)

was shaken at 25�C for 5 h with 25 mL of 1 mol L)1

NaOAc adjusted to pH 5.0 with HOAc. The sus-

pension was then centrifuged and filtered as done in

step (1).

3 Iron ⁄ manganese oxides bound Fe (Ox-Fe): Fifty mL

of 0.04 M NH2OHHCl in 25% (v ⁄ v) HOAc were

added to the residue from (2). The mixture was

placed into a water bath at 95 ± 3�C with occasional

agitation for 5 h, and the suspension was then centri-

fuged and filtered as in step (1).

4 Organic matter bound Fe (Om-Fe): Three mL of

0.02 mol L)1 HNO3 and 5 mL of 30% H2O2 adjus-

ted to pH 2.0 with HNO3 were added to the residue

from (3) and the mixture was placed in a water bath

at 85 ± 2�C for 2 h with intermittent agitation. An

additional 5 mL of 30% H2O2 adjusted to pH 2.0

with HNO3 were then added and the mixture was

kept in the water bath at 85 ± 2�C for another 3 h

with intermittent agitation. After cooling, 50 mL of

1 mol L)1 NH4NO3 at pH 7.0 was added, and the

suspension was shaken for 2 h before centrifugation

and filtering.

5 Minerals bound Fe (Min-Fe): The residue from Step

(4) was heated to dryness at 180�C, then 0.5000 g

residue (<0.25 mm) was placed in a Teflon beaker on

a hot plate, and digested with a mixture of HNO3–

HClO4–HF. The digest was transferred into a

100-mL flask.

Between each successive extraction, the supernatant

solution was removed and stored in a polyethylene bottle

at 4�C. The residue was washed once with 10 mL of de-

ionized water before proceeding to the next step in the

extraction procedure. All glassware was soaked in 14%

HNO3 (v ⁄ v) and rinsed with deionized water prior to use.

Reagents used in the extraction were analytical grade. All

soil extracts and digests were analyzed for Fe using atomic

absorption spectrometry (SpectrAA-220 Zeeman; Varian

Inc., Palo Alto, CA, USA).

Statistical analysis

Two-way analysis of variance (ANOVA) was conducted to

test landscape position and soil depth main effects and

interaction effects on soil properties, DTPA-Fe and Fe frac-

tions. Correlation analysis, path analysis, and factor analy-

sis were performed to identify the relationships between

DTPA-Fe and soil properties and among Fe fractions. Path

analysis has advantages in partitioning correlations into

direct and indirect effects with an attempt to differen-

tiate correlation and causation compared with a multiple

regression analysis. This technique also features multiple

linear regressions and generates standardized partial

regression coefficients (path coefficients) (Knoke and

Bohrnsted 1994). Path analysis was used to partition the

correlation coefficients between DTPA-Fe and soil proper-

ties and between DTPA-Fe and Fe fractions into direct and

indirect coefficients. The factor analysis technique, which

allows a considerable reduction in the number of variables

and the detection of structure in the relationships between

variables has been successfully used for evaluation of heavy

metals and their relations with metal fractions by Maiz

et al. (2000). In the present study, factor analysis was used

to group Fe fractions and DTPA-Fe into a few factors.

These factors, although statistically constructed, can be

assessed with respect to the availability of each Fe fraction.

Factor analysis was performed by evaluating principal

components and computing the eingenvectors. After-

wards, the rotation of the principal components was car-

ried out by the varimax normalized method. The results

were presented as factor loadings of the rotated matrix.

The factor plot was therefore portrayed according to the

factor loadings of factor 1 and 2. All statistical analyses

were performed using SAS software (SAS Institute Inc

2000).

RESULTS

Profile distribution of soil properties andDTPA-Fe

Soil organic matter, pH, and CEC varied significantly

with soil depth at the selected landscape positions

(Table 1, Fig. 2). As expected, organic matter content

decreased with soil depth, whereas soil pH and CEC only

slightly changed. The decrease in organic matter content

with soil depth was more pronounced at the gully bottom

than at the other three landscape positions. Plateau soil

was higher in organic matter and CEC but lower in pH

compared with soils at the other landscape positions.

� 2010 Japanese Society of Soil Science and Plant Nutrition

Soil iron fractionation and availability 619

Conversely, compared with other positions, the soils in

the gully bottom had higher CEC and lower pH and

organic matter.

The distribution of DTPA-Fe in the soil profiles varied

significantly with landscape positions (Table 1, Fig. 2).

The largest DTPA-Fe was observed in the plateau soils,

and the smallest in the gully bottom soils. The mean val-

ues of DTPA-Fe in plateau soils were about two to three

times larger than in the gully bottom soils. In addition, the

DTPA-Fe varied significantly with depth in the plateau

and the gully bottom soils but only slightly in the slope

and the terrace soils. In the plateau and the gully bottom,

the DTPA-Fe was significantly larger in the 0–20 cm layer

than in the deeper soil.

Profile distribution of Fe fractions

Concentrations of Fe fractions varied greatly with soil

depth in the study area (Table 2). Average concentra-

tions of Ex-Fe, Carb-Fe, and Om-Fe decreased with soil

depth. The concentrations of Ex-Fe and Carb-Fe were

<0.5 and 5 mg kg)1, respectively. These two fractions

account for <0.05% of the total amount of Fe. The con-

centration of Om-Fe ranged from 14 to 102 mg kg)1

with a proportion of <0.5% of the total Fe. In addition,

the contents of Ox-Fe varied from 0.89 to 7.92 g kg)1,

corresponding to 3.5–27.3% of the total Fe. The Ox-Fe

concentration was larger in the 20–60 cm soil layer than

in the 0–20 and 60–80 cm soil layers. The fraction of

Table 1 The analysis of variance (ANOVA) results of soil properties, diethylene thiamine pentacetic acid (DTPA)-Fe and Fe fractions atdifferent landscape positions and soil depths

pH OM CEC DTPA-Fe Ex-Fe Carb-Fe Ox-Fe Om-Fe Min-Fe

F

LP 20.9 9.29 41.9 34.3 10.0 18.0 14.4 27.2 13.5

SD 3.78 4.24 6.66 0.49 2.11 0.92 9.28 10.4 5.19

LP · SD 20.0 1.36 14.8 2.46 2.32 10.1 9.66 3.62 2.81

P

LP <0.0001 0.0003 <0.0001 <0.0001 0.0002 <0.0001 <0.0001 <0.0001 <0.0001

SD 0.0366 0.0259 0.0048 0.6208 0.1428 0.4108 0.0010 0.0005 0.0130

LP · SD <0.0001 0.2706 <0.0001 0.0525 0.0640 <0.0001 <0.0001 0.0101 0.0314

Carb-Fe, Carbonate bound Fe; CEC, Cation exchange capacity; Ex-Fe, Exchangeable Fe; LP, landscape position; Min-Fe, Minerals bound Fe; OM, organicmatter; Om-Fe, Organic matter bound Fe; Ox-Fe, Oxides bound Fe; SD, soil depth.

0

20

40

60

80

pH

Soil

dept

h (c

m)

0

20

40

60

80

Catrion exchange capacity (cmol kg–1)

Soil

dept

h (c

m)

0

20

40

60

80

Organic matter (mg kg–1)

Soil

dept

h (c

m)

PlateauSlopeGully bottomTerrace

0

20

40

60

80

8.1 8.3 8.5 10 15 20 25 30

0 5 10 15 20 0 2 4 6DTPA-Fe (mg kg–1)

Soil

dept

h (c

m)

Figure 2 Profile distribution of soil pH, organic matter, and cation exchange capacity at selected landscape positions.

� 2010 Japanese Society of Soil Science and Plant Nutrition

620 X. Wei et al.

Min-Fe ranged between 72.5 and 96.4% of the total Fe

in the studied area.

The profile distribution of Fe fractions was significantly

influenced by landscape position, soil depth and their inter-

active effect (Table 1, Fig. 3). Among the soil profiles, the

lowest Ex-Fe was observed in plateau soils. In the 0–60 cm

soil layer, Ex-Fe was also lower in gully bottom soils than

that in slope and terrace soils. Besides, Ex-Fe changed

slowly with soil depth in gully bottom and plateau soils,

but varied markedly in slope and terrace soils. Carbonate

bound Fe decreased rapidly with soil depth in terrace soils

but slowly in soils at the other three landscape positions. In

general, the Carb-Fe concentration followed the order of

terrace > plateau > slope > gully bottom in the 0–60 cm

layer, but had a slightly different order of plateau >

slope > terrace > gully bottom in 60–80 cm layer. The

lowest Ox-Fe was found in gully bottom soils where it var-

ied markedly with soil depth. In gully bottom soils, Ox-Fe

in the 0–60 cm layer was nearly twice that in the depth of

60–80 cm layer. Among the four landscape positions, the

plateau soils had the largest Ox-Fe concentration, which

increased gradually with soil depth. The Ox-Fe concentra-

tion was somewhat larger in slope than in terrace, and

changed slightly with soil depth in both soils. Similarly, the

lowest concentration of Om-Fe occurred in gully bottom

in the 0–80 cm soil layer, while the largest concentration

occurred at different soil depths, which varied with land-

scape positions. The Min-Fe concentration changed signifi-

cantly with soil depths in gully bottom soils, whereas it

changed slowly with soil depth in plateau, slope and ter-

race soils. Relatively, gully bottom soil contained more

Min-Fe except for the 20–40 cm soil layer than the soils at

other landscape positions, and plateau soils had the least

Min-Fe.

DISCUSSION

Relationships between DTPA-Fe and soilproperties

The content of DTPA-Fe in soils is controlled by soil phys-

icochemical properties (e.g. pH, organic matter and CEC)

and is correlated to Fe fractions. The result of path analy-

sis indicated that organic matter posed the largest positive

direct influence on DTPA-Fe, while pH exerts a negative

influence on the concentration of DTPA-Fe (Table 3).

These results were in agreement with results reported in

the literature (Li et al. 2007; Moreno-Caselles et al. 2005;

Sharma et al. 2000, 2004; Wei et al. 2006). The relations

shown in Table 3 are also supported by the profile distri-

bution of DTPA-Fe, pH, and organic matter at different

landscape positions. In our study, the plateau soil with the

largest organic matter and smallest pH had the highest

DTPA-Fe. The gully bottom soil with small organic mat-

ter had the smallest DTPA-Fe concentration among all of

the selected landscape positions, though the pH of gully

bottom soil was also small. It is thus obvious that the

influence of soil organic matter on DTPA-Fe concentra-

tion was greater than that of pH in the study area due

likely to the narrow range of soil pH. This result is consis-

tent with that observed in some Alfisols of Punjab by

Sharma et al. (2005).

Profile distribution of Fe fractions

The fractional distribution of Fe in the studied soils was

similar to that in other Chinese soils. Shao et al. (1995)

found that the ranges of Om-Fe, Ox-Fe, and Min-Fe were

7.4–29.6, 2.7–4.8, and 22.2–33.1 g kg)1, respectively in

agricultural soils in Gansu Province, China. Similarly, Lu

Table 2 Fractional distribution of Fe in soils in Wangdonggou watershed

Soil layer Ex-Fe (mg kg)1) Carb-Fe (mg kg)1) Ox-Fe (g kg)1) Om-Fe (mg kg)1) Min-Fe (g kg)1)

0–20 cm

Mean (SE) 0.28 (0.14) 2.76 (0.77) 4.51 (1.25) 54.1 (17.6) 22.5 (2.41)

Range 0.08–0.65 1.78–4.43 1.26–6.22 14.4–84.5 19.1–26.8

CV 0.51 0.28 0.28 0.33 0.11

20–40 cm

Mean (SE) 0.27 (0.11) 2.57 (0.66) 4.67 (1.49) 50.6 (17.7) 21.6 (3.36)

Range 0.00–0.49 1.62–3.67 0.89–7.05 17.1–84.5 11.4–25.8

CV 0.41 0.26 0.32 0.35 0.16

40–60 cm

Mean (SE) 0.25 (0.07) 2.45 (0.70) 4.69 (1.21) 50.7 (19.2) 22.6 (2.85)

Range 0.08–0.33 1.38–3.67 1.89–7.03 17.1–102 18.6–29.1

CV 0.30 0.28 0.26 0.38 0.13

60–80 cm

Mean (SE) 0.23 (0.10) 2.35 (0.67) 4.62 (1.81) 49.5 (20.6) 22.2 (1.75)

Range 0.08–0.49 1.51–3.89 0.90–7.92 18.5–89.3 19.6–25.8

CV 0.45 0.29 0.39 0.42 0.08

Carb-Fe, Carbonate bound Fe; Ex-Fe, Exchangeable Fe; Min-Fe, Minerals bound Fe; Om-Fe, Organic matter bound Fe; Ox-Fe, Oxides bound Fe.

� 2010 Japanese Society of Soil Science and Plant Nutrition

Soil iron fractionation and availability 621

et al. (1993) reported that the amounts of Om-Fe, Ox-Fe,

and Min-Fe in four calcareous soils were 3.3–41.4, 0.3–

1.8 mg kg)1, and 2.6–31.2 g kg)1, respectively in Sichuan

Province, China. Xing and Zhu (2003) studied Fe frac-

tional distribution in 25 Chinese soils, including Camb-

isol, and found that 23 soils had Om-Fe concentrations

<1.0% of the total Fe, whereas Min-Fe in nine soils

accounted for more than 70% of the total soil Fe. In their

investigation, the proportions of Om-Fe, Ox-Fe, and

Min-Fe in Cambisol were 0.1, 34.1, and 85.7%, respec-

tively, which were similar to our findings in this study.

On the Loess Plateau, soil water regimes and soil

physicochemical properties vary greatly with landscape

positions (Wei and Shao 2007), resulting in large variation

of soil Fe fractions (Fig. 3). Landscape position effects on

the soil profile distributions of Carb-Fe can be attributed

Plateau Slope Gully bottom Terrace

1 2 3 4 5

0–20 cm

20–40 cm

40–60 cm

60–80 cm

0.1 0.2 0.3 0.4 0.5

0–20 cm

20–40 cm

40–60 cm

60–80 cm

0 14 28 42 56 70

0–20 cm

20–40 cm

40–60 cm

60–80 cm

0 1 2 3 4 5 6

0–20 cm

20–40 cm

40–60 cm

60–80 cm

10 15 20 25 30

0–20 cm

20–40 cm

40–60 cm

60–80 cm

Exechangeable iron (mg kg–1 Carbonate bound iron (mg kg–1)

Oxides bound iron (g kg–1 gkgm(noridnuobrettamcinagrO) –1)

Mineral bound iron (g kg–1)

)

Figure 3 Profile distribution of iron fractions at selected landscape positions.

Table 3 Path coefficient and correlation coefficients of soilproperties to diethylene thiamine pentacetic acid (DTPA)-Fe

Direct path

coefficients

Indirect path coefficients

rpH CEC OM

pH )0.123 )0.075 0.031 )0.166

CEC 0.193 0.048 )0.003 0.237

OM 0.369 )0.010 )0.002 0.357

CEC, Cation exchange capacity; OM, organic matter; r, correlation coef-ficient; r0.01 = 0.215, n = 141.

� 2010 Japanese Society of Soil Science and Plant Nutrition

622 X. Wei et al.

to carbonate distributions in the profiles because soil car-

bonate concentration is usually affected by the amount of

leaching water and soil pH, which change with location

and depth of soils in the landscape. In gully bottom, run-

off water accumulates leading to larger soil water content

and lower soil pH compared with other landscape posi-

tions (Fig. 2). The high water content favors dissolution

of soil carbonate by changing the carbonic acid equilib-

rium (Mulder and Cresser 1994; Rogovska et al. 2007;

Zuo et al. 2007), and eventually results in low concentra-

tions of carbonate and Carb-Fe (Fig. 3).

The solubility of Fe in oxides is governed by soil reduc-

tion conditions, which are partly controlled by soil water

regime (Fiedler and Sommer 2004; Tong et al. 1987;

Zhang et al. 2003). The persistent wet soil condition may

facilitate the formation of reducing conditions (Bohrerova

et al. 2004; Clay et al. 1992; Han et al. 2001; Porter et al.

2004). As a result, dissolution and release of Fe from soil

metal oxides increases as observed in gully bottom soil,

which had lower Ox-Fe concentrations than other soils.

After release, a part of the Fe may move deeper into the

soil, while the remaining Fe can be transformed into other

Fe fractions depending on the capacity of binding sites

available in the different soil components. In contrast,

soils in plateau, terrace and slope are oxidized and such a

condition favors the formation and retention of Fe oxides.

Iron in soils can be strongly complexed by organic mat-

ter because Fe-organic matter complexes have a very high

chemical stability constant (Mackowiak et al. 2001).

Many researchers have reported that the content of

Om-Fe is closely related with organic matter content

(Agbenin 2003; Darke and Walbridge 2000; Young et al.

2006; Zhou et al. 2003). This relationship was also

detected in our study (r = 0.654, P < 0.0001, n = 141),

which explained the observations that Om-Fe and organic

matter shared similar profile distributions at each land-

scape position.

Relationships between Fe fractions and DTPA-Feand its indications to Fe availability

In order to further reveal the relationship between Fe frac-

tion and its availability, we analyzed the contribution of

each Fe fraction to Fe availability through correlation

analysis and path analysis (Table 4). The results showed

that Ex-Fe was positively related to DTPA-Fe concentra-

tion at P < 0.01. In view of the fact that Ex-Fe is the most

available Fe in soils for plant absorption (Agbenin 2003;

Xue et al. 2006; Young et al. 2006; Zhou et al. 2003), we

can safely conclude that Ex-Fe is the most direct source of

available Fe in soils, which is in agreement with other

findings (Agbenin 2003; Xue et al. 2006; Young et al.

2006; Zhou et al. 2003). However, in our study, the con-

tent of Ex-Fe in soils was <0.7 mg kg)1, while the content

of DTPA-Fe ranged from 1.3 to 5.2 mg kg)1 with an

average value of 3.3 mg kg)1, nearly six times the Ex-Fe

content. The great difference between Ex-Fe and DTPA-

Fe suggests that Ex-Fe only contributes about one sixth of

the available Fe and that other Fe fractions must also con-

tribute. This assumption is supported by the relatively

small direct path coefficient of Ex-Fe to DTPA-Fe (0.227).

Carbonate bound Fe (Carb-Fe) represents the associated

or the co-precipated Fe fraction that has been occluded

into carbonates (Kabata-Pendias 2000; Xing and Zhu

2003). The presence of carbonate may decrease the Ex-Fe

fraction because Ex-Fe is apt to be precipitated or

occluded by carbonates in calcareous soils (Rogovska

et al. 2007; Wei et al. 2005;Zuo et al. 2007), resulting in

reduction of Fe availability in soil. In the present study,

Carb-Fe was negatively related to DTPA-Fe (Table 4). Its

very small direct path coefficient indicated that Carb-Fe

had negligible contribution to soil Fe availability.

Organic matter bound-Fe (Om-Fe) was positively

related with DTPA-Fe at a highly significant level

(P < 0.01). This relationship is in line with the well-

accepted conclusion that available Fe in soils is greatly

determined by Om-Fe (Agbenin 2003; Wei et al. 2005;

Xing and Zhu 2003; Young et al. 2006; Zhou et al.

2003). In nature, the contribution of Om-Fe to soil Fe

availability is subject to the interactions between organic

matter and oxides. Because most Om-Fe can be absorbed

by plants (Mackowiak et al. 2001), Om-Fe can be

assumed to be a major direct source of available Fe.

In our study, Ox-Fe was associated with the largest

direct path coefficient and the largest positive correlation

Table 4 Path coefficient and correlation coefficients of Fe fractions to diethylene thiamine pentacetic acid (DTPA)-Fe

Fraction

Direct path

coefficients

Indirect path coefficients

rEx-Fe Carb-Fe Ox-Fe Om-Fe Min-Fe

Exe-Fe 0.227 )0.021 0.120 )0.037 0.044 0.333

Carb-Fe 0.049 )0.097 )0.008 0.026 )0.007 )0.037

Ox-Fe 0.812 0.034 )0.001 )0.143 0.050 0.753

Om-Fe )0.213 0.040 )0.006 0.544 )0.003 0.361

Min-Fe )0.227 )0.044 0.002 )0.180 )0.003 )0.453

Carb-Fe, Carbonate bound Fe; Ex-Fe, Exchangeable Fe; Min-Fe, Minerals bound Fe; Om-Fe, Organic matter bound Fe; Ox-Fe, Oxides bound Fe;r, correlation coefficient; r0.01 = 0.215, n = 141.

� 2010 Japanese Society of Soil Science and Plant Nutrition

Soil iron fractionation and availability 623

coefficient to DTPA-Fe (Table 4), suggesting that Ox-Fe

was another source of available Fe in soils. The reason for

this is probably that oxides in soils can be reduced, and Fe

bound into this component will be released into soil solu-

tions under proper soil conditions (such as low soil pH,

high organic matter and CEC), providing available Fe into

soils and increasing DTPA-Fe content (Xing and Zhu

2003; Xue et al. 2006). However, it is worth noting that

Ox-Fe is an indirect rather than a direct source of avail-

able Fe in soils because the release of Ox-Fe is slow in well

drained soils (Xue et al. 2006).

Iron bound into minerals (Min-Fe) cannot be absorbed

by plants and is therefore not readily available for plant

growth. The release and transformation of Min-Fe into

other Fe fractions is very slow and can hardly occur in

natural soil environments, meaning it contributes least to

Fe availability, as indicated by its small direct and indirect

path coefficients (Table 4).

Factor analysis was conducted to clarify the relation-

ships between Fe fractions and DTPA-Fe. The results in

Figure 4 show that Ex-Fe was solely classified into a

group because it is the direct source of available Fe in

soils. However, the low contents and small contributions

of Ex-Fe to available Fe makes Ex-Fe have a relatively

long distance from DTPA-Fe in Figure 4, indicating its

low contribution to Fe availability. As discussed above,

Om-Fe is the major direct source of available Fe and Ox-

Fe is an indirect source of available Fe. The influence of

Om-Fe on DTPA-Fe is also influenced by Ox-Fe. These

two Fe fractions are thus classified into the same group

with DTPA-Fe. Since Min-Fe and Carb-Fe are not signifi-

cantly correlated to DTPA-Fe, they plot further away

from DTPA-Fe in Figure 4.

The profile distribution of DTPA-Fe in soils of various

landscape positions were related with the profile distribu-

tions of Fe fractions. Since Ox-Fe and Om-Fe are the major

sources of available Fe in soils, a relatively high content of

these two fractions results in a relatively high content of

DTPA-Fe in plateau soils, whereas the lower contents

of the two fractions lead to a lower DTPA-Fe in gully

bottom soils. Although Ex-Fe is the direct source of avail-

able Fe in soils, the low content causes a weak relationship

between Ex-Fe and DTPA-Fe. The higher content of Min-

Fe often indicates high amounts of primary and secondary

minerals in soils. As a result, the strong association of Fe

onto minerals decreases concentrations of DTPA-Fe. In

the present study, Min-Fe is highest in gully bottom soils

and lowest in plateau soils, corresponding to the lowest

DTPA-Fe in gully bottom soils and highest in plateau soils.

Conclusions

The soil profile distributions of DTPA-Fe varied greatly at

selected landscape positions in the loessial gully region.

The highest and lowest DTPA-Fe contents were observed

in the plateau and gully bottom soils, respectively. The

mineral bound Fe accounted for about 73 to 96% of the

total Fe in this study area. The proportions of Ex-Fe,

Carb-Fe, Ox-Fe, and Om-Fe were <28%. Among the five

Fe fractions, Ex-Fe and Min-Fe were higher in slope, ter-

race, and gully bottom soils than in plateau soils, and

Carb-Fe, Ox-Fe, and Om-Fe contents were higher in pla-

teau, slope, and terrace soils than in gully bottom soils.

Exchangeable Fe is the direct source of available Fe, but

has limited contribution to Fe availability due to its low

content. Oxide bound Fe is the largest indirect source of

available Fe, thereby contributing most to Fe availability

in soils. Organic matter bound Fe is the major direct

source of available Fe. The different profile distributions

of available Fe at the selected landscape positions are

affected by Fe fractional distribution and organic matter.

ACKNOWLEDGMENT

This study was supported by the National Key Basic

Research Special Foundation Project (2007CB106803),

National Natural Science Foundation of China

(40801111), Program for Youthful Talents in Northwest

A & F University, Chinese Academy of Sciences Visiting

Professorship for Senior International Scientists (2009Z2-

37), West Light Foundation of the Chinese Academy of

Sciences, and Japan Society for the Promotion of Science-

Chinese Academy of Sciences (JSPS-CAS) Core-University

Program ‘‘Researches on Combating Desertification and

Developmental Utilization in Inland China’’. The authors

thank Dr Isaac Shainberg of the Institute of Soil, Water

and Environmental Sciences, the Agricultural Research

Ex-Fe

–1.0

–0.5

0.0

0.5

1.0

–1.0 –0.5 0.0 0.5 1.0Factor 1

Fact

or 2

Om-Fe

Ox-Fe

DTPA-Fe

Carb-Fe

Min-Fe

Figure 4 Factor plot of diethylene thiamine pentacetic acid (DTPA)extractable iron and iron fractions.

� 2010 Japanese Society of Soil Science and Plant Nutrition

624 X. Wei et al.

Organization, Israel for help in improving the quality of

the manuscript.

REFERENCES

Abadia J 1995: Iron Nutrition in Soils and Plants. Kluwer Aca-

demic Publishers, Dordrecht.

Agbenin JO 2003: The distribution and transformation of iron

and manganese in soil fractions in a savanna Alfisol under

continuous cultivation. Nutr. Cycl. Agroecosys., 66, 259–

270.

Barton LL, Abadia J 2006: Iron Nutrition in Plants and Rhizo-

spheric Microorganisms. Kluwer Academic Publishers,

Dordrecht.

Bohrerova Z, Stralkova R, Podesvova J, Bohrer G, Pokorny E

2004: The relationship between redox potential and nitri-

fication under different sequences of crop rotations. Soil Till.

Res., 77, 25–33.

Clay DE, Clapp CE, Linden DR, Molina JAE 1992: Tillage influ-

ence on redox potential following rainfall. Soil Till. Res.,

22, 211–219.

Connolly EL, Guerinot ML 2002: Iron stress in plants. Genome

Biol., 3, 1024.1–1024.4.

Darke AK, Walbridge MR 2000: Al and Fe biogeochemistry in a

floodplain forest: implications for P retention. Biogeochem-

istry, 51, 1–32.

FAO ⁄ ISRIC ⁄ ISSS 1998: World Reference Base for Soil

Resources. World soil resources reports, Rome.

Fiedler S, Sommer M 2004: Water and redox conditions in

wetland soils-their influence on pedogenic oxides and

morphology. Soil Sci. Soc. Am. J., 68, 326–335.

Han FX, Banin A, Triplett GB 2001: Redistribution of heavy

metals in arid-zone soils under a wetting-drying cycle soil

moisture regime. Soil Sci., 166, 18–28.

Kabata-Pendias A 2000: Trace Elements in Soils and Plants,

3rd edn. CRC Press, Boca Raton.

Knoke D, Bohrnsted GW 1994: Statistics for Social Data Analy-

sis, 3rd edn. F. E. Peacock Publishers, Itaska.

Li H, Futch SH, Stuart RJ, Syvertsen JP, McCoy CW 2007: Asso-

ciations of soil iron with citrus tree decline and variability of

sand, soil water, pH, magnesium and Diaprepes abbreviatus

root weevil: two-site study. Environ. Exp. Bot., 59, 321–

333.

Lindsay WL, Norvell WA 1978: Development of a DTPA soil

test for zinc, iron, manganese and copper. Soil Sci. Soc. Am.

J., 42, 421–428.

Lu SH, Liu BH, Hu S, Xiang ZJ, Zhou ZY 1993: Studies on

Fe forms in calcareous soils as affected by different tillage

measurements. In Role of Sulphur, Magnesium and

Micronutrients in Balanced Plant Nutrition, Ed S Hu,

pp. 518–522, Publishing House of Sichuan Scientific Univer-

sity, Sichuan, China (in Chinese).

Mackowiak CL, Grossl PR, Bugbee BG 2001: Beneficial effects

of humic acid on micronutrient availability to wheat. Soil

Sci. Soc. Am. J., 65, 1744–1750.

Maiz I, Arambarri I, Garcia R, Millan E 2000: Evaluation of

heavy metal availability in polluted soils by two sequential

extraction procedures using factor analysis. Environ.

Pollut., 110, 3–9.

Moreno-Caselles J, Moral R, Perez-Murcia MD, Perez-Espinosa

A, Paredes C, Agullo E 2005: Fe, Cu, Mn, and Zn input

and availability in calcareous soils amended with the solid

phase of pig slurry. Commun. Soil Sci. Plant Anal., 36,

525–534.

Mulder J, Cresser MS 1994: Soil and soil solution chemistry.

In Biogeochemistry of Small Catchments: A Tool for Envi-

ronmental Research, Eds B Moldan and J Cerny, pp. 107–

131, John Wiley & Sons Ltd., Chichester, UK.

Page AL, Miller RH, Kenney DR 1982: Methods of Soil Analy-

sis, Part 2. Agronomy Monographs, vol. 9. American Soci-

ety of Agronomy, Madison.

Plekhanova IO 2007: Transformation of Fe, Mn, Co, and Ni

compounds in humic podzols at different moisture.

Biol. Bull., 34, 67–75.

Porter GS, Bajita-Locke JB, Hue NV, Strand D 2004: Manganese

solubility and phytotoxicity affected by soil moisture, oxy-

gen levels, and green manure additions. Commun. Soil Sci.

Plant Anal., 35, 99–116.

Rogovska NP, Blackmer AM, Mallarino AP 2007: Relationships

between soybean yield, soil pH, and soil carbonate concen-

tration. Soil Sci. Soc. Am. J., 71, 1251–1256.

SAS Institute Inc 2000: SAS Procedures Guide. Version 8. SAS

Inst. Inc, Cary, NC.

Shao YT, Zhen QX, Liu SD 1995: Study on forms and availabil-

ity of Cu, Zn, Mn and Fe of principal agricultural soils

in Gansu Province. Acta Pedologica Sin., 32, 423–429

(in Chinese).

Sharma BD, Arora H, Kumar R, Nayyar VK 2004: Relationships

between soil characteristics and total and DTPA-extractable

micronutrients in Inceptisols of Punjab. Commun. Soil Sci.

Plant Anal., 35, 799–818.

Sharma BD, Mukhopadhyay SS, Arora H 2005: Total and

DTPA-extractable micronutrients in relation to pedogenesis

in some Alfisols of Punjab, India. Soil Sci., 170, 559–572.

Sharma BD, Mukhopadhyay SS, Sidhu PS, Katyal JC 2000:

Pedospheric attributes in distribution of total and DTPA-

extractable Zn, Cu, Mn and Fe in Indo-Gangetic plains.

Geoderma, 96, 131–151.

Shuman LM 1985: Fractionation method for soil micronutrients.

Soil Sci., 140, 11–22.

Tessier A, Campbell PGC, Bisson M 1979: Sequential extraction

procedure for the speciation of particulate trace metals.

Anal. Chem., 51, 844–851.

Tong YA, Chaney RL, Korcak RF, Fan F, Faust M 1987: Influ-

ence of soil moisture level on apple iron chlorosis develop-

ment in a calcareous soil. Plant Soil, 104, 85–92.

Wei XR, Hao MD, Shao MA 2005: Effects of long-term crop-

ping on the forms and the availability of micronutrients in

dryland soils on the Loess Plateau. Acta Ecologica Sinica,

27, 3196–3203 (in Chinese).

Wei XR, Hao MD, Shao MA, Gale WJ 2006: Changes in soil

properties and the availability of soil micronutrients after

18 years of cropping and fertilization. Soil Till. Res., 91,

120–130.

� 2010 Japanese Society of Soil Science and Plant Nutrition

Soil iron fractionation and availability 625

Wei XR, Shao MA 2007: Distribution of soil properties as

affected by landforms in watershed of loessial gully region.

J. Nat. Resour., 22, 946–953 (in Chinese).

Xing GX, Zhu JG 2003: Soil Chemistry of Trace Elements and

Rare Earth Elements. Science Press, Beijing (in Chinese).

Xue N, Seip HM, Guo J, Liao B, Zeng Q 2006: Distribution of

Al-, Fe- and Mn-pools and their correlation in soils from

two acid deposition small catchments in Hunan, China.

Chemosphere, 65, 2468–2476.

Young KC, Maurice PA, Hersman LE 2006: Acquisition of Fe

from various natural organic matter isolates by aerobic

pseudomonad bacteria. Geomicrobiol. J., 23, 183–188.

Yu CZ, Peng L, Liu YH, Dai MJ, Peng XL 1991: Content and

distribution of trace elements and fertilizer efficiency in soils

of loessial region. Acta Pedologica Sin., 28, 317–326

(in Chinese).

Zhang LY, Zhang XF, Zhai H 2002: Effects of soil factors

on plant chlorosis due to iron deficit. Chin. J. Soil Sci., 33,

74–77 (in Chinese).

Zhang CE, Zheng FL 2002: Temporal and spatial change charac-

teristics of elements in reclaimed slope forest land. Chin. J.

Appl. Ecol., 13, 672–674 (in Chinese).

Zhang FS, Zheng Y, Li L 2003: Iron availability as affected by

soil moisture in intercropped peanut and maize. J. Plant

Nutr., 26, 2425–2437.

Zhou QX, Gibson CE, Stewart BM 2003: Chemical forms

and extractability of iron in sediments of three contrast-

ing lakes of China and UK. J. Environ. Sci. (China), 15,

728–733.

Zhou XM, Guo JX, Zhao J 2004: Dynamics of nutrient element

iron in soil-plant ecosystem of the Songnen Plain Leymus

chinensis grassland. Chin. J. Appl. Ecol., 15, 2250–2254

(in Chinese).

Zuo Y, Ren L, Zhang F, Jiang RF 2007: Bicarbonate concentra-

tion as affected by soil water content controls iron nutrition

of peanut plants in a calcareous soil. Plant Physiol. Bio-

chem., 45, 357–364.

� 2010 Japanese Society of Soil Science and Plant Nutrition

626 X. Wei et al.