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Composition and Organic CarbonDistribution of Organomineral Complexin Black Soil under Different Land Usesand Management SystemsXueying Hou a b , Xiaozeng Han b , Haibo Li a b & Baoshan Xing ca Northeast Institute of Geography and Agricultural Ecology, ChineseAcademy of Sciences , Harbin, Chinab Graduate School of Chinese Academy of Sciences , Beijing, Chinac Department of Plant, Soil, and Insect Sciences , University ofMassachusetts , Amherst, Massachusetts, USAPublished online: 04 May 2010.

To cite this article: Xueying Hou , Xiaozeng Han , Haibo Li & Baoshan Xing (2010) Composition andOrganic Carbon Distribution of Organomineral Complex in Black Soil under Different Land Uses andManagement Systems, Communications in Soil Science and Plant Analysis, 41:9, 1129-1143, DOI:10.1080/00103620903430016

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Communications in Soil Science and Plant Analysis, 41:1129–1143, 2010Copyright © Taylor & Francis Group, LLCISSN: 0010-3624 print / 1532-2416 onlineDOI: 10.1080/00103620903430016

Composition and Organic Carbon Distributionof Organomineral Complex in Black Soil underDifferent Land Uses and Management Systems

XUEYING HOU,1,2 XIAOZENG HAN,2 HAIBO LI,1,2

AND BAOSHAN XING3

1Northeast Institute of Geography and Agricultural Ecology, Chinese Academyof Sciences, Harbin, China2Graduate School of Chinese Academy of Sciences, Beijing, China3Department of Plant, Soil, and Insect Sciences, University of Massachusetts,Amherst, Massachusetts, USA

Organomineral complexes form the basis of soil fertility and have significant effects onthe soil environment. In this research, we aimed to study the composition and organiccarbon (C) distribution of organomineral complexes in a black soil under differentland uses and management by means of ultrasonic dispersion and particle assortment.The results showed that the fine sand–size complex (20–200 μm) was dominant underdifferent land uses and management. Silt-size (2–20 μm) and fine sand-size contentincreased with nitrogen and phosphorus application (NP) and NPM (NP together withorganic manure) treatment, whereas clay-size (0–2 μm) content decreased. The contentof <20-μm complex in GL (grassland) was less than in BL (bareland), and >20-μmcomplex showed the opposite trend. The silt-size content increased with the increaseof SOC (soil organic C). A negative relationship was observed between the clay-sizecomplex content and SOC content. Land-use change resulted in different dynamics inC sequestration in soil. The content of <20-μm complex in GL was more than in NPand NPM; GL has potential to sequester more C than tilled soil because of the stabil-ity of SOC stored in the <20-μm fraction. Long-term application of organic manureand vegetation restoration increased the OC (organic carbon) content of all sizes ofcomplexes; the OC contents of clay-size complex were in the order GL > NPM >NP > BL > NF (no fertilizer applied) and increased the proportion of OC in >20-μmcomplexes, indicating that OC content in sand-size fractions increased with total SOCcontent.

Keywords Carbon sequestration, land use, organomineral complex, SOM

Introduction

Soil organic matter (SOM) is an important component of soil structure. The SOM affectssoil properties such as aggregate stability, soil temperature and moisture, and microbialpopulation and, most important, provides nutrients for plant growth (Aurélie et al. 2007;Chandrika, Nirmal, and Kunal 1984). Decomposition of SOM gives rise to decline of soil

Received 11 April 2008; accepted 25 February 2009.Address correspondence to Xiaozeng Han, Chinese Academy of Sciences, Northeast Institute

of Geography and Agroecology, Harbin, Heilongjiang, China. E-mail: hanxz@cern.ac.cn

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1130 X. Hou et al.

fertility and increase of atmospheric carbon dioxide (CO2). Thus, SOM play a criticalrole in soil quality, agriculture sustainability, and mitigation of negative environmentaleffects.

Jastrow (1996) reported that less than 20% of soil organic carbon (SOC) was storedin mass aggregates in soil; the others were associated with mineral materials. Humus isa by-product of heterogeneous organic compounds produced by microbial metabolismand is the oldest form of organic matter in soils. When it forms, it interacts with inor-ganic surfaces in soil and often presents as organomineral complex (Sergey, George, andLeonid 1999), indicating its significant resistance to microbial degradation. Jastrow (1996)found that significant proportions of additional organic-matter input were rapidly associ-ated with minerals, which then promoted the formation of stable macroaggregates. Thismight contribute to the physical protection of OC (organic carbon). Humus decomposedrapidly when removed from its mineral matrices and exposed to microbial attack, indicat-ing that minerals are key to humus stabilization (Gramss, Ziegenhange, and Sorge 1999).The ability of SOM to adsorb decreased with increase in the mineral size and shrink-ing surface area, and SOC content decreased with damage to the organomineral complex(Van Aeen and Kuikman 1990). Soil humus is physically protected in organomineralcomplexes, enhancing the SOC resistance to soil microorganism decomposition, accel-erating the soil C sequestration in soil, and reducing the release of greenhouse gases.Ogranomineral complexes form and alter as a result of pedogenic processes and thro-pogenic impacts on soil (Schulton and Leinweber 2000), thus forming the basis of soilfertilities and exerting important effects on physical, chemical, and biological proper-ties of soil and loss and accumulation of SOM (Xiong and Jiang 1983). Different sizesof organomineral complexes showed different SOM dynamics. Christensen (1987) per-formed an aerobic incubation experiment and observed that SOM degraded in the ordersand > clay > whole soil > silt. Land-use changes and soil management strongly influ-enced the composition of different sizes of organomineral complexes. Shi, Zhang, andLin (2002) reported that chemical fertilizers or chemical fertilizers together with organicmanure increased the content of clay-size and silt-size complexes and decreased the contentof sand-size complexes in fluvo-aquic soil and red paddy soil. By contrast, Wei, Chen, andXie (1995) showed that chemical fertilizer application or chemical fertilizer with organicmanure had no significant effect on content of clay-size complexes in purple paddy soilbut decreased the content of 2- to 10-μm fractions. Chemical fertilizer application reducedthe content of 10- to 250-μm complexes, and inorganic fertilizer application together withorganic manure showed the opposite trend. This may be attributed to the properties ofdifferent soil species.

Black soils (Mollisol), the most fertile and productive soils, are distributed mainly inJiLin and Heilongjiang Provinces in northeast China. Black soil has high humus contentcompared to other soil species in China and is the main soil type used to grow maize andsoybean in northeast China (Han and Wang 2006). In recent years, intensive cultivation hasled to deterioration of soil properties, SOC loss, and associated yield suppression (Liu etal. 2005). Proper management of black soil to improve its soil properties has drawn greatattention. Information related to how land-use changes and soil management affect theorganomineral complex and to the dynamics of the distribution of SOM in different sizesof organomineral complexes in black soil is limited. Our research is aimed at examining thecomposition of organomineral complexes and OC distribution in black soil under differentland uses and soil management systems.

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Composition of Organomineral Complex 1131

Materials and Methods

Study Site Descriptions

The study was carried out at National Field Research Station of Agroecosystem of ChineseAcademy of Science in Hailun County, Heilongjiang Province, China. The area is locatedat latitude 47◦ 26’ N and longitude 126◦ 38’ E at an altitude of 240 m. The mean annualtemperature is 1.5 ◦C, and annual precipitation is 550 mm. The duration of the frost-freeperiod is about 130 days. The soil is a black soil derived from loamy loess.

Soil sampling was performed from the following sites: (i) grassland (GL), formerlycropped, but fallowed in 1985 with the grass Leymus chinesis as the dominant species;(ii) bareland (BL), like grassland, fallowed in 1985, but the grasses were eliminated peri-odically during the plant growth stage; and (iii) cropland: three treatments were included interms of long-term site experiment established in 1993, in which no fertilizer was applied(NF); nitrogen and phosphorus fertilizer was applied (NP), and NP fertilizer together withorganic manure (NPM) was selected. Every treatment has four replications. Crops grown inthe agroecosystem were wheat (Triticum aestivum L. cv. ‘Long 4083’), corn (Zea mays L.cv. ‘Haiyu 6’), and soybean [Glycine max (Merrill.) L. cv. ‘Heinong 35’] in rotation. Sucha rotation represented the main cropping system in the region. The crops were grown oneseason per year. The fertilizers were applied at the following rates (kg ha−1) for each crop:(i) 120 kg nitrogen (N) hm−2 and 24 kg phosphorus (P) hm−2 as urea and ammoniumphosphate [(NH4)2HPO4], respectively, with or without 15,000 pig manure for wheat;(ii) 32.3 kg N hm−2 and 36 kg P hm−2 with or without 15,000 pig manure for soybean;and (iii) 150 kg N hm−2 and 32.75 kg P hm−2 with or without 30,000 pig manure for corn.The pig manure contained average total N, P, and potassium (K) concentrations of 22.1,2.6, and 2.4 g kg−1, respectively, and an average organic P of 2.3 g kg−1. Other nutrientsincluding K were at adequate levels in the soil and thus were not applied as fertilizers. Thecultivated crop was soybean in 2006 when sampling.

Soil samples were collected from 0- to 20-, 20- to 30-, 30- to 40-cm soil layers fromsoil profiles. More than 10 spots were mixed one composite sample, which was then air-dried and homogenized.

Laboratory Analysis

Organomineral Complex. Different sizes of organomineral complexes were determinedaccording to Gavinelli et al. (1995), involving successive wet-sieving procedures of the soilfollowing washing and sedimentation steps. The following fractions were separated: coarsesand–size complex (>200 μm); fine sand–size complex (20–200 μm); silt-size complex(2–20 μm), and clay-size complex (0–2 μm). The soil samples were dispersed by ultrason-ication with water–soil 4:1, wet-sieved through 200-μm meshes (extracted through densityfractionation in water), sedimented and decanted to obtain fractions 2–20 and 0–2 μm, andcentrifuged. All the particle-size fractions were dried at 40 ◦C, weighed, finely ground inan agate mortar, and passed through a 100-eye mesh for further analysis. One fractionationwas carried out for each soil sample (i.e., three replications for each treatment).

The content of coarse sand–size complex in soil only accounted for 1.2–1.9%, almosta mixture of plant debris and minerals. Because many plant residues and organics mixedwith coarse sand–size complexes could not be separated clearly, this size complex was notdetermined.

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1132 X. Hou et al.

Particle-Size Analysis. The protocol for particle-size analysis (Klute 2005) was as follows:soil samples were dry-sieved through 2-mm mesh. Fifteen g of air-dried soil samples weremixed with 20 mL of hydrogen peroxide (H2O2, 30 vol.) for 6 h at room temperature. Then15 mL sodium acetate (NaOAc; 0.25 mol L−1) was added during 6 h at room temperature.The volume in the beaker was brought up to 200 mL with purified water, and the suspensionwas gently boiled for 40 min. Sedimentation/decantation was used to obtain fractions of0–2 μm, 2–20 μm, 20–200 μm, and >200 μm, which were oven dried at 105 ◦C andweighed.

Total C and N in Whole Soil and Organomineral Complex. Carbon and N content of soilsand organomineral complexes were determined using dry combustion on a Vario EL CHNelemental analyzer (Heraeus Elementar Vario EL, Hanau, Germany). Total C is equivalentto organic C because there are no carbonates present in this soil type.

Statistical Analysis

Duncan’s multiple range test at P < 0.05 and P < 0.01 levels was used to compare themeans among treatments. All the analyses were conducted using SAS (SAS Institute Inc.,Cary, N.C.) and Excel 2000.

Results and Discussion

Effect on the Content of SOC under Different Land Uses and Management Styles

Under different land uses, SOC concentration in the upper mineral soil (0–20 cm)decreased in the order GL (32.42 g kg−1) > NF (26.99 g kg−1) > BL (26.54 g kg−1). Allthree treatments had no fertilizer input. The SOC of GL increased by 20.12% compared toNF and was 21.48% more than BL. The SOM sharply decreased when grassland changedto tilled land. Chan (1997) indicated that the POC (particle-size organic carbon) contentwas first decreased in cropland and then increased when converted to grassland again bycomparing grassland and agricultural soil. In addition, Wang, Amundson, and Trumbore(1999) showed that SOC content decreased by 22% when virgin grassland was reclaimedto cropland, and all the SOC was meanly distributed at the surface layer (0–30 cm). It wassimilar for grassland and cropland at the 30-cm soil layer. The SOC in NF increased by1.69% compared to BL; this increased part of SOC in NF may result from decompositionof root exudes and plant residues. The SOC content in NF was lower than in BL and GLat 20- to 30-cm soil layers and might be due to tillage. The SOC content in GL increasedby 6.64% compared to NPM, and SOC content in BL increased by 10.22% compared withNF, indicating that tillage was the main factor controlling the change of SOC content at the20- to 30-cm soil layer. Tillage fractures, inverts, and opens the soil, allowing rapid O2 andCO2 exchange with simultaneous incorporation of crop residues into the soil; as a result,SOM was susceptible to microorganism decomposition. Intensive agricultural practiceswill decrease SOM content. However, SOC content increased by 7–30% after 10 years ofconservation tillage as compare to convention tillage (Yang and Kay 2001).

Under different land fertilization management styles, SOC concentration decreased inthe order NPM (33.29 g kg−1) > NP (29.32 g kg−1) > NF (26.99 g kg−1) and declined withthe depth of soil. The SOC concentrations of NP and NPM increased 10.47% and 25.43%respectively, as compared to NF. The SOC content was significantly different betweenNF and both NP and NPM (P < 0.01). Chemical fertilizer applied with organic mineral

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Composition of Organomineral Complex 1133

influenced soil properties and soil C pools because of an increase of aboveground biomass,crop yield, and radical mass. The SOM content in the corn belt of black soil in northeastChina was lower than in the Mollisol corn belt in America, due to the removal of rootsand plant residues from cropland (Yang 2000). Trumbore, Davidson, and Barbosa (1995)reported that the decrease of SOC was attributed to less OM input or removal of plantresidues. In the present experiment, SOC content increased 2.76% more in NPM than inGL, owing to organic manure application for NPM. Appropriate land management mayincrease SOC content in agricultural soil (Gregorich 1995). In croplands, input chemicaland manure fertilizer increased SOC of the 20- to 30-cm layer.

The SOC content at the 30- to 40-cm soil layer was not significantly different underdifferent land uses and management methods, due to the lack of roots and physicaldisturbance.

Effect of Different Land Uses and Management on Organomineral Complexes and OCContent

We can see in Table 2 that the clay-size complex content of five plots was less than itsclay-size mineral content, but silt-size and fine sand–size complexes were more than thecorresponding fractions, indicating smaller complexes combined and formed the biggercomplexes. The formation of a bigger complex resulted in decline of small complexes(Aurélie et al. 2007; Zhao et al. 1993).

Effect of Different Land Uses on Organomineral Complexes and OC Content

In the surface layers (0–20 cm), clay-size complexes decreased in the order NF > GL >

BL, and OC content of clay-size complexes was in the order GL > BL > NF. Differencesin �Mass of clay-size complexes were significant (P < 0.01) among the three experimentalplots (Figure 1). Clay-size �Mass in NF was more than in other plots. The increased partwas mostly the clay-size minerals, and the increased amount of mineral was more than thedecreased amount of OC in clay-size complexes. The increase of clay-size mineral in NFwas attributed to the degeneration of bigger complexes. Clay-size complex content in BLwas less than in GL; the decreased part was that of OC in clay-size complex, which wasmore than the increased amount of mineral in BL. Chandrika, Nirmal, and Kunal (1984)showed that when the amount of clay was small, humus of some layers were not directlyattached to the surface of the inorganic matter. With the increase of the clay concentration,more surfaces would be available for humus adsorption, whereby the extent of mutilayerformation decreases and then forms a unilayer. Humus indirectly bonded to the clay wouldbe readily available for microbial attack. The NF and BL had no organic-material inputs;increased content of clay-size complex was due to the increase of clay-size mineral. Thismight be beneficial for physical protection of SOC content, but less SOC content of thesetwo soils could not improve soil properties as compared to GL.

Silt-size complex decreased in the order BL > GL > NF. No organic input into theecosystem resulted in C loses by decomposition and mineralization of OC. The SOCdecreased by 44.56% and 42.15% in BL and NF, respectively, as compared with the orig-inal soil sample, which was collected before the experiment was arranged. Fine sand–sizecomplexes lost OC and then became smaller fractions. The OC content in silt-size com-plexes was more stable than in other complexes (Christensen 1987), indicating that SOCin BL was more stable than in other plots and that release of greenhouse gases was rel-atively small. The silt-size complex concentration of NF was less than BL by 13.42%.

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Tabl

e1

Soil

orga

nic

carb

on(S

OC

),so

ilpH

,and

part

icle

-siz

ean

alys

isat

the

expe

rim

enta

lplo

tsun

der

diff

eren

tlan

dus

esat

diff

eren

tdep

ths

(mea

nan

dst

anda

rder

ror

inpa

rent

hese

sfo

rth

ree

repl

icat

es)

Part

icle

-siz

ean

alys

is(%

)

Para

met

era

Dep

th(c

m)

pH<

2μ

m2–

20μ

m20

–200

μm

C(g

kg−1

)N

(gkg

−1)

C/N

ratio

BL

0–20

6.15

41.3

5(0

.3)

28.7

8(0

.4)

29.3

2(0

.4)

26.5

4(0

.4)

1.89

(0.2

)14

.04

(0.4

)20

–30

6.38

41.9

7(0

.3)

29.7

1(0

.3)

27.6

7(0

.3)

23.6

2(0

.3)

1.38

(0.1

)17

.10

(0.3

)30

–40

6.55

43.8

6(0

.4)

29.8

7(0

.5)

25.7

6(0

.4)

18.3

8(0

.2)

1.10

(0.2

)16

.72

(0.2

)G

L0–

206.

3642

.54

(0.2

)26

.49

(0.3

)30

.42

(0.5

)32

.37

(0.5

)2.

38(0

.2)

13.5

9(0

.5)

20–3

06.

5844

.57

(0.6

)28

.16

(0.4

)26

.89

(0.3

)24

.24

(0.3

)1.

43(0

.2)

16.9

0(0

.6)

30–4

06.

6343

.32

(0.5

)27

.68

(0.4

)28

.45

(0.2

)19

.50

(0.1

)1.

27(0

.3)

15.3

6(0

.2)

NF

0–20

7.06

40.8

5(0

.2)

25.6

6(0

.1)

32.3

8(0

.5)

26.9

9(0

.9)

1.86

(0.2

)14

.49

(0.5

)20

–30

6.25

42.6

7(0

.4)

28.7

5(0

.5)

27.7

9(0

.3)

21.4

3(0

.5)

1.38

(0.1

)15

.53

(0.3

)30

–40

6.30

44.2

0(0

.5)

28.2

0(0

.4)

27.0

1(0

.3)

16.1

3(0

.3)

0.94

(0.2

)17

.14

(0.2

)N

P0–

206.

5741

.81

(0.4

)25

.59

(0.2

)31

.79

(0.4

)29

.32

(0.7

)2.

09(0

.2)

14.0

6(0

.3)

20–3

06.

2045

.64

(0.4

)26

.50

(0.4

)27

.19

(0.4

)22

.42

(0.3

)1.

58(0

.2)

14.1

8(0

.4)

30–4

06.

3644

.87

(0.5

)25

.72

(0.5

)28

.66

(0.3

)16

.12

(0.3

)1.

02(0

.3)

15.8

6(0

.3)

NPM

0–20

5.74

41.7

3(0

.3)

23.0

9(0

.5)

33.8

3(0

.6)

33.2

9(0

.9)

2.54

(0.2

)13

.09

(0.3

)20

–30

6.08

43.0

7(0

.5)

24.3

2(0

.5)

31.8

0(0

.5)

22.7

3(0

.2)

1.60

(0.1

)14

.20

(0.4

)30

–40

6.21

43.3

1(0

.4)

27.6

6(0

.3)

28.1

2(0

.5)

18.3

7(0

.2)

1.27

(0.5

)14

.50

(0.2

)

aB

arel

and

(BL

),gr

assl

and

(GL

),no

fert

ilize

rap

plie

d(N

F),

nitr

ogen

and

phos

phor

usfe

rtili

zer

appl

ied

(NP)

,an

dN

Pfe

rtili

zer

amen

ded

with

orga

nic

mat

eria

ls(N

PM).

1134

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Tabl

e2

Car

bon

cont

ent,

C/N

ratio

,and

the

cont

ento

for

gano

min

eral

com

plex

esun

der

diff

eren

tlan

dus

esan

dm

anag

emen

tatd

iffe

rent

dept

hs(m

ean

and

stan

dard

erro

rin

the

pare

nthe

ses

for

thre

ere

plic

ates

)

0–20

cm20

–30

cm30

–40

cm

Para

met

era

Frac

tion

(μm

)M

ass

(%)

gC

kg−1

com

plex

C/N

ratio

Mas

s(%

)g

Ckg

−1co

mpl

exC

/N

ratio

Mas

s(%

)g

Ckg

−1co

mpl

exC

/N

ratio

BL

20–2

0047

.8(0

.2)

12.1

(0.1

)14

.0(0

.4)

46.9

(0.1

)5.

4(0

.3)

10.5

(0.2

)44

.2(0

.4)

4.2

(0.2

)9.

7(0

.2)

2–20

35.0

(0.3

)36

.1(0

.1)

13.5

(0.3

)31

.5(0

.2)

38.0

(0.2

)15

.8(0

.1)

30.0

(0.2

)25

.2(0

.2)

14.4

(0.2

)<

216

.1(0

.7)

44.5

(0.4

)12

.5(0

.3)

20.0

(0.3

)39

.7(0

.2)

15.5

(0.1

)24

.5(0

.3)

33.5

(0.2

)14

.0(0

.2)

Rec

over

yb98

.9%

96.5

%98

.3%

94.9

%98

.7%

95.9

%G

L20

–200

49.7

(0.3

)20

.9(0

.1)

12.0

(0.5

)47

.0(0

.1)

7.0

(0.1

)8.

1(0

.2)

44.9

(0.2

)4.

9(0

.3)

9.4

(0.2

)2–

2032

.5(0

.2)

38.8

(0.4

)13

.2(0

.3)

33.2

(0.2

)40

.0(0

.2)

15.3

(0.2

)30

.0(0

.1)

28.6

(0.3

)12

.9(0

.2)

<2

16.5

(0.2

)55

.0(0

.5)

12.6

(0.3

)18

.7(0

.5)

39.9

(0.2

)14

.3(0

.6)

24.1

(0.2

)34

.5(0

.3)

14.9

(0.5

)R

ecov

eryb

98.7

%99

.1%

98.8

%98

.9%

99.0

%98

.1%

NF

20–2

0050

.5(0

.6)

13.3

(0.8

)12

.8(0

.6)

44.7

(0.2

)6.

0(0

.3)

9.6

(0.3

)40

.7(0

.2)

3.1

(0.3

)7.

3(0

.3)

2–20

30.3

(0.7

)38

.5(0

.9)

13.5

(0.4

)36

.0(0

.3)

32.7

(0.4

)14

.4(0

.4)

36.5

(0.3

)22

.0(0

.3)

14.5

(0.1

)<

217

.6(0

.5)

40.6

(0.7

)12

.4(0

.3)

17.5

(0.6

)36

.3(0

.3)

13.0

(0.6

)21

.4(0

.2)

29.3

(0.2

)15

.1(0

.2)

Rec

over

yb98

.3%

94.4

%98

.2%

97.0

%98

.6%

98.0

%N

P20

–200

50.9

(0.6

)14

.8(0

.9)

14.1

(0.8

)49

.4(0

.3)

6.8

(0.1

)10

.9(0

.4)

45.9

(0.2

)3.

0(0

.1)

9.9

(0.2

)2–

2032

.1(0

.8)

39.1

(0.8

)13

.7(0

.5)

31.0

(0.2

)36

.8(0

.3)

15.5

(0.3

)28

.5(0

.2)

21.2

(0.2

)12

.9(0

.2)

<2

15.1

(0.3

)50

.5(0

.8)

12.4

(0.3

)18

.3(0

.5)

38.4

(0.4

)13

.3(0

.6)

24.7

(0.2

)34

.0(0

.3)

13.1

(0.3

)R

ecov

eryb

98.1

%94

.4%

98.7

%97

.4%

98.7

%97

.5%

NPM

20–2

0051

.3(0

.7)

15.1

(0.6

)13

.1(0

.6)

50.6

(0.4

)7.

1(0

.2)

6.3

(0.3

)46

.7(0

.2)

4.5

(0.2

)7.

0(0

.4)

2–20

34.4

(0.6

)38

.4(0

.5)

12.8

(0.3

)30

.9(0

.2)

38.3

(0.4

)13

.7(0

.2)

28.5

(0.1

)25

.3(0

.2)

14.5

(0.3

)<

212

.9(0

.3)

52.5

(0.7

)10

.4(0

.4)

17.0

(0.8

)39

.1(0

.2)

15.3

(0.2

)24

.1(0

.2)

36.2

(0.1

)13

.2(0

.1)

Rec

over

yb98

.6%

83.3

%98

.5%

97.0

%99

.2%

98.2

%

aB

arel

and

(BL

),gr

assl

and

(GL

),no

fert

ilize

rapp

lied

(NF)

,nitr

ogen

and

phos

phor

usfe

rtili

zera

pplie

d(N

P),a

ndN

Pfe

rtili

zera

men

ded

with

orga

nic

mat

eria

ls(N

PM).

bR

elat

ions

hip

betw

een

soil

Can

dth

esu

nof

orga

nom

iner

alco

mpl

ex.

1135

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1136 X. Hou et al.

–30.0

–25.0

–20.0

–15.0

–10.0

–5.0

0.0

5.0

10.0

15.0

20.0

25.0

BL

GL

NF

2µm

2–20 µm

0–20cm 20–30cm

2µm 2µm

30–40cm

A B C

A A B

a a a

a

b b

A B

C

a bc

A AB

B

A

B

C

A B

C

Mass

20–200 µm 2–20 µm 20–200 µm 2–20 µm 20–200 µm

Figure 1. Difference in the �mass at different depths under different land uses. For example,clay-size �mass = clay-size organomineral complex content (%) – OC content of clay-sizeorganomineral complex – clay-size mineral content × (100 − soil organic carbon). Bareland (BL),grassland (GL), and no fertilizer applied (NF). Letters on the top of bars indicate significantdifferences at the 5% level; capital letters on the top of bars indicate significant differences at the1% level.

Both soils had no organic input, but the activities of tillage in NF improved the aerationof soil, increasing the decomposition of SOM. From a 20-year long-term experiment, Hanand Wang (2006) showed that SOC content of humin in eight different treatments wasalmost the same. Additionally, Wang et al. (2006) showed that about 13% of the initial SOCdecomposed during a 13-year period of maize cultivation in black soil, and the turnover rateof SOC was 8% year−1. The part of labile SOC was subject to microorganism decomposi-tion. This is why the content of silt-size complex in NF was less than in BL. The silt-sizecomplex content of GL was less than in BL even if soils in BL and GL were not tilledduring the past 20 years. The OC concentration increased in GL, making some silt-sizecomplexes become fine sand-size complexes. The increased amount of SOC in silt-sizecomplexes was less than the decreased part of mineral of this size. The other reason is finesand–size complexes in BL might lose SOM and became silt-size complexes.

The content of fine sand–size complex decreased in the order GL > BL > NF,and differences among these soil plots were not significant. Tillage could improve theharmonization among water, air, and nutrition of soil, increasing soil aeration and soil per-meability on clay loam and making soil suitable for crop production. The SOC was theimportant source of nutrition for plant growth and microorganism C. Tillage also allowedrapid air exchange to accompany simultaneous incorporation of crop residues into soil andmade SOC accessible to microorganism attack and greater microbial processing (Fang,Yang, and Zhang 2003). Intensive agriculture decreased SOM content, led to degeneration

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Composition of Organomineral Complex 1137

of soil quality, and did not help soil C sequestration. It also made the fine sand–sizecomplex become a small-size complex in NF as compared to BL, which has the sameSOC content but no tillage for 20 years.

Land-use changes affect the content of clay-size, silt-size, and fine sand–size com-plexes. Significant differences were observed among three plots at the 20- to 30-cm soillayer. Silt-size complex content in NF was much more than in other plots at 20–30 cm, andfine sand–size complex content was the least. This might be due to soil tillage. Silt-size�Mass in NF under the surface soil layer was more than the other’s (Figure 1); this wasbecause the bigger complex lost SOM, which was then incorporated into small-size parti-cles. More clay-size and silt-size complexes, and less fine sand–size complex were foundat the 30- to 40-cm soil layer than in upper soil layers. Although different sized complexeshad different mineral contents, with increasing soil fertility, appropriate sized complex dis-tribution could improve soil structure. The SOM input also made small complexes becomebigger ones in GL, as compared with BL.

The difference was significant for the three samples plots at 30–40 cm deep, indicatingthat land-use changes still had distinct effects on the 30- to 40-cm soil layer. Tillage alsomade fine sand–size complexes in NF become small ones in this soil layer, so we can seethe silt-size complex in NF was more than in BL and GL. The SOM was the main factorcontrolling complexes and particle-size distribution for BL and GL at the 30- to 40-cm soillayer, because in this layer there were few plant roots and lack of disturbance. The 20- to200-μm size �Mass of GL was less than BL (Figure 1) because GL has more minerals20–200 μm than BL.

Effect of Different Land Management Styles on Organomineral Complexes and OCContent

In the surface layers (0–20 cm), clay-size complexes decreased in the order NF > NP >

NPM, whereas OC content of clay-size complexes was in the order NPM > NP > NF. Thedecreased part was that of minerals for NP and NPM, which was more than the increasedamount of OC in clay-size complex as compared with NF. The differences among NF, NP,and NPM were significant at P < 0.01. The NP and NPM decreased the content of clay-sizecomplexes, and clay-size complex in NPM decreased greatly compared to NP. Xu and Shen(2000) showed similar results. Shi, Zhang, and Lin (2002) had opposite results, probablydue to lack of clay-size minerals in the tested soil. The increased part of clay-size complexmay be organic fractions. The cation exchange capacity (CEC) decreased with increasingparticle size. Brogowski et al. (1976) indicated that about 90–95% of exchangable cationssuch as calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+) werestored in the organomineral complexes (<20 μm), with the majority adsorbed by clay-sizefraction. Song, Han, and Tang (2007) studied the changes in P fractions, adsorption, anddesorption in the same plots as we did, showing that adsorption of P in GL was more thanin BL and decreased in the order NF > NP > NPM, which might improve the capacityof soil P on clay-size complex. Zhao et al. (1993) reported that soil of poor fertility hadmore clay-size complexes than that of good-fertility soil, and the clay-size complex contentincreased with tillage period (Yu et al. 2004).

Silt-size complex decreased in the order NPM > NP > NF. Differences in the con-tent of silt-size complex were significant (P < 0.01) among the three experimental plots.Long-term fertilization (chemical and manure) increased SOC and then increased the con-centration of OC in silt-size complex in NPM and NP. The clay-size complex and SOMincorporated into bigger complexes gave rise to an increase of the content of silt-size

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1138 X. Hou et al.

−40.0

−30.0

−20.0

−10.0

0.0

10.0

20.0

30.0

BA

C

C

AB

aa

a

ba

a

CB

A

abba

NFNPNPM

cb

a

A

A

C

B

BAB

Mass 0–20cm 20–30cm 30–40cm

2µm

2–20 µm

2µ 2m µm

20–200 µm 2–20 µm 20–200 µm 2–20 µm 20–200 µm

Figure 2. Difference in the �mass at different depths under different land management schemes.For example, clay-size �mass = clay-size organomineral complex content (%) – OC content ofclay-size organomineral complex – clay-size mineral content × (100 − soil organic carbon). Nofertilizer applied (NF), nitrogen and phosphorus fertilizer applied (NP), and NP fertilizer amendedwith organic materials (NPM). Letters on the top of bars indicate significant differences at the 5%level; capital letters on the top of bars indicate significant differences at the 1% level.

complexes. Silt-size �Mass in NPM is more than in the others (Figure 2), indicating thatlong-term application of organic manure may be beneficial for forming silt-size complexes.The NP and NPM increased the content of silt-size complex, and Shi, Zhang, and Lin(2002) and Wei, Chen, and Xie (1995) showed similar results. Because OM was incorpo-rated into soil for NPM, the silt-size complex content was more than NP and NF. Xu andShen (2000) showed that 2- to 10-μm complexes increased after 14 years of fertilization,and N and P were mainly stored in 2- to 10-μm fractions, indicating that this particle-sizefraction played an important role in cycling, transforming, and keeping soil nutrition.

The content of fine sand–size complex decreased in the order NPM > NP > NF. Long-term fertilization increased the OC concentration in both fine sand–size complex and soil(Table 2), which might be because the �Mass of NP was more than in NPM in both 0- to20-cm and 20- to 30-cm soil layers (Figure 2), probably due to changes of soil pH bysoil fertilization, which had potential to form the silt-size complex. Soils of different pHvalues might form different types of organomineral complexes, when pH varied from 4.8to 6.6 and increased stability of SOM (Balesdent et al. 1998). The other reason was thatthe structure and composition of SOM in NPM and NP might be significantly different andform different size complexes.

In the 20- to 30-cm soil layer, as a result of lost SOC, fine sand–size complex incropland became small-size particles, and there was an increase in the content of clay-size and silt-size complexes because of lost SOC. Chemical and manure fertilizer wouldslow down this trend. In the 30- to 40-cm soil layer, more fine sand–size complexes of NF

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Composition of Organomineral Complex 1139

became smaller ones. Different SOC contents of NP and NPM made them have the samesilt-size complex content, but NPM had more fine sand-size complexes than NP.

Relationship of SOC, Organomineral Complex, and OC Concentration

Figure 3 shows that the content of clay-size complex had negative correlation with SOC(r = –0.92∗∗, P = 0.01, n = 14), while positive correlation was observed between finesand–size complex and SOC (r = 0.81∗∗, P = 0.01, n = 14). Silt-size complex contentincreased with increasing SOC content, but it was not significant.

The OC content of every size complex increased with SOC content. The OC contentof clay-size, silt-size, and fine sand–size complexes was highly correlated with total SOCcontent (Figure 4).

The SOC Distribution in Organomineral Complexes

Figure 5 shows that the distribution of SOC in <20-μm complexes decreased in the orderBL > NF > NP > GL > NPM at surface layer. Clay- and silt-size complexes were con-sidered the stable C pool of soil. With decomposition of SOM in particle-size fractions,the turnover times of the SOM in different size complexes showed the order coarse sand >

clay > silt-size complexes (Christensen 1987). That SOC is mainly distributed in <20-μmfractions in the BL and NF soil might explain why soil C pools were stabilized. The inputof organic materials increased proportion in >20 μm, suggesting that when SOC contentincreased because of land-use change, SOC concentration in each size complex increasedand then the content of SOC in the >20-μm fraction also increased. The SOC contentin the <20-μm fraction increased with the decrease of SOC content at 20–30 cm and30–40 cm. This is because when the adsorption of SOM on clay reached equilibrium,SOM content of bigger minerals increased (Zhao et al. 1997). This was beneficial for sta-bilizing soil aggregates and harmonizing the absorption, reservation, and release of soil

y = 2.80x2 – 20.15x + 50.47, r = 0.92**

y = –0.57x2 + 4.64x + 24.23,r = 0.45

y = –2.03x2 + 14.36x + 25.49, r = 0.81**

0

10

20

30

40

50

60

1.5 2.0 2.5 3.0 3.5

Soil organic carbon content(g kg–1)

Org

ano–

min

eral

com

plex

con

tent

(%)

<2 µm

2–20 µm

20–200 µm

Figure 3. Correlation of SOC concentration and OC content of clay-size, silt-size, and fine sand–sizecomplexes.

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1140 X. Hou et al.

y = 0.25x2 – 0.30x + 0.12, r = 0.95**

y = –1.19x2 + 6.88x – 5.91, r = 0.97**

y = 0.32x2 – 0.26x + 2.78, r = 0.97**

0

1

2

3

4

5

6

1.5 2.0 2.5 3.0 3.5Soil organic carbon content(g kg–1)

OC

con

tent

of

orga

no–m

iner

al c

ompl

ex(g

kg–1

)

<2 µm

2–20 µm

20–200 µm

Figure 4. Correlation of SOC and clay-size, silt-size, and fine sand–size complexes.

0%

20%

40%

60%

80%

100%

BL

0–20

cm

BL

20–3

0cm

BL

30–4

0cm

GL

0–20

cm

GL

20–3

0cm

GL

30–4

0cm

NF0

–20c

m

NF2

0–30

cm

NF3

0–40

cm

NP0

–20c

m

NP2

0–30

cm

NP3

0–40

cm

NPM

0–20

cm

NPM

20–3

0cm

NPM

30–4

0cm

Org

anic

car

bon

dist

ribu

tion

(%)

A

20–200 µm

2–20 µm

<2 µm

Figure 5. Organic C distribution in clay-size, silt-size, and fine sand–size complexes.(A) Undertermined organic C; the concentration of total SOC is shown in Table 1. Bareland (BL),grassland (GL), no fertilizer applied (NF), nitrogen and phosphorus fertilizer applied (NP), and NPfertilizer amended with organic materials (NPM).

water and nutrition. Roscoe and Buurman (2003) compared SOM dynamics in tillage andno-tillage soil, indicating that free light fraction was more sensitive to land-use change.The SOC stored in this fraction in tillage soil was much more than in no-tillage soil; as aconsequence, SOC in tillage soil was rapidly decomposed. Light fraction decreased whennatural grassland converted to agroecosystems (Christensen 1996). Balesdent, Wagner, andMariotti (1988) reported that native soil has significantly larger OC content in particle-sizecomplexes than in the corresponding complexes of cropland. Organomineral complexesfrom manured soils had more OC content than those from unfertilized soils and chemi-cally fertilized soils (Christensen 1992). Both natural vegetation restoration and long-termfertilization increased SOM, along with an increase of SOM content in every size complex,and an increase in the proportion of SOC distributed in >20-μm complexes.

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Composition of Organomineral Complex 1141

The C/N Ratio

The C/N ratio showed the order coarse sand > fine sand > silt > clay (Table 2). Theresult was shared by Aurélie et al. (2007) and Freixo et al. (2002). The different resultsachieved by other researchers may be due to the variation of climate conditions. Annualmean temperature is 1.5 ◦C and long frozen period is 170–180 days year−1 in the studysites. The C/N ratio in clay-size complex was the lowest in the three sizes of complexesand increased with increasing particle size, indicating that the structure and compositionof organics in the four sizes of complexes are different. Turchenek and Oades (1979) gavean explanation for lower C/N ratio in small-size complexes: these complexes containedmaterials with low C/N ratio, such as bacterium and root exudes. Coarse-size complexescontained materials with high C/N ratio, such as small roots and plant litter. Similarly,Hassink (1995) reported that C/N ratio in the heavy fraction (clay-size complex) was lowerthan in the light fraction (organics that do not combine with minerals). Xu and Shen (2000)suggested that the low C/N in clay-size complex was due to absorption of NH4

+, resultingin increased N content. Zhao et al. (2005) suggested that the humification index in smallcomplexes was more than in bigger ones, which has a pronounced effect on C/N ratios ofsoil humus in different sized complexes.

Conclusion

Although OC of particle-size fractions was stabile in NF and BL relative to other plots,total SOM content decreased in these two soils, degrading soil properties. The SOC con-tent of GL was more than BL, and its OC content of three sizes of complexes showedsignificant differences; however, the content of organomineral complexes of GL and BLlooked the same. This needs further study. In cropland, fertilization increases the contentof silt-size and fine sand–size complexes and decreases the clay-size complex content.Long-term fertilization and vegetation restoration increased the OC content of all sizes ofcomplexes. Long-term application of organic manure increased the content of >20-μmcomplexes compared to NF with removal of vegetation. The NPM increased content of>20-μm complexes and SOC content, which was available for microbe decompositionand released nutrients for plant growth. Conversion of land use changed dynamics of Csequestration in soil. The contents of <20-μm complexes in GL were more than in NPMand NP, and the decomposition of OM in <20-μm complexes was slower than in >20-μmcomplexes, so the soil C sequestration was greater in GL than in NP and NPM.

Acknowledgments

This research was supported by the Knowledge Innovation Project of the Chinese Academyof Sciences (KZCX2-YW-407, KSCX2-YW-N-002) and the National Basic ResearchProgram of China (2005CB121101, 2005CB121103).

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