Dynamics of soil organic carbon and its fractions after

abandonment of cultivated wetlands in northeast China

Zhang Jinbo a, Song Changchun b,*, Wang Shenmin c

a State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences,

No. 71 East Beijing Road, Nanjing 210008, Chinab Northeast Institute of Geography and Agroecology, Chinese Academic Science, Changchun Jilin 130012, China

c Department of Resources and Tourism Sciences, Nanjing University, 210093, China

Received 23 August 2006; received in revised form 8 June 2007; accepted 11 August 2007

www.elsevier.com/locate/still

Soil & Tillage Research 96 (2007) 350–360

Abstract

Soil organic carbon (SOC) and its different labile fractions are important in minimizing negative environmental impacts and

improving soil quality. However, very little is known of the dynamics of SOC and its labile fractions after the cultivated wetlands

have been abandoned in northeast China. The objectives of this study were (1) to estimate the dynamics of SOC after the

abandonment of cultivated soil, (2) to investigate the most sensitive fraction for detecting changes in organic C due to the

abandonment of cultivated soil, and (3) to explore the key factors affecting the dynamics of soil C after the abandonment of

cultivated soil in the freshwater marsh region of northeast China. Our results showed that the abandonment of cultivated wetlands

resulted in an increase in SOC and the availability of C. The SOC content increased to 31, 44, and 107 g kg�1 after these cultivated

wetlands were abandoned for 1, 6, and 13 years, respectively, as compared to an SOC content of 28 g kg�1 in the soil that had been

cultivated on for 9 years. In northeast China, where a cultivated wetland was abandoned, the initial regeneration of SOC pools was

considerably rapid and in accordance with the Boltzmann equation. An analysis of the stepwise regression indicated that the

dynamics of SOC (g kg�1) can be quantitatively described by a linear combination of the root density and the mean soil temperature

5 cm underground in the growing season, as expressed by the following relationship: TOC = 0.008 root density �3.264T + 96.044

(R2 = 0.67, n = 9, p < 0.05. T is the mean soil temperature 5 cm underground in the growing season), indicating that approximately

67% of the variability in SOC can be explained by these two parameters. The root biomass was the key factor affecting SOC

concentration according to the observation made during the recovery of cultivated soil that was abandoned. Soil temperature

indirectly influenced the SOC concentration by affecting soil microbial activity. The abandonment of cultivated wetlands resulted in

an increase in the light-fraction organic C (LF-OC), microbial biomass C (MBC), and dissolved organic C (DOC) concentration.

The rate of increase in LF-OC was considerably higher than that in SOC and HF-OC. Similarly, the rate of increase in MBC was also

considerably higher than that in SOC in cultivated soils abandoned for 4–8 years. However, the rate of increase in DOC was far

lower than that in SOC. The R2 value for the correlation between the increments of the LF-OC and SOC was significantly higher

than that for the correlation between DOC and MBC (0.99 vs. 0.90), indicating that LF-OC was the most sensitive fraction for

detecting changes in organic C due to the abandonment of cultivated soil.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Agricultural abandonment; Labile organic C; Northeast China; Soil organic C; Wetlands

* Corresponding author.

E-mail addresses: zhangjb@mail.issas.ac.cn (Z. Jinbo),

songcc@neigae.ac.cn (S. Changchun).

0167-1987/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2007.08.006

1. Introduction

Soil organic matter (SOM) is a major reservoir of

organic carbon and is estimated at approximately

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360 351

1550 Pg, which is twice the amount of C in the

atmosphere (Raich and Potter, 1995). Since the pool of

C in the atmosphere is considerably smaller than in the

soil, a small relative change in the amount of C in the soil

will have a substantial influence on the C content in the

atmosphere (Bruce et al., 1999). Thus, an understanding

of the dynamics of SOM is fundamental to evaluating the

role of soil as a C source or sink (Lal, 2004). The change

in land use has significantly affected the carbon cycles

both regionally and globally (Kirschbaum, 2000; Lal,

2002). Much work has focused on the effects of the

conversion of natural soil to cropland or pasture on C

storage (Liu and Ma, 2000; Saggar et al., 2001; Ghani

et al., 2003; Song et al., 2004; Zhang et al., 2003).

However, very little is known of the dynamics of SOM

after agricultural abandonment. There is some evidence

that the abandonment of agriculture and the subsequent

regeneration of forests may return C storage to the

preagricultural levels although the rate of recovery

depends on the time frame one considers and whether the

land was previously used as cropland or pasture (Moraes

et al., 1996; Post and Kwon, 2000; Guo and Gifford,

2002; Templer et al., 2005). However, Gao (1997)

reported that climate was the controlling factor affecting

the dynamics of SOC after the abandonment of an

agricultural land. They reported that abandonment led to

an increase in SOC in a favorable climate that decreased

during unfavorable climate conditions.

SOC contains fractions with a rapid turnover rate as

well as fractions with a slower turnover rate (Schimel

et al., 1985). The labile fractions of organic C, such as

microbial biomass C (MBC) and dissolved organic C

(DOC), can respond rapidly to changes in C supply.

These components have therefore been suggested as

early indicators of the effects of land use on SOM

quality (Gregorich et al., 1994) and as important

indicators of soil quality. Dissolved organic matter is an

important labile fraction since it is the main energy

source for soil microorganisms a primary source of

mineralizable N, P, and S and it influences the

availability of metal ions in the soil by forming soluble

complexes (Stevenson, 1994). Soil microbial biomass is

the ‘‘eye of the needle’’ through which all organic

material that enters the soil must pass (Martens, 1995).

Soil microorganisms play a key role in the energy flows,

nutrient transformations, and element cycles in the

environment (Tate, 2000). Recently, there has been

increased interest in the importance of microbiological

properties as the indicators of change in the soil quality

(Saggar et al., 2001). However, few studies have

focused on the dynamics of labile organic C after

agricultural abandonment.

The Sanjiang Plain in northeast China is one of the

largest freshwater marsh regions and the most

extensively tilled region in China for the past 50 years.

Since the 1950s, there have been three periods of

extensive tillage in this region when approximately

3.8 Mha of land was tilled. With human interference

during the past half century, the ecosystem in the

Sanjiang Plain has changed significantly. Converting

the native wetland to agricultural soil resulted in distinct

changes in the soil water content and temperature (Song

et al., 2004) leading to a rapid decrease in the labile

organic C concentration and SOM (Zhang et al., 2006a,

2007). The losses in SOC were rapid during the initial

5–9 years of cultivation. Subsequent losses were

considerably slower (Zhang et al., 2006a). Fortunately,

since the 1990s, the government has established a new

policy that forbids the conversion of intact wetland soil

to cultivated soil and implements the abandonment of

cultivated soil. However, little knowledge exists on the

dynamics of SOC and the labile fractions of organic C

after the abandonment of cultivated wetlands in

northeast China. We hypothesized that the SOC content

increases and that the labile fractions of organic C, such

as DOC, light fraction (LF) C, and MBC, respond

rapidly to the abandonment of cultivated soil; these are

the early indicators of the dynamics of SOM following

the abandonment of cultivated wetlands.

The objectives of this study were (1) to estimate the

dynamics of soil C following the abandonment of

cultivated soil, (2) to investigate the most sensitive

fraction for detecting changes in the organic C due to

the abandonment of cultivated soil, and (3) to explore

the key factors affecting the dynamics of soil C after the

abandonment of cultivated soil in this freshwater marsh

region of northeast China.

2. Materials and methods

2.1. Site characteristics and sampling

The study site is located at the Sanjiang Mire

Wetland Experimental Station, Chinese Academy of

Sciences, Tongjiang City, Heilongjiang Province,

China, at approximately 478350N, 1338310E (Fig. 1).

The average altitude is 55.4–57.9 m. The mean annual

temperature is 1.9 8C with an average frost-free period

of 125 days. The mean annual precipitation is 550–

560 mm with the precipitation in July and August

accounting for more than 65% of the total precipitation.

In May 2003, we selected three adjacent sites within

a radius of 1 km, in which soybean (Glycine max Merr)

was planted continuously before abandonment (Fig. 1).

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360352

Fig. 1. The position of this study site in the northeast China. The sketch map of China was cited from National geomatics center of China site.

The average altitude is between 55.6 and 56 m. Site 1

was previously a wetland dominated by Deyeuxia

angustifolia (D. angustifolia); it was converted into a

farmland and cultivated upon for 9 years, which was

abandoned after sampled in May, 2003. Site 2 was a

farm field abandoned for 4 years after being in

cultivation for 10 years and previously a D. angustifolia

wetland. Site 3 was abandoned cultivated soil that

abandoned 13 years after converted D. angustifolia

wetland to cultivated soil for about 8 years. We selected

D. angustifolia-intact wetland soil neighboring the sites

1, 2, and 3 for use as reference. The parent material is

the Quaternary Period sediment at these sites. The soils

at all sites were classified as Albaquic Paleudalfs with

silty clay texture. In sites 1, 2, and 3, the sand content is

19, 23, and 25%, respectively; the silt content is 62, 60,

and 56%, respectively; and the clay content is 19, 17,

and 19%, respectively. Our previous results showed that

the physical, biological, and chemical properties of soil,

such as the amount of water-stable macroaggregate and

microaggregate, the bulk density, soil porosity, water

capacity, pH value, SOC content, soil organic N content,

DOC, MBC, and basal respiration, reached a new

equilibrium state after the conversion of natural wetland

soil to cultivated soil for approximately 9–15 years

(Zhang, 2006b). In this study, we selected three adjacent

sites within a radius of 1 km, in which soybean (Glycine

max Merr) was planted continuously before abandon-

ment (Fig. 1) and was cultivated for 8, 9, and 10 years.

Therefore, we assumed that all the three sites had the

same soil conditions with similar management history

and that all the changes in the physical, biological, and

chemical properties of soil were mainly attributable to

the duration of abandonment.

Three plots (40 m � 40 m) were arbitrarily estab-

lished at each field. For each plot, 20 cores (0–10 cm

depths) were taken in May 2003, 2004, and 2005,

respectively. Thus, we gained durations of abandon-

ment soil samples, which abandoned 0, 1, 2, 4, 5, 6, 13,

14, 15 years. Meanwhile, we randomly sampled three

soil cores to measure the bulk density and porosity. The

field-moist cores in each plot were pooled and sieved

(<2 mm) soon after collection and were split up into

two subsamples. One subsample was stored at 4 8C for

MBC and DOC analyses. The other sample was later

air-dried for SOC and density fractionation analyses.

2.2. Density fractionation

The LF was separated by flotation in a NaI solution

(1.7 g cm�3). In brief, 100 g of the sample was placed

in a 1 l beaker with 500 ml of NaI solution, gently

shaken by hand, and ultrasonicated at 400 J ml�1 with

a calibrated Vibracell VCX 600 probe-type model.

The supernatant was aspirated with a vacuum pump,

centrifuged (15 min, 3500 rpm), and filtered through a

membrane filter. The fraction recovered on the filter

was washed first with 100 ml of 0.01 M CaCl2 and

then with 200 ml of distilled water. The sediment from

the centrifuge tubes was replaced in the beakers,

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360 353

resuspended in NaI, and gently shaken by hand. The

same procedure was repeated twice as described

above. The three subfractions were combined, oven-

dried at 50 8C, and stored for analysis. This fraction

was called the LF. The sediment from the centrifuge

tubes and the beaker was the heavy fraction (HF), and

it was washed once with 0.01 M CaCl2 and approxi-

mately 10 times with distilled water, oven-dried at

50 8C, and weighed (Roscoe and Buurman, 2003). The

C concentration in the total soil and the fractions were

determined using a FLASH1112 CNS Analyzer. The C

concentration in the fractions was calculated using the

followed equation:

Pw ¼weight of fraction

weight of soil(1)

where Pw is the weight of the fraction separated from

100 g soil and the weight of soil is 100 g.

Fraction C concentration

¼ C concentration in fraction� Pw (2)

2.3. Soil MBC

The soil MBC was determined by a fumigation–

extraction method (Vance et al., 1987). The fumigated

and non-fumigated soils were extracted with 0.5 mol/l

K2SO4 by shaking at 30 rpm for 30 min (soil:extractant

ratio, 1:5), and the extracts were analyzed for C using

high-temperature combustion (TOC-VCPH analyzer,

Shimadzu, Kyoto, Japan). The MBC was calculated

using the following equation (Lu, 2000):

MBC ¼ microbial-C flush

0:38(3)

where the microbial-C flush was the C obtained from

the fumigated samples minus the C from non-fumigated

samples.

Microbial quotient ðMQÞ ¼ MBC

SOC(4)

2.4. DOC measurement

Moist soil samples (equivalent to 10 g oven-dried

weight) from the field were weighed into 40-ml

polypropylene centrifuge tubes. The samples were

extracted with 30 ml of distilled water for 30 min on an

end-over-end shaker at approximately 230 rpm and

centrifuged for 20 min at 8000 rpm. All the supernate

was filtered through a 0.45-mm filter into separate vials

for C analysis (Ghani et al., 2003). The extracts were

analyzed for C using high-temperature combustion

(TOC-VCPH analyzer, Shimadzu, Kyoto, Japan).

2.5. SOC analyses

The SOC was determined by wet combustion.

2.6. Water-stable macroaggregation

A water-stable macroaggregation of soil was

separated by passing three 25 g fragmented and air-

dried soil samples through a 0.25 mm sieve and

agitating for 60 s with a Ro-tap sieve shaker in water.

The aggregate remaining on the sieve was collected,

oven-dried at 50 8C, and weighed.

2.7. Rate of SOC increase

According to the equation in Figs. 3, 6 and 7, the rate

of increase in the organic C fraction was derived using

the following formula:

Ri ¼Ciþ1 � Ci

Ci

Ci+1 � Ci is the increment in C concentration; Ci+1 the

C concentration in the soil abandoned for i + 1 years; Ci

the C concentration in the soil abandoned for i years; Ri

is the rate of increase in organic C from i to i + 1 years.

2.8. Statistics

Statistical analysis was carried out with factor,

correlation, and regression analyses using the SPSS

software package for Windows. Figures were drawn

using the Origin 7.5 software. For all analyses where

p < 0.05, the stepwise tested values and the correlation

were considered to be statistically significant.

3. Results

3.1. Changes in vegetation, biomass, and physical

properties of soil

During the initial 1–2 years following abandonment,

the vegetation was gramineous. After 4 years, D.

angustifolia recovered completely. During the aban-

donment, there was no significant difference in the

aboveground biomass. However, the root density

increased significantly. The thickness of the litter layer

also increased significantly (Table 1).

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360354

Table 1

Changes of vegetation, litter layer and biomass after abandonment of cultivated soil in the freshwater marsh region of northeast China

Time of abandonment (a) Vegetation Litter

layer (cm)

Aboveground

biomass (g/m2)

Roots density

(g/m2)

1 Gramineous 0 501.9 � 43.3 30.6 � 5.3

2 Gramineous, associated Deyeuxia angustifolia 0.1 � 0.1a 531.0 � 33.1 50.9 � 10.6

4 Deyeuxia angustifolia 1.5 � 0.5 430.5 � 50.8 1589.8 � 412.0

5 Deyeuxia angustifolia 2.0 � 0.9 410.6 � 34.0 2757.9 � 398.3

6 Deyeuxia angustifolia 2.2 � 0.7 420.6 � 43.7 2989.0 � 408.0

13 Deyeuxia angustifolia, associated Salicifolia 5.5 � 1.4 561.9 � 65.0 4364.1 � 591.5

14 Deyeuxia angustifolia, associated Salicifolia 5.7 � 1.3 565.5 � 43.6 4265.8 � 664.8

15 Deyeuxia angustifolia, associated Salicifolia 6.1 � 1.1 552.3 � 72.5 4293.7 � 679.4

Intact Deyeuxia

angustifolia marsh

Deyeuxia angustifolia, associated Salicifolia 6.5 � 1.3 530.8 � 71.6 7319.1 � 1376.7

a The averages and standard deviation in the bracket (n = 6).

After the abandonment of cultivated soil, the amount

of water-stable macroaggregate significantly increased

(Fig. 2a). The bulk density decreased significantly

(Fig. 2b). In the soil abandoned for 6 and 15 years, the

bulk density decreased to 0.91 and 0.66 g cm�3 (vs. 1.07

g cm�3 in the cultivated soil), respectively. The soil poro-

sity and water capacity increased clearly (Fig. 2c and d).

Fig. 2. Changes of soil physical properties after abandonment of cultivated

figures are means of three replicates; bars represent standard deviation.

3.2. Dynamics of SOC after abandonment of

cultivated soil

The SOC content increased to 31, 44, and 107 g kg�1

after the abandonment of soil for 1, 6, and 13 years,

respectively, as compared with the SOC content of

28 g kg�1 in the soil cultivated for 9 years. It increased

soil in the freshwater marsh region of northeast China. The points in

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360 355

Fig. 3. Dynamics of SOC and ratio of total organic C to total nitrogen after abandonment of cultivated soil in the freshwater marsh region of

northeast China. SOC is soil organic C, SOC/TN is ratio of total organic C to total nitrogen. The points in figures are means of three replicates; bars

represent standard deviation.

significantly after the abandonment of cultivated soil in

accordance with the Boltzmann equation (Fig. 3). The

ratio of the total organic C to total nitrogen also

increased with the period of abandonment (Fig. 3b).

Fig. 4. Changes of light and heavy fraction C after abandonment of cultiva

light fraction organic C, HFOC is heavy fraction organic C. The point

deviation.

3.3. Changes of light and heavy fraction C

The abandonment of cultivated soil resulted in a

rapid increase in the light-fraction organic C (LF-OC)

ted soil in the freshwater marsh region of northeast China. LFOC is

s in figures are means of three replicates; bars represent standard

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360356

Fig. 5. Rate of increase of labile organic C fractions after abandonment of cultivated soil in the freshwater marsh region of northeast China. SOC is

soil organic C, LF-OC is light fraction organic C, HF-OC is heavy fraction organic C, DOC is dissolved organic C, MBC is microbial organic C.

These are modeled curves.

and heavy-fraction organic C (HF-OC) concentrations

and the SOC concentration (Fig. 4a and b). The LF-OC

and HF-OC concentrations increased to 5 and

32 g kg�1, and 8 and 36 g kg�1 after being abandoned

for 2 and 6 years, respectively, as compared with the LF-

OC and HF-OC concentrations of 3 and 24 g kg�1,

respectively, in the soil cultivated for 9 years. Similar to

the increase in SOC, the LF-OC and HF-OC

Fig. 6. Dynamics of dissolved organic C after abandonment of cultivated soil

C, DOC is dissolved organic C. The points in figures are means of three r

concentrations increased in accordance with the

Boltzmann equation after the abandonment of culti-

vated soil.

However, the rate of increase in LF-OC concentra-

tion was considerably higher than that of the SOC and

HF-OC fractions ( p < 0.001) (Fig. 5a). Further, the rate

of increase in the HF-OC concentration was much lower

than that of the SOC concentration ( p < 0.001).

in the freshwater marsh region of northeast China. SOC is soil organic

eplicates; bars represent standard deviation.

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360 357

Therefore, LF-OC was the most sensitive fraction and

the most appropriate for detecting changes in organic C

due to the abandonment of cultivated soil.The C

accumulated in the HF ranged between 69 and 90%

(Fig. 4d). The highest proportion was observed in the

cultivated soil, and the proportion decreased signifi-

cantly after the abandonment of the cultivated soil.

After 15 years of abandonment, the proportion

decreased to approximately 69%. In contrast, the LF

showed a different pattern (Fig. 4c). Approximately

11% of the total C was in the LF in the soil cultivated for

10 years. Upon abandonment, this proportion increased

significantly. After 15 years of abandonment, the

proportion increased to approximately 31%.

3.4. Dynamics of DOC

Abandonment of the cultivated soil resulted in an

increase in DOC and SOC concentrations (Fig. 6a). The

DOC concentration increased to 181, 212, and

355 mg kg�1 after abandonment of the soil for 2, 6,

and 13 years, respectively, as compared with the DOC

concentration of 150 mg kg�1 in the soil cultivated for 9

years. However, the rate of increase in the DOC

concentration was far lower than that of the SOC

concentration after the abandonment of the cultivated

soil ( p < 0.05) (Fig. 5c). Therefore, the ratio of

concentrations of DOC to SOC decreased obviously

after the abandonment of the cultivated soil (Fig. 6b).

3.5. Changes in the concentration and activity of

microbial biomass C

Abandonment of the cultivated soil resulted in an

increase in MBC (Fig. 7a). The MBC concentration

increased to 660, 1378, and 2905 mg kg�1 after

abandonment of the soil for 2, 6, and 13 years,

Fig. 7. Dynamics of microbial properties after abandonment of cultivated so

biomass C, MQ is microbial quotient. The points in figures are means of t

respectively, as compared with the MBC concentration

of 500 mg kg�1 in the soil cultivated for 9 years. During

the initial 1–2 years of abandonment, the rate of

increase in the MBC concentration was lower than that

in the SOC concentration. The rate of increase in the

MBC concentration was considerably higher than that

in the SOC concentration ( p < 0.05) during the 3–9

years of abandonment. The subsequent rate of increase

in the MBC concentration was much lower than that in

the SOC concentration ( p < 0.05) (Fig. 5d). Therefore,

during the initial 1–2 years of abandonment, the

changes in microbial quotient were not significant.

During the 3–8 year abandonment, the microbial

quotient increased significantly. Subsequent increase

was slow (Fig. 7b).

4. Discussions

4.1. Dynamics of soil C after abandonment of

cultivated soil

Our results revealed that SOC and the labile fraction

C contents could rapidly increase, after abandoned

cultivated soil and regenerating wetland soils in the

freshwater marsh region of northeast China. Guggen-

berger and Zech (1999) also reported that the plentiful

input of organic matter to the soil resulted in LF-OC

concentration rapidly increase in abandoned pasture

soil and regenerating forest soils.

During the 3–9 year abandonment, the increase rate

of microbial biomass C was much higher than that of

SOC ( p < 0.05). Generally, if a soil being cultivated,

the microbial quotient will decrease, whereas the

reverse is true in soils with enlarging SOC pool. The

changes of microbial quotient indicated accumulation

of SOC and the increase in the availability of organic C

after cultivated soil abandoned. The increase in MBC

il in the freshwater marsh region of northeast China. MBC is microbial

hree replicates; bars represent standard deviation.

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360358

and microbial quotient were indicative of a shift in the

state of equilibrium of the abandoned soil system.

Saggar et al. (2001) also reported that microbial

biomass C and microbial quotient rapidly increase in

abandoned pasture soil.

The dissolution of microbial lysates (Kaiser and

Zech, 2001) and plant root exudates (Yano et al., 2000)

can increase the amount of DOC. Our results showed

the relationship of DOC with MBC, the thickness of

the litter layer, and the root density (R2 = 0.99, 0.58,

0.62, respectively) (Table 2). The rate of increase in

the DOC concentration was considerably lower than

that in the SOC concentration. The microbial

community increased rapidly, and the microbes divert

ample C to the new microbial biomass, causing soil C

sequestration in the microbes; this results in a lower

increase in the DOC during the recovery of the

cultivated soil.

When the intact D. angustifolia wetland is used as a

reference in this region, the time required for the

abandoned sites to completely recover their SOC and

C pools in their fractions can be estimated using

the regression equation (Figs. 3, 6 and 7). It would

require approximately 40, 60, 30, 35, and 30 years

from the time of abandonment to recover their total

organic C, ratio of total organic C to total nitrogen,

LF-OC concentration, DOC, and MBC pools, respec-

tively, to the levels that existed before interference.

Rhoades et al. (2000) and Templer et al. (2005) reported

that it would require approximately 9 and 20 years,

respectively, from the time of abandonment by pasture

sites to recover the SOM pools in tropical forests to

the levels that existed before interference; these values

were lower than those obtained in our results. However,

Knops and Tilman (2000) predicted that it would

require 180 and 230 years, respectively, to reach 95% of

Table 2

Relationship between the measured variables

Bd P Wc LFC DOC

Ma 0.96 0.92 0.91 0.98 0.99

Bd 0.94 0.97 0.95 0.97

P 0.93 0.89 0.91

Wc 0.88 0.93

LFC 0.99

DOC

MBC

L

Ab

R

Ma presents water-stable macroaggregate; Bd presents bulk density; P prese

organic C; DOC presents dissolved organic C; MBC presents microbial biom

roots concentration; T presents the mean temperature below ground 5 cm i

the preagricultural amounts of soil N and C in

Minnesota (458240N, 938120W).

4.2. The factors affecting dynamics of SOC

In order to determine the factors affecting the

dynamics of SOC, we applied a stepwise regression

method (SPSS) to determine the relationship of SOC

with the various variables measured. We hypothesized

that the various variables measured were correlated

among themselves (multicollinearity). In order to

reduce multicollinearity, a cross-correlation analysis

was first applied. It showed that there were obvious

linear correlations between water-stable macroaggre-

gate, bulk density, porosity, water capacity, LF-OC,

DOC, MBC, and basal respiration (R2 ranged from 0.88

to 0.99, Table 2). A one-to-one correlation between the

increments in water-stable macroaggregate, bulk

density, porosity, water capacity, LF-OC, DOC,

MBC, and basal respiration was also significant

(Table 3). These clearly indicated a multicollinearity

problem. Hence, these variables were excluded, and the

litter, aboveground plant biomass, root density, and the

mean temperature 5 cm underground in the growing

season were selected to apply a stepwise regression. An

analysis of the stepwise regression indicated that the

dynamics of SOC (g kg�1) can be quantitatively

described by a linear combination of root density and

mean soil temperature 5 cm underground in the growing

season, as expressed by the following relationship:

TOC ¼ 0:008 Root density� 3:264T

þ 96:044 ðR2 ¼ 0:67; n ¼ 9; p< 0:05Þ:

T is the mean soil temperature 5 cm underground in the

growing season.

MBC L Ab R T

0.98 0.56 0.16 0.58 0.70

0.97 0.74 0.20 0.73 0.70

0.94 0.63 0.30 0.55 0.72

0.95 0.79 0.26 0.73 0.57

0.98 0.54 0.13 0.62 0.62

0.99 0.58 0.17 0.62 0.64

0.64 0.23 0.65 0.77

0.32 0.86 0.37

0.11 0.55

0.55

nts porosity; Wc presents water capacity; LFC presents light fraction

ass C; L presents litter; Ab presents aboveground biomass; R presents

n the growing season. The values in the table were R2.

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360 359

Table 3

Relationship between increments of the measured variables

DMa DBd DP DWc DLFC DDOC DMBC DL DAb DR

DSOC 0.89 0.86 0.61 0.55 0.99 0.90 0.90 0.25 0.12 0.12

DMa 0.89 0.70 0.73 0.89 0.99 0.90 0.13 0.13 0.01

DBd 0.88 0.73 0.86 0.84 0.84 0.24 0.09 0.06

DP 0.84 0.59 0.64 0.70 0.32 0.10 0.08

DWc 0.55 0.68 0.71 0.24 0.25 0.02

DLFC 0.91 0.91 0.24 0.12 0.11

DDOC 0.92 0.15 0.14 0.03

DMBC 0.38 0.31 0.15

DL 0.42 0.82

DAb 0.15

DSOC presents increment of soil organic C; DMa presents increment of water-stable macroaggregate; DBd presents increment of bulk density; DP

presents increment of porosity; DWc presents increment of water capacity; DLFC presents increment of light fraction organic C; DDOC presents

increment of dissolved organic C; DMBC presents increment of microbial biomass C; DL presents increment of litter; DAb presents increment of

aboveground biomass; DR presents increment of roots concentration. Increment of the measured variables = Vi+1 � Vi, Vi+1 is values of variables in

i + 1 years, and Vi is values of variables in i years.

The R2 value of 0.67 indicates that approximately

67% of the variability in SOC can be explained by these

two parameters. The coefficient of 0.008, on the other

hand, suggests that the SOC content will be increased

by 0.008 g with an increase in the root biomass of 1 g.

The root biomass was the key factor affecting the SOC

concentration during the abandonment of cultivated soil

toward recovery. The soil temperature indirectly

influenced the SOC concentration via the soil microbial

activity.

The organic C content in soils depends on the

balance between the C input and the decomposition

rates (Saggar et al., 2001; Huang et al., 2002). After the

abandonment of cultivated soil, the input of matter to

the soil increased due to the disappearance of

cultivation and vegetation restoration (Zhang et al.,

2003; Song et al., 2004). The recovery of the water-

stable macroaggregate may have protected the organic

matter against microbial decomposition and facilitated

the organic C accumulation. SOM accumulation is

conducive for the recovery of physical properties and

facilitates the improvement of the soil structure. It

would require approximately 60 years from the time of

abandonment to recover the nitrogen pools that existed

before interference; this time period is longer than that

required to recover total SOC. Nitrogen is the limiting

factor during the recovery of cultivated soil. Knops and

Tilman (2000) obtained the same results.

Our result showed that it would take approximately

40 years from the time of abandonment to recover the

total organic C levels that existed before interference.

However, Knops and Tilman (2000) predicted that it

would take 180 and 230 years, respectively, to recover

95% of the amounts of soil N and C that existed before

agriculture in Minnesota (458240N, 938120W). Our

study site lies at 478350N, 1338310E, at the latitude

identical to that in the study sites of Knops and Tilman.

The vegetation at both the sites was herbage, and the

climate at these sites is similar. The soil texture is very

different. In Minnesota, the soil is sandy and the sand

content ranged from 92 to 97%; further, the clay content

is very low, which is not favorable for the accumulation

of organic matter. In our study site, the soil is silty clay.

The high clay and silt contents protect the organic

matter against decomposition, favoring their accumula-

tion. Therefore, the soil texture appears to be an

important factor that affecting the time required for

SOC recovery.

The R2 value in correlation between the increments

in the LF-OC and SOC concentrations was significantly

higher than DOC and MBC concentrations (0.99 vs.

0.90) and other variables, indicating that the LF-OC was

the most sensitive fraction for detecting changes in

organic C due to the abandonment of cultivated soil.

Freixo et al. (2002), Swanston et al. (2002), and Roscoe

and Buurman (2003) also reported that the LF-OC

fraction was the most dynamic and sensitive to the

changes in organic C.

5. Conclusion

Our results demonstrated that the abandonment of

the cultivated soil resulted in an increase in the SOC

content and the availability of organic C in the

freshwater marsh region of northeast China. Approxi-

mately 67% of the variability in SOC can be explained

by the root density and the mean soil temperature 5 cm

underground during the growing season. The root

Z. Jinbo et al. / Soil & Tillage Research 96 (2007) 350–360360

biomass was the key factor affecting the SOC

concentration during the abandonment of cultivated

soil for recovery. The soil temperature indirectly

influenced the SOC concentration by affecting the soil

microbial activity. LF-OC was the most sensitive

fraction for the detection of the changes in organic C

following the abandonment of cultivated soil. This

study suggested that in the freshwater marsh region of

northeast China where cultivated soil was being

abandoned, the initial regeneration of soil C pools

was considerably rapid, but further work is required to

address the long-term rate of recovery in this region.

Acknowledgements

This work was founded by National Natural Science

Foundation of China (40471124, 40431001), Chinese

Academy of Sciences. Thanks the reviewers for their

times and advices very much.

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