dynamics and stabilization of soil organic carbon after nineteen years of afforestation in...

Dynamics and stabilization of soil organic carbon afternineteen years of afforestation in valley-type savannah insouthwest China

G. TANG1,2, K. LI

1 ,2, Y. SUN1,2 & C. ZHANG

1,2

1Research Institute of Resource Insects, Chinese Academy of Forestry, Kunming 650224, China, and 2Yuanmou Desertification

Ecosystem Research Station, State Forestry Administration of China, Kunming 650224, China

Abstract

Serious concerns about carbon (C) sequestration capacity and the stabilization of sequestered C in

forested soils have emerged in the context of global climate change. The organic C in soil and in soil

fractions at four sampling times in Acacia auriculiformis plantations afforested in 1991 were

investigated with a combination of density fractionation and acid hydrolysis techniques. The results

showed that the accumulation of C in the forested soils had accelerated because the afforestation of

wasteland with A. auriculiformis. The C accumulation rates of the surface and subsurface soils

averaged 0.38 and 0.17 t C/ha/yr, respectively, during the 19 yr following the afforestation. The

percentage of organic C in heavy fraction relative to total soil organic C at the surface soil was 71%

in 2003. This value was significantly (P < 0.05) higher than that in 2010 (68%). The chemical

recalcitrant C index of light fraction was significantly (P < 0.05) higher than that of heavy fraction in

2003 regardless of soil depth, but both decreased with time. ca. 58–68% of the newly sequestered C

was protected by physical mechanism, and 41–50% was transferred into the acid nonhydrolysable

fraction during the 12–19 yr after the trees were planted. The chemical stability of the physically

protected C remained lower than that of the unprotected C following the afforestation in the valley-

type savannah. However, both the stability values showed a decline with time.

Keywords: Soil organic carbon, carbon dynamics, carbon stabilization, afforestation, soil fractionation,

valley-type savannah, dry-hot valley

Introduction

The estimates of the total global soil organic C (SOC) in the

top one meter are converging on a value of ca. 1500 Gt C

(Eswaran et al., 1993; Batjes, 1996). This value is ca. 2.6

times that in the biotic pool and twice that in the

atmospheric pool. Consequently, the SOC pool has the

potential to greatly impact the global climate by acting as

either a source or a sink of atmospheric CO2 (Kirschbaum,

2000).

The effects of afforestation or reforestation on soil C

pools varied. Previous studies have reported accumulation

(Trouve et al., 1994; Lopez-Ulloa et al., 2005; Niu & Duiker,

2006; Gr€unzweig et al., 2007; Laik et al., 2009), losses

(Lopez-Ulloa et al., 2005; Richards et al., 2007), or no net

change (Epron et al., 2009; Marin-Spiotta et al., 2009),

resulting from the establishment of trees. Most studies have

focused on the quantity of SOC occurring in plantations.

However, information about this quantity may not be

sufficiently comprehensive to provide an understanding of

the dynamics of C quality, such as C stabilization. Soil

organic C is protected by a variety of physical and chemical

mechanisms, which produce C pools with different residence

times. Soil C sequestration is defined as a biotic process,

whereby the atmospheric CO2 is transferred into long-lived

soil C pools (Lal, 2004). Changes in total SOC with changes

in land use or management can be explained, in part, by the

way in which C is allocated to different fractions of soil

organic matter. A number of physical and chemical methods

have been developed to separate the bulk pool into fractions

with different chemical composition and/or location in the

soil matrix (e.g. Janzen et al., 1992; Rovira & Vallejo, 2002;

Silveira et al., 2008). Soil organic matter is a complex of

organic materials with different C components like

polysaccharides, lipid, lignin, humus, charcoal and so on.Correspondence: K. Li. E-mail: [email protected]

Received March 2012; accepted after revision November 2012

48 © 2013 The Authors. Journal compilation © 2013 British Society of Soil Science

Soil Use and Management, March 2013, 29, 48–56 doi: 10.1111/sum.12015

SoilUseandManagement

Some C components present in soil, such as polysaccharides,

are considered chemically ‘labile’ because they are easily

utilized and then degraded by soil microorganisms when

they are not protected by physical processes. Therefore, the

labile fraction is expected to respond most rapidly to

environmental changes. Some chemical recalcitrant

compounds accumulate in soil as the decomposition of soil

organic matter or exotic organic materials and are of

important for C sequestration owing to their long residence

times (Rovira & Vallejo, 2002; Sollins et al., 2006; Silveira

et al., 2008). Soil organic compounds could be fractionated

in groups with similar chemical recalcitrance by different

acidity and/or solution temperatures via acid hydrolysis

approaches (Rovira & Vallejo, 2002; Swanston et al., 2005;

Wang et al., 2005; Sollins et al., 2006; Silveira et al., 2008).

Valley-type savannah is a unique type of savannah that

develops in river valleys, such as those of the rivers in the

Hengduan Mountains, southwest China, and the total area

of this type is ca. 3.0 9 106 ha in China (Jin, 2002). The

plant community developed in Yuanmou County (25°23′–26°06′N, 101°35′–102°06′E), a dry-hot valley of the Jinsha River

(the upper reach of the Yangtze River), is a representative

example of this savannah type (Zhang, 1992; Jin, 2002). The

quantity of SOC under various land uses in these dry-hot

valleys has been investigated (He et al., 1997; Guo et al.,

2007; Tang et al., 2010), but there has been no study of the

C sequestration capacity or the stabilization of the

sequestered C in soils following afforestation. In dry-hot

valleys of China, many areas of valley-type savannah are

experiencing afforestation or reforestation. Acacia

auriculiformis has been introduced and planted widely in

valley-type savannah during the last 20 yr for their multiple

uses, such as water and soil conservation.

The objectives of this study were to investigate (i) C

dynamics, (ii) the physical and chemical stability of SOC,

and (iii) the stabilization of sequestered C in forested soil

during 19 yr of afforestation.

Materials and methods

Study sites

The study was conducted in Yuanmou County, Yunnan

Province, southwest China. The mean temperature of the

study area is 21.9 °C, with a mean maximum of 27.1 °C in

May and a mean minimum of 14.5 °C in December (data

from Yuanmou Meteorological Station over 60 yr since

1950). The mean annual precipitation is 634 mm, ca. 92% of

which falls from June through October. The annual potential

evaporation is 3911 mm, ca. 6.2 times the precipitation. The

relative mean annual humidity is 53%. The vegetation in the

county (below 1600 m a.s.l.) consists of grasses and shrubs

with dispersed single trees or clusters of trees. The dominant

grass species are Heteropogon contortus, Bothriochloa pertusa,

Eulalia speciosa, Cymbopogon distans and Imperata cylindica,

and the native woody plants are Phyllanthus emblica,

Dodonaea viscose, Sophora viciifolia, Terminalia franchetii,

Quercus franchetii, Acacia farnesiana and Vitex negundo. The

typical soils are classified as Ferralic Arenosols according to

the FAO Taxonomy (FAO-UNESCO, 1988).

The plantations investigated in this study were located in

the forest region of the Yuanmou Desertification Ecosystem

Research Station, State Forestry Administration of China

(25°40′N and 101°51′E, 1100–1120 m a.s.l.) in Yuanmou

County. In 1991, we selected a 25-ha area of wasteland on a

barren hill for study, which is typical of the valley-type

savannah. The wasteland had not been cultivated or planted

as planting industry for at least 20 yr, and the site

conditions of the land were relatively uniform. The

wasteland was located at a mid-slope landscape position

with a slope gradient of ca. 7–12° and was experienced light

erosion owing to water flow (Tang et al., 2010). The native

vegetation was H. contortus with sparse D. viscose and

P. emblica. The vegetation was disturbed by human

activities, such as grazing and mowing for livestock, which is

a local custom for the heavy human population pressure in

this region. Six multiple-purpose tree species were planted in

the wasteland at a stocking density of 1333–2000 trees per

ha in May–June 1991. The experiment was arranged in a

randomized block design with six treatments replicated five

times. Each replicated plot covered an area of 0.81 ha. All of

these plantations were protected from anthropogenic

disturbances by barbed-wire fences and did not show any

replacement of existing vegetation by any species during

their growth period. Only data for A. auriculiformis

plantations are reported in this study, for not all data about

the other five plantations were collected. The trees grow well

and could naturally regenerate, in part, after 15 yr of

afforestation. To obtain soil basic properties, soil samples

were randomly collected from each of the replicated plots, of

the future forestland, in 1991 (Table 1).

One permanent quadrat with an area of 400 m2

(20 m 9 20 m) was established in each replicated plot in

May 1996 for long-term investigation and sampling,

including soil and litter sampling. In total, five permanent

quadrats were established in A. auriculiformis plantations.

Sample collection

In our research, soil samples were collected four times, and

the sampling times were in April 1991, May 1997, April 2003

and May 2010. The litter horizon was removed prior to soil

sampling. Soil samples from the five replicated plots in 1991

or five permanent quadrats in 1997, 2003 and 2010 were

collected randomly using a stainless cylinder (5 cm diameter)

from two depths (0–15 and 15–30 cm). One soil composite

sample representing each replication was prepared by mixing

12–15 undisturbed soil cores from the respective soil depths

© 2013 The Authors. Journal compilation © 2013 British Society of Soil Science, Soil Use and Management, 29, 48–56

Dynamics and stabilization of soil organic carbon 49

within each plot or quadrat. In total, five composite samples

(one from each plot or quadrat) were collected from each

depth interval. One bulk density sample was taken using a

soil core sampler for each depth from each plot or quadrat.

After the removal of visible plant residues by hand, the soil

samples were air-dried, sieved through a 2-mm mesh and

then stored at 4 °C. The subsamples were ground through a

0.25-mm mesh prior to the determination of SOC.

Litter samples were collected biweekly from five randomly

located baskets over a period of 12 months every 3 yr in

each of the five permanent quadrats beginning in May 1996.

The samples were oven-dried at 70 °C for the calculation of

dry mass of litter fall. Up to 2010, annual litter fall in

A. auriculiformis plantations was measured five times

(Fig. 1).

Laboratory analysis

The physical and chemical fractions of the soil samples

collected in 2003 and 2010 were isolated with a combination

of density fractionation and acid hydrolysis techniques.

Because fractionation methods were not developed at that

time, especially acid hydrolysis techniques, soil samples

collected in 1991 and 1997 were not fractionated. Two

density fractions, the light fraction (LF) and heavy fraction

(HF), were separated from the bulk soil following a modified

version of Janzen et al. (1992). Briefly, 25 g of air-dried soil

was placed in a preweighed, 250-mL centrifuge bottle with

80 mL of NaCl solution adjusted to a density of 1.7 g/mL

and gently shaken by hand for ca. 30 s to form a

suspension. The suspension was sonicated for 10 min at

200 J/mL and then centrifuged for 10 min at 4200 r/min at

room temperature (ca. 25 °C). Floating material was

siphoned from the centrifuge bottle and then rinsed with

deionized water on Whatman G/F filter paper. This

procedure was repeated three times. The material collected

on the filter paper, designated as the LF, was washed with

deionized water into a preweighed container. The residue

remaining in the centrifuge bottle (HF) was rinsed repeatedly

by adding deionized water, sonicated and centrifuged.

To quantify the chemical stability of the C in the density

fractions (LF and HF), an acid hydrolysis approach

modified from Rovira & Vallejo (2002) was used. In brief,

an aliquot of the separated density fractions (1000 mg for

HF, 200 mg for LF) was hydrolysed in a 50-mL sealed

Pyrex centrifuge tube with 1 mL of 2.5 M H2SO4 in a boiling

water bath for 30 min. The hydrolysate was recovered by

centrifugation and siphoning. The residue was rinsed with

20 mL of deionized water. The material in the rinse water

was recovered by centrifugation and added to the

hydrolysate. The unhydrolysed residue was dried at 60 °C.An aliquot of 13 M H2SO4 (2 mL) was added to the tube

and placed in an end-over-end shaker overnight. After the

acid was diluted to 1 M with deionized water, the residue

was hydrolysed for 3 h in a boiling water bath. The

hydrolysate was recovered as described earlier. After being

washed, the residue was the acid nonhydrolysable fraction

(ANF), taken as chemical recalcitrance (Wang et al., 2005).

Table 1 Soil properties in Acacia auriculiformis plantations at different sampling times

Soil depth (cm)

Sampling

time

Soil bulk

density (g/cm3)

Mass ratio

of soil particles

(>2 mm) to soil Soil pH SOC (g/kg) Soil N (g/kg)

0–15 1991 1.68 a 0.047 a 5.99 a 2.82 � 0.202 d 0.22 � 0.015 d

1997 1.67 a 0.048 a 6.14 a 3.26 � 0.272 c 0.25 � 0.020 c

2003 1.62 a 0.046 a 6.25 a 4.45 � 0.293 b 0.36 � 0.025 b

2010 1.59 a 0.047 a 6.24 a 6.15 � 0.464 a 0.52 � 0.037 a

15–30 1991 1.72 a 0.047 a 6.20 a 2.73 � 0.116 c 0.21 � 0.008 c

1997 1.72 a 0.047 a 6.37 a 2.88 � 0.142 c 0.22 � 0.011 c

2003 1.69 a 0.046 a 6.40 a 3.42 � 0.154 b 0.27 � 0.011 b

2010 1.68 a 0.046 a 6.42 a 4.11 � 0.207 a 0.33 � 0.015 a

Data are mean � standard deviation of five replicates, and different letters indicate significant differences for the means in the same columns for

each given soil depth according to time series analysis (LSD test, P < 0.05).

0

1

2

3

4

1996

-199

7

1999

-200

0

2002

-200

3

2005

-200

6

2008

-200

9

Investigation time

Litte

r fa

ll in

sta

nd (

t/ha/

yr)

Figure 1 Litter fall in Acacia auriculiformis plantations at different

sampling periods. Error bars represent the standard deviation of

replicates (n = 5).


50 G. Tang et al.

All soil fractions (LF, HF and ANFs, see Table 2) from

each separation method were oven-dried at 60 °C to a

constant weight and then weighed. These fractions were

pulverized to pass through a 0.25-mm mesh and analysed for

C concentration using a VarioEL elemental analyser (Vario-

MAX C/N, Elemental Co., Germany).

Soil bulk density was determined using the method

proposed by Blake & Hartge (1986). Soil pH was measured

with a combination electrode (soil-to-water ratio 1:2). Soil

organic C and total N were determined by a dry combustion

method at ca. 630 °C using a VarioEL elemental analyser.

Data analysis and statistics

In this study, we defined the difference of soil C densities at

two sample times as newly sequestered C. For example, the

subtraction of SOC in 2010 to that in 2003 within the same

quadrat could be considered as newly sequestered C during

2003–2010. In this study, we only consider the net change in

SOC density at two times and do not consider the turnover

of all organic C including the ‘original’ organic C and the

newly sequestrated C during the study period.

The five composite samples for each depth increment were

used as replicates to analyse the various soil parameters. All

data were tested for normality and homogeneity of variance.

Significant differences in the soil parameters among the

different sampling times were tested by time series analysis

(LSD test) because samples from the replicated plots or the

permanent quadrats at different time intervals are not

independent from each other. A paired samples t-test was

applied to analyse the difference in the recalcitrant C index

between the LF and HF for each given sampling time.

A statistical software package (SPSS 16.0) was used.

Results

Dynamics of soil organic C

Plantation ages had significant (P < 0.01) effects on the SOC

density of the surface (0–15 cm) and subsurface (15–30 cm)

soils except for the differences in SOC density between 1991

and 1997 (Fig. 2a). In 2010, SOC densities of the surface

and subsurface soils averaged 13.98 and 9.87 t C/ha,

respectively. These values represented increases of 7.19 and

3.15 t C/ha, respectively, compared with the initial values for

the former wasteland in 1991.

Afforestation by A. auriculiformis increased the SOC

density of the soils (Fig. 2). During the entire study period

(i.e. from 1991 to 2010), the average C accumulation rates of

the afforested soils were 0.38 and 0.17 t C/ha/yr at the

surface and subsurface soil layers, respectively (Fig. 2b).

SOC density of the surface soil increased at an average rate

of 0.17 t C/ha/yr during the initial 6 yr after the trees were

planted (i.e. from 1991 to 1997) and then showed improved

rates of 0.43 and 0.53 t C/ha/yr during 1997–2003 and 2003–

2010, respectively. The dynamics of SOC at the subsurface

soil generally resembled the dynamics of the surface soil, but

the increase rates were equivalent to 41–48% of the

corresponding rates at the surface soil at any given sampling

time (Fig. 2b).

Distribution of organic C in density fractions

The organic C in both LF and HF (LF-C and HF-C)

increased significantly (P < 0.05) during 2003–2010

(Table 3). For example, the LF-C density of the surface soil

Table 2 Parameters used in the text and their calculations

Parameters Calculation

Dry mass ratio of

HF to soil (%)

= dry mass of HF (g)/mass

of tested soil (g) 9 100

Dry mass ratio of

LF to soil (%)

= dry mass of LF (g)/mass of

tested soil (g) 9 100

HF-C content (g/kg) = dry mass ratio of HF to soil

(%) 9 C concentration in HF

(g/kg)/100

LF-C content (g/kg) = dry mass ratio of LF to soil

(%) 9 C concentration in LF

(g/kg)/100

HF-C/SOC ratio (%) = HF-C density (t C/ha)/SOC density

(t C/ha) 9 100

LF-C/SOC ratio (%) = LF-C density (t C/ha)/SOC density

(t C/ha) 9 100

Dry mass ratio of

ANF to HF (%)

= dry mass ratio of ANF in HF

(g)/mass of tested HF (g) 9 100

Dry mass ratio of

ANF to LF (%)

= dry mass ratio of ANF in LF

(g)/mass of tested LF (g) 9 100

HF-ANC content (g/kg) = day mass ratio of ANF to HF

(%) 9 C concentration in ANF

(g/kg)/100

LF-ANC content (g/kg) = dry mass ratio of ANF to LF

(%) 9 C concentration in ANF

(g/kg)/100

IANC of HF = HF-ANC density (t C/ha)/HF-C

density (t C/ha) 9 100

IANC of LF = LF-ANC density (t C/ha)/LF-C

density (t C/ha) 9 100

C density in soil

(or in fractions) (t C/ha)

= d 9 h 9 c 9 (1-d)/10, where, d is soil

depth (cm), h is soil bulk density

(g/cm), c is C content in soil

(or in fractions) (g/kg), d is mass

ratio of soil particles (>2 mm) to

soil.

HF, heavy fraction; LF, light fraction; HF-C, organic C in heavy

fraction; LF-C, organic C in light fraction; SOC, soil organic C;

ANF, acid nonhydrolysable fraction; ANC, organic C in acid

nonhydrolysable fraction; HF-ANC, acid nonhydrolysable C in

heavy fraction; LF-RC, acid nonhydrolysable C in light fraction;

ICRC, chemical recalcitrant C index.



in 2010 increased by 1.45 t C/ha compared with that in

2003. The dry mass of LF constituted <0.80% of the dry

mass of the surface soil and ca. 0.25% of the subsurface soil.

However, the LF-C density was � 23.34–30.11% of the total

SOC. The dry mass ratio of LF to soil, the LF-C density,

the HF-C density and the LF-C/SOC ratio all increased

significantly (P < 0.05) with time, whereas the HF-C/SOC

ratio decreased significantly (P < 0.05). The dry mass ratio

of HF to soil showed no obvious change during 2003–2010

(P = 0.248).

Acid nonhydrolysable C in density fractions

The soil samples collected in 2003 and 2010 were analysed to

determine the acid nonhydrolysable C (ANC) density in the

density fractions and the chemical recalcitrant C index

(ICRC) (Table 4). The ANC density in LF (LF-ANC) varied

substantially from 1.05 t C/ha at the subsurface soil in 2003

to 2.28 t C/ha at the surface layer in 2010. The ANC density

in HF (HF-ANC) ranged from 3.21 t C/ha at the subsurface

layer in 2003 to 5.04 t C/ha at the surface layer in 2010. The

LF-ANC and HF-ANC densities of the surface soil in 2010

were both significantly (P < 0.05) higher than those in 2003.

Similarly, the LF-ANC and HF-ANC densities of the

subsurface soil also increased significantly (P < 0.05) during

2003–2010. Irrespective of sampling time, the LF-ANC

density was significantly (P < 0.05) lower than the HF-ANC

density. For example, the LF-ANC density was equivalent to

45.24% of the HF-ANC density at the surface soil in 2010.

The percentage of total ANC relative to SOC declined

slightly with the plantation age (Table 4).

The ICRC of LF at the surface and subsurface layers in

2010 averaged 54.16 and 52.30, respectively (Table 4). These

values were slightly higher than those of HF at the

corresponding layer in 2010. At both soil depths, however,

the ICRC of LF was significantly (P < 0.05) higher than

indices of HF in 2003. As the age of the trees increased, the

ICRC of LF decreased significantly (P < 0.05), whereas no

significant reduction in the ICRC of HF was observed

(Table 4).

Discussion

C sequestration in afforested soils

The SOC density at surface soil layer in the 19-yr-old

A. auriculiformis plantations afforested on wasteland was

13.98 t C/ha in a valley-type savannah, southwest China

4

8

12

16

1991 1997 2003 2010Investigation time

Soi

l org

anic

C d

ensi

ty (

tC/h

a)Surface soil

Subsurface soil

0.00

0.25

0.50

0.75

1991-1997 1997-2003 2003-2010 1991-2010Investigation time

C a

ccum

ulat

ion

rate

(tC

/ha/

yr)

Surface soil

Subsurface soil

(a) (b)

Figure 2 Soil organic C density (a) and C accumulation rate (b) at surface (0–15 cm) and subsurface (15–30 cm) soils at different sampling

times. Error bars represent the standard deviation of replicates (n = 5).

Table 3 Distribution of organic C in density fractions at surface and subsurface soils

Soil depth

(cm)

Sampling

time

Dry mass

ratio of LF

to soil (%)

Dry mass

ratio of HF

to soil (%)

LF-C density

(t C/ha)

HF-C density

(t C/ha)

LF-C/SOC

ratio (%)

HF-C/SOC

ratio (%)

0–15 2003 0.60 � 0.057 b 97.72 � 1.015 a 2.76 � 0.27 b 7.37 � 0.58 b 26.77 � 2.18 b 71.48 � 1.83 a

2010 0.78 � 0.082 a 97.58 � 0.696 a 4.21 � 0.21 a 9.51 � 0.80 a 30.11 � 1.18 a 68.02 � 1.28 b

15–30 2003 0.25 � 0.015 a 98.77 � 1.075 a 1.93 � 0.12 b 6.27 � 0.30 b 23.34 � 1.13 a 75.82 � 1.00 a

2010 0.26 � 0.013 a 98.18 � 1.073 a 2.39 � 0.22 a 7.36 � 0.34 a 24.21 � 1.51 a 74.57 � 1.12 a

LF, light fraction; HF, heavy fraction; LF-C, organic C in light fraction; HF-C, organic C in heavy fraction. Data are mean � standard

deviation of five replicates, and different letters indicate significant differences for the means in the same columns according to time series

analysis (LSD test, P < 0.05).


52 G. Tang et al.

(Fig. 2). This value was comparable to SOC densities in

forested soils in dry-hot valleys previously reported by others

(Guo et al., 2007; Tang et al., 2010) and to those in typical

savannah elsewhere (Trouve et al., 1994; Epron et al., 2009).

For example, Tang et al. (2010) reported that the topsoil C

densities in four types of plantations ranged from 8.45 to

14.78 t C/ha after 8 yr of afforestation in a dry-hot valley.

At a country scale, however, our results of SOC densities

were substantially lower than the average value for surface

soils of the whole country (26.7 t C/ha) and for top one-

meter soil layers of forestland (143.3 t C/ha) in China (Xie

et al. 2004, Yu et al., 2007). Our results therefore concur

with others in showing that the forestland in dry-hot valleys

was probably a low C density region in China, which is

possibly because of the particular environment in these

regions, such as under water shortages, high air

temperatures, low soil fertility and low litter fall (Guo et al.,

2007; Tang et al., 2010).

The C sequestration rates of the topsoil approximated 0.06

–0.10 t C/ha/yr for the principal terrestrial ecosystems (Fang

et al., 2007) and 0.03 t C/ha/yr for forestland (Piao et al.,

2009) in China during last two decades. Paul et al. (2002)

reported an even lower rate (on average 0.14 t C/ha/yr) after

reviewing 43 published studies on 204 sites worldwide. We

observed that the C accumulation rate of the surface soil

was 0.38 t C/ha/yr during nearly two decades of the

afforestation on wasteland in the valley-type savannah of

China.

The C accumulation rate of the surface soil during the last

stage of our study (i.e. 2003–2010) was obviously higher

than that in the medium term or initial stages (Fig. 2b). This

result suggested that the gain of C at the surface soil

accelerated. We can suggest two possible explanations for

this difference. The input litter fall into the wasteland were

rather small (0.05 t/ha/yr) before afforestation (Tang et al.,

2010). A cumulative increase in the litter fall throughout the

19-yr period was observed (Fig. 1). The received plant litter,

in part, generated SOC through microbial degradation.

Moreover, a large number of dead and under decomposed

roots as well as root exudation might have contributed to

the increase in SOC. For example, Gao et al. (2012) studied

the biomass of 9-yr-old A. auriculiformis pure plantations

adjacent to our study area and reported that the root

biomass was ca. 3.18 t/ha. The other possible reason is that

the plant residues of A. auriculiformis might have contained

higher levels of cellulose than before. The lower decay rate

of this material would produce a hysteresis effect or delay

the formation of SOC resulting from plant residues (Li

et al., 2006; Tang et al., 2010).

Capacity of soil to protect C

Organic C combines with soil mineral particles to form

organo–mineral complexes or is concealed within micro-

aggregates to make the organic C partly unavailable to

microflora. The underlying mechanism is the formation of

physical barriers between substrates and decomposers, which

generally causes the stability of C in HF (HF-C). Compared

with HF-C, the organic C in LF (LF-C) is characterized as

chemically and visually more plant- or litter-like. It is

commonly considered more sensitive to change in climate,

land use and management practices owing to its relatively

rapid turnover rates (Janzen et al., 1992; Wang et al., 2005,

2009; Tan et al., 2007; Silveira et al., 2008; Wick & Tiessen,

2008). Christensen (1992) summarized the mass ratios of LF

to soil at the upper 10- to 15-cm layer of ten forested sites in

a temperate zone and found that the dry mass of the LFs

accounted for 1.8–14.7% of the total soil. We observed

relatively small ratios of LF to soil at the surface layer

(0.60–0.78%, Table 3). This result could be explained by the

long-term lack of incorporation of organic materials into

wasteland soils and the arid and hyperthermic climate in this

region. Tang et al. (2010) reported that arid conditions and

hyperthermia in dry-hot valleys would be more favourable

for the decomposition of labile C, such as unprotected LF-C,

than for that of stable C.

The HF-C density of the surface soil was significantly

higher in 2010 than in 2003, whereas a significantly lower

Table 4 Acid nonhydrolysable C density and chemical recalcitrant C index in fractions at surface and subsurface soils

Soil depth

(cm)

Sampling

time

ANC in LF

(t C/ha)

ANC in HF

(t C/ha)

Total ANC

(t C/ha) ICRC of LFa ICRC of HFa

ANC/SOC

ratio (%)

0–15 2003 1.57 � 0.13 a 3.93 � 0.26 a 5.51 � 0.33 a 56.88 � 1.05 a1 53.32 � 0.84 a2 53.44 � 0.64 a

2010 2.28 � 0.12 a 5.04 � 0.39 a 7.33 � 0.43 a 54.16 � 1.92 a1 53.00 � 0.61 a1 52.43 � 1.99 a

15–30 2003 1.05 � 0.05 b 3.21 � 0.26 b 4.26 � 0.27 b 54.40 � 1.29 b1 51.20 � 2.72 a2 51.51 � 2.17 a

2010 1.25 � 0.11 b 3.67 � 0.21 b 4.92 � 0.27 b 52.30 � 1.70 a1 49.86 � 0.88 b1 49.85 � 1.11 b

ANC, acid nonhydrolysable C; LF, light fraction; HF, heavy fraction; total ANC, sum of ANC in LF and HF; ICRC, chemical recalcitrant C

index; SOC, soil organic C. Data are mean � standard deviation of five replicates. Different letters indicate significant differences for the

means in the same columns in a given soil depth according to time series analysis (LSD test, P < 0.05). aDifferent numbers followed by letters

indicate significant differences for means in the same rows between the two columns according to the paired samples t-test procedure

(P < 0.05).



value of HF-C/SOC ratio was found in 2010 compared with

in 2003. This result is consistent with previous work showing

the preferential entrance of C into LF (Billings, 2006;

Richards et al., 2007; Tan et al., 2007; Marin-Spiotta et al.,

2009).

Chemical recalcitrance of soil organic C

Marin-Spiotta et al. (2009) found that ca. 5% of LF-C at

the topsoil after 80 yr of reforestation originated from the

land use present at the site before reforestation. They

suggested that recalcitrant material was present in the LF.

Swanston et al. (2005) and Richards et al. (2007) both

reported similar results. Wang et al. (2005) hypothesized that

the higher turnover rates of LF were not a consequence of

the lower chemical recalcitrance of LF-C but of the free-C

that was more readily available to soil microbes owing to the

shortage of physical protection. They further hypothesized

that the higher stability of HF was not a consequence of the

higher recalcitrance of HF-C but the result of the

inaccessibility of HF-C. In our research, SOC pools were

distinguished as acid hydrolysable fraction and acid

nonhydrolysable fraction via acid hydrolysis approaches. In

general, the acid hydrolysable fraction is largely comprised

of proteins, nucleic acids and polysaccharides and some

carboxyl C, while acid nonhydrolysable residue typically

contains mainly lignin and related compounds, along with

fats, waxes, resins and suberins (Paul et al., 1997; Rovira &

Vallejo, 2002; Silveira et al., 2008). Those acid

nonhydrolysable fractions were generally taken as chemical

recalcitrance (Wang et al., 2005; Sollins et al., 2006). We

found that the chemical recalcitrance of HF-C was lower

than that of LF-C irrespective of sampling time (Table 4).

This finding suggested that the chemical stability of the

protected C was lower than that of the unprotected C in our

study. One probable reason for this outcome is that a rapid

turnover rate of labile C (acid hydrolysable fraction)

compared with ANC in LF resulted in the relative

accumulation of ANC in LF as the decomposition of soil

organic matter or exotic organic materials (Billings, 2006;

Gr€unzweig et al., 2007; Marin-Spiotta et al., 2009). A second

probable reason is that the soil microbes had limited

opportunities to access and then utilize HF-C because

physical protection made it inaccessible. The presence of

these barriers potentially stimulated the microbes and

consequently increased the utilization of the chemical

recalcitrant C in HF in a situation involving a deficiency of

energy and C sources (Billings, 2006; Gr€unzweig et al.,

2007). The ICRC of HF was significantly lower than that of

LF in 2003 but not in 2010. This result might also be a

consequence of the incorporation of a large quantity of litter

fall and root biomass including live, dead and under

decomposed roots and root exudation. The fresh and readily

decomposed organic materials entering the system ‘diluted’

the proportion of LF-ANC relative to LF-C (i.e. the ICRC of

LF) as a result of the preferential entrance of exogenous

organic materials into LF (Gr€unzweig et al., 2007; Richards

et al., 2007; Marin-Spiotta et al., 2009).

Stabilization of newly sequestered C in afforested soils

The stabilization of newly sequestered C is the key aspect of

C sequestration in soils (Lal, 2004; Swanston et al., 2005).

This principle is especially valid for forested soils in

seasonally arid regions in subtropical zone (Richards et al.,

2007). In our study, the newly sequestered C protected by

physical mechanisms contributed to 58 and 68% of the

overall newly sequestered SOC at the surface soil (3.67 t C/ha)

and at the subsurface soil (1.60 t C/ha), respectively, during

12–19 yr after afforestation. Therefore, most of the newly

sequestered C (i.e. 58–68%) was physically protected and

therefore became more physically stable.

The increments in LF-ANC and HF-ANC densities at the

surface layer during 2003–2010 constituted 49 and 52% of

the newly sequestered C, respectively, in the corresponding

density fractions at the same period (Tables 3 and 4).

Moreover, ca. 42–43% of the total newly sequestered C in

the density fractions was chemically stable at the subsurface

layer (Tables 3 and 4). It appeared that the quantities of the

newly sequestered ANC and the newly sequestered labile C

(acid hydrolysable fraction) were comparable at the surface

layer in both density fractions, whereas slightly more newly

sequestered labile C was observed at the subsurface layer.

The total newly sequestered ANC (LF-ANC + HF-ANC) at

the surface and subsurface soils was ca. 50 and 41%,

respectively, relative to the overall newly sequestered SOC at

the corresponding soil layers.

Conclusions

The C accumulation rates of the soils in the A. auriculiformis

plantations increased following the afforestation of

wasteland during nearly two decades in a valley-type

savannah in southwestern China. About 58–68% of newly

sequestered C in soils was physically protected (i.e. into

heavy fraction), or ca. 41–50% achieved chemical stability

(i.e. into acid nonhydrolysable fraction) during the period

of 12–19 yr after afforestation. Without disturbance, the

chemical stability of the physically protected C was less than

that of the unprotected C, but in both cases, the chemical

stability declined with time.

Acknowledgments

This work was supported by the National Natural Science

Foundation of China (31100462), the Research Institute of

Resource Insects, Chinese Academy of Forestry


54 G. Tang et al.

(riricaf201001M, riricaf2012007M) and the Special Fund for

Forestry Research in the Public Interest (201104002-3-2). We

thank Prof. Tong Chengli for his useful discussion. We also

appreciate anonymous reviewers for their helpful comments

in improving the manuscript.

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