effects of forest conversion on soil labile organic carbon fractions and aggregate stability in...

10
REGULAR ARTICLE Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China Yusheng Yang & Jianfen Guo & Guangshui Chen & Yunfeng Yin & Ren Gao & Chengfang Lin Received: 11 September 2008 / Accepted: 3 February 2009 / Published online: 19 March 2009 # Springer Science + Business Media B.V. 2009 Abstract Soil labile fractions play an important role in improving soil quality due to its ability of maintaining soil fertility and minimizing negative environmental impacts. The objective of this study was to evaluate the effects of forest transition (conversion of natural broadleaf forests into mono- culture tree plantations) on soil labile fractions (light fraction organic carbon, particulate organic carbon, and microbial biomass carbon). Soil samples were collected from a natural forest of Castanopsis kawakamii Hayata (NF) and two adjacent 36-year- old monoculture plantations of C. kawakamii (CK) and Cunninghamia lanceolata Lamb. (Chinese fir) (CF) at Xinkou Experimental Forestry Centre, south- eastern China. In the 0100 cm depth, the light fraction organic carbon (LFOC), particulate organic carbon (POC) and microbial biomass carbon (MBC) were significantly lower in the CK and CF than in the NF (P <0.05). Generally, LFOC, POC and MBC contents declined consistently with profile depth. Significant differences in LFOC, POC and MBC concentrations between the native forest and two plantations were detected at 040 cm depth, especial- ly the top 10 cm, whereas there was less change below 40 cm, indicating that labile fraction losses due to forest transition mainly occurred in the surface soils. The three indices of labile organic carbon were closely correlated, suggesting they are interrelated properties. Labile fractions (LFOC, POC and MBC) were more sensitive indicators of SOC change resulting from the forest transition. We also found that forest types significantly affected the water stable aggregate >0.25 mm content (WSA) at the 010 cm depth. It suggested that converting old-growth native forest to intensively-managed plantations would re- duce labile organic C, which may be attributed to a combination of factors including quantity of litter materials, microbial activity and management distur- bances, which would change greatly with the forest conversion. How long these changes would persist needs the further study. Keywords Natural forest . Monoculture plantation . Labile fractions . Aggregation Introduction Soil organic carbon (SOC) plays an essential role in determining the physical and chemical characteristics of a soil and therefore in determining its fertility. Currently, there has been an additional interest in the role of SOC as a potential sink for atmospheric CO 2 (Post and Kwon 2000). Meanwhile, SOC is recog- Plant Soil (2009) 323:153162 DOI 10.1007/s11104-009-9921-4 Responsible Editor: Ingrid Koegel-Knabner. Y. Yang (*) : J. Guo : G. Chen : Y. Yin : R. Gao : C. Lin Key Laboratory of Humid Subtropical Eco-geographical Process of the Ministry of Education, College of Geographical Science, Fujian Normal University, Fuzhou 350007, China e-mail: [email protected]

Upload: yusheng-yang

Post on 15-Jul-2016

214 views

Category:

Documents


2 download

TRANSCRIPT

Page 1: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

REGULAR ARTICLE

Effects of forest conversion on soil labile organic carbonfractions and aggregate stability in subtropical China

Yusheng Yang & Jianfen Guo & Guangshui Chen &

Yunfeng Yin & Ren Gao & Chengfang Lin

Received: 11 September 2008 /Accepted: 3 February 2009 /Published online: 19 March 2009# Springer Science + Business Media B.V. 2009

Abstract Soil labile fractions play an important rolein improving soil quality due to its ability ofmaintaining soil fertility and minimizing negativeenvironmental impacts. The objective of this studywas to evaluate the effects of forest transition(conversion of natural broadleaf forests into mono-culture tree plantations) on soil labile fractions (lightfraction organic carbon, particulate organic carbon,and microbial biomass carbon). Soil samples werecollected from a natural forest of Castanopsiskawakamii Hayata (NF) and two adjacent 36-year-old monoculture plantations of C. kawakamii (CK)and Cunninghamia lanceolata Lamb. (Chinese fir)(CF) at Xinkou Experimental Forestry Centre, south-eastern China. In the 0–100 cm depth, the lightfraction organic carbon (LFOC), particulate organiccarbon (POC) and microbial biomass carbon (MBC)were significantly lower in the CK and CF than in theNF (P<0.05). Generally, LFOC, POC and MBCcontents declined consistently with profile depth.Significant differences in LFOC, POC and MBCconcentrations between the native forest and twoplantations were detected at 0–40 cm depth, especial-

ly the top 10 cm, whereas there was less changebelow 40 cm, indicating that labile fraction losses dueto forest transition mainly occurred in the surfacesoils. The three indices of labile organic carbon wereclosely correlated, suggesting they are interrelatedproperties. Labile fractions (LFOC, POC and MBC)were more sensitive indicators of SOC changeresulting from the forest transition. We also foundthat forest types significantly affected the water stableaggregate >0.25 mm content (WSA) at the 0–10 cmdepth. It suggested that converting old-growth nativeforest to intensively-managed plantations would re-duce labile organic C, which may be attributed to acombination of factors including quantity of littermaterials, microbial activity and management distur-bances, which would change greatly with the forestconversion. How long these changes would persistneeds the further study.

Keywords Natural forest . Monoculture plantation .

Labile fractions . Aggregation

Introduction

Soil organic carbon (SOC) plays an essential role indetermining the physical and chemical characteristicsof a soil and therefore in determining its fertility.Currently, there has been an additional interest in therole of SOC as a potential sink for atmospheric CO2

(Post and Kwon 2000). Meanwhile, SOC is recog-

Plant Soil (2009) 323:153–162DOI 10.1007/s11104-009-9921-4

Responsible Editor: Ingrid Koegel-Knabner.

Y. Yang (*) : J. Guo :G. Chen :Y. Yin : R. Gao :C. LinKey Laboratory of Humid Subtropical Eco-geographicalProcess of the Ministry of Education,College of Geographical Science, Fujian Normal University,Fuzhou 350007, Chinae-mail: [email protected]

Page 2: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

nized to consist of various fractions varying in degreeof decomposition, recalcitrance, and turnover rate(Huang et al. 2008). For example, light fractionorganic carbon (LFOC) is characterized by the rapidmineralization due to the labile nature of its constit-uents and to the lack of protection by soil colloids(Turchenek and Oades 1979). Particulate organiccarbon (POC) is biologically available and a sourceof C and energy for soil microorganisms (Gregorichand Janzen 1996). Although the absolute C amount inmicrobial biomass (MBC) is small, the microbes arethe most important labile C pool since they are vitalfor SOC dynamics and nutrient cycling (Powlson etal. 1987). As SOC is a heterogeneous mixture oforganic substances, the different forms or fractions ofSOC might have different effects on soil fertility andquality. Additionally, soil C contains a large propor-tion of the nutrient holding capacity of most soils andcontributes to important structural properties such asaggregate stability (Reeves 1997). Accumulatingevidence suggests that these labile fractions of organiccarbon and macroaggregates (>250 μm soil particles)might have significant effects on soil quality and are,therefore, more sensitive indicators of the effects ofland use or management practices compared withSOC (Von Lutzow et al. 2000; He et al. 2008).

The effects of land use change on carbon storageare of increasing concern in the context of interna-tional policy agenda on greenhouse gas emissionsmitigation, and the first of all are those associatingwith the conversion of native forest into agriculturalsystems, especially in the tropical region. However,the effects of conversion of natural forest to forestplantations have been less assessed. In many areas ofsoutheastern China, large areas of native broadleafforest have been cleared, followed by prescribedburning and converted to pure plantation forests inorder to meet the growing demands of timber, fuelmaterial, and other forest products due to the rapidhuman population growth in the last a few decades.However, soil degradation caused by slash burningaffects soil physical, chemical and biological proper-ties (Yang 1998). The changes provoked by the firehave received much attention in the subtropical regionof China, where edaphic and climatic factors, highsoil slope values and meteorological conditions(abundant high-intensity rainfall events in the summerperiod just after fires) tend to increase runoff anderosion processes in the surface soil horizon (Yang

1998). Practices of forest soil management aretherefore necessary in order to improve soil qualityfor forest conversion.

During the 1960s, part of natural Castanopsiskawakamii Hayata forest at National Nature Reserveof Xiaohu in Fujian Province was clear-cut toestablish a series of pure conifer and broadleaf treeplantations such as Chinese fir (Cunninghamia lan-ceolata Lamb.), C. kawakamii, Ormosia xylocarpaChun, Merr. And Chen, Castanopsis carlesii Chun,Cyclobalanopsis glauca Oerst. and Phoebe bournei(Hemsl.) Yang. These plantations and the adjacentnatural forest had homogeneous substrate (similarmineralogy, depths, and horizon). Differences in SOCstorage, forest floor properties, litterfall and fine rootdynamics among this natural forest of C. kawakamii(NF) and two plantations of C. kawakamii (CK) andChinese fir (CF) have been examined in detail (Yanget al. 1993, 2004a, b; Chen et al. 2005b). Also,changes of SOC concentration and storage with soildepth in the NF, CK and CF forests had been reported(Chen et al. 2005b). However, there are very fewstudies on the effects of forest conversion on thelevels of labile C fractions, as well as distribution oflabile organic C through the soil profile depth in thisregion. In this paper, we focused on two plantationforests of C. kawakamii (CK) and Chinese fir (CF)and an adjacent relict native C. kawakamii forest (NF)as a control to estimate effects of forest conversion onsoil labile organic C and soil structural stability. Therelationships among labile organic C fractions, waterstable aggregates and total SOC were also investigat-ed. We hypothesized that soil water stable aggregationand labile C fractions would be greater in the NF thanin the CK and CF plantation forests due to greaterroot biomass and richer litterfall (in terms of nutrientcontent and diversity of species) generally found inthe native versus plantation forests.

Materials and methods

Study sites

The study was carried out in the Xiaohu work-area ofthe Xinkou Experimental Forestry Centre of FujianAgricultural and Forestry University, Sanming,Fujian, China (26°11′30″ N, 117°26′00″ E). It bordersthe Daiyun Mountain on the southeast, and the Wuyi

154 Plant Soil (2009) 323:153–162

Page 3: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

Mountain on the northwest. The region has a mid-subtropical monsoonal climate with a mean annualtemperature of 19.1°C and a mean annual precipita-tion of 1,749 mm, and the growing season isrelatively long with an annual frost-free period ofaround 330 days. The soils are red soils (HumicPlanosol, FAO) developed from an acidic sandy shale(Yang et al. 2004b).

Selected forest characteristics and some propertiesof the surface soil (0–20 cm) from three adjacent siteshad been investigated in January 1999 (Table 1)(Yang et al. 2004a, b; Chen et al. 2005b). In additionto C. kawakamii, the overstory of NF also containedother tree species, such as Pinus massoniana D. Don,Schima superba Gardn. and Champ., Lithocarpusglaber (Thunb.) Nakai, Symplocos caudate Wall. exA. DC, Randia cochinchinensis (Lour.) Merr., andSymplocos stellaris Brand. In 1966, part of this NFwas clear-cut, slashed and burned. In 1967, the soilwas prepared by digging holes and then 1-year-oldseedlings of C. kawakamii and C. lanceolata (Chinesefir) seedlings were planted with density of 3,000 treesper hectare. The area of each plantation is more than20 ha. The plantation forests were managed withsimilar practices, such as weed-controlling and fertil-

izing during the first 3 years, and thinning twicebetween 10–15 year old. The normal rotation length is30 years for Chinese fir (CF) and 40 years for C.kawakamii (CK), respectively.

Soil sampling and analyses

In January 1999, five replicated plots (20×20 m) wereestablished for each forest type. In April 2002, six soilcores per plot were collected at six depths (0–10, 10–20, 20–40, 40–60, 60–80 and 80–100 cm) using an8 cm bulk density corer and then pooled by depth.Some of the fresh soil was passed through a 2 mmsieve to remove rocks and plant roots. Sub-samples ofthis soil were taken for analysis of microbial biomassC within 3 days, and the remainder was air-dried andground for SOC and C fractionation. The rest of thefresh soil sample was passed through a 5 mm sieveand immediately air-dried for later analysis ofaggregate size structure.

LFOC was separated by flotation on NaI solutionwith a density of 1.70 g cm−3 using a modification ofthe method described by Gregorich and Janzen(1996). Twenty-five grams of the <2 mm air-driedsoil was weighed into a 100 mL plastic centrifuge

Parameter Forest type

NFb CK CF

Forest characteristics

Canopy coverage (%) 80 90 65

Mean tree agea (year) ∼150 33 33

Mean tree height (m) 24.3 18.9 21.9

Mean tree diameter at breast height (cm) 42.2 24.2 23.3

Stand density (stem ha−1) 255 875 1,117

Stand volume (m3 ha−1) 398.3 412.4 425.9

Biomass of shrub layer (t ha−1) 10.12 0.78 1.99

Biomass of herb layer (t ha−1) 0.87 0.29 2.48

Standing crop of forest floor (t ha−1) 7.7 7.4 3.2

Soil properties (top 0–20 cm)

Bulk density (g cm−3) 0.93 1.10 1.20

Organic matter (g kg−1) 46.0 29.8 29.5

Total N (g kg−1) 1.88 1.12 1.12

Total P (g kg−1) 0.36 0.31 0.29

Annual litterfall (t ha−1) 11.01 9.54 5.47

Fine-root biomass (t ha−1) 4.94 3.20 1.49

Annual fine-root mortality (t ha−1) 8.63 5.15 2.49

Table 1 Forest characteris-tics and soil properties in anatural forest and twoplantation forests

NF natural forest of Casta-nopsis kawakamii, CK C.kawakamii plantation forest,CF Chinese fir (Cunning-hamia lanceolata)plantation foresta Age in 1999 since plantationbCastanopsis kawakamii isonly involved in the NF

Plant Soil (2009) 323:153–162 155

Page 4: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

tube and 50 mL NaI liquid solution was added to thetube. After gently shaking by hand for 30 s, the tubewas placed in an ice bath and sonicated for 15 min at400 W and 25°C with a tank-type ultrasonic disinte-grator KQ400DB, and then centrifuged at 3,000 rpmfor 10 min. The floating material was poured into aplastic bottle with a filter paper. This process (withoutsonication) was repeated twice, the supernatantmaterial was poured into previous plastic bottle withthe same filter paper. The material collected on thefilter was washed three times with 0.01 M CaCl2 toremove excess NaI, then again washed three timeswith deionized water, dried at 65°C for 12 h, weighedand finely ground for determination of organic C.

POC (53–2,000 μm) was determined with neces-sary modifications based on the method described byCambardella and Elliot (1992). Twenty grams of the<2 mm air-dried soil was dispersed with 100 mL of5 g L−1 sodium hexametaphosphate solution by handshaking the mixture for 15 min and then set on areciprocal shaker (90 r min−1) for 15 h. The dispersedsoil sample was passed subsequently through a 53 μmsieve and rinsed thoroughly with distilled water, thematerial remaining on the sieve, defined as the POCfraction, was dried at 50°C for 12 h, weighed andfinely ground.

MBC was determined by the fumigation–extrac-tion method (Vance et al. 1987). The 25-g fresh soil<2 mm was fumigated with chloroform for 24 h andextracted with 100 mL 0.5 M K2SO4, shaken for30 min and filtered through a membrane filter with0.45 μm pores, while the unfumigated control soilwas also extracted in the same manner and MBC wascalculated as the difference in extractable C beforeand after fumigation using a Kc factor of 0.38.Organic C in the extracts was measured using aTOC Analyzer (Elementar Analysensysteme GmbH,Germany).

SOC, LFOC and POC contents were determinedby dry combustion with an elemental analyzer(ELEMEMTAR Vario EL III), and expressed asorganic C mass in the whole soil or fractions to thewhole soil mass.

Soil water stable aggregates >0.25 mm (WSA)were determined by wet sieving procedure (Cambardellaand Elliot 1993). About 20 g of the air-dried soilpassed through a 5 mm sieve was weighed and wetsieved for 10 min using sieves of 2, 1, 0.5 and0.25 mm aperture in a 2-L cylindrical container. A

stroke length of 38 mm at a frequency of 30 strokesper minute was used. The different aggregates sizeswere dried at 50°C for 12 h and weighed. Thepercentage of soil water stable aggregate >0.25 mmwas calculated.

Statistical analyses

Differences in labile fractions among the forests weretested with ANOVA. We acknowledge that a limita-tion of this study, as with many other paired-sitestudies, was the pseudo-replication used for ANOVA.Least significant difference (LSD, P<0.05) was usedto separate the means when differences were signif-icant (PROC GLM, SAS Institute Inc., Cary, NC).The relationships among the different fractions wereevaluated by regression analysis using SPSS 13.0statistical package (SPSS Inc. 2004).

Results

Labile fraction organic C

Average concentrations of LFOC, POC and MBC alldecreased with soil depth. In the three forests, LFOCand MBC concentrations were significantly higher at0–10 and 10–20 depths compared with other soildepths (Table 2). However, the difference in POCconcentration through the soil profile under the CFwere not significant among the depths of 0–10, 10–20and 20–40 cm.

After the conversion of NF into two plantations ofCK and CF, the LFOC, POC and MBC concentrationsfor the entire 0–100 cm profile were markedlyreduced (P<0.05) (Table 2). Significant differencesin LFOC, POC and MBC concentrations between NFand two plantations of CK and CF were also detectedat 0–10, 10–20 and 20–40 cm depths (Table 2). In the0–10 cm layer, the concentration of LFOC in NF was41% and 53% higher than those in CK and CF, whilethe concentrations of POC and MBC in NF were 2.2–4.8 times and 1.6–1.9 times than those of CK and CF,respectively. There was no significant difference inMBC concentration between CK and CF plantationsat 0–40 cm depth. POC concentrations at all soildepths were significantly higher under the CKplantation than under the CF plantation, whilesignificantly higher LFOC concentration occurred in

156 Plant Soil (2009) 323:153–162

Page 5: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

the topsoil (0–20 cm) under the CK when comparedwith the CF.

The proportion of labile fraction organic C to SOCdecreased with the increasing soil depth at all sites(Fig. 1). The differences in the proportions of LFOC,POC and MBC to SOC through the soil profile weresignificant for each forest. As a general trend, theproportions of LFOC, POC and MBC to SOCdecreased in the order of NF > CK > CF, and theconcentrations of labile C pools in each forest soildecreased in the order: POC > LFOC > MBC(Table 2).

Soil water stable aggregate distribution

Soil water stable aggregate content (WSA) >0.25 mmat the 0–10 cm depth was greater in NF than those inCK and CF plantations (Fig. 2). The NF hadsignificantly higher percentage of >5.0 and 2.0–5.0 mm in comparison with the two plantations,whereas the percentage of 0.25–0.5 mm under CFwas markedly higher than those of NF and CK(Fig. 2).

Correlation analysis

There were positive relationships between the per-centage of soil water stable aggregate 2–5 mm andSOC, LFOC and POC (P<0.01). Correlation analysisshowed that LFOC, POC and MBC were closelycorrelated with SOC (P<0.01), and significant corre-lations were also found among LFOC, POC and MBC(P<0.01) (Table 3).

Discussion

The three adjacent sites used for this study had thesame vegetative cover (i.e., NF) prior to the estab-lishment of CK and CF plantations, and the soils weredeveloped from the same basaltic parent material. Assuch, differences in soil labile organic C fractionsand aggregate stability among the sites are assumedto be the result of the land-use change. Thedifferences between the NF and the two plantationforest soils may reflect the impact of the change intree species, the ensuing difference in the quality of

Forest types Soil depths (cm) LFOC (g kg−1 soil) POC (g kg−1 soil) MBC (mg kg−1 soil)

NF 0–10 8.95±1.13 Aa 11.94±0.98 Aa 990.62±136.71 Aa

10–20 4.73±1.56 Ba 6.24±0.79 Ba 665.72±85.21 Ba

20–40 0.47±0.24 Ca 1.84±0.26 Ca 218.24±33.88 Ca

40–60 0.32±0.31CDa 1.11±0.23 Da 108.90±11.11 Da

60–80 0.15±0.13 CDa 0.78±0.19 Da 98.50±10.65 Da

80–100 0.05±0.03 Da 0.38±0.13 Ea 89.21±7.32 Da

0–100 1.41±0.11 a 2.44±0.19 a 281±25 a

CK 0–10 5.25±1.21 Ab 5.48±0.86 Ab 600.90±74.51 Ab

10–20 3.15±1.16 Bb 3.59±0.57 Bb 379.49±47.82 Bb

20–40 0.20±0.23 Cb 1.22±0.41 Cb 140.23±32.57 Cb

40–60 0.09±0.14 Cb 0.96±0.37 CDa 72.21±10.93 Da

60–80 0.05±0.11 Cb 0.75±0.21 CDa 62.47±11.31 Da

80–100 0.01±0.03 Ca 0.33±0.17 Da 56.12±5.60 Da

0–100 0.83±0.06 b 1.48±0.13 b 196±19 b

CF 0–10 4.18±1.31 Ac 2.46±1.10 Ac 523.53±68.58 Ab

10–20 1.96±1.12 Bc 1.82±0.96 ABc 293.60±35.53 Bb

20–40 0.20±0.13 Cb 0.62±0.35 ACc 114.63±31.46 Cb

40–60 0.05±0.06 Cb 0.31±0.28 BCb 41.01±10.65 Db

60–80 0.01±0.05 Cb 0.28±0.21 BCb 35.66±11.89 Db

80–100 0.01±0.03 Ca 0.18±0.±19 Cb 26.31±2.08 Db

0–100 0.62±0.04 c 0.68±0.05 c 152±15 c

Table 2 Distribution char-acteristics of labile fractionorganic C for adjacent nat-ural forest of Castanopsiskawakamii and two planta-tions of C. kawakamii andChinese fir

Numbers are mean ± stan-dard deviations. Valueswithin the same columnfollowed by the differentcapital letter for each foresttype or small letter for eachsoil depth are significantlydifferent (P<0.05) amongdifferent depths or forestsrespectively according toLSD test (n=5)

LFOC light fraction organicC, POC particulate organic C,MBC microbial biomass C,NF natural forest of Casta-nopsis kawakamii, CK C.kawakamii, CF Chinese fir

Plant Soil (2009) 323:153–162 157

Page 6: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

organic matter input, the effect of disturbanceduring CK and CF establishment and subsequentsilvicultural practices, and changes in microclimate.It is acknowledged that pseudo-replication is alimitation of this study and as such the experimentmay be viewed as a case study.

Effects of forest conversion on soil labile C fractions

The amount of organic C in the soil results from thenet balance between the rate of organic materialinputs and rate of mineralization in SOC (Golchin etal. 1994; Gregorich and Janzen 1996; Post and Kwon2000). Relative sizes of labile C pools in variousecosystems and their responses to disturbance couldhave important implications in understanding SOCstability. Also, identification of such fractions mayserve as an indicator or even as a verification tool forSOC changes in terms of accounting for C stocks inthe Kyoto Protocol. Currently, interest in the effects ofSOC fractions on soil quality indicators of physical,chemical and properties under different land-use typesis increasing (Campbell et al. 1999; Chan et al. 2002).

For this study, it was hypothesized that theconversion of natural forest into the two plantationforests can alter the size of soil labile C pools. Ashypothesized, levels of LFOC, POC and MBC in the0–100 cm soil profile significantly declined after theforest conversion (Table 2). These results are inaccordance with the finding of others (Chen et al.2004a; Xu et al. 2008). In addition to the differencesin microclimatic conditions, explanations given forthe differences between NF and two plantations ofCK and CF have included differences in the groundvegetation cover, quantity and quality of organicmatter inputs to soils (Table 1). In addition, manage-ment practices such as clear-cutting, slash burningand soil preparation not only resulted in a directdepletion of SOC, but also led to a decrease ofphysical protection of organic C by soil mineralparticles and soil aggregates (Yang et al. 2005). Thesefactors also lowered labile C in the plantationscompared with those in NF. But the nature and detailof such disturbance occurring approximately 36 yrago are not available for appropriate comparisons.

0

5

10

15

20

25

30

35

0-10 10-20 20-40 40-60 60-80 80-100

Soil depth (cm)

0-10 10-20 20-40 40-60 60-80 80-100

Soil depth (cm)

0-10 10-20 20-40 40-60 60-80 80-100

Soil depth (cm)

LFO

C/S

OC

(%

)

NF

CK

CF

a

bc

dd

de e

ab

c

e e e

b

d

0

5

10

15

20

25

30

35

40

45

50

PO

C/S

OC

(%

)

NF

CK

CF

a

b

c d

d

f

b

c d

d

f

d

de ef f f

0

0.5

1

1.5

2

2.5

3

3.5

4

MB

C/S

OC

(%

)

NF

CK

CF

a

b b

c c c

a

c c

d d d

a

c c

e de e

�Fig. 1 Proportion of labile fraction organic C to soil organic C(SOC) through the soil profile in adjacent natural forest ofCastanopsis kawakamii (NF) and two plantations of C.kawakamii (CK) and Chinese fir (CF). a The proportion oflight fraction organic C to SOC (LFOC/SOC) through the soilprofile; b the proportion of particulate organic C to SOC (POC/SOC) through the soil profile; and c the proportion of microbialbiomass C to SOC (MBC/SOC) through the soil profile. Pointsin figure are means (n=5). Points in each figure with thedifferent letters are significantly different at P<0.05 showingeffect of both depth and forest type

158 Plant Soil (2009) 323:153–162

Page 7: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

The magnitudes of LFOC, POC and MBC revealeda clear decline with soil depth (Table 2). Because theyare strongly related to root C inputs (Gale et al. 2000;Wander and Yang 2000), and other organic residuesare often accumulated at the soil surface. In addition,losses of labile fraction organic C after foresttransition mainly occurred in 0–10 cm topsoil. Ourresults are consistent with the previous findings (Guoand Gifford 2002; Chen et al. 2004a; Sarkhot et al.2008).

Generally, LFOC accounts for 1–25% of SOC incultivated agricultural soils (Janzen et al. 1992), butas much as 63% of SOC in forest soils (Boone 1994).In contrast to these data, the proportions of LFOC toSOC in the 0–100 cm profile were much lower(Fig. 1). The values at 0–10 cm in our study rangedfrom 23% to 30% and slightly lower than that ofDouglas fir plantation (40%) at top 0–15 cm depth(Cromack et al. 1999), but close to the average values(∼32%) at 0–10 cm depth of natural secondary forestsin warm temperate region reported by Wu et al.(2002). In the three forests, POC accounted for 14–40% of the SOC at 0–10 cm depth (Fig. 1), this wasslightly higher than those obtained by Cambardella

and Elliot (1992), which ranged between 18% and25%, but similar to that of native grassland soils (20–39%). In general, the POC usually represents asmaller proportion (10–20%) of the SOC in warmand wet tropical and subtropical regions (Bayer et al.2002). However, Franzluebbers and Arshad (1997)found a higher proportion of about 50% under dry orcold climate. The smaller proportions POC to SOCand LFOC to SOC in our study were probably relatedto the warm and wet subtropical climate which highlyfavored for biological decomposition of recent organ-ic material inputs, leading to less accumulation ofPOC and LFOC (Chen et al. 2004b). Meanwhile, theratio of MBC to SOC indicates the proportion oforganic C that may be readily metabolized. It usuallyfalls within the range of 1–4% (Sparling 1992).However, because of differences in soil and manage-ment practices, variations in sampling date andanalytical methods, wider ranges of MBC/SOC ratiofrom 0.27% to 7.0% have been reported (Insam et al.1989; Omay et al. 1997). Our study showed that theratio of MBC to SOC ranged from 0.7–3.3% (Fig. 1),which lies within the range reported. In comparisonwith forest soils of subtropical Australia (0.6–2.5%)

Water stable aggregates (size mm) MBC SOC POC

>5 2–5 1–2 0.5–1 0.25–0.5 >0.25

LFOC 0.305 0.882 0.615 0.016 0.203 0.409 0.981 0.913 0.975

POC 0.682 0.987 0.689 0.268 0.183 0.576 0.978 0.954

SOC 0.273 0.802 0.559 0.021 0.220 0.399 0.982

Table 3 Coefficients of de-termination (R2) betweenthe various soil organic Cpools (n=5)

LFOC light fraction organicC, POC particulate organic C,MBC microbial biomass C,SOC soil organic C

0

10

20

30

40

50

60

70

>5.0 2.0-5.0 1.0-2.0 0.5-1.0 0.25-0.5 >0.25

Aggregate size (mm)A

ggre

gate

siz

e di

strib

utio

n (%

)

NF

CK

CF

a b c

ab

c

aba

a ab b b

a

aab

b

Fig. 2 Distribution ofwater-stable aggregates(WSA) among differentaggregate-size classes to0–10 cm soil depth underadjacent natural forest ofCastanopsis kawakamii(NF) and two plantations ofC. kawakamii (CK) andChinese fir (CF). Thin barsare standard errors (n=5)and bars that share thedifferent letter for eachaggregate-size class are sig-nificantly different amongforest types (P<0.05)

Plant Soil (2009) 323:153–162 159

Page 8: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

(Chen et al. 2005a), our results showed higherproportions of MBC to SOC at the 0–10 cm depth(2.9–3.3%). Also, MBC to SOC ratios in different soillayers under NF were higher than under CK and CF(Fig. 1). The result can be explained on the basis ofthe fact that more diversified organic substrateproduction and input in NF support more inter-dependent food web which allows the maintenanceof higher MBC per unit soil organic C (Anderson andDomsch 1989).

The LFOC, POC and MBC have been proposed tobe used as indicators to evaluate the effect of differentsoil management practices because these fractionsmay precede future changes of SOC (Chen et al.2004a; He et al. 2008; Xu et al. 2008). In our study,significant differences in LFOC, POC and MBCresulting from the land use change were observedamong the forests. The LFOC, POC and MBCconcentration values for the entire 0–100 cm profileranged from 0.62 to 1.41 g kg−1 (a 2.3-folddifference), 0.68 to 2.44 g kg−1 (a 3.6-fold differ-ence), and 152 to 281 mg kg−1 (a 1.8-fold difference)among the three forest soils (Table 2), while SOCcontent ranged from 7.45 to 10.50 g kg−1 (a 1.4-folddifference) (Chen et al. 2005b). Also, significantcorrelations among labile fraction organic C andSOC were found. This demonstrates that it isdesirable to understand the impacts of labile Cpools on soil quality under different land uses ormanagement practices.

Effects of forest conversion on soil structural stability

Soil structure, a good indicator of soil quality, isusually strongly related to SOC content and controlssoil C stabilization capacity following land useconversion (Six et al. 2002). In this study, theconversion of the natural forest into the forestplantations led to a decline in soil physical structureparticularly in the surface soils (0–10 cm). After theconversion, soil physical structure degraded andvegetative cover was limited, thus the soil was likelyto have low infiltration rates and prone to erosion.Results from our soil physical analyses suggested thataggregate structure of soils in the CK and CF did notrecover in comparison with that of the natural forestsoil and content of the larger aggregates >2 mm wassignificantly affected by POC or LFOC (Table 3).According to Tisdall and Oades (1982), stability of

macroaggregates (>0.25 mm) is controlled by thetemporary forms of organic C and as such is moresensitive to management practices. Chan et al. (2001)also reported that there are strong correlationsbetween soil labile organic C and soil aggregatestability. Taken together, our results clearly demon-strated much greater losses of organic C in the formof POC and LFOC as a result of land-use changes andsuggested an association of the loss in water stabilityof >2 mm with the losses of POC and LFOC.

Conclusions

The levels of labile organic C fractions declinedconsistently with the soil depth, and significantchange was found at the 10 cm depth, suggestinglabile C fraction losses mainly occurred in the topsoildue to the forest transition. Our results clearlyillustrated that losses of labile C pools following theconversion of the natural forest into the two planta-tions were more pronounced than those of SOC in thewhole soil. Namely, POC, LFOC and MBC arerelatively sensitive indicators of future changes inSOC under different types of land conversion. It canbe partially attributed to changes of aboveground andbelowground biomass and litter due to the land-usechange or management practices. However, becauseplots were localized in the same stand, the experimentwas not truly replicated, and differences between theNF and two plantations of CK and CF should not beassigned to differences in land use without caution.Also, it is worthwhile to note that this is a case studyat 36 years following planting and future studiesshould incorporate other space-for-time observationsin other ecosystems to further support these findings.

The implications from this study are that plantationestablishment systems may lead to a reduction inlabile organic C in soils compared to the native forest,and this could negatively impact C cycling as well asthe prospects for sustainable management. Thussustainability-oriented practices that ensure soil sta-bility and biological productivity should be applied inthe management of plantation forests.

Acknowledgements This research was funded by the Ministryof Education of China through the Supporting Program forUniversity Key Teacher and by the Key Basic Research Projectof Fujian Province (2000F004).

160 Plant Soil (2009) 323:153–162

Page 9: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

References

Anderson TH, Domsch KH (1989) Ratios of microbialbiomass carbon to total organic carbon in arable soils.Soil Biol Biochem 21:471–479. doi:10.1016/0038-0717(89)90117-X

Bayer C, Mielniczuk J, Martin-Neto L, Ernani PR (2002)Stocks and humification degree of organic matter fractionsas affected by no-tillage on a subtropical soil. Plant Soil238:133–140. doi:10.1023/A:1014284329618

Boone RD (1994) Light-fraction soil organic matter: origin andcontribution to net nitrogen mineralization. Soil BiolBiochem 26:1459–1468. doi:10.1016/0038-0717(94)90085-X

Cambardella CA, Elliot ET (1992) Particulate soil organic-matter changes across a grassland cultivation sequence.Soil Sci Soc Am J 56:777–783

Cambardella CA, Elliot ET (1993) Carbon and nitrogendistribution in aggregates of cultivated and native grass-land soils. Soil Sci Soc Am J 57:1071–1076

Campbell MR, Biederbeck VO, McConkey BG (1999) Soilquality-effect of tillage and fallow frequency: soil organicmatter quality as influenced by tillage and fallow frequencyin a silt loam in southwestern Saskatchewan. Soil BiolBiochem 31:1–7. doi:10.1016/S0038-0717(97)00212-5

Chan KY, Bowman A, Oates A (2001) Oxidizible organiccarbon fractions and soil quality changes in an OxicPaleustalf under different pasture leys. Soil Sci 166:61–67.doi:10.1097/00010694-200101000-00009

Chan KY, Heenan DP, Oates A (2002) Soil carbon fractions andrelationship to soil quality under different tillage andstubble management. Soil Tillage Res 63:133–139.doi:10.1016/S0167-1987(01)00239-2

Chen CR, Xu ZH, Mathers NJ (2004a) Soil carbon pools inadjacent natural and plantation forests of subtropicalAustralia. Soil Sci Soc Am J 68:282–291

Chen GS, Yang YS, Xie JS, Li L, Gao R (2004b) Soilbiological changes for a natural forest and two plantationsin subtropical China. Pedosphere 14(3):297–304

Chen CR, Xu ZH, Zhang SL, Keay P (2005a) Soluble organicnitrogen pools in forest soils of subtropical Australia. PlantSoil 277:285–297. doi:10.1007/s11104-005-7530-4

Chen GS, Yang YS, Xie JS, Guo JF, Gao R, Qian W (2005b)Conversion of a natural broad-leafed evergreen forest intopure plantation forests in a subtropical area: effects oncarbon storage. Ann For Sci 62:659–668. doi:10.1051/forest:2005073

Cromack K Jr, Miller RE, Helgerson OT, Smith RB, AndersonHW (1999) Soil carbon and nutrients in a coastal OregonDouglas-fir plantation with red alder. Soil Sci Soc Am J 63(1):232–239

Franzluebbers AJ, Arshad MA (1997) Particulate organiccarbon content and potential mineralization as affectedby tillage and texture. Soil Sci Soc Am J 61:1382–1386

Gale WJ, Cambardella CA, Bailey TB (2000) Root-derivedcarbon and the formation and stabilization of aggregates.Soil Sci Soc Am J 64:201–207

Golchin A, Oades JM, Skjemstad JO, Clarke P (1994) Study offree and occluded particulate organic matter in soils bysolid state 13C CP/MAS NMR spectroscopy and scanning

electron microscopy. Aust J Soil Res 32:285–309.doi:10.1071/SR9940285

Gregorich EG, Janzen HH (1996) Storage of soil carbon in thelight fraction and macro organic matter. In: Carter MR,Stewart BA (eds) Advances in soil science. Structure andorganic matter storage in agricultural soils. CRC Lewis,Boca Raton, pp 167–190

Guo LB, Gifford RM (2002) Soil carbon stocks and land usechange: a meta analysis. Glob Change Biol 8:345–360.doi:10.1046/j.1354-1013.2002.00486.x

He Y, Xu ZH, Chen CR, Burton J, Ma Q, Ge Y, Xu JM (2008)Using light fraction and macroaggregate associated organicmatters as early indicators for management-induced changesin soil chemical and biological properties in adjacent nativeand plantation forests of subtropical Australia. Geoderma147:116–125. doi:10.1016/j.geoderma.2008.08.002

Huang ZQ, Xu ZH, Chen CR, Boyd S (2008) Changes in soilcarbon during the establishment of a hardwood plantationin subtropical Australia. For Ecol Manage 254:46–55

Insam H, Parkinson D, Domsch KH (1989) Influence ofmacroclimate on soil microbial biomass. Soil Biol Bio-chem 21:211–221. doi:10.1016/0038-0717(89)90097-7

Janzen HH, Campbell CA, Brandt SA, Lafond GP, TownleySmith L (1992) Light fraction organic matter in soils fromlong term crop rotations. Soil Sci Soc Am J 56:1799–1806

Omay AB, Rice CW, Maddux LD, Gordon WB (1997) Changes insoil microbial and chemical properties under long-term croprotation and fertilization. Soil Sci Soc Am J 61:1672–1678

Post WM, Kwon KC (2000) Soil carbon sequestration andland-use change: processes and potential. Glob ChangeBiol 6:317–327. doi:10.1046/j.1365-2486.2000.00308.x

Powlson DS, Brookes PC, Christensen BT (1987) Measure-ment of soil microbial biomass provides an early indica-tion of changes in total soil organic matter due to strawincorporation. Soil Biol Biochem 19:159–164.doi:10.1016/0038-0717(87)90076-9

Reeves DW (1997) The role of soil organic matter inmaintaining soil quality in continuous cropping systems.Soil Tillage Res 43:131–167. doi:10.1016/S0167-1987(97)00038-X

Sarkhot DV, Jokela EJ, Comerford NB (2008) Surface soilcarbon size–density fractions altered by loblolly pinefamilies and forest management intensity for a Spodosolin the southeastern US. Plant Soil 307:99–111.doi:10.1007/s11104-008-9587-3

Six J, Conant RT, Paul EA, Paustian K (2002) Stabilizationmechanisms of soil organic matter: Implications for C-saturation of soils. Plant Soil 241:155–176. doi:10.1023/A:1016125726789

Sparling GP (1992) Ratio of microbial biomass carbon to soilorganic carbon as a sensitive indicator of changes in soilorganic matter. Aust J Soil Res 30:195–207. doi:10.1071/SR9920195

SPSS Inc (2004) SPSS 13.0. SPSS Inc., ChicagoTisdall JM, Oades JM (1982) Organic matter and water stable

aggregates in soil. J Soil Sci 33:141–163. doi:10.1111/j.1365-2389.1982.tb01755.x

Turchenek LW, Oades JM (1979) Fractionation of organo-mineral complexes by sedimentation and density techni-ques. Geoderma 21:311–343. doi:10.1016/0016-7061(79)90005-3

Plant Soil (2009) 323:153–162 161

Page 10: Effects of forest conversion on soil labile organic carbon fractions and aggregate stability in subtropical China

Vance ED, Brookes PC, Jenkinson DS (1987) An extractionmethod for measuring soil microbial biomass C. SoilBiol Biochem 19:703–707. doi:10.1016/0038-0717(87)90052-6

Von Lutzow M, Leifeld J, Kainz M, Kogel-Knabner I, MunchJC (2000) Indications for soil organic matter quality insoils under different management. Geoderma 105:243–258. doi:10.1016/S0016-7061(01)00106-9

Wander MM, Yang Z (2000) Influence of tillage on thedynamics of loose- and occluded-particulate and humifiedorganic matter fractions. Soil Biol Biochem 32:1551–1560. doi:10.1016/S0038-0717(00)00031-6

Wu JG, Zhang XQ, Wang YH, Xu DY (2002) The effects ofland use changes on the distribution of soil organic carbonin physical fractionation of soil. Scientia Silvae Sinicae 38(4):19–29. In Chinese with English abstract

Xu ZH, Ward S, Chen CR, Blumfield T, Prasolova N, LiuJX (2008) Soil carbon and nutrient pools, microbialproperties and gross nitrogen transformations in adjacentnatural forest and hoop pine plantations of subtropicalAustralia. J Soils Sediments 8:99–105. doi:10.1065/jss2008.02.276

Yang YS (1998) Studies on the sustainable management ofChinese fir plantations. China Forestry Press, Beijing,pp 118–126. In Chinese

Yang YS, Li ZW, Liu AQ (1993) Studies on soil fertility fornatural forest of Castanopsis kawakamii replaced bybroadleaf plantation. J Northeast For Univ 21(5):14–21.In Chinese with English abstract

Yang YS, Chen GS, Lin P, Xie JS, Guo JF (2004a) Fine rootdistribution, seasonal pattern and production in fourplantations compared with a natural forest in subtropicalChina. Ann For Sci 61:617–627. doi:10.1051/forest:2004062

Yang YS, Guo JF, Chen GS, Lin RY, Cai LP, Lin P (2004b)Litterfall, nutrient return, and leaf-litter decomposition infour plantations compared with a natural forest insubtropical China. Ann For Sci 61:465–476 doi:10.1051/forest:2004040

Yang YS, Guo JF, Chen GS, Xie JS, Gao R, Li Z, Jin Z (2005)Carbon and nitrogen pools in Chinese fir and evergreenbroadleaved forests and changes associated with fellingand burning in mid-subtropical China. For Ecol Manage216(1–3):216–226

162 Plant Soil (2009) 323:153–162