effects of tree species mixture on soil organic carbon stocks and greenhouse gas fluxes in...

Forest Ecology and Management xxx (2012) xxx–xxx

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Forest Ecology and Management

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Effects of tree species mixture on soil organic carbon stocks and greenhousegas fluxes in subtropical plantations in China

Hui Wang a, Shirong Liu a,⇑, Jingxin Wang b, Zuomin Shi a, Lihua Lu c, Ji Zeng c, Angang Ming c,Jixin Tang c, Haolong Yu c

a Key Laboratory of Forest Ecology and Environment, China’s State Forestry Administration, Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry,No. 2 Dongxiaofu, Haidian District, Beijing 100091, Chinab Division of Forestry and Natural Resources, West Virginia University, P.O. Box 6215, Morgantown, WV 26506-6125, USAc The Experimental Center of Tropical Forestry, Chinese Academy of Forestry, Guangxi Youyiguan Forest Ecosystem Research Station, Pingxiang, Guangxi 532600, China

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:Soil carbonSoil�atmosphere trace gas exchangesMixed plantationForest conversionSubtropical China

0378-1127/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.foreco.2012.04.005

⇑ Corresponding author. Tel.: +86 10 62889311; faxE-mail address: [email protected] (S. Liu).

Please cite this article in press as: Wang, H., etplantations in China. Forest Ecol. Manage. (201

a b s t r a c t

Indigenous broadleaf plantations are increasingly being developed as a prospective silvicultural approachfor substituting coniferous plantations in subtropical China. Three plantations of monoculture and mixedPinus massoniana and Castanopsis hystrix were selected to examine soil organic carbon (SOC) stocks andtemporal and spatial patterns of the main greenhouse gases fluxes for understanding the effects of mixedforests on soil carbon and nitrogen (N) cycling processes. We found that SOC stock in 0–20 cm layer in themixed plantation was 14.3% higher than that in the P. massoniana, and 8.1% higher than that in the C.hystrix plantations. Differences in SOC stock among the plantations were attributed to soil N stock andleaf litterfall input. Soil CO2 and N2O fluxes in the mixed plantation displayed the seasonal trends, whilesoil CH4 flux did not show the seasonal trend. The seasonal variations in soil CO2 and N2O emissions werepositively related to soil temperature and moisture. Mean soil CO2 and N2O emissions (53.2 mg C m�2 h�1

and 5.21 lg N m�2 h�1, respectively) were significantly higher in the mixed plantation than in the P. mas-soniana plantation, while they were lower than in the C. hystrix plantation. Mean soil CH4 uptake(38.4 lg C m�2 h�1) was significantly higher in the mixed plantation than in the C. hystrix plantation,while it is similar to that in the P. massoniana plantation. Variations in soil CO2 flux among the plantationswere influenced by fine root biomass, leaf litterfall mass, soil N stock and soil C:N ratio. Differences in soilN2O flux among the plantations could be attributed to the differences in soil N stock, soil NO3

��N contentand soil C:N ratio. Soil respiration rate and soil NO3

��N content could account for variations in soil CH4

flux among the plantations. This study confirms that the mixed plantation has a higher SOC stock thanthe monoculture plantations, and there is an increase in amount of GHG absorbed by the soil of mixedplantations compared to C. hystrix plantations. Therefore, a mixture of C. hystrix versus P. massoniana,could be a better silvicultural approach for SOC sequestration than monoculture C. hystrix plantationfor substituting P. massoniana plantations in subtropical China.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Carbon sequestration in vegetation and soil is recognized as amechanism that can mitigate atmospheric carbon dioxide (CO2)accumulation (Janzen, 2004). Afforestation and reforestation areconsidered important tools for sequestering atmospheric CO2 bywhich to offset greenhouse gases (GHGs) emissions from fossilfuels; however, establishment of plantations necessarily involvesseveral silvicultural treatments that may impact soil carbon

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sequestration and its relative stability (Maillard et al., 2010). Theenhanced production and reduced consumption of naturally occur-ring GHGs such as CO2, nitrous oxide (N2O) and methane (CH4), areresponsible for approximately 90% of the global warming and cli-mate change phenomenon (Solomon et al., 2007). A considerableamount of atmospheric GHGs is produced and consumed throughsoil processes (Tang et al., 2006). However, there remain consider-able uncertainties about the effects of forest management activitieson soil carbon sequestration and greenhouse gas fluxes (Post andKwon, 2000; Johnston et al., 2004; Kelliher et al., 2006).

Forest soil organic carbon (SOC) is influenced by the complexinteractions of climate, soil type, management, and tree species(Lal, 2005). A growing body of evidence has demonstrated that for-est species composition will influence soil carbon turnover due to

re on soil organic carbon stocks and greenhouse gas fluxes in subtropicaloreco.2012.04.005

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Table 1Diameter of tree measured at breast height (DBH), tree height, stem density and soiltexture of the three plantations in subtropical China.

Plantation types Pinusmassoniana

Mixedplantation

Castanopsishystrix

DBH (cm) 24.6 27.3 24.9Tree height (m) 17.2 18.1 17.8Stem density (trees

ha�1)404 400 415

Sand (%) 57 62 59Silt (%) 8 7 7Clay (%) 35 31 34

2 H. Wang et al. / Forest Ecology and Management xxx (2012) xxx–xxx

its different microclimates at the forest floor (Berger et al., 2002).These effects have been attributed to the fact that tree speciescould potentially alter size and physiochemical properties of car-bon additions in litter from the aboveground and belowgroundflora and fauna, distribution of the root systems of plants in the soilprofile, distribution of carbon within the soil matrix and its inter-action with clay surfaces (Oades, 1988; Berger et al., 2002). Severalstudies have shown that mixed forests may influence soil carbonand nitrogen (N) concentrations and stocks (Berger et al., 2002;Borken and Beese, 2005; Tang et al., 2006).

Afforestation and reforestation can greatly affect soil GHGsfluxes by changing key physical and chemical properties that influ-ence soil nutrients and carbon cycling and microbial activity (Paulet al., 2002; Merino et al., 2004; Kelliher et al., 2006). Tree speciesare considered to alter soil chemical, physical (e. g. moisture andtemperature) and biological processes through their root system,crown structure, foliage, leaf structure and litter quality (Borkenand Beese, 2006; Jonard et al., 2007; Ullah et al., 2008). Thus conver-sion of monoculture forests to mixed forests is closely related to theGHGs reduction of afforestation. Mean CH4 uptake rates in themixed and the pure beech stand ranged between 18 and 48 lgC m�2 h�1 during 2.5 years and were about twice as large as thatin the pure spruce stand (Borken and Beese, 2006). There were sig-nificant differences in annual mean N2O fluxes in broadleaf, mixedand pine forests (0.08 mg N2O m�2 h�1, 0.06 mg N2O m�2 h�1,0.05 mg N2O m�2 h�1, respectively) (Tang et al., 2006). A largevariation in soil CO2 fluxes was observed in the pure and mixedstands of European beech and Norway spruce (Borken and Beese,2005).

Plantations are being established at an increasing rate through-out much of the world, and now account for 5% of global forest cov-er (FAO, 2001). There is also growing recognition of theconservation value of plantations in reducing logging pressure onnatural forests, sequestering carbon, and restoring degraded lands(Kelty, 2006). In China, the total plantation area reached 6.2 � 107

ha, accounting for 31.8% of the total forest area of China, rankingthe first in the world (Department of Forest Resources Manage-ment, SFA, 2010). South China is an appropriate area for developingplantations, because of plenty of solar radiation and water re-sources. The plantation area in south China occupied 63% of the to-tal plantation of China (SFA (State Forestry Administration), 2007).However, most of these plantations were planted with single conif-erous tree species (e.g. Pinus massoniana and Cunninghamia lanceo-lata) and exotic tree species (Eucalyptus) (SFA, 2007), leading to alack of biodiversity, ecosystem stability and soil fertility (Penget al., 2008). Therefore, indigenous valuable broadleaf and mixedplantations, which can supply valuable timber, biodiversity andecosystem services (Carnevalea and Montagnini, 2002; Liang,2007), are increasingly being developed as a prospective silvicul-tural management approach for substituting coniferous planta-tions in subtropical China as well as in other countries (Borkenand Beese, 2006; Vesterdal et al., 2008). Forest conversion couldalter SOC and N stocks and fluxes of future forests by changes inthe amount and morphology of forest floors, which partly controlthe sink and source strength for GHGs. However, there is a lackof knowledge about SOC stocks and soil�atmosphere trace gas ex-changes in mixed forests in comparison to pure stands since moststudies were conducted in pure stands.

The objectives of the study were (i) to assess the effects oftree species mixture on SOC stocks and soil CO2, CH4 and N2Ofluxes and (ii) to investigate their controlling factors in themonoculture and mixed plantations of P. massoniana and C. hys-trix in subtropical China. We have chosen mature stands withsimilar management history and site conditions to assess thelong-term effects of forest conversion on soil carbon sequestra-tion and GHGs fluxes.

Please cite this article in press as: Wang, H., et al. Effects of tree species mixtuplantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.f

2. Materials and methods

2.1. Site description

The study area is located at the Experimental Center of TropicalForestry, Chinese Academy of Forestry (22�100 N, 106�500 E), Ping-xiang City, Guangxi Zhuang Autonomous Region, China, which be-longs to the subtropical region. Annual rainfall is approximately1400 mm, occurring primarily from April to September. Annualmean temperature is 21 �C with a mean monthly minimum tem-perature of 12.1 �C, and a mean monthly maximum temperatureof 26.3 �C. The study site soil of sandy texture was formed fromgranite, classified as red soil in Chinese soil classification, equiva-lent to oxisol in the USDA Soil Taxonomy (Liang and Wen, 1992;State Soil Survey Service of China, 1998; Soil Survey Staff of USDA,2006). Three adjacent plantations of monoculture and mixed P.massoniana and C. hystrix were selected based on their similartopography, soil texture, stand age, and management history. P.massoniana and C. hystrix are the major indigenous tree speciesfor afforestation in subtropical regions. These three plantationswere established in 1983 after a clearcut on a C. lanceolata planta-tion site with an elevation of 550 m. Historically, the study site wasoccupied by a subtropical evergreen forest, and then C. lanceolataplantation was established in 1950s after a clearcut. The standcharacteristics in this study are summarized in Table 1. Six sam-pling plots (20 m � 20 m each) were randomly set up in thesethree plantations, respectively.

2.2. Soil, litterfall and fine root sampling and measurements

Soil samples were collected from the soil surface down to20 cm. A total of six soil cores were collected using an 8.7 cm diam-eter stainless steel core from each plot and bulked to one compos-ite sample. Soil samples were air dried at room temperature(25 �C), and then were passed through a 2 mm mesh sieve to re-move coarse living roots and gravel and ground with a mill to passthrough a 0.25 mm mesh sieve before chemical analysis. Mean-while six soil pits were sampled to measure bulk density in eachplot.

Litterfall was collected monthly from each of five litter traps(1 m � 1 m) with a mesh size of 1 mm in each plot from October2008 to September 2009 and sorted into categories of leaf, smallwoody material, and miscellaneous material (Fang et al., 2007).Litterfall samples were oven dried at 65 �C and weighed.

Sequential soil coring method was used to investigate fine root(diameter < 2 mm) biomass. A total of 12 soil cores from the soilsurface down to 20 cm were collected using an 8.7 cm diameterstainless steel core from each plot on six dates at 2 month intervalsfrom October 2008 to September 2009 (Hendricks et al., 2006). Liveand dead root fragments were subsequently separated by visualinspection as described by Vogt and Persson (1991). Fine root sam-ples were oven dried at 65 �C and weighed. The total fine root



H. Wang et al. / Forest Ecology and Management xxx (2012) xxx–xxx 3

biomass was estimated by the average of fine root biomass of thesix sample dates during the year (Janssens et al., 2002).

Soil, litterfall and fine root samples were analyzed for total or-ganic carbon by the dichromate oxidation method (Nelson andSommers, 1996). Total N was analyzed using the Kjeldahl method(Bremner, 1996). Soil was removed and extracted using a 1 mol L�1

KCl solution (1:4, soil: 1 M KCl, m/v) while shaken for 1 h and thenfiltered (Whatman 42, UK). Subsamples were removed to colori-metrically determine NO3

��N and NH4+-N concentrations. Soil

pH was measured in a 1 mol L�1 KCl solution using a glass elec-trode. Particle size distribution was measured using the hydrome-ter method (DSNR, 2002).

2.3. Soil CO2, CH4 and N2O measurements

Soil CO2, CH4 and N2O fluxes were measured using the staticchamber and gas chromatography techniques (Wang and Wang,2003). One static chamber was established in each plot at the startof the experiment (August 15, 2008). Sampling rings were installed2–3 months before the first sampling campaign, as used in the pre-vious studies (Zhang et al., 2008b; Bréchet et al., 2009). Each cham-ber was a 25-cm-diameter ring anchored 5 cm into the soilpermanently. During flux measurements, a 30-cm-high chambertop was attached to the ring and a fan (about 8 cm in diameter)was installed on the top wall of each chamber to ensure good mix-ing of the air when collected (Mo et al., 2008). Air was sampledfrom each chamber between 09:00 and 10:00 o’clock at each sam-pling date. Diurnal studies showed that fluxes of soil N2O, CH4 andCO2 measured from 09:00 to 10:00 o’clock were close to dailymeans in similar forests (Tang et al., 2006; Mo et al., 2008). Fluxesof soil N2O, CH4 and CO2 were measured monthly during the exper-iment (October 2008 to September 2009). The sampling time inthese three plantations was the same in each month in order tocompare the differences in GHGs flux among plantations with dif-ferent tree species in the study. Gas samples were collected with100 ml plastic syringes at 0, 15 and 30 min intervals after chamberclosure and stored in sealed gas sampling bags. N2O, CH4 and CO2

concentrations in the samples were analyzed within 48 h using gaschromatography (Agilent 4890D, Agilent Co., Santa Clara, CA, USA).The gas chromatography was equipped with an electron capturedetector for N2O analysis and a flame ionization detector for CH4

and CO2 analysis. Gases fluxes were calculated from the linearregression of concentration vs. time using the data points fromeach chamber to minimize the negative effect of chamber closureon N2O, CH4 and CO2 fluxes (Magill et al., 1997; Tang et al.,2006). Coefficients of determination (R2) for all linear regressionswere > 0.96 (p < 0.01).

2.4. Micro-environmental data measurements

Air temperature of the chamber headspace, air temperature at1.5 m above ground and atmospheric pressure were measuredsimultaneously. Soil temperature and moisture at 5 cm below soilsurface were monitored at each chamber while gas samples werecollected. Soil temperature was measured using a digital ther-mometer. Volumetric soil moisture (cm3 H2O cm�3 soil) was mea-sured simultaneously using MPKit soil moisture gauge (NTZT Inc.,Nantong, China). Volumetric soil moisture values were convertedinto values of water filled pore space (WFPS) by the followingformula:

WFPS ½%� ¼ Vol½%�1� bd ½g cm�3 �

2:65 ½g cm�3 �

where bd is bulk density, Vol is volumetric water content and 2.65is the density of quartz.


2.5. Statistical analysis

To assess differences among these three plantations, results ofSOC stock, soil N stock, soil C:N ratio and soil CO2, CH4 and N2Ofluxes were analyzed using one-way ANOVA. Multiple comparisonsof means among plantations were performed using Duncan test.The relationships between soil GHGs fluxes and soil temperatureand WFPS were examined using regression modeling techniques.Pearson linear correlation analysis was used to examine the rela-tionships between SOC stock and leaf litterfall mass and soil N stock,and relationships between soil CO2, CH4 and N2O fluxes and theirinfluencing factors. All variables were of normal distribution andhomogeneity. All analyses were performed using SPSS 13.0 forwindows. Statistical significant differences were set with pvalues < 0.05.

3. Results

3.1. Effects of forest types on SOC and soil N stocks

SOC stock and soil N stock in 0–20 cm layer varied with planta-tion types (Fig. 1a and b). There were significant differences in SOCstock among these three plantations (p < 0.05). SOC stock in 0–20 cm layer in the mixed plantation was 14.3% higher than thatin the P. massoniana, and 8.1% higher than that in the C. hystrixplantations. Soil N stock was significantly lower in the P. massoni-ana plantation than in the C. hystrix and mixed plantations(p < 0.05). Across these three plantations, SOC stock was positivelycorrelated to leaf litterfall mass and soil N stock (Fig. 2a and b).

3.2. Seasonality of soil CO2, CH4 and N2O fluxes

Soil CO2 and N2O fluxes in the mixed plantation displayed sea-sonal trends with the highest value in August, in the hot-humidseason and lowest value in January, in the cool-dry season. How-ever, soil CH4 flux did not show a seasonal trend. Seasonal changesin soil CO2 and N2O emissions in the mixed plantation were posi-tively related to changes in soil temperature and WFPS (Fig. 3a, band e–f). Soil CH4 flux in the mixed plantation was positively re-lated to WFPS, indicating that the absolute magnitude of soil CH4

uptake reduced with increased WFPS (Fig. 3c and d).

3.3. Effects of vegetation types on soil CO2, CH4 and N2O fluxes

There were significant differences in mean soil CO2 emissionamong the plantations (p < 0.05) (Fig. 4a). Lowest mean soil CO2

emission was found in the P. massoniana plantation, followed bythe mixed and C. hystrix plantations (Fig. 4a). Mean soil CO2 emis-sion was 67% and 36% higher in the C. hystrix plantation than in theP. massoniana and mixed plantations.

Mean soil CH4 uptake differed among the plantations (Fig. 4b).The soil of C. hystrix plantation assimilated least CH4, followed bythe mixed and P. massoniana plantation soils (Fig. 4b). Mean soilCH4 uptake was 20% and 24% lower in the C. hystrix plantation thanin the mixed and P. massoniana plantations.

There were significant differences in mean soil N2O emissionamong the plantations (Fig. 4c). Highest mean soil N2O emissionwas observed in the C. hystrix plantation, followed by the mixedand P. massoniana plantations (Fig. 4c). Mean soil N2O emissionwas 51% and 24% higher in the C. hystrix plantation than in themixed and P. massoniana plantations.

3.4. Explanation in variations in soil CO2, CH4 and N2O fluxes

Leaf litterfall mass, fine root biomass and soil N stock were pos-itively correlated with soil CO2 emission (Fig. 5a–c), while soil C:N



0

10

20

30

40

50

60

70

C. hystrixMixed plantationP. massoniana

a

(a)

ba

Soil

orga

nic

C s

tock

s (M

g ha

-1)

0

1

2

3

4

Mixed plantationP. massoniana C. hystrix

(b)b

Soil

nitr

ogen

sto

cks

(Mg

ha-1

)

a

b

Fig. 1. SOC stock (a) and soil N stock (b) from the soil surface down to 20 cm in the P. massoniana, mixed and C. hystrix plantations. Error bars indicate standard error (n = 6).

Leaf litterfall mass (g m-2 yr-1)

150 200 250 300 350

Soil

C s

tock

s (M

g ha

-1)

40

45

50

55

60

65(a)R2=0.22P<0.05

Soil N stocks (Mg ha-1)

2.0 2.5 3.0 3.5 4.0

Soil

C s

tock

s (M

g ha

-1)

40

45

50

55

60

65R2=0.33P<0.05

(b)

Fig. 2. Relationships between SOC stock (0–20 cm) and leaf litterfall mass and soil N stock (0–20 cm) in the P. massoniana, mixed and C. hystrix plantations.


ratio showed a negative relationship with soil CO2 emission(Fig. 5d). Both soil NO3-N content and soil respiration were posi-tively correlated with soil CH4 flux, that is to say, the absolute mag-nitude of soil CH4 uptake reduced with increased soil CO2 emissionand soil NO3-N content among the plantations (Fig. 6a and b).There were positive relationships between soil N2O emission andsoil N stock and soil NO3-N content (Fig. 7a and b), and negativerelationship between soil N2O emission and soil C:N ratio (Fig. 7c).

4. Discussion

4.1. SOC and soil N stocks

Our study demonstrated that SOC stock in 0–20 cm layer wassignificantly higher in the mixed plantation than in the monocul-ture plantations (Fig. 1a). This indicates that mixed forest has an ef-fect on the quantity of SOC, with the higher SOC accumulation inthe mixed plantation than in the conifer and broadleaf plantations.It was also reported that mixed species plantations have a poten-tial to improve carbon sequestration in soil (Kaye et al., 2000; Reshet al., 2002). Some studies reported that SOC stock was relativelylower under coniferous species compared to broadleaf species(Russell et al., 2007; Wang et al., 2010a). Other results showed thatSOC stock was generally larger under coniferous species thanbroadleaf species (Augusto et al., 2002; Kasel and Bennett, 2007;Schulp et al., 2008). Although most studies mentioned above showdifferent effects of vegetation types on SOC stock, there is no con-sensus on the specific effects of coniferous and broadleaf tree spe-cies on SOC. Our findings provide support that mixed plantation


can accumulate more SOC relative to monoculture coniferous andbroadleaf plantations in this region.

Vegetation types can alter SOC stock through several key fac-tors, including litter inputs through litterfall and root turnover(Chen et al., 2004; Jandl et al., 2007), litter quality (Vesterdalet al., 2008), and soil chemistry (Blagodatskaya and Anderson,1998; Mulder et al., 2001; Beets et al., 2002). In this study, SOCstock was positively correlated with leaf litterfall mass and soil Nstock (Fig. 2a), indicating a higher quantity of SOC in the mixedplantations than in the P. massoniana and C. hystrix plantationscould be attributed to aboveground litterfall and soil nutrient sta-tus. First, SOC accumulation is driven by site factors increasing or-ganic carbon input and inhibiting organic carbon decomposition(Jandl et al., 2007). Our result is consistent with results from otherstudies, in which litter mass drove SOC accrual in forests (Russellet al., 2004, 2007). Moreover, high soil N concentration stimulatestree growth, which potentially increases carbon inputs into soilsthrough litterfall and rhizo deposition, and promotes SOC seques-tration by decreasing decomposition rates of old litter and recalci-trant soil organic matter by suppression of soil microbes and bychemical stabilization (Jandl et al., 2007; Mo et al., 2008). Further-more, litterfall-derived carbon at the soil surface can be incorpo-rated into the mineral horizons by leaching of dissolved organiccarbon or particulate organic matter (Montané et al., 2010). Leaflitter decomposition rate of P. massoniana was significantly fasterin the mixed plantation than in the P. massoniana plantation, andleaf litter decomposition rate of C. hystrix was also significantly fas-ter in the mixed plantation than in the C. hystrix plantation (unpub-lished). Hence, the faster decomposition rate of P. massoniana andC. hystrix in the mixed stand than in the monoculture stands could



(a)

(c)

(e)

(b)

(d)

(f)

-2 -2

-2

-2

-2

-2

Fig. 3. Relationships between soil CO2, CH4, and N2O fluxes, soil temperature and soil water filled pore space, WFPS in the P. massoniana, mixed and C. hystrix plantations(n = 72).


result in more carbon incorporation into the mineral soil in themixed plantation in this study.

4.2. Soil CO2 flux

Mean soil CO2 emission rate of 56.2 mg C m�2 h�1 measured inthis study is similar to that measured in temperate forests (Wanget al., 2006), subtropical forests (Tang et al., 2006) and tropical rainforests (Sotta et al., 2004). Soil CO2 emission was higher in themixed plantation than in the P. massoniana plantation, while itwas lower than in the C. hystrix plantation (Fig. 4a). This indicatesthat the mixed forest has an effect on soil respiration rate, whichincreases as the proportion of broadleaf trees increases. Tanget al. (2006) also found that soil respiration rate was higher in


broadleaf evergreen forest than in the mixed and pine forests insubtropical China. Soil respiration was highest in pure beech standand lowest in pure spruce stand among adjacent pure and mixedstands of European beech and Norway spruce at Solling, Germany(Borken and Beese, 2005).

Soil CO2 emission, as the result of soil respiration generatesmainly from autotrophic (root) and heterotrophic (microbial) activ-ity. Much of the spatial variations in soil respiration obtained acrossforest types were explained by differences in the quantity and qual-ity of litter, root biomass and soil properties (Epron et al., 2006). Ourstudy demonstrated that soil CO2 emission increased as leaf litter-fall mass, fine root biomass and soil N stock increased (Fig. 5a–c),while decreased with increased soil C:N ratio (Fig. 5d). Plant litter,as the main source of soil organic matter, is the substrate of soil



0

20

40

60

80

100

C. hystrixMixed plantationP. massoniana

c

(a)

b

aC

O2-C

flux

(mg

m-2

h-1

)

0

-10

-20

-30

-40

-50

Mixed plantationP. massoniana C. hystrix

(b)

b

CH

4-C fl

ux (u

g m

-2 h

-1)

a a

0

2

4

6

8

10

Mixed plantation C. hystrix

(c)N

2O-N

flux

(ug

m-2

h-1

)

cb

a

P. massoniana

Fig. 4. Mean soil CO2, CH4, and N2O fluxes in the P. massoniana, mixed and C. hystrix plantations. Error bars indicate standard error (n = 6).

Leaf litterfall mass (g m-2 yr-1)

150 200 250 300 350

Soil

CO

2-C (m

g m

-2 h

-1)

20

40

60

80

100(a)R2=0.25P<0.05

Fine roots biomass (g m-2)

100 200 300 400 500

Soil

CO

2-C

(mg

m-2

h-1

)

20

40

60

80

100(b)R2=0.79P<0.05

Soil nitrogen stock (Mg ha-1)2.0 2.5 3.0 3.5 4.0

Soil

CO

2-C

(mg

m-2

h-1

)

20

40

60

80

100(c) R2=0.46P<0.05

Soil C/N 12 14 16 18 20

Soil

CO

2-C (m

g m

-2 h

-1)

20

40

60

80

100(d) R2=0.48P<0.05

Fig. 5. Relationships between soil CO2 flux and leaf litterfall mass, fine roots biomass, soil N stock and soil C:N ratio (0–20 cm) in the P. massoniana, mixed and C. hystrixplantations.


Please cite this article in press as: Wang, H., et al. Effects of tree species mixture on soil organic carbon stocks and greenhouse gas fluxes in subtropicalplantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005


Soil NO3-N content (mg kg-1)

1.0 1.5 2.0 2.5 3.0

Soil

CH

4-C

flu

x (u

g m

-2 h

-1)

-50

-45

-40

-35

-30

-25

-20

(a)R2=0.38P<0.05

Soil CO2-C (mg m-1 h-1)

20 40 60 80

Soil

CH

4-C

flu

x (u

g m

-2 h

-1)

-50

-45

-40

-35

-30

-25

-20

(b)R2=0.37P<0.05

Fig. 6. Relationships between soil CH4 flux and soil NO3-N content (0–20 cm) and soil CO2 efflux in the P. massoniana, mixed and C. hystrix plantations.

Soil nitrogen stock (Mg ha-1)

2.0 2.5 3.0 3.5 4.0

Soil

N2O

-N fl

ux(u

g m

-2 h

-1)

2

3

4

5

6

7

8(a)R2=0.35P<0.05

Soil NO3-N content (mg kg-1)

1.0 1.5 2.0 2.5 3.0

Soil

N2O

-N fl

ux(u

g m

-2 h

-1)

2

3

4

5

6

7

8(b) R2=0.49P<0.05

Soil C/N

12 14 16 18 20

Soil

N2O

-N fl

ux(u

g m

-2 h

-1)

2

3

4

5

6

7

8(c) R2=0.38P<0.05

Fig. 7. Relationships between soil N2O flux and soil N stock, soil NO3-N content and soil C:N ratio (0–20 cm) in the P. massoniana, mixed and C. hystrix plantations.


microbial metabolic activity, and thus influences soil respiration(Zhang et al., 2005). Soil heterotrophic respiration decreased withaboveground litter input decreased (Sheng et al., 2010). Raich andSchlesinger (1992) found that soil respiration was positively corre-lated to litterfall mass. There was a positive relationship betweensoil respiration and leaf litterfall mass (Fig. 5a). Thus litter inputwas an important factor regulating soil respiration in this study,as observed in some studies that compared different vegetationtypes (Adachi et al., 2006; Sheng et al., 2010). Moreover, land usechange can influence root input in belowground, and thus changeautotrophic respiration derived from roots (Hertel et al., 2009). Inthis study, fine root biomass was higher in the C. hystrix plantationthan in the mixed and P. massoniana plantations, and there was apositive correlation between soil respiration and fine root biomass(Fig. 5b). Therefore, fine root biomass was another primary factordriving the differences in the plantations in this study. Soil CO2


emission decreased with fine root biomass decreased among landuse types in mid-subtropical region (Sheng et al., 2010). Wanget al. (2006) also observed that soil respiration was higher in forestswith higher fine root biomass in temperate zone. In addition,numerous studies emphasized that the enhanced soil N concentra-tion can increase soil labile organic matter and substrate of respira-tion, and thus increase soil microbial activity and root biomass(Zhang et al., 2005). Soil C:N ratio, as a good indicator of substratequality, was an important factor representing soil nutrient avail-ability (Ren et al., 2006). Soil CO2 emission was positively correlatedto soil N stock, while negatively correlated to soil C:N ratio in thisstudy (Fig. 5c and d). Hence, difference in soil respiration rateamong the plantations was also attributed to soil nutrient status.The study in tropical forests showed the similar result that soil res-piration rate was positively related to soil N content (Werner et al.,2007).




The temporal variation in soil CO2 emission in the mixed plan-tation coincided with the variations in soil temperature and mois-ture (Fig. 3a and b). The temporal variations in soil respiration inthe P. massoniana and C. hystrix plantations were also correlatedto soil temperature and moisture (Wang et al., 2010b). These re-sults indicate that soil temperature and moisture exert significanteffects on the temporal variations in soil CO2 emissions in thesesubtropical plantations. The previous studies in subtropical forestssupported our results (Tang et al. 2006; Sheng et al., 2010).

4.3. Soil CH4 flux

Soil CH4 measurements indicated a consistent net soil con-sumption of CH4 (i.e. negative CH4 flux) in these three plantations.Mean soil CH4 uptake rate of �35.7 lg C m�2 h�1 measured in theplantations is similar to that measured in other forests (Verchotet al., 2000; Borken and Beese, 2006; Tang et al., 2006; Fest et al.,2009). Mean soil CH4 uptake was significantly lower in the C. hys-trix plantation than in the mixed and P. massoniana plantations(Fig. 4b), as observed in some studies that compared coniferouswith broadleaf forests (Xu et al., 1995; McNamara et al., 2008).However, in several studies soil CH4 uptake was higher in broadleafforests than in mixed and coniferous forests (Tang et al., 2006; Bor-ken and Beese, 2006). Although most mentioned studies show dif-ferent effects of vegetation types on soil CH4 flux, there is noconsensus on the specific effect of tree species and silvicultural ap-proach on soil CH4 flux. Our findings provide support that soils ofconiferous and mixed plantations can assimilate more CH4 relativeto broadleaf plantation in the subtropical region.

Soil CH4 flux is controlled by CH4 producing methanogens oper-ating at anaerobic conditions and CH4 consuming methanotrophsthat depend on oxygen as a terminal electron acceptor (Topp andPattey, 1997). Activity and population sizes of these microbes aredependent on a multitude of soil factors, like soil temperature,moisture, pH, substrate availability, and aeration of soil profile (Ver-chot et al., 2000; Merino et al., 2004; Reay and Nedwell, 2004; Wer-ner et al., 2007). Significant relationships have been reportedbetween inorganic N stocks, N availability indices or rates of nitrifi-cation and soil CH4 flux rates in forests (Castro et al., 1994). It hasalso been suggested that ammonium, nitrate and cations associatedwith nitrate are the main factors producing the inhibitory effect onsoil CH4 oxidation (Keller et al., 1990; Jang et al., 2006). Soil NO3-Ncontent was higher in the C. hystrix plantation than in the mixed andP. massoniana plantations, and soil CH4 uptake decreased with in-creased soil NO3-N content (Fig. 6a). Therefore, soil NO3-N contentwas a major factor driving soil CH4 uptake in this study. Further-more, the high soil respiration rates can create anaerobic micrositesas O2 is consumed, resulting in CH4 production in soil (Verchot et al.,2000). Thus soils should consume less CH4 when CO2 production byroot and microbial respiration is high. In this study, the lower meansoil CH4 uptake in the C. hystrix plantation than in the P. massonianaand mixed plantations could also be attributed to the higher meansoil CO2 efflux in the C. hystrix plantation.

The temporal variation in soil CH4 flux in the mixed plantationdisplayed dependency on soil WFPS (Fig. 3c and d). The temporalvariations in soil CH4 fluxes in the P. massoniana and C. hystrix plan-tations were also correlated to soil WFPS (Wang et al., 2010b).These results are similar with other studies in tropical and temper-ate forests, where soil CH4 uptake rates were negatively related tosoil moisture (Castro et al., 2000; Verchot et al., 2000).

4.4. Soil N2O flux

Mean soil N2O emission of 5.3 lg N m�2 h�1 measured in thesethree plantations agrees with estimates from other forest studies(Merino et al., 2004; Rosenkranz et al., 2006; Livesley et al.,


2009). Soil N2O emission was higher in the mixed plantation thanin the P. massoniana plantation, while it was lower than in the C.hystrix plantation (Fig. 4c), revealing the effects of mixed foreston soil N2O emission. Tang et al. (2006) also found that soil N2Oemission was higher in broadleaf evergreen forest than in themixed and pine forests in subtropical China.

The general soil N2O emission potential is largely controlled bysoil pH (Stevens et al., 1997), soil carbon and N stocks (Li et al.,2005), soil inorganic N contents (Merino et al., 2004) and C:N ratioof litter and soil (Booth et al., 2005; Werner et al., 2007). In thisstudy, soil N2O emission was positively related to soil N stock andsoil NO3-N content, while negatively related to soil C:N ratio(Fig. 7). Nitrifying bacteria convert ammonium (NH4

+) to nitriteand nitrate (NO3

�) under aerobic conditions and can produce N2Oas a byproduct of this conversion pathway (Livesley et al., 2009).Soil N2O emission is controlled by soil nitrification rate (Ambuset al., 2006), and the main product of nitrification, NO3-N content(Livesley et al., 2009). Thus soil NO3-N content was an importantfactor regulating soil N2O emission in this study, as observed inthe other study that soil N2O emission rate increased as soil NO3-N content increased under different vegetation types (Livesleyet al., 2009). Moreover, soil C:N ratio usually can be used as animportant indicator of soil nutrient availability and soil quality(Ren et al., 2006). The decrease of soil N status and availability couldreduce soil N cycling, and thus lead to the decline in soil N2O emis-sion (Werner et al., 2007; Zhang et al., 2008a). In this study, soil Nstock was higher in the C. hystrix plantation than in the mixed andP. massoniana plantations, and soil C:N ratio was lower in the C. hys-trix plantation than in the mixed and P. massoniana plantations.Hence, differences in the magnitude of soil N2O emission amongthe plantations could also be explained by soil N stock and C:N ratio.

The temporal variations in soil N2O emissions were attributedto the temporal variations in soil temperature and moisture(Fig. 3e and f). The temporal variations in soil N2O emissions inthe P. massoniana and C. hystrix plantations were also correlatedto soil temperature and moisture (Wang et al., 2010b). Similar re-sults were reported in other subtropical forests (Tang et al., 2006;Liu et al., 2008).

5. Conclusion

SOC stock in 0–20 cm layer in the mixed plantation was 14.3%higher than that in the P. massoniana, and 8.1% higher than that inthe C. hystrix plantations. The differences in SOC stock among theplantations were attributed to soil N stock and leaf litterfall input.Mean soil CO2 and N2O emissions were significantly higher in themixed plantation than in the P. massoniana plantation, while theywere lower than in the C. hystrix plantation. Mean soil CH4 uptakewas significantly higher in the mixed plantation than in the C. hys-trix plantation, while it was similar to that in the P. massonianaplantation. This study confirms that the mixed plantation has ahigher SOC stock than the monoculture plantations, and there isan increase in amount of GHGs absorbed by the soil of mixed plan-tations compared to C. hystrix plantations. Therefore, conifer-broad-leaf mixed plantation could be a better silvicultural mode for soilcarbon sequestration than monoculture broadleaf plantation forsubstituting large coniferous plantations in subtropical China.

Acknowledgements

We are grateful to Riming He, Hai Chen, Zhongguo Li, XuemanHuang, Yuan Wen, Jia Xu, Lu Zheng and En Liu for their assistancein field sampling and data collection. We also gratefully acknowl-edge the support from the Chinese Academy of Forestry’s Experi-mental Center of Tropical Forestry. This study was funded by the




Ministry of Finance (200804001 and 201104006), the Ministry ofScience and Technology (2011CB403205 and 2012BAD22B01),China’s National Natural Science Foundation (31290223 and31100380) and Institute of Forest Ecology, Environment andProtection, Chinese Academy of Forestry Foundation (CAFRI-FEEP201104), and was supported by the CFERN & GENE AwardFunds on Ecological Paper.

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