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

Forest Ecology and Management xxx (2012) xxx–xxx

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Effects of tree species mixture on soil organic carbon stocks and greenhouse gas 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, China b Division of Forestry and Natural Resources, West Virginia University, P.O. Box 6215, Morgantown, WV 26506-6125, USA c 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 carbon Soilatmosphere trace gas exchanges Mixed plantation Forest conversion Subtropical China

a b s t r a c t Indigenous broadleaf plantations are increasingly being developed as a prospective silvicultural approach for substituting coniferous plantations in subtropical China. Three plantations of monoculture and mixed Pinus massoniana and Castanopsis hystrix were selected to examine soil organic carbon (SOC) stocks and temporal and spatial patterns of the main greenhouse gases fluxes for understanding the effects of mixed forests on soil carbon and nitrogen (N) cycling processes. We found that 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 in the C. hystrix plantations. Differences in SOC stock among the plantations were attributed to soil N stock and leaf litterfall input. Soil CO2 and N2O fluxes in the mixed plantation displayed the seasonal trends, while soil CH4 flux did not show the seasonal trend. The seasonal variations in soil CO2 and N2O emissions were positively related to soil temperature and moisture. Mean soil CO2 and N2O emissions (53.2 mg C m2 h1 and 5.21 lg N m2 h1, respectively) were significantly higher in the mixed plantation than in the P. massoniana plantation, while they were lower than in the C. hystrix plantation. Mean soil CH4 uptake (38.4 lg C m2 h1) 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 plantations were influenced by fine root biomass, leaf litterfall mass, soil N stock and soil C:N ratio. Differences in soil N2O flux among the plantations could be attributed to the differences in soil N stock, soil NO3N content and soil C:N ratio. Soil respiration rate and soil NO3N content could account for variations in soil CH4 flux among the plantations. This study confirms that the mixed plantation has a higher SOC stock than the monoculture plantations, and there is an increase in amount of GHG absorbed by the soil of mixed plantations 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 plantation for substituting P. massoniana plantations in subtropical China. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Carbon sequestration in vegetation and soil is recognized as a mechanism that can mitigate atmospheric carbon dioxide (CO2) accumulation (Janzen, 2004). Afforestation and reforestation are considered important tools for sequestering atmospheric CO2 by which to offset greenhouse gases (GHGs) emissions from fossil fuels; however, establishment of plantations necessarily involves several silvicultural treatments that may impact soil carbon

⇑ Corresponding author. Tel.: +86 10 62889311; fax: +86 10 62884229. E-mail address: [email protected] (S. Liu).

sequestration and its relative stability (Maillard et al., 2010). The enhanced production and reduced consumption of naturally occurring GHGs such as CO2, nitrous oxide (N2O) and methane (CH4), are responsible for approximately 90% of the global warming and climate change phenomenon (Solomon et al., 2007). A considerable amount of atmospheric GHGs is produced and consumed through soil processes (Tang et al., 2006). However, there remain considerable uncertainties about the effects of forest management activities on soil carbon sequestration and greenhouse gas fluxes (Post and Kwon, 2000; Johnston et al., 2004; Kelliher et al., 2006). Forest soil organic carbon (SOC) is influenced by the complex interactions of climate, soil type, management, and tree species (Lal, 2005). A growing body of evidence has demonstrated that forest species composition will influence soil carbon turnover due to

0378-1127/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.foreco.2012.04.005

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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its different microclimates at the forest floor (Berger et al., 2002). These effects have been attributed to the fact that tree species could potentially alter size and physiochemical properties of carbon additions in litter from the aboveground and belowground flora and fauna, distribution of the root systems of plants in the soil profile, distribution of carbon within the soil matrix and its interaction with clay surfaces (Oades, 1988; Berger et al., 2002). Several studies have shown that mixed forests may influence soil carbon and nitrogen (N) concentrations and stocks (Berger et al., 2002; Borken and Beese, 2005; Tang et al., 2006). Afforestation and reforestation can greatly affect soil GHGs fluxes by changing key physical and chemical properties that influence soil nutrients and carbon cycling and microbial activity (Paul et al., 2002; Merino et al., 2004; Kelliher et al., 2006). Tree species are considered to alter soil chemical, physical (e. g. moisture and temperature) and biological processes through their root system, crown structure, foliage, leaf structure and litter quality (Borken and Beese, 2006; Jonard et al., 2007; Ullah et al., 2008). Thus conversion of monoculture forests to mixed forests is closely related to the GHGs reduction of afforestation. Mean CH4 uptake rates in the mixed and the pure beech stand ranged between 18 and 48 lg C m2 h1 during 2.5 years and were about twice as large as that in the pure spruce stand (Borken and Beese, 2006). There were significant differences in annual mean N2O fluxes in broadleaf, mixed and pine forests (0.08 mg N2O m2 h1, 0.06 mg N2O m2 h1, 0.05 mg N2O m2 h1, respectively) (Tang et al., 2006). A large variation in soil CO2 fluxes was observed in the pure and mixed stands of European beech and Norway spruce (Borken and Beese, 2005). Plantations are being established at an increasing rate throughout much of the world, and now account for 5% of global forest cover (FAO, 2001). There is also growing recognition of the conservation value of plantations in reducing logging pressure on natural 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, ranking the first in the world (Department of Forest Resources Management, SFA, 2010). South China is an appropriate area for developing plantations, because of plenty of solar radiation and water resources. The plantation area in south China occupied 63% of the total plantation of China (SFA (State Forestry Administration), 2007). However, most of these plantations were planted with single coniferous tree species (e.g. Pinus massoniana and Cunninghamia lanceolata) and exotic tree species (Eucalyptus) (SFA, 2007), leading to a lack of biodiversity, ecosystem stability and soil fertility (Peng et al., 2008). Therefore, indigenous valuable broadleaf and mixed plantations, which can supply valuable timber, biodiversity and ecosystem services (Carnevalea and Montagnini, 2002; Liang, 2007), are increasingly being developed as a prospective silvicultural management approach for substituting coniferous plantations in subtropical China as well as in other countries (Borken and Beese, 2006; Vesterdal et al., 2008). Forest conversion could alter SOC and N stocks and fluxes of future forests by changes in the amount and morphology of forest floors, which partly control the sink and source strength for GHGs. However, there is a lack of knowledge about SOC stocks and soilatmosphere trace gas exchanges in mixed forests in comparison to pure stands since most studies were conducted in pure stands. The objectives of the study were (i) to assess the effects of tree species mixture on SOC stocks and soil CO2, CH4 and N2O fluxes and (ii) to investigate their controlling factors in the monoculture and mixed plantations of P. massoniana and C. hystrix in subtropical China. We have chosen mature stands with similar management history and site conditions to assess the long-term effects of forest conversion on soil carbon sequestration and GHGs fluxes.

2. Materials and methods 2.1. Site description The study area is located at the Experimental Center of Tropical Forestry, Chinese Academy of Forestry (22°100 N, 106°500 E), Pingxiang City, Guangxi Zhuang Autonomous Region, China, which belongs to the subtropical region. Annual rainfall is approximately 1400 mm, occurring primarily from April to September. Annual mean temperature is 21 °C with a mean monthly minimum temperature of 12.1 °C, and a mean monthly maximum temperature of 26.3 °C. The study site soil of sandy texture was formed from granite, classified as red soil in Chinese soil classification, equivalent 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 similar topography, soil texture, stand age, and management history. P. massoniana and C. hystrix are the major indigenous tree species for afforestation in subtropical regions. These three plantations were established in 1983 after a clearcut on a C. lanceolata plantation site with an elevation of 550 m. Historically, the study site was occupied by a subtropical evergreen forest, and then C. lanceolata plantation was established in 1950s after a clearcut. The stand characteristics in this study are summarized in Table 1. Six sampling plots (20 m  20 m each) were randomly set up in these three plantations, respectively.

2.2. Soil, litterfall and fine root sampling and measurements Soil samples were collected from the soil surface down to 20 cm. A total of six soil cores were collected using an 8.7 cm diameter stainless steel core from each plot and bulked to one composite sample. Soil samples were air dried at room temperature (25 °C), and then were passed through a 2 mm mesh sieve to remove coarse living roots and gravel and ground with a mill to pass through a 0.25 mm mesh sieve before chemical analysis. Meanwhile six soil pits were sampled to measure bulk density in each plot. 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 October 2008 to September 2009 and sorted into categories of leaf, small woody 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 soil surface down to 20 cm were collected using an 8.7 cm diameter stainless steel core from each plot on six dates at 2 month intervals from October 2008 to September 2009 (Hendricks et al., 2006). Live and dead root fragments were subsequently separated by visual inspection as described by Vogt and Persson (1991). Fine root samples were oven dried at 65 °C and weighed. The total fine root

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

Pinus massoniana

Mixed plantation

Castanopsis hystrix

DBH (cm) Tree height (m) Stem density (trees ha1) Sand (%) Silt (%) Clay (%)

24.6 17.2 404

27.3 18.1 400

24.9 17.8 415

57 8 35

62 7 31

59 7 34

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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

biomass was estimated by the average of fine root biomass of the six sample dates during the year (Janssens et al., 2002). Soil, litterfall and fine root samples were analyzed for total organic carbon by the dichromate oxidation method (Nelson and Sommers, 1996). Total N was analyzed using the Kjeldahl method (Bremner, 1996). Soil was removed and extracted using a 1 mol L1 KCl solution (1:4, soil: 1 M KCl, m/v) while shaken for 1 h and then filtered (Whatman 42, UK). Subsamples were removed to colorimetrically determine NO3N and NH4+-N concentrations. Soil pH was measured in a 1 mol L1 KCl solution using a glass electrode. Particle size distribution was measured using the hydrometer method (DSNR, 2002). 2.3. Soil CO2, CH4 and N2O measurements Soil CO2, CH4 and N2O fluxes were measured using the static chamber and gas chromatography techniques (Wang and Wang, 2003). One static chamber was established in each plot at the start of the experiment (August 15, 2008). Sampling rings were installed 2–3 months before the first sampling campaign, as used in the previous studies (Zhang et al., 2008b; Bréchet et al., 2009). Each chamber was a 25-cm-diameter ring anchored 5 cm into the soil permanently. During flux measurements, a 30-cm-high chamber top 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 mixing of the air when collected (Mo et al., 2008). Air was sampled from each chamber between 09:00 and 10:00 o’clock at each sampling date. Diurnal studies showed that fluxes of soil N2O, CH4 and CO2 measured from 09:00 to 10:00 o’clock were close to daily means in similar forests (Tang et al., 2006; Mo et al., 2008). Fluxes of soil N2O, CH4 and CO2 were measured monthly during the experiment (October 2008 to September 2009). The sampling time in these three plantations was the same in each month in order to compare the differences in GHGs flux among plantations with different tree species in the study. Gas samples were collected with 100 ml plastic syringes at 0, 15 and 30 min intervals after chamber closure and stored in sealed gas sampling bags. N2O, CH4 and CO2 concentrations in the samples were analyzed within 48 h using gas chromatography (Agilent 4890D, Agilent Co., Santa Clara, CA, USA). The gas chromatography was equipped with an electron capture detector for N2O analysis and a flame ionization detector for CH4 and CO2 analysis. Gases fluxes were calculated from the linear regression of concentration vs. time using the data points from each chamber to minimize the negative effect of chamber closure on N2O, CH4 and CO2 fluxes (Magill et al., 1997; Tang et al., 2006). Coefficients of determination (R2) for all linear regressions were > 0.96 (p < 0.01). 2.4. Micro-environmental data measurements Air temperature of the chamber headspace, air temperature at 1.5 m above ground and atmospheric pressure were measured simultaneously. Soil temperature and moisture at 5 cm below soil surface were monitored at each chamber while gas samples were collected. Soil temperature was measured using a digital thermometer. Volumetric soil moisture (cm3 H2O cm3 soil) was measured simultaneously using MPKit soil moisture gauge (NTZT Inc., Nantong, China). Volumetric soil moisture values were converted into values of water filled pore space (WFPS) by the following formula:

WFPS ½% ¼

3

2.5. Statistical analysis To assess differences among these three plantations, results of SOC stock, soil N stock, soil C:N ratio and soil CO2, CH4 and N2O fluxes were analyzed using one-way ANOVA. Multiple comparisons of means among plantations were performed using Duncan test. The relationships between soil GHGs fluxes and soil temperature and WFPS were examined using regression modeling techniques. Pearson linear correlation analysis was used to examine the relationships between SOC stock and leaf litterfall mass and soil N stock, and relationships between soil CO2, CH4 and N2O fluxes and their influencing factors. All variables were of normal distribution and homogeneity. All analyses were performed using SPSS 13.0 for windows. Statistical significant differences were set with p values < 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 plantation types (Fig. 1a and b). There were significant differences in SOC stock among these three plantations (p < 0.05). 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 in the C. hystrix plantations. Soil N stock was significantly lower in the P. massoniana plantation than in the C. hystrix and mixed plantations (p < 0.05). Across these three plantations, SOC stock was positively correlated 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 seasonal trends with the highest value in August, in the hot-humid season and lowest value in January, in the cool-dry season. However, soil CH4 flux did not show a seasonal trend. Seasonal changes in soil CO2 and N2O emissions in the mixed plantation were positively related to changes in soil temperature and WFPS (Fig. 3a, b and e–f). Soil CH4 flux in the mixed plantation was positively related 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 emission among the plantations (p < 0.05) (Fig. 4a). Lowest mean soil CO2 emission was found in the P. massoniana plantation, followed by the mixed and C. hystrix plantations (Fig. 4a). Mean soil CO2 emission was 67% and 36% higher in the C. hystrix plantation than in the P. massoniana and mixed plantations. Mean soil CH4 uptake differed among the plantations (Fig. 4b). The soil of C. hystrix plantation assimilated least CH4, followed by the mixed and P. massoniana plantation soils (Fig. 4b). Mean soil CH4 uptake was 20% and 24% lower in the C. hystrix plantation than in the mixed and P. massoniana plantations. There were significant differences in mean soil N2O emission among the plantations (Fig. 4c). Highest mean soil N2O emission was observed in the C. hystrix plantation, followed by the mixed and P. massoniana plantations (Fig. 4c). Mean soil N2O emission was 51% and 24% higher in the C. hystrix plantation than in the mixed and P. massoniana plantations.

Vol½% 3

bd ½g cm  1  2:65 ½g cm3 

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

3.4. Explanation in variations in soil CO2, CH4 and N2O fluxes Leaf litterfall mass, fine root biomass and soil N stock were positively correlated with soil CO2 emission (Fig. 5a–c), while soil C:N

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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

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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).

150

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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 positively correlated with soil CH4 flux, that is to say, the absolute magnitude of soil CH4 uptake reduced with increased soil CO2 emission and soil NO3-N content among the plantations (Fig. 6a and b). There were positive relationships between soil N2O emission and soil N stock and soil NO3-N content (Fig. 7a and b), and negative relationship 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 was significantly higher in the mixed plantation than in the monoculture plantations (Fig. 1a). This indicates that mixed forest has an effect on the quantity of SOC, with the higher SOC accumulation in the mixed plantation than in the conifer and broadleaf plantations. It was also reported that mixed species plantations have a potential to improve carbon sequestration in soil (Kaye et al., 2000; Resh et al., 2002). Some studies reported that SOC stock was relatively lower under coniferous species compared to broadleaf species (Russell et al., 2007; Wang et al., 2010a). Other results showed that SOC stock was generally larger under coniferous species than broadleaf species (Augusto et al., 2002; Kasel and Bennett, 2007; Schulp et al., 2008). Although most studies mentioned above show different effects of vegetation types on SOC stock, there is no consensus on the specific effects of coniferous and broadleaf tree species on SOC. Our findings provide support that mixed plantation

can accumulate more SOC relative to monoculture coniferous and broadleaf plantations in this region. Vegetation types can alter SOC stock through several key factors, including litter inputs through litterfall and root turnover (Chen et al., 2004; Jandl et al., 2007), litter quality (Vesterdal et al., 2008), and soil chemistry (Blagodatskaya and Anderson, 1998; Mulder et al., 2001; Beets et al., 2002). In this study, SOC stock was positively correlated with leaf litterfall mass and soil N stock (Fig. 2a), indicating a higher quantity of SOC in the mixed plantations than in the P. massoniana and C. hystrix plantations could be attributed to aboveground litterfall and soil nutrient status. First, SOC accumulation is driven by site factors increasing organic carbon input and inhibiting organic carbon decomposition (Jandl et al., 2007). Our result is consistent with results from other studies, in which litter mass drove SOC accrual in forests (Russell et al., 2004, 2007). Moreover, high soil N concentration stimulates tree growth, which potentially increases carbon inputs into soils through litterfall and rhizo deposition, and promotes SOC sequestration by decreasing decomposition rates of old litter and recalcitrant soil organic matter by suppression of soil microbes and by chemical stabilization (Jandl et al., 2007; Mo et al., 2008). Furthermore, litterfall-derived carbon at the soil surface can be incorporated into the mineral horizons by leaching of dissolved organic carbon or particulate organic matter (Montané et al., 2010). Leaf litter decomposition rate of P. massoniana was significantly faster in the mixed plantation than in the P. massoniana plantation, and leaf litter decomposition rate of C. hystrix was also significantly faster in the mixed plantation than in the C. hystrix plantation (unpublished). Hence, the faster decomposition rate of P. massoniana and C. hystrix in the mixed stand than in the monoculture stands could

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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

-2

(c)

-2

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(f)

-2

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(e)

(b)

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5

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 the mixed plantation in this study. 4.2. Soil CO2 flux Mean soil CO2 emission rate of 56.2 mg C m2 h1 measured in this study is similar to that measured in temperate forests (Wang et al., 2006), subtropical forests (Tang et al., 2006) and tropical rain forests (Sotta et al., 2004). Soil CO2 emission was higher in the mixed plantation than in the P. massoniana plantation, while it was lower than in the C. hystrix plantation (Fig. 4a). This indicates that the mixed forest has an effect on soil respiration rate, which increases as the proportion of broadleaf trees increases. Tang et al. (2006) also found that soil respiration rate was higher in

broadleaf evergreen forest than in the mixed and pine forests in subtropical China. Soil respiration was highest in pure beech stand and lowest in pure spruce stand among adjacent pure and mixed stands of European beech and Norway spruce at Solling, Germany (Borken and Beese, 2005). Soil CO2 emission, as the result of soil respiration generates mainly from autotrophic (root) and heterotrophic (microbial) activity. Much of the spatial variations in soil respiration obtained across forest types were explained by differences in the quantity and quality of litter, root biomass and soil properties (Epron et al., 2006). Our study demonstrated that soil CO2 emission increased as leaf litterfall 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

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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P. massoniana Mixed plantation C. hystrix 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).

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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. hystrix plantations.

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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

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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.

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Soil C/N 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 with aboveground litter input decreased (Sheng et al., 2010). Raich and Schlesinger (1992) found that soil respiration was positively correlated to litterfall mass. There was a positive relationship between soil respiration and leaf litterfall mass (Fig. 5a). Thus litter input was an important factor regulating soil respiration in this study, as observed in some studies that compared different vegetation types (Adachi et al., 2006; Sheng et al., 2010). Moreover, land use change can influence root input in belowground, and thus change autotrophic respiration derived from roots (Hertel et al., 2009). In this study, fine root biomass was higher in the C. hystrix plantation than in the mixed and P. massoniana plantations, and there was a positive correlation between soil respiration and fine root biomass (Fig. 5b). Therefore, fine root biomass was another primary factor driving the differences in the plantations in this study. Soil CO2

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

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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The temporal variation in soil CO2 emission in the mixed plantation coincided with the variations in soil temperature and moisture (Fig. 3a and b). The temporal variations in soil respiration in the P. massoniana and C. hystrix plantations were also correlated to soil temperature and moisture (Wang et al., 2010b). These results indicate that soil temperature and moisture exert significant effects on the temporal variations in soil CO2 emissions in these subtropical plantations. The previous studies in subtropical forests supported our results (Tang et al. 2006; Sheng et al., 2010). 4.3. Soil CH4 flux Soil CH4 measurements indicated a consistent net soil consumption of CH4 (i.e. negative CH4 flux) in these three plantations. Mean soil CH4 uptake rate of 35.7 lg C m2 h1 measured in the plantations is similar to that measured in other forests (Verchot et al., 2000; Borken and Beese, 2006; Tang et al., 2006; Fest et al., 2009). Mean soil CH4 uptake was significantly lower in the C. hystrix plantation than in the mixed and P. massoniana plantations (Fig. 4b), as observed in some studies that compared coniferous with broadleaf forests (Xu et al., 1995; McNamara et al., 2008). However, in several studies soil CH4 uptake was higher in broadleaf forests than in mixed and coniferous forests (Tang et al., 2006; Borken and Beese, 2006). Although most mentioned studies show different effects of vegetation types on soil CH4 flux, there is no consensus on the specific effect of tree species and silvicultural approach on soil CH4 flux. Our findings provide support that soils of coniferous and mixed plantations can assimilate more CH4 relative to broadleaf plantation in the subtropical region. Soil CH4 flux is controlled by CH4 producing methanogens operating at anaerobic conditions and CH4 consuming methanotrophs that depend on oxygen as a terminal electron acceptor (Topp and Pattey, 1997). Activity and population sizes of these microbes are dependent on a multitude of soil factors, like soil temperature, moisture, pH, substrate availability, and aeration of soil profile (Verchot et al., 2000; Merino et al., 2004; Reay and Nedwell, 2004; Werner et al., 2007). Significant relationships have been reported between inorganic N stocks, N availability indices or rates of nitrification and soil CH4 flux rates in forests (Castro et al., 1994). It has also been suggested that ammonium, nitrate and cations associated with nitrate are the main factors producing the inhibitory effect on soil CH4 oxidation (Keller et al., 1990; Jang et al., 2006). Soil NO3-N content was higher in the C. hystrix plantation than in the mixed and P. massoniana plantations, and soil CH4 uptake decreased with increased soil NO3-N content (Fig. 6a). Therefore, soil NO3-N content was a major factor driving soil CH4 uptake in this study. Furthermore, the high soil respiration rates can create anaerobic microsites as O2 is consumed, resulting in CH4 production in soil (Verchot et al., 2000). Thus soils should consume less CH4 when CO2 production by root and microbial respiration is high. In this study, the lower mean soil CH4 uptake in the C. hystrix plantation than in the P. massoniana and mixed plantations could also be attributed to the higher mean soil CO2 efflux in the C. hystrix plantation. The temporal variation in soil CH4 flux in the mixed plantation displayed dependency on soil WFPS (Fig. 3c and d). The temporal variations in soil CH4 fluxes in the P. massoniana and C. hystrix plantations were also correlated to soil WFPS (Wang et al., 2010b). These results are similar with other studies in tropical and temperate forests, where soil CH4 uptake rates were negatively related to soil moisture (Castro et al., 2000; Verchot et al., 2000). 4.4. Soil N2O flux Mean soil N2O emission of 5.3 lg N m2 h1 measured in these three 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 than in the P. massoniana plantation, while it was lower than in the C. hystrix plantation (Fig. 4c), revealing the effects of mixed forest on soil N2O emission. Tang et al. (2006) also found that soil N2O emission was higher in broadleaf evergreen forest than in the mixed and pine forests in subtropical China. The general soil N2O emission potential is largely controlled by soil 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 ratio of litter and soil (Booth et al., 2005; Werner et al., 2007). In this study, soil N2O emission was positively related to soil N stock and soil NO3-N content, while negatively related to soil C:N ratio (Fig. 7). Nitrifying bacteria convert ammonium (NH4+) to nitrite and nitrate (NO3) under aerobic conditions and can produce N2O as a byproduct of this conversion pathway (Livesley et al., 2009). Soil N2O emission is controlled by soil nitrification rate (Ambus et al., 2006), and the main product of nitrification, NO3-N content (Livesley et al., 2009). Thus soil NO3-N content was an important factor regulating soil N2O emission in this study, as observed in the other study that soil N2O emission rate increased as soil NO3N content increased under different vegetation types (Livesley et al., 2009). Moreover, soil C:N ratio usually can be used as an important indicator of soil nutrient availability and soil quality (Ren et al., 2006). The decrease of soil N status and availability could reduce soil N cycling, and thus lead to the decline in soil N2O emission (Werner et al., 2007; Zhang et al., 2008a). In this study, soil N stock was higher in the C. hystrix plantation than in the mixed and P. massoniana plantations, and soil C:N ratio was lower in the C. hystrix plantation than in the mixed and P. massoniana plantations. Hence, differences in the magnitude of soil N2O emission among the plantations could also be explained by soil N stock and C:N ratio. The temporal variations in soil N2O emissions were attributed to the temporal variations in soil temperature and moisture (Fig. 3e and f). The temporal variations in soil N2O emissions in the P. massoniana and C. hystrix plantations were also correlated to soil temperature and moisture (Wang et al., 2010b). Similar results 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 in the C. hystrix plantations. The differences in SOC stock among the plantations were attributed to soil N stock and leaf litterfall input. Mean soil CO2 and N2O emissions were significantly higher in the mixed plantation than in the P. massoniana plantation, while they were lower than in the C. hystrix plantation. Mean soil CH4 uptake was significantly higher in the mixed plantation than in the C. hystrix plantation, while it was similar to that in the P. massoniana plantation. This study confirms that the mixed plantation has a higher SOC stock than the monoculture plantations, and there is an increase in amount of GHGs absorbed by the soil of mixed plantations compared to C. hystrix plantations. Therefore, conifer-broadleaf mixed plantation could be a better silvicultural mode for soil carbon sequestration than monoculture broadleaf plantation for substituting large coniferous plantations in subtropical China. Acknowledgements We are grateful to Riming He, Hai Chen, Zhongguo Li, Xueman Huang, Yuan Wen, Jia Xu, Lu Zheng and En Liu for their assistance in field sampling and data collection. We also gratefully acknowledge the support from the Chinese Academy of Forestry’s Experimental Center of Tropical Forestry. This study was funded by the

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 subtropical plantations in China. Forest Ecol. Manage. (2012), http://dx.doi.org/10.1016/j.foreco.2012.04.005

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