Mixed-species plantation with Pinus massoniana and Castanopsis hystrix accelerates C loss in recalcitrant coniferous litter but slows C loss in labile broadleaf litter in southern China

Forest Ecology and Management 422 (2018) 207–213

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Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco

Mixed-species plantation with Pinus massoniana and Castanopsis hystrix accelerates C loss in recalcitrant coniferous litter but slows C loss in labile broadleaf litter in southern China

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Hui Wanga, Shirong Liua, , Jingxin Wangb, Yeming Youc, Yujing Yanga, Zuomin Shia, Xueman Huangc, Lu Zhengd, Zhaoying Lid, Angang Mingd, Lihua Lud, Daoxiong Caid 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 Division of Forestry and Natural Resources, West Virginia University, P.O. Box 6215, Morgantown, WV 26506, USA c College of Forestry, Guangxi University, Nanning, Guangxi 530004, China d Experimental Center of Tropical Forestry, Chinese Academy of Forestry, Guangxi Youyiguan Forest Ecosystem Research Station, Pingxiang, Guangxi 532600, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Carbon chemical composition Close-to-nature forest Litter decomposition Mixed-species plantation Subtropical China

Litter decomposition varies with forest type in complex ways and is significantly dependent on tree species diversity. A field litterbag experiment was conducted in monospecific and mixed-species plantations of Pinus massoniana and Castanopsis hystrix in subtropical China, to examine the effects of litter mixing and changing stand environment on the litter decomposition rate and the composition-specific litter organic C loss rate of P. massoniana and C. hystrix. Admixing with the lower-quality P. massoniana litter slowed C. hystrix litter decomposition and recalcitrant C loss, whereas admixing with the higher-quality C. hystrix litter hastened P. massoniana litter decomposition in the mixed-species plantation. The P. massoniana litter exhibited a faster decomposition rate in the mixed-species plantation than in the P. massoniana monospecific plantation. The same trend was found in the loss rates of aromatic C and carbonyl C from P. massoniana litter. The greater soil organic matter content and nitrogen availability in the mixed-species plantation than in the P. massoniana monospecific plantation could have resulted in the increased rates of decomposition and organic C loss in P. massoniana litter. This study highlights that maintaining mixed conifer-broadleaf forests and/or adopting close-to-nature forest management could accelerate the decomposition of recalcitrant coniferous litter, facilitating C sequestration and mitigating the C emissions derived from the labile broadleaf litter.

1. Introduction Litter decomposition is essential for biogeochemical cycles and the formation of new soil organic matter (Setiawan et al., 2016). Decomposition rate is determined by interactions between resource quality and decomposers, both controlled by the environment (climatic and soil conditions) (Hobbie et al., 2006). Models of litter decomposition show a satisfactory match with measured carbon (C) levels (Jenkinson and Rayner, 2006), but the actual chemical form and behavior of organic C during litter decomposition remains ambiguous (Ono et al., 2013). There is much evidence now pointing to higher productivity and ecological stability in mixed-species forests (Liang et al., 2016; Pretzsch et al., 2017). It is promising and of practical relevance to convert pure coniferous plantations into mixed-species plantations, and these have been increasingly developed in many countries, including China

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Corresponding author. E-mail address: [email protected] (S. Liu).

https://doi.org/10.1016/j.foreco.2018.04.024 Received 22 December 2017; Received in revised form 6 April 2018; Accepted 11 April 2018 0378-1127/ © 2018 Elsevier B.V. All rights reserved.

(Vesterdal et al., 2008; Lu et al., 2009). Greater biological diversity produces litter mixtures of varying quality and more diverse microhabitats that directly influence litter decomposition rates. However, the effects on the chemical composition of decomposing organic C when mixing species in litter is still not clear. Coniferous tree species differ from broadleaf tree species in many functional traits such as leaf structure, photosynthetic capacity, hydraulic network, tissue composition, and litter chemistry, and these differences may influence ecosystem functioning (e.g. litter decomposition and, subsequently, the accumulation of organic C in soils) (Binkley and Giardina, 1998; Augusto et al., 2015). In general, higherquality litter can stimulate the decomposition of more recalcitrant litter, and conversely, litter decomposition can be slowed by an admixture of lower-quality litter (Fyles and Fyles, 1993; McTiernan et al., 1997; Salamanca et al., 1998). Admixing with a nitrogen (N)-fixing

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2. Methods

species (Alnus formosana) improves the decomposition of eucalyptus (Eucalyptus grandis) litter (Wu et al., 2014). By comparison, admixing with a fast-decomposing species (Pseudopanax simplex) did not accelerate the decomposition of any of its neighbor species, but decreased the mass loss of both Pseudowintera colorata and Nothofagus menziesii litters in a New Zealand forest (Hoorens et al., 2010). The transfer of N among the different litters has been documented as an important mechanism in the interaction between decomposing litters (Berglund and Ågren, 2012; Berglund et al., 2013; Handa et al., 2014). An increased litter mass loss may also be correlated with the creation of a more diverse and abundant decomposer community in a mixed-species stand (Hansen and Coleman, 1998; Gartner and Cardon, 2004; Makkonen et al., 2013). Conversely, the less decomposable litter slows the adjacent litter decomposition by releasing inhibitory compounds such as phenolics (Prescott et al., 2000). Therefore, the effect of such mixture on litter decomposition, particularly on composition-specific C loss, is not yet easily predicted. Mixed-species management can affect the stand environment by changing the forest structure and tree species composition. Trees could promote soil communities that are particularly capable of degrading the litter they encounter most often, and thus litter could decompose faster when placed in the habitat from which it was derived than it would in a foreign habitat. This has been termed the “home field advantage” (HFA) of litter decomposition (Chomel et al., 2015; Veen et al., 2015). Spruce litter exhibits a faster decomposition rate in monospecific plantations than in mixed-species plantations (Chomel et al., 2015). Low-quality litters that contain highly recalcitrant (such as lignin or tannins) or toxic (such as secondary metabolites, terpenoids, and phenolics) compounds may generate a larger HFA because fewer soil communities can degrade these compounds (Ayres et al., 2009; Strickland et al., 2009). Faster decomposition of litter in its ‘‘home’’ could also be due to an overall greater functional ability of organisms to decompose litter rapidly, regardless of environment and substrates (Keiser et al., 2014). However, in the decomposition of litter organic C, it is not known whether an HFA exists between the monospecific and mixed-species forests. C dynamics during litter decomposition have been described in a variety of forest ecosystems and provided insights into C flow in soils (Ono et al., 2013). Recent studies have reported that the chemical composition of organic C changes during litter decomposition in several monospecific forests (Ono et al., 2013). We also found that litter decomposition and the loss of O-alkyl C, aromatic C, and carbonyl C from Castanopsis hystrix litter were faster than they were from Pinus massoniana litter in in situ decomposition experiments (Wang et al., 2010; Wang et al., 2013). Soil moisture and litter quality played critical roles in regulating C composition during the decomposition process (Wang et al., 2010; Wang et al., 2013). To the best of our knowledge, no previous study has explored how mixed tree species affect the dynamics of the litter’s organic C chemical composition during decomposition in plantations. Therefore, we conducted an in situ litter decomposition experiment using the coniferous species (P. massoniana) and the broadleaf species (C. hystrix), in a subtropical plantation ecosystem, to investigate the effects of litter mixing and changing stand environment on the litter decomposition rate and the composition-specific litter organic C loss rate of P. massoniana and C. hystrix. We hypothesized that (i) mixing of P. massoniana and C. hystrix litters would accelerate the litter decomposition and the loss of litter organic C in P. massoniana litter, and slow the litter decomposition and loss of litter organic C in C. hystrix litter; (ii) the litter decomposition rate and composition-specific litter organic C loss rate of P. massoniana and C. hystrix would differ between the monospecific plantations and mixed-species plantation.

2.1. Site location The decomposition experiment, using litterbags, was conducted across a P. massoniana monospecific plantation, a C. hystrix monospecific plantation, and a mixed plantation with P. massoniana and C. hystrix, in Guangxi Youyiguan Forest Ecosystem Research Station at the Experimental Center of Tropical Forestry, Chinese Academy of Forestry (22°05′ N, 106°86′ E), on the outskirts of Pingxiang City, Guangxi Zhuang Autonomous Region, southern China. The region is characterized by a typical subtropical monsoon climate, with an annual mean precipitation of 1300 mm. The average annual temperature is 22.3 °C. The loamy textured soil in these plantations was formed from a granitic parent geological material, and is classified as a Ferrosol in the Chinese system of soil classification, equivalent to an Oxisol in the USDA Soil Taxonomy (USDA, 1996). The elevation of the study site is about 550 m a.s.l. These plantations were established in 1983 after the clear-cutting of a Cunninghamia lanceolata monospecific plantation, and have similar topography, soil texture, and silviculture history. The diameter of the trees at breast height, total tree height, and stem density of the P. massoniana plantation were 24.6 cm, 17.2 m, and 404 trees ha−1 respectively; those of the C. hystrix plantation were 24.9 cm, 17.8 m, and 415 trees ha−1 respectively; and those of the mixed plantation were 27.3 cm, 18.1 m, and 400 trees ha−1 respectively. The basic ecological monitoring work was in full compliance with the “Observation Methodology for Long-term Forest Ecosystem Research” of the National Standards of the People's Republic of China (GB/T 33027-2016).

2.2. Experiment design Three plots (each 20 m × 20 m) were randomly established in each plantation type. The freshly fallen litters of P. massoniana and C. hystrix used for the litterbag experiment were collected in the respective plantations in March 2008. Litters were collected using air-dried litter traps and nylon mesh placed above the surface of forest floor. The litterbags were made of polyethylene, and measured 250 mm × 250 mm, with a mesh size of 1 mm. Each single-species litter bag was filled with 12 g of air-dried mass of P. massoniana or C. hystrix litter, and each mixed-species litterbag was filled with 6 g of air-dried mass of P. massoniana and 6 g of air-dried mass of C. hystrix litter. The in situ litter decomposition began on April 15, 2008. 20 P. massoniana litterbags were placed back in each plot of the P. massoniana stand, and 20 C. hystrix litterbags were placed back in each plot of the C. hystrix stand. 20 P. massoniana litterbags, 20 C. hystrix litterbags, and 20 mixed litterbags were placed back in each plot of the mixed stand. Thus, there were five types of litter bags: (1) P. massoniana litter in the monospecific P. massoniana plantation, (2) P. massoniana litter in the mixed-species plantation, (3) C. hystrix litter in the monospecific C. hystrix plantation, (4) C. hystrix litter in the mixed-species plantation, and (5) a mixture of P. massoniana litter and C. hystrix litter in the mixed-species plantation. Litterbags were randomly retrieved at 3-month intervals from July 2008 to April 2009. At each interval (3, 6, 9, and 12 months), five replicate litterbags of each decomposing litter type were randomly collected from each plot. The decomposed litter fragments in the litterbags were picked out with tweezers to remove the contaminated soil particulates as carefully as possible, and dried in an oven at 50 °C for 48 h before weighing (Ostertag et al., 2008). The litter fragments in mixture litterbags were carefully separated into P. massoniana or C. hystrix litter. The samples of the five litterbags of each type were combined and ground in a mill to pass through a 0.25 mm sieve before chemical analysis. Subsamples of the initial and decomposed litter were dried at 105 °C, and all results were reported on a 105 °C basis.

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2.3. Litter chemical analysis

Table 1 Litter chemical characteristics of Pinus massoniana and Castanopsis hystrix (Mean values with SE, p < 0.05).

Litter chemical properties were analyzed after each field sampling. The major C chemical compositions were analyzed using a solid-state 13 C cross polarization and magic angle spinning (CPMAS) NMR technique to quantitatively assess how the decomposition processes varied between litter types (Ono et al., 2013). The 13C CPMAS NMR spectra of the samples were obtained at a frequency of 100.64 MHz on a Bruker AVANCE III 400 spectrometer (Bruker, Karlsruhe, Germany). Samples were packed in a ZrO2 rotor (od = 7 mm) and spun at 5 kHz at the magic angle. A single contact time of 1 ms was applied with a recycle delay of 1 s. Transients (20,000) were collected for all samples. Chemical shift values were referenced externally to glycine at 176.03 ppm. The 13C CPMAS NMR spectra were divided into four chemical regions that were assigned to specific organic C functional groups (Spielvogel et al., 2006): 0–45 ppm, alkyl C (lipids, cutin, and suberin); 45–110 ppm, O-alkyl C (carbohydrates, cellulose, hemicellulose, and methoxyl C); 110–160 ppm, aromatic C (lignin, tannin, olefins, and aromatic compounds); and 160–220 ppm, carbonyl C (carboxylic acid, amide, and ketone groups). The corresponding areas under the curve of the above four regions were quantified by integration. Alkyl C is found in the side chains of amino acids and alicyclic C is found in resins and lipids. O-alkyl C is found in cellulose, hemicellulose, polysaccharides, and amines, while aromatic C is in tannins and lignins, and carbonyl C is the carboxylic C in tannins and resin acids (Ono et al., 2013).

C (g kg−1) N (g kg−1) C/N Alkyl C (%) O-alkyl C (%) Aromatic C (%) Carbonyl C (%)

P. massoniana

C. hystrix

P-value

522.8 (0.4) 15.4 (0.23) 34.0 (0.5) 17.99 (0.07) 52.04 (0.21) 22.24 (0.26) 7.73 (0.08)

485.0 (0.7) 17.1 (0.17) 28.4 (0.3) 18.55 (0.39) 55.77 (0.38) 18.90 (0.35) 6.79 (0.12)

< 0.001 0.004 0.001 0.227 0.001 0.001 0.003

12 months during decomposition), X0 is the initial litter or C mass, Xt denotes the litter or C mass remaining at time t, and e is the base of the natural logarithm. One-way ANOVA was used to analyze the differences in soil properties among the three plantations, and the k values of the total litter and individual organic C chemical compositions among the decomposed litter types. The differences in the initial litter chemical characteristics between P. massoniana and C. hystrix were examined using ttest. Multiple comparisons of means were performed using Duncantests. Data for all variables were normally distributed and had homogeneous variance. The level of statistical significance was set at p < 0.05. All analyses were performed using SPSS 19.0 for Windows.

2.4. Soil sampling and measurements

3. Results

Mineral soil samples were collected in April 2008 to determine soil properties. Six soil cores of 8.7 cm diameter were randomly collected from a depth of 10 cm in each plot. The six cores from each plot were combined and sieved through a 2 mm sieve to carefully remove the plant material, roots, and gravel to minimize the influence of plant residues on both the chemical and microbial analyses. The fresh soil samples were measured for available N, pH, and microbial biomass C (MBC). Soil was extracted using a 1 mol L−1 KCl solution (1:4, soil: 1 M KCl, m/v), shaken for 1 h, and then filtered (Whatman 42, UK) to colorimetrically determine the NO3−-N and NH4+-N concentrations. Soil pH was measured in a 1 mol L−1 KCl solution using a glass electrode. MBC was measured by fumigation-extraction, using 0.5 mol L−1 K2SO4 as the extracting agent; a conversion factor of 2.64 was used to convert the extracted C to MBC (Vance et al., 1987). Soil organic C (SOC) and litter organic C were analyzed using the dichromate oxidation method, and soil total N was analyzed using the Kjeldahl method (Sparks et al., 1996). Once a month, during litter decomposition, six measurements of the soil temperature and soil moisture at a depth of 5 cm below the surface were randomly collected in each plot. Soil temperature was measured using digital thermometers (Harvesting Science and Technology Co., Ltd, BJ, China). Volumetric soil moisture (% v/v) was measured simultaneously using an MPKit-B (NTZT Inc., Nantong, JS, China).

3.1. Litter chemistry and stand properties Initial litter chemical characteristics differed significantly between the P. massoniana and C. hystrix species (p < 0.05, Table 1). The P. massoniana litter had a higher organic C content and C/N ratio, and higher proportions of aromatic C and carbonyl C in the total organic C than the C. hystrix litter (p < 0.05, Table 1). The C. hystrix litter had higher total N content and a higher proportion of O-alkyl C than the P. massoniana litter (p < 0.05, Table 1). Some stand properties differed significantly among the three plantations (p < 0.05, Table 2). Soil temperature was significantly higher in the P. massoniana plantation than in the mixed-species and C. hystrix plantations (p < 0.05, Table 2). The SOC content in the P. massoniana plantation was significantly lower than in the C. hystrix and mixedspecies plantations (p < 0.05, Table 2). Concentrations of NH4+ and NO3− in the soil varied significantly among the three plantations Table 2 Stand properties of the three plantations (Pinus massoniana monospecific plantation, Castanopsis hystrix monospecific plantation, and mixed-species plantation with P. massoniana and C. hystrix). Different lowercase letters of the same row represent statistically significant differences (p < 0.05; Duncan’s HSD). (Mean values with SE, n = 3). Plantation types

P. massoniana plantation

C. hystrix plantation

Mixed-species plantation

DBH (cm) Tree height (m) Stem density (trees ha−1) Temperature (°C) Moisture (%) pH (KCl) Organic C (g kg−1) Total N (g kg−1) C/N NH4+ (mg kg−1) NO3− (mg kg−1) Microbial biomass C (mg kg−1)

23.0 (1.9) a 17.1 (1.3) a 416 (37) a

24.5 (3.1) a 18.2 (1.7) a 391 (20) a

26.8 (1.0) a 17.8 (2.2) a 397 (56) a

23.14 (0.13) b 18.2 (1.4) a 3.73 (0.01) a 26.46 (0.21) a 1.41 (0.02) a 18.74 (0.27) a 5.16 (0.19) ab 1.95 (0.19) a 344.3 (19.0) a

22.23 (0.13) a 19.7 (0.5) a 3.77 (0.02) a 29.53 (0.14) b 1.37 (0.10) a 21.83 (1.54) a 6.29 (0.34) b 2.38 (0.28) ab 285.8 (27.2) a

22.33 (0.14) a 18.5 (1.1) a 3.79 (0.02) a 28.47 (0.71) b 1.44 (0.05) a 19.84 (1.21) a 4.12 (0.59) a 3.32 (0.54) b 324.9 (48.3) a

2.5. Statistical analyses The remaining mass of each C type after decomposition was calculated as the remaining mass of the total organic C multiplied by the proportions of each C type obtained from the 13C CPMAS NMR spectra after each decomposition period. The following equation was used for the estimation: Mass of each C type at time t = (total litter mass at time t) × (organic C content at time t) × (relative proportion of each C type at time t based on the integrated peak area in the 13C NMR spectral regions). The decomposition rates of both total litter and each C type were determined by fitting needle/leaf mass loss data to a simple negative exponential model: Xt/X0 = e−kt, where k represents the decomposition coefficient (Olson, 1963), t is the sampling time (0, 3, 6, 9, and 209

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H. Wang et al. P. massoniana litter in monospecific P. massoniana plantation P. massoniana litter in the mixed-species plantation P. massoniana litter in mixture in the mixed-species plantation C. hystrix litter in monospecific C. hystrix plantation C. hystrix litter in the mixed-species plantation C. hystrix litter in mixture in the mixed-species plantation

Mass remaining (%)

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4. Discussion The differences in litter chemistry between the P. massoniana and C. hystrix litter in the present study indicated that the quality of C. hystrix litter was higher than that of P. massoniana litter. The environmental conditions in terms of soil temperature, SOC content, and N availability differed among the three plantations. We found that mixing of the P. massoniana and C. hystrix litters hastened the litter decomposition of P. massoniana but slowed the litter decomposition of C. hystrix in the mixed-species plantation. Litter mixing also slowed the loss rates of aromatic C and carbonyl C from C. hystrix litter during decomposition. Initially, our results verified the hypothesis (i) that mixing of the P. massoniana and C. hystrix litters would accelerate litter decomposition and the loss of litter organic C in P. massoniana litter and slow the loss of litter organic C in C. hystrix litter in the mixed-species plantation. The non-significant effect of litter mixing on the litter organic C loss of P. massoniana was the only inconsistency with our hypothesis. Our data also indicates that the litter decomposition rate and the litter organic C loss rate of P. massoniana were faster in the mixed-species plantation than in the P. massoniana monospecific plantation, which partly supported our second hypothesis (ii) that the litter decomposition rate and the organic C loss rate of P. massoniana litter would differ between the monospecific and mixed-species plantations. However, the rates of litter decomposition and litter organic C loss in C. hystrix did not differ between the monospecific and mixed-species plantations. In the present study, mixing litter with a different tree species was found to be a critical factor regulating the pattern of litter decomposition by changing the chemical characteristics of the litter mixture. An admixture of higher-quality litter is expected to promote the decomposition of more recalcitrant litter, or vice versa (McTiernan et al., 1997). This is consistent with the present study, which demonstrated the specific effect that a species has on its neighbor (Hättenschwiler et al., 2005; Ball et al., 2008). The transfer of N from C. hystrix litter to P. massoniana litter could be a potential mechanism in the accelerated decomposition of P. massoniana litter and the slowed decomposition of C. hystrix litter in mixed litter. The overall mass loss rate for the mixedspecies litter was estimated as 0.997 ± 0.08 year−1, which was not significantly different from the expected litter decomposition rate (1.043 ± 0.06 year−1) for single-species P. massoniana and C. hystrix litter, estimated using the method for detecting additive or non-additive responses in mixed-species litter (Hui and Jackson, 2009), pointing to an observed additive effect. This effect was explained through the balance between the synergistic effect of litter mixing on P. massoniana litter decomposition and the antagonistic effect of litter mixing on C. hystrix litter decomposition (Fig. 3), and not through negligible litter mixing. Moreover, mixed litter from diverse tree species possibly increases the microhabitat complexity, thus enhancing the biomass and community diversity of the decomposers (Milcu et al., 2006; Wardle et al., 2006). An admixture of Acer monspessulanum and Cotinus coggygria litters with Quercus pubescens demonstrated a positive effect on decomposers, particularly fungal biomass and detritivores diversity, consequently improving litter decomposition efficiency (Santonja et al., 2017). Similarly, more bacterial diversity is observed in mixed litter of alder and eucalyptus than in pure eucalyptus litter during decomposition (Wu et al., 2014). However, in the present plantation ecosystem, mixing the species P. massoniana and C. hystrix did not significantly change the soil microbial biomass. Soil biodiversity can increase with higher-quality litter inputs into the ecosystem (Hättenschwiler et al., 2005). Thus, the potential difference in the composition of the soil microbial community between single-species and mixed-species litters might drive the observed changes in the litter decomposition of P. massoniana and C. hystrix. Soil microbial community and site environment differ among different forests, especially coniferous and broadleaf forests. In the present study, the litter mixing decomposition experiment was carried out in a mixed-species plantation, and thus our results could only indicate the effects of litter mixing on litter decomposition in

80

60

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20 0

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Decomposition period (month) Fig. 1. Mass loss patterns during the decomposition of litter in six litter types (Pinus massoniana litter in monospecific P. massoniana plantation, P. massoniana litter in mixed-species plantation, P. massoniana litter in mixed-species litterbags in mixed plantation, Castanopsis hystrix litter in monospecific C. hystrix plantation, C. hystrix litter in mixed-species plantation, and C. hystrix litter in mixed-species litterbags in mixed plantation). Error bars indicate standard error (p < 0.05).

(p < 0.05, Table 2). 3.2. Remaining masses and k values for litter and organic C chemical compositions Litter mass loss rates slowed down exponentially with time for all litter types (Fig. 1). The masses of individual organic C chemical compositions also decreased, and their mass loss rates slowed down during litter decomposition (Fig. 2). The effect of litter mixing on litter decomposition was revealed by comparing the decomposition rates of P. massoniana and C. hystrix litters between the single-species litterbags and the mixed-species bags in the mixed-species plantation. The litter mass loss rate of P. massoniana in the mixed-species litterbags was significantly faster than in the single-species litterbags (p < 0.05, Fig. 3), whereas the litter mass loss rate of C. hystrix in the mixed-species litterbags was significantly slower than it was in the single-species litterbags (p < 0.05, Fig. 3). The loss rates of aromatic C and carbonyl C from C. hystrix litter in the mixedspecies litterbags were much lower than they were in the single-species litterbags (p < 0.05, Fig. 3). No significant differences were observed in the loss rates of any organic C chemical composition from P. massoniana litter between the single-species and mixed-species litterbags (Fig. 3). The effect of stand environment on litter decomposition was shown by comparing the decomposition rates of P. massoniana or C. hystrix litter between the monospecific and mixed-species plantations. The decomposition rate for the P. massoniana litter in the mixed-species plantation was significantly higher than that in the P. massoniana monospecific plantation (p < 0.05, Fig. 3), whereas the decomposition rate for the C. hystrix litter in the mixed-species plantation was not different from that in the C. hystrix monospecific plantation (Fig. 3). The loss rates for aromatic C and carbonyl C from P. massoniana litter in the mixed-species plantation were significantly higher than those in the P. massoniana monospecific plantation (p < 0.05, Fig. 3). There were no significant differences in the loss rates of any organic C chemical compositions from C. hystrix litter between the mixed-species plantation and the C. hystrix monospecific plantation (Fig. 3). 210

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(a) P. massoniana litter in monospecific P. massoniana plantation O-alkyl C Aromatic C Carbonyl C Alkyl C

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(c) P. massoniana litter in mixture in the mixed-species plantation

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(d) C. hystrix litter in monospecific C. hystrix plantation O-alkyl C Aromatic C Carbonyl C Alkyl C

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(e) C. hystrix litter in the mixed-species plantation O-alkyl C Aromatic C Carbonyl C Alkyl C

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(f) C. hystrix litter in mixture in the mixed-species plantation

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O-alkyl C Aromatic C Carbonyl C Alkyl C

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Fig. 2. Mass loss patterns during the decomposition of different carbon types in six litter types (Pinus massoniana litter in monospecific P. massoniana plantation, P. massoniana litter in mixed-species plantation, P. massoniana litter in mixed-species litterbags in mixed-species plantation, Castanopsis hystrix litter in monospecific C. hystrix plantation, C. hystrix litter in mixed-species plantation, and C. hystrix litter in mixed-species litterbags in mixed plantation). Error bars indicate standard error (p < 0.05).

of recalcitrant compounds (e.g. aromatic structures) in the P. massoniana litter could have slowed the C decomposition in the C. hystrix litter. The soil microbial community involved in the decomposition of the specific organic C could also affect the composition-specific loss rates of C in litter mixture. Stable isotope labelling may be applied in the future to further examine the role of diverse microbial communities on litter C chemical composition decomposition. The lower decomposition rates for aromatic C and carbonyl C likely imply slower releases of these recalcitrant C fractions into the atmosphere. There is accumulating evidence that litter tends to decompose faster in the habitat from which it was derived than in other habitats (Veen et al., 2015). However, in the present study, the faster litter decomposition rate for P. massoniana was in the mixed-species plantation

the mixed-species plantation. Furthermore, composition-specific loss rates of C would be useful to predict the C dynamics in forest ecosystems and to validate C compartment models in forest soils (Ono et al., 2013). Previous studies state that aromatic C accumulates during litter decomposition, and carbonyl C is formed by hydrolysis and oxidation during decomposition (Lemma et al., 2007; Ono et al., 2013). In the present study, litter mixing slowed the loss of aromatic C and carbonyl C from C. hystrix litter in the subtropical plantation. However, the loss of phenolics increased with the number of plant species in the litter mixture in a Mediterranean oak forest (Santonja et al., 2017). These results indicate that the effects of litter mixing on the composition-specific loss rates of litter organic C is still uncertain among different forest ecosystems. The high proportion

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P. massoniana litter in monospecific P. massoniana plantation P. massoniana litter in the mixed-species plantation P. massoniana litter in mixture in the mixed-species plantation C. hystrix litter in monospecific C. hystrix plantation C. hystrix litter in the mixed-species plantation C. hystrix litter in mixture in the mixed-species plantation

Decomposition constant (k)

2.0

community could be further determined in the next step to explore the specific decomposer community involved in litter decomposition. Previous studies explored the C-specific fate of different litter types during in situ decomposition (Lemma et al., 2007; Mathers et al., 2007; Ono et al., 2013). The present study revealed a positive effect of stand nutrient characteristics on the composition-specific loss rates of organic C in the same litter type. The faster losses of aromatic C and carbonyl C in the mixed-species plantation than in the P. massoniana monospecific plantation suggested higher input rates of the recalcitrant C compositions into the soil, which could, consequently, affect the quality of the soil organic matter.

d c bc

1.5 c

1.0

b

c

bc

b

a

ab ab

ab

ab

b

c c

cd

b

abc

bc ab

bc

ab

b

a

a

bc bc

a a

5. Conclusions

0.5

0.0

Total litter

Alkyl C

O-alkyl C

Aromatic C

Mixing the species with P. massoniana and C. hystrix in the subtropical plantation altered the litter decomposition rate and litter organic C composition-specific loss rates through mixing diverse litter and improving soil nutrients. Admixing with lower-quality P. massoniana litter slowed C. hystrix litter decomposition and recalcitrant C loss from C. hystrix litter, whereas admixing with higher-quality C. hystrix litter hastened P. massoniana litter decomposition in the mixed-species plantation. The better soil nutrient condition in the mixed-species plantation promoted P. massoniana litter decomposition and recalcitrant C loss from P. massoniana litter. These findings highlight that mixed conifer-broadleaf forests should be sustained to hasten the decomposition of recalcitrant coniferous litter, facilitating SOC sequestration and mitigating the C loss derived from labile broadleaf litter.

Carbonyl C

Fig. 3. Differences in the mean values of decomposition constants (k values) of total litter and four carbon types (alkyl C, O-alkyl C, aromatic C, carbonyl C) across the litter types (Pinus massoniana litter in monospecific P. massoniana plantation, P. massoniana litter in mixed-species plantation, P. massoniana litter in mixed-species litterbags in mixed-species plantation, Castanopsis hystrix litter in monospecific C. hystrix plantation, C. hystrix litter in mixed-species plantation, and C. hystrix litter in mixed-species litterbags in mixed plantation). Error bars indicate standard error. Different lowercase letters of the same category represent statistically significant differences (p < 0.05; Duncan’s HSD).

Acknowledgements rather than in the P. massoniana monospecific plantation is opposite to the previous finding that spruce litter decomposed faster at its home field than in a mixed forest under a boreal climate (Chomel et al., 2015). Warmer and wetter conditions favor higher activity by soil organisms and faster litter breakdown (Trofymow et al., 2002). In the warm subtropical plantation in this study, specific decomposers may have a less substantial influence on recalcitrant litter decomposition than they do in conditions that favor slow litter decomposition (Veen et al., 2015). Furthermore, numerous responses of ecosystem processes (e.g. soil microbial feedbacks) to global change depend on soil C and N availability (Bradford et al., 2014). Soil N enrichment stimulates the growth and dominance of basidiomycete fungi (Crowther et al., 2015), and warming-induced labile C reduction alters heterotrophic microbes (Crowther and Bradford, 2013). Decomposers may be limited by nutrients at home field and respond strongly to higher-quality litter inputs from elsewhere, which could lead to an accelerated litter breakdown away from home (Gartner and Cardon, 2004). In the present study, the better nutrient availability, in terms of the higher SOC and NO3− contents in the mixed-species plantation than in the P. massoniana monospecific plantation, could have resulted in the faster P. massoniana litter decomposition in the mixed-species plantation. The slower loss rates of aromatic C and carbonyl C from P. massoniana litter in the P. massoniana monospecific plantation than in the mixed-species plantation likely resulted from the abundant recalcitrant lignin and tannins derived from conifers in the P. massoniana plantation soil. Once the condition of available nutrients in the soil is improved, the decomposition rates for recalcitrant C compositions might increase. In the present study, SOC content and NO3−-N availability were much higher in the mixed-species plantation than in the P. massoniana monospecific plantation. Conversely, few stand properties differed between the mixed-species plantation and the C. hystrix monospecific plantation, which could have led to the non-significant difference in the litter decomposition rate and the loss rate of any litter organic C chemical composition between the mixed-species plantation and the C. hystrix monospecific plantation. Soil microbes also play a key role in litter decomposition, but in the present study there was no difference in soil microbial biomass C among the plantations. Soil microbial

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