Comparisons of litterfall, litter decomposition and nutrient return in a monoculture Cunninghamia lanceolata and a mixed stand in southern China

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Forest Ecology and Management 255 (2008) 1210–1218 www.elsevier.com/locate/foreco

Comparisons of litterfall, litter decomposition and nutrient return in a monoculture Cunninghamia lanceolata and a mixed stand in southern China Qingkui Wang a,b, Silong Wang a,*, Yu Huang c a

Huitong Experimental Station of Forest Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China b Huitong National Station of Forest Ecology, Huitong 418307, China c Science and Technology Department of Hunan Province, Changsha 410000, China Received 26 April 2007; received in revised form 8 October 2007; accepted 13 October 2007

Abstract Litter production, leaf litter decomposition and nutrient return were compared in a monoculture Cunninghamia lanceolata and a mixed stand in southern China. The mean annual litter production was significantly higher 24% in the mixed stand than the monoculture C. lanceolata stand. Within the mixed stand, about 38% of the total litterfall was from broadleaved tree Michelia macclurei. The litterfall was concentrated during the cool and dry period (November–March) and about 65% of total litterfall occurred during this period. The mass loss of leaf litter was positively correlated with N and P content and negatively correlated with C/N ratio. The decomposition rate of leaf litter in the pure stand was increased in the order: C. lanceolata < mixture of C. lanceolata and M. macclurei < M. macclurei. Soil conditions also affected litter decomposition. The decomposition rate of mixed litter was slightly, but not significantly, faster in the mixed stand than the pure stand. N concentration in all leaf litters increased over time during decomposition, whereas the remaining amount of N decreased. The returns of N, P and K via leaf litter were significantly higher in mixed stand than pure stand, but the returns of C, Ca and Mg between both stands did not differ significantly. The percent contribution of M. macclurei leaf litter to total nutrient return varied from 23% to 79% in the mixed stand. Our results indicated that introduction of broadleaved tree into monoculture coniferous stand could increase litter production, nutrient return and thus it had advantages in degraded soil restoration and sustainable land management. # 2007 Elsevier B.V. All rights reserved. Keywords: Litter production; Litter decomposition; Nitrogen release; Monoculture plantation; Soil restoration

1. Introduction Cunninghamia lanceolata, an important coniferous timber species, has been widely planted for more than 1000 years in southern China due to its fast growth and good timber quality (Wu, 1984). Planting area has reached 12 million ha, accounting for about 24% of all forested area in China, and the system of successive cropping has been widely used (Chen and Wang, 2004). In recent decades, however, this practice has led to remarkably decline in soil fertility and timber productivity because of nutrient depletion and deterioration of physical, chemical and biochemical activity (Feng et al., 1988; Chen

* Corresponding author. Tel.: +86 24 8397 0344. E-mail address: [email protected] (S. Wang). 0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2007.10.026

et al., 1990) which resulted from successive cropping, a shortrotation time of about 20 years, whole-tree harvest and site preparation. Fortunately, some scientists have found that planting mixed forest of C. lanceolata and broadleaved tree could improve the quality of forest land and the productivity of C. lanceolata stands (Feng et al., 1988; Chen et al., 1990). However, little information is known about the ecological properties of these ecosystems to avoid failures and optimize the use of soil and other resources in subtropical China. The litter on the forest floor acts as input–output system of nutrient and the rates at which forest litter falls and subsequently, decomposes contribute to the regulation of nutrient cycling and primary productivity, and to the maintenance of soil fertility in forest ecosystems (Olson, 1963; Singh et al., 1999; Fioretto et al., 2003; Onyekwelu et al., 2006; Pandey et al., 2007). Therefore, it is critical to understand the

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amount and pattern of litterfall in these forest ecosystems. Although numerous studies on amount and pattern of litterfall in plantations has been conducted in subtropical region (Zhang et al., 1993; Parrotta, 1999; Liao et al., 2006; Pandey et al., 2007), few attempts have been made to comparatively measure litter in pure and mixed plantations under similar climatic and edaphic conditions in subtropical China. Litter decomposition is a major pathway for providing organic and inorganic elements for the nutrient cycling processes and controls nutrient return to the forest ecosystem. The decomposition of litter is primarily influenced by the environmental conditions in which decay takes place, the chemical quality of leaf litter, and the nature and abundance of decomposing organisms present (Swift et al., 1979; Polyakova and Billor, 2007). Despite many studies carried out on litter decomposition, most of them have considered the decomposition of litter of a given species in isolation from other species in one forest stand in subtropical forest ecosystems (Rai and Proctor, 1986; Adams and Angradi, 1996; Sundarapandian and Swamy, 1999). Litter decay of each species present has the potential to be affected by the presence of litters from coexisting plant species. In recent years, researchers have only specifically begun to examine potential interactions among leaves of different species during decomposition. However, these studies have not shown consistent effects of litter mixing on decomposition rates, which are positive, negative, or neutral (Hansen, 2000; Hector et al., 2000; Wardle et al., 2003; Polyakova and Billor, 2007). Information is urgently needed about litter mixing effects of C. lanceolata and broadleaved tree on decomposition rates in different forest stands. In this paper, we hypothesized that litterfall, litter decomposition rate and nutrient cycling could increase in ecosystems when broadleaved tree species (e.g. Michelia macclurei) is added to pure C. lanceolata stands, and that litter could decompose more rapidly in the mixed stand than the pure coniferous stand. Therefore, in the study covering a 6-year period, litterfall, leaf litter decomposition and nutrient return were investigated in a monoculture C. lanceolata and a mixed stand in subtropical China. In addition, soil chemical and biochemical characteristics were also investigated in both stands. The aims of the present investigation were conducted: (a) to investigate the impact of broadleaved tree species on the amount and pattern of litterfall, litter decomposition and nutrient return in C. lanceolata plantations and (b) to determine effects of litter mixing and stand quality on decomposition rate.

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2. Materials and methods 2.1. Study area This study was conducted at Huitong Experimental Station of Forest Ecology, Chinese Academy of Sciences (latitude 268400 –278090 N and longitude 1098260 –1108080 E), Hunan Province, China. This experimental Station lies at the transition zone from the Yunnan-Guizhou plateau to the low mountains and hills on the southern side of the Yangtze River at an altitude of 300–1100 m above mean sea level. The climate of this region is humid mid-subtropical monsoon with mean annual temperature of 15.8 8C with a mean minimum of 1.9 8C in January and a mean maximum of 29 8C in July. Therefore, four seasons are divided as follows, spring (March–May), summer (June–August), autumn (September–November) and winter (December–February). The mean annual precipitation was 1200 mm of which about 67% occurred between April and August. The mean relative humidity varied from 34% to 93% during the study period (Fig. 1). The native vegetation being evergreen broadleaved forest typical of the subtropics, with the major species component of Castanopsis spp. and Lithocarpus spp., has almost been extirpated by human activities, and C. lanceolata has become the major forest community. After clear-cutting of native broadleaved forest at an altitude of 480–560 m above mean sea level in autumn of 1982 and slash burning in winter, a pure C. lanceolata stand and a mixed stand of C. lanceolata and M. macclurei in total 10 ha were established in early spring of 1983. Planting density was 2000 trees ha1 in the both stands. The ratio of C. lanceolata to M. macclurei in the mixed stand was 4:1. Tree density was 940 stems ha1 C. lanceolata in pure stand, and 657 stems ha1 C. lanceolata and 265 stems ha1 M. macclurei in mixed stand at the investigation. Five replicate plots were selected in each of the stands giving 10 permanent plots of 15 m  20 m in size. Chen et al. (2000) documented that the background values, including soil profile characteristics, textures and mineral composition, were almost identical in the different plots. All the plots were situated in areas of similar soil moisture class, topography, aspect, slope position. The common management practices used in the early stage was: weeding and chemical fertilizer in surface soil around 1 m2 area of tree trunk. According to US Taxonomy, the soil is Oxisol developed from slate and shale. The soils in both stands have developed from the same parent material. Forest soil is 50–70 cm deep and

Fig. 1. Rainfall and temperature from December 1998 to November 2004 in Huitong Experimental Station of Forest Ecology, in Hunan Province, China.

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reddish in colour. Prior to the experiment, soils were sampled at random locations in each permanent plot and analyzed in February 1983. The soil texture is medium-clayey loam (sand 32%, silt 22%, clay 46%) and soil pH ranges from 4.8 to 5.0 (Chen et al., 2000). Soil organic carbon, total N and P were 19.4, 1.8 and 0.16 g kg1. 2.2. Litter and soil collection Litterfall was measured from December 1998 to November 2004 in the pure C. lanceolata stand and the mixed stand of C. lanceolata and M. macclurei within the permanent plots. Ten litter traps, each of 1 m2 area, were randomly placed in each plot. One hundred traps were totally placed in both stands. Each trap consisted of 1-mm mesh nylon netting (on a wooden frame) suspended from a wire hoop and was raised 50 cm above the ground. The litterfall was collected at 15-day intervals. The collected litter at each time was oven-dried at 75 8C to constant weight. The dried litter was combined by month and plot at the end of each month. The surface soil (0–20 cm) in each stand was sampled by a 45-mm-diameter hand auger near each trap in June 2003. Visible roots and organic residues were removed at the sampling time, and then each sample was divided into two parts. One was stored at 4 8C for analysis of microbial biomass. The other part of the soil was air-dried, and then was ground, and stored in plastic bags before analysis. 2.3. Leaf litter decomposition Leaf litter decomposition was studied using the standard litter-bag technique (Falconer et al., 1933). Freshly fallen leaf litter was collected and air-dried for decomposition study in December 2002. C. lanceolata and M. macclurei and their mixture were placed in pure C. lanceolata stand to determine the litter quality effects on decomposition rate. The mixture of C. lanceolata and M. macclurei was also placed in the mixed stand of C. lanceolata and M. macclurei to determine effect of stand quality on decomposition rate. In mixed species litterbags, the ratio of C. lanceolata to M. macclurei was 3:2, which was approximately close to the actual leaf litter values in the mixed stand in this study. Fifteen grams of airdried leaf litters were placed inside 15 cm  20 cm plastic bags with 1.0 mm mesh. Litterbags of each treatment (five plots) were randomly placed on the forest floor in March 2003. The bags were attached to the forest floor by metal pins to prevent movement and to ensure contact between the bags and the litter layer. Litterbags of each treatment were randomly collected at 2, 4, 8, 12 and 18 months after placement of bags, respectively. The materials recovered from the litterbags were air-dried and carefully brushed to remove attached soil particles, and finally were oven-dried at 75 8C to achieve a constant weight. 2.4. Litter and soil analysis The oven-dried litter samples were ground and sieved through a 0.5 mm mesh and analyzed for total C, N, P, K, Ca and Mg

concentrations. C was measured using the dry combustion method (Nelson and Sommers, 1982). Total N concentration was determined by the micro-Kjeldahl method by digesting 0.5 g samples in 10 ml concentrated H2SO4, using a catalyst mixture (CuSO4, K2SO4 and selenium powder) and distillation. To measure P, 0.2 g litter samples were digested in 10 ml triacid mixture (nitric, perchloric and sulphuric acid; 5:1:1) and then cooled. Total P was determined in digested samples colorimetrically using the ammonium molybdate stannus chloride method (Olsen and Sommers, 1982). Contents of K, Ca and Mg in the digested solution were determined with a flame atomic absorption spectrophotometer (Hitachi Z-8100, Tokyo, Japan) following HClO4–HNO3 digestion (Jones and Case, 1990). Soil pH was measured using a mixture of soil and deionized water (1:2.5, w/v) with a glass electrode; total organic carbon content was determined by potassium dichromate oxidation and total nitrogen (N) by the semimicro-Kjeldahl method; total phosphorus (P) and available P were measured colorimetrically; total K and available K by flame photometry; bulk density was determined by the core method. Microbial biomass (C, N and P) were determined by fumigation extraction method. Microbial biomass C was calculated by using Wu et al. (1990): microbial C ¼ K EC  2:2 Microbial N was calculated by using Brookes et al. (1985): microbial N ¼ K EN  2:2 Microbial biomass P was determined by ammonium molybdate stannous chloride method and calculated by Brookes et al. (1982): microbial P ¼ K EP  2:5 KEC, KEN and KEP are the difference between C, N and P extracted from fumigated and unfumigated soils. Basal respiration was determined by measuring CO2 evolution. Field-moist soil sample (equivalent to 40 g dry weight) was placed in 250 ml air-tight glass vessel, moistened to 40% water-holding capacity and incubated in the dark at 28 8C. Vials containing 15 ml 0.1 M NaOH were placed inside the vessels and the CO2 produced from the soil was determined by titration of carbonate with 0.05 M HCl. The value of CO2 evolved in a glass vessel without soil was subtracted from the value of CO2 evolved from the soil sample. The metabolic quotient (qCO2, Anderson and Domsch, 1993) was calculated by dividing the hourly basal respiration rate by the corresponding microbial C (Cmic) ((mg CO2-C h1) (g Cmic)1). Invertase, acid phosphatase, proteinase, urease and polyphenoloxidase activities were assayed in triplicate air-dried soils as described by Guan (1986). 2.5. Statistical analysis Nutrient use efficiency (NUE) was calculated according to Vitousek (1984): NUE ¼

litterfall mass ðg m2 year1 Þ nutrient content in litterfall ðg m2 year1 Þ

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The remaining mass for each period (X1) was determined and compared to the initial mass values (X0) using the formula:   X1 % MR ¼  100 X0 Annual decomposition constant (k) of leaf litter was estimated with a single exponential decomposition model by the equation (Olson, 1963):   Wt ln ¼ kt W0 where W0 is the original mass of litter, Wt the amount of liter remaining after time t, t the time (year) and k is the decomposition rate (year1). As proposed by Olson (1963), the time required for 50% and 95% mass loss and nutrient release was calculated as t50% = 0.693/k and t95% = 3/k. Data were analyzed using repeated measures ANOVA. Means were compared using Tukey’s Honestly Significant Difference tests for differences in litter production, leaf litter decomposition and nutrient return and soil properties between the pure and mixed stands. In all analyses, p < 0.05 was the criterion for significant differences. 3. Results 3.1. Litterfall Annual litterfall at the pure and mixed stands varied from 244 to 788 and 445 to 1041 g m2 year1, respectively (Fig. 2). The mean annual litterfall at the mixed stand (699 g m2 year1) was higher 24% than that at the pure stand (565 g m2 year1). Litter from M. macclurei contributed about 38% of the total litterfall at the mixed stand. The contribution of leaf litter portion in total litterfall was similar at both pure and mixed stands, ranging between 63% and 68% (Table 1).

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Seasonal variation in litterfall at the mixed stand (variation coefficence 41%) was higher than that at the pure stand (variation coefficence 30%), although the difference in seasonal litterfall was not significant (Table 1). About 67% of total litterfall at the mixed stand occurred in cool and dry period (November–March), whereas about 63% at the pure stand. 3.2. Leaf litter decomposition Mass loss of leaf litter was significantly positively correlated (r > 0.97, p < 0.05) with the time elapsed in months (Fig. 3). The decomposition rate of leaf litter in the pure C. lanceolata stand was increased in the order: C. lanceolata < mixture of C. lanceolata and M. macclurei < M. macclurei. The mass loss of mixture of C. lanceolata and M. macclurei observed in litterbags was slightly lower than that expected based on mass losses from component species decomposing alone. The decomposition rate of the mixture was slightly, but not significantly, faster in the mixed stand than the pure C. lanceolata stand during decomposition. The relative mass loss trends amongst species observed during the 1st year were continued through the second rainy season (Fig. 3). Evidently, the decay rate coefficient (k) calculated on the basis of 540 days observation ranged from 0.71 in C. lanceolata to 0.99 in M. macclurei (Table 2). The differences in the decay rate coefficient and half-life (t50%) and 95% mass loss (t95%) periods of decomposing leaf litter samples were significantly only between C. lanceolata and M. macclurei. The concentrations of C and N in decomposing leaf litters were also depicted in Fig. 3. During decomposing, C concentration in all leaf litters decreased over time, whereas N concentration increased. N concentration of mixed litter of C. lanceolata and M. macclurei was lowest among decomposing leaf litters in the pure C. lanceolata stand. M. macclurei leaf litter had the highest N concentration at the end of decomposition. Surprisingly, N concentration of mixture was

Fig. 2. Monthly litterfall from Cunninghamia lanceolata and Michelia macclurei in a monoculture C. lanceolata and mixed with M. macclurei stands.

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Table 1 Seasonal variations in leaf and non-leaf litterfall from December 1998 to November 2004 in a monoculture Cunninghamia lanceolata and mixed with Michelia macclurei stands (g m2) Monoculture stand (C. lanceolata)

Winter (December–February) Spring (March–May) Summer (June–August) Autumn (September–November)

Mixed stand

Leaf

Non-leaf

114  51 108  50 60  41 73  60

61  23 71  39 32  34 47  39

C. lanceolata

M. macclurei

Leaf

Non-leaf

Leaf

Non-leaf

85  24 107  76 39  24 35  17

64  29 57  43 25  19 24  9

51  16 69  44 48  15 39  11

14  8 22  17 13  10 63

Litterfall in different seasons was not significantly different at 5% level according to the Tukey’s Honestly Significant Difference test.

slightly, but not significantly, higher in the mixed stand than the pure C. lanceolata stand. The ratio of C to N in studied leaf litters decreased over time during decomposition and declined by 39–47% compared to initial values (Fig. 3). The remaining amount of C and N in decomposing litters decreased over time (Fig. 3), which followed the pattern of weight loss. The amount of C released ranged from about 69%

(in C. lanceolata leaf litter) to about 80% (in M. macclurei leaf litter) when decomposing under pure stand. The amount of N released was lower and ranged from 47% to 66% in the leaf litter of C. lanceolata and M. macclurei under pure plantation floor, respectively. The remaining amount of C in mixed leaf litter of C. lanceolata and M. macclurei was slightly, but not significantly, lower in the mixed stand than the pure stand.

Fig. 3. Percentage of remaining litter mass, C and N, C:N ratio and C and N concentration in leaf litter in a monoculture C. lanceolata or mixed with M. macclurei stands during experimental period. C. lanceolata (CC), M. macclurei (MC), and mixture of C. lanceolata and M. macclurei (HC) under the monoculture C. lanceolata stand; mixture of C. lanceolata and M. macclurei under the mixed stand of C. lanceolata and M. macclurei (HH).

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Table 2 Decomposition parameters and time required for different levels of decay (t50% and t95% mass loss) of leaf litter in a monoculture C. lanceolata and mixed with M. macclurei stands CC

MC

HC

HH

Mass k (year1) t50% (year) t95% (year)

0.71  0.06 a 0.97  0.08 b 4.22  0.36 b

0.99  0.09 b 0.70  0.07 a 3.02  0.28 a

0.78  0.11 ab 0.89  0.07 ab 3.85  0.30 ab

0.81  0.10 ab 0.85  0.10 ab 3.70  0.45 ab

Carbon k (year1) t50% (year) t95% (year)

0.78  0.08 a 0.89  0.09 b 3.86  0.39 b

1.07  0.13 b 0.65  0.08 a 2.80  0.33 a

0.85  0.14 ab 0.82  0.07 ab 3.55  0.31 ab

0.89  0.10 ab 0.78  0.09 ab 3.37  0.38 ab

Nitrogen k (year1) t50% (year) t95% (year)

0.43  0.07 a 1.62  0.28 b 7.03  1.23 b

0.72  0.14 b 0.97  0.19 a 4.18  0.34 a

0.52  0.12 ab 1.33  0.15 ab 5.75  0.64 ab

0.47  0.12 ab 1.48  0.34 ab 6.40  1.45 ab

Values followed by different letter within the same row are different significant at 5% level according to the Tukey’s Honestly Significant Difference test. C. lanceolata (CC), M. macclurei (MC), and mixture of C. lanceolata and M. macclurei (HC) under the monoculture C. lanceolata plantation; mixture of C. lanceolata and M. macclurei under the mixed plantation of C. lanceolata and M. macclurei (HH).

Table 3 Nutrient concentrations (NC, g kg1) and nutrient use efficiency (NUE) of leaf litter in a monoculture C. lanceolata and mixed with M. macclurei stands Monoculture stand NC

N P K Ca Mg

Mixed stand NUE

12.1  0.6 0.29  0.03 0.54  0.04 9.37  0.7 1.75  0.21

C. lanceolata

29 1223 657 38 203

M. macclurei

NC

NUE

NC

NUE

13.4  0.8 0.35  0.04 0.63  0.08 8.76  0.92 1.69  0.23

20 761 423 30 158

16.4  0.9 1.85  0.11 3.01  0.19 3.42  0.24 1.31  0.08

13 112 69 61 158

However, the remaining amount of N in mixed leaf litter was higher in the mixed stand than the pure stand, which was opposite to C(Table 3). 3.3. Nutrient return through leaf litter The C return of leaf litter varied from 167 g m2 year1 (in pure stand) to 213 g m2 year1 (in mixed stand) (Table 4). C was generally returned to soil in the highest amount, followed by N with a range of 4.3–7.0 g m2 year1 and Ca with a range of 3.0–3.3 g m2 year1. Meanwhile, the returns of P, K and Mg through leaf litter were much smaller than those for C, N and Ca. No significant differences existed in the returns of C, Ca

and Mg between pure and mixed stands. The percent contribution of M. macclurei leaf litter to total nutrient return ranged from 23% (Ca) to 79% (P) in the mixed stand. Nutrient use efficiency (NUE) of C. lanceolata leaf litter in mixed stand was lower in comparison with that in pure stand. 3.4. Soil properties Bulk density of the surface soils (0–20 cm) of the two stands was similar (Table 5). Total organic C, N, P and K in mixed stand was slightly, but not significantly, higher than that in pure stand. Significant differences ( p < 0.05) between the two stands existed for pH, available P and K.

Table 4 Nutrient return (g m2 year1) via leaf litter to forest soil in a monoculture C. lanceolata and mixed with M. macclurei stands C

N

P

K

Ca

Mg

Monoculture stand C. lanceolata

167  36 a

4.3  1.1 a

0.10  0.04 a

0.19  0.05 a

3.32  0.86 a

0.62  0.26 a

Mixed stand C. lanceolata M. macclurei

122  31 91  18

3.6  0.8 3.4  0.7

0.09  0.03 0.38  0.12

0.17  0.03 0.62  0.21

2.33  0.71 0.71  0.28

0.45  0.22 0.27  0.13

213  47 a

7.0  1.3 b

0.48  0.13 b

0.79  0.29 b

3.04  0.87 a

0.72  0.30 a

Total

Values followed by different letter within the same column are different significantly at 5% level according to the Tukey’s Honestly Significant Difference test.

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Table 5 Physical and chemical properties of the surface soil (0–20 cm) in a monoculture C. lanceolata and mixed with M. macclurei stands in the study area Properties 3

Bulk density (g cm ) pH (H2O) Total organic C (g kg1) Total N (g kg1) Total P (g kg1) Available P (mg kg1) Total K (g kg1) Available K (mg kg1)

Monoculture stand

Mixed stand

1.06 (0.13) 4.1 (0.1) a 20.31 (2.78) 1.78 (0.13) 0.14 (0.01) 1.23 (0.26) 12.85 (1.69) 58.98 (5.23)

0.98 (0.15) a 4.6 (0.2) b 21.86 (3.37) a 1.91 (0.17) a 0.17 (0.02) a 1.96 (0.33) b 13.29 (2.34) a 90.34 (10.17) b

a a a a a a a

Values followed by different letter within the same row are different significant at 5% level according to the Tukey’s Honestly Significant Difference test.

Microbial biomass (C, N and P) and basal respiration of surface soil were significantly higher in the mixed stand than the pure stand (Table 6). The metabolic quotient (qCO2) was slightly lower in the mixed stand than the pure stand. The surface soil of the mixed stand displayed higher enzyme activities except polyphenoloxidase than the pure stand did. There were significant differences in acid phosphatase, proteinase, invertase and polyphenoloxidase between both stands. 4. Discussions The mean annual litterfall in the present study was significantly higher in the mixed stand than the pure stand. This difference in litter production between the two stands could be mainly attributed to species composition. Sundarapandian and Swamy (1999) reported that tree species composition was important for litter production within the same climate range. In the mixed stand, M. macclurei contributed about 38% of the total litter production. This result was in agreement with the results reported by Liao et al. (2000) for the mixed plantation of C. lanceolata and M. macclurei at Guangxi Province and Zhang et al. (1993) for mixed plantation of Pinus massoniana and M. macclurei in subtropical China. Increase in litter production may also be explained by a mechanism called the competitive production principle (Kelty, 2006). C. lanceolata and M. macclurei have

substantially different characteristics such as foliar phenology, shade tolerance, crown structure and root depth and phenology. These species may use site resources more efficiently in producing materials, resulting in greater biomass than would occur in monocultures of the component species (Binkley et al., 1992). The litterfall in the present study was concentrated during the cool and dry period (November–March) of the year and about 65% of total litterfall occurred during this period. Pattern of litterfall in this study was comparable with other results in subtropical forest ecosystems (Zhang et al., 1993; Sundarapandian and Swamy, 1999; Pandey et al., 2007). The tendency of litterfall to be concentrated in the cool and dry season is related to a combination of decline in temperature and lowered soil moisture during this period. Pascal (1988) had also reported that a heavy litterfall of leaf occurred during the dry season in evergreen forests of Attappadi, Western Ghats, India. This pattern can be explained by annual cycles of moisture and temperature. Leaf fall would occur to avoid seasonal moisture and temperature stress during dry period (Jackson, 1978). Water stress could cause to synthesize abscissic acid in the foliage of plants which could stimulate senescence of leaves and other parts (Moore, 1980). Litter quality has been considered as an important factor controlling the decomposition rate (Singh et al., 1999; Sundarapandian and Swamy, 1999; Ribeiro et al., 2002; Tateno et al., 2007). C. lanceolata leaf litter had a slower decomposition rate than M. macclurei did in the same stand, which also suggested that leaf litter quality was an important factor for decomposition. This may be mainly because the leaf C:N ratio, which is a good indicator of the decomposition rate (Swift et al., 1979; Sundarapandian and Swamy, 1999; Tripathi et al., 2006; Tateno et al., 2007), was lower in M. macclurei than in C. lanceolata. Initial N and P content of leaf litter may be major factors influences on decomposition rate in this study. The initial N and P contents of C. lanceolata leaf litter were significantly lower than that of M. macclurei leaf litter. The decomposition rate was positively correlated with N (r = 0.924, P < 0.05) and P (r = 0.902, P < 0.05) content and negatively correlated with C/N ratio (r = 0.881, P < 0.05). Many studies

Table 6 Microbial properties and enzyme activities of the surface soil (0–20 cm) in a monoculture C. lanceolata and mixed with M. macclurei stands in the study area

Microbial properties Microbial C (mg kg1) Microbial N (mg kg1) Microbial P (mg kg1) Basal respiration (g CO2-C kg1 soil h1) qCO2 ((mg CO2-C h1) (g Cmic)1) Enzyme activities Urease (mmol NH3 g1 dry weight soil h1) Acid phosphatase (mg p-nitrophenol g1 soil h1) Proteinase (mmol NH3 g1 soil h1) Polyphenoloxidase (ml 0.005 mol l1 I2 g1 soil h1) Invertase (mg dextrose g1 soil d1)

Monoculture stand

Mixed stand

350 (57) a 56 (11) a 12 (4) a 0.62 (1.3) a 1.77 (0.06) a

539 (133) b 98 (24) b 17 (6) b 0.84 (1.7) b 1.56 (0.09) a

0.11 (0.02) a 83.4 (12.3) a 1.02 (0.13) a 1.05(0.07) b 24.56 (3.26) a

0.14 (0.03) a 122.3 (17.1) b 1.41(0.19) b 0.55 (0.08) a 37.17 (5.61) b

Values followed by different letter within the same row are different significant at 5% level according to the Tukey’s Honestly Significant Difference test.

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have also suggested that initial N and P contents in leaf litter are good indicators of the decomposition rate (Sundarapandian and Swamy, 1999; Yang et al., 2004; Tateno et al., 2007). The initial lignin/N ratio and soluble labile C of leaf litter are also controlling the earlier phases of decomposition (Harmon et al., 1990; Singh et al., 1999; Ribeiro et al., 2002; Yang et al., 2004; Tripathi et al., 2006). Yang et al. (2004) reported that Fokienia hodginsii leaf litter with lower lignin/N ratio and higher water soluble matter had a higher decay rate than C. lanceolata leaf litter did. Tripathi et al. (2006) found that lignin/N ratio was significantly negatively correlated with mass loss at all stages of litter decomposition. Harmon et al. (1990) stated that the rapid mass loss was strongly correlated with the soluble labile C content during the early decomposition phase. The water soluble matter content of M. macclurei leaf litter was significantly higher than that of C. lanceolata leaf litter (data not shown). However, in this study, the lignin content was not determined. Site conditions (e.g. soil moisture, temperature and fertility) also affect litter decomposition (Swift et al., 1979; Singh et al., 1999; Tripathi et al., 2006; Pandey et al., 2007). Decomposition rate of mixed leaf litter of C. lanceolata and M. macclurei was slightly, but not significantly, faster in mixed stand than pure stand. This result is in agreement with the observations of Pandey et al. (2007) who found that decomposition rate of Quercus leaf litter was slightly faster at the mixed forest than pure plantation site at the later stage of litter decomposition and Ozalp et al. (2007) who reported that water tupelo leaves on the Big and Little Bull Creeks side decomposed faster than on the Pee Dee River side probably due to microenvironmental factors such as temperature and moisture availability. Difference in decomposition rate between both stands may be primarily due to the difference in site fertility. Some studies have shown that leaf litter decay faster on more nutrient-rich stands (Swift et al., 1979). Soil availability nutrient concentrations were significantly higher in mixed stand than pure stand. Soil nutrient availability can influence decomposer biomass, microbial and enzyme activities (Raubuch and Beese, 1995; Rejma´nkova´ and Sirova´, 2007). Higher enzyme activities and microbial biomass were found in the mixed stand, which suggested that soil availability nutrient in the mixed stand was richer than that of pure stand. Plants in nutrient-poor stands are often characterized by high nutrient resorption leading to low litter quality and decomposition rates (Rejma´nkova´ and Sirova´, 2007), which in turn, result in low amount of nutrient return to soil and litter. Additionally, broadleaved tree may change forest microenvironment to promote decomposition, which was in favor of increasing the microbe activity on the forest floor (Singh et al., 1999; Tripathi et al., 2006). Annual returns of N, P and K to soil systems via leaf litter were far greater in mixed stand than in pure stand, as a result of both higher litter mass and higher nutrient concentrations in litter. Within the mixed stand, broadleaved tree made a greater contribution to total returns of N, P and K than C. lanceolata did. This result was consistent with the results reported by Parrotta (1999) who found that the rates of nutrient return for N, P and K were generally highest in mixed species plantations than in

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single-species plantation. In mixed stand, rapid decomposition of leaf litter accelerates nutrient return to soil, which in turn, promotes plant growth and increases litter production. 5. Conclusions Introduction of broadleaved tree species into the monoculture plantation increased litter production. The mean annual litterfall of mixed stand was significantly higher 24% than that of pure C. lanceolata stand. The fastest rate of mass loss of leaf litter occurred in M. macclurei and the slowest rate in C. lanceolata. The decomposition rate of mixed leaf litter of the two species did not differ significantly between mixed and pure stands, but was slightly higher in mixed stand. N concentration in all leaf litter increased over time during decomposing, whereas N net release occurred constantly. Broadleaved tree increased nutrient returns to soil. Returns of N, P and K via leaf litter were significantly higher in mixed stand than pure stand. This study suggests that mixed forests of C. lanceolata and broadleaved tree species increase litter production and nutrient returns, and are helpful to restore soil fertility of degraded monoculture forests. Acknowledgements This work was supported by the Chinese Academy of Sciences Program (KZCX2-YW-405) and the Knowledge Innovation Program of the Chinese Academy of Sciences. We are grateful to the editor and anonymous reviewers for their helpful comments and suggestions for improving this manuscript. We also thank Xiuyong Zhang for help in collecting samples. References Adams, M.B., Angradi, T.R., 1996. Decomposition and nutrient dynamics of hardwood leaf litter in the Fernow whole-watershed acidification experiment. Forest Ecol. Manage. 83, 61–69. Anderson, T.H., Domsch, K.H., 1993. The metabolic quotient for CO2 (qCO2) as a specific activity parameter to assess the effects of environmental conditions, such as pH, on the microbial biomass of forest soils. Soil Biol. Biochem. 25, 393–395. Binkley, D., Dunkin, K.A., DeBell, D., Ryan, M.G., 1992. Production and nutrient cycling in mixed plantations of Eucalyptus and Albizia in Hawaii. Forest Sci. 38, 393–408. Brookes, P.C., Powlson, D.S., Jenkinson, D.S., 1982. Phosphorus in soil microbial biomass. Soil Biol. Biochem. 14, 319–329. Brookes, P.C., Kragt, J.F., Powlson, D.S., Jenkinson, D.S., 1985. Chloroform fumigation and release of soil nitrogen: the effect of fumigation time and temperature. Soil Biol. Biochem. 17, 831–835. Chen, C.Y., Wang, S.L., 2004. Ecology of Mixed Plantation Forest. Science Press, Beijing, p. 3 (in Chinese). Chen, C.Y., Zhang, J.W., Zhou, C.L., Zheng, H.Y., 1990. Researches on improving the quality of forest land and the productivity of artificial Cunninghamia lanceolata stands. J. Appl. Ecol. 1 (2), 97–106 (in Chinese). Chen, C.Y., Liao, L.P., Wang, S.L., 2000. Ecology of Chinese Fir Plantation Forest. Science Press, Beijing, pp. 1–39, 84–96 (in Chinese). Falconer, G.J., Wright, J.W., Beall, H.W., 1933. The decomposition of certain types of fresh litter under field conditions. Am. J. Bot. 20, 196–203. Feng, Z.W., Chen, C.Y., Zhang, J.W., 1988. A coniferous broad-leaved mixed forest with higher productivity and ecological harmony in subtropics—

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