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Litter decomposition in two subalpine forests during the freeze–thaw season
Acta Oecologica 36 (2010) 135e140
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Original article
Litter decomposition in two subalpine forests during the freezeethaw season Fuzhong Wu, Wanqin Yang*, Jian Zhang, Renju Deng Faculty of Forestry, Sichuan Agricultural University, 625014 Ya'an, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2009 Accepted 3 November 2009 Published online 25 November 2009
Mass loss and nutrient release of forest litter during the freezeethaw season could play an essential role in C and nutrient cycling in cold regions, but few studies in some key ecosystems have been available. In order to characterize litter decomposition during the freezeethaw season in a subalpine forest region of western China, a field experiment using the litterbag method was conducted on the decomposition of foliar litter of two dominant species, fir (Abies faxoniana) and birch (Betula platyphylla) under their respective forests. Over the freezeethaw season following leaf-fall, about 18% and 20% of mass, 13% and 14% of lignin, 30% and 26% of cellulose, 14% and 21% of C, 30% and 27% of N, 17% and 15% of P, and 17% and 13% of K were lost from fir and birch litters, respectively. The lost mass and components accounted for more than 64% and 65% of mass, 72% and 69% of lignin, 75% and 60% of cellulose, 49% and 59% of C, 56% and 71% of N, 62% and 37% of P, and 38% and 37% of K in 1 year net loss rate of fir and birch litter, respectively. In addition, the loss of mass, lignin, cellulose and component bio-elements during the freezeethaw season correlated closely with the initial substrate type and the levels of the individual bioelements. The results demonstrated that litter decomposition during the freezeethaw season contributes significantly to the first year decomposition in these subalpine forests. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Freezeethaw Mass loss Nutrient release Litter quality Subalpine forest
1. Introduction Foliar litter decomposition is one of the most important ecological processes dominating C and nutrient cycling in the forests, which is the main input of organic C in forest soil (Melillo et al., 1989; Berg et al., 1993; Kalbitz et al., 2000; Herman et al., 2008). Measurements of litter decomposition in cold ecosystems during winter and early spring have been limited because of methodological limitations and the low activity of soil organisms (Uchida et al., 2005). In the past, it has been assumed that litter decomposition in winter could be ignored. However, more and more researchers have noted that microbial activity does not exactly cease in winter (Coxson and Parkinson, 1987; Clein and Schimel, 1995; Robinson, 2001; Schimel and Mikan, 2005). Some studies have observed significant mass loss and nutrient release from litter during winter in cool-temperate forests and other northern ecosystems (Gosz et al., 1973; Melillo et al., 1982; Hobbie and Chapin, 1996). Moreover, simulated laboratory experiments have also shown that repeated freezeethaw cycles exhibit significant effects on litter physical structure, and subsequently on decomposition rates (Hurst et al., 1985; Taylor and Parkinson, 1988; Freppaz et al., 2007; Henry, 2007). Consequently, the factors
* Corresponding author. Tel.: þ86 835 2882132; fax: þ86 835 2882016. E-mail address: [email protected] (W. Yang). 1146-609X/$ e see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2009.11.002
associated with freezing and thawing have an important role in litter decomposition during the cold winter (Edwards et al., 2007) in many alpine and subalpine regions. In general, litterfall occurs mainly just before the freezeethaw season. Nutrient release from litter in the cold winter can be favorable to plant nutrition and growth in the following growing season (Freppaz et al., 2007; Berg and McClaugherty, 2008), and the changed litter quality caused by freezing and thawing events might influence the later processes of litter decomposition (Weintraub et al., 2007). There are a variety of mechanisms that can induce mass loss and nutrient release from litter during the freezeethaw season. First, microbial activity does not completely cease when soil is frozen (Brooks et al., 1997). Biological decomposition might be responsible for much of the mass loss and nutrient release during the cold season (Uchida et al., 2005). Second, freezing events during winter have a destructive effect on the physical structure of litter (Schimel and Clein, 1996; Groffman et al., 2001), breaking down the recalcitrant components as lignin or resin canal in the litter. This increases the substrate availability to microorganisms, and consequently increases mass loss and nutrient release. Third, thawing events, associated with the increase of temperature and moisture during the early spring, can directly increase microbial activity, and concomitant leaching can induce large releases of nutrients (Sharma and Ambasht, 1987; O'Connell, 1988; Lemma et al., 2007). Fourth, the fresh litter that occurs in autumn (Norden and Berg, 1990; Don and Kalbitz, 2005) is often characterized with relative
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Table 1 Characteristics of substrate fir (Abies faxoniana) and birch (Betula platyphylla) litter under their respective (FF and BF) forests. Forest
C (g kg1)
N (g kg1)
P (g kg1)
K (g kg1)
Lignin (L) (g kg1)
Cellulose (g kg1)
C/N
N/P
L/N
FF BF
515.63 472.22
9.19 11.13
1.335 1.394
5.52 4.22
418.31 432.03
294.5 275.9
56.10 42.42
6.88 7.94
45.52 38.84
richer labile C in the early stage (Yang et al., 2004; Lemma et al., 2007), resulting in a subsequent rapid rate of litter decomposition during winter. Nevertheless, litter decomposition is tightly related to the substrate quality (Martínez-Yrízar et al., 2007; Zhang et al., 2008; Papa et al., 2008) and many other biotic and abiotic factors (Melillo et al., 1982; Taylor et al., 1989; Bird et al., 2008). The details of the processes and mechanisms of litter decomposition during the freezeethaw season have not been sufficiently documented. Fir and birch forests are two representative forests in many subalpine zones as them in Western China, and play important roles in regulating regional climate and conserving water and soil (Yang et al., 2005). The dynamics surrounding freezing development and subsequent thawing over about half of a year in winter and early spring may have significant effects on the belowground processes in these forests (Wang et al., 2004; Yang et al., 2006). But most studies have tended to focus on the litter decomposition in the plant growing season (Wu et al., 2005; Yang et al., 2006). Far less information has been available on litter decomposition during the freezeethaw period between two growing seasons in these subalpine forests. Therefore, based on the hypothesis that litter decomposition in the freezeethaw season could significantly contribute to the decomposition over the entire year, the changes in litter quality during the freezeethaw season and the entire first year were studied by a field experiment using the litterbag method under fir and birch forests. The objectives were to 1) characterize the fir and birch litter decomposition rate during the freezeethaw season, 2) quantify the percentages of litter decomposition in the freezeethaw season in relation to the rest of the first year, and further explore the contributions of seasonal freezeethaw cycles to litter decomposition. 2. Materials and methods 2.1. Study site This study was conducted in the Wanglang National Nature Reserve (103 550 e104100 E, 32 490 e33 020 N, 2300e4980 m a.s.l.), 1.5
which is located in Pingwu county, western Sichuan, China. The climatic and vegetation features were described by Wang et al. (2004). The mean annual temperature is 2.9 C, the annual cumulative temperature is (10 C) 1056.5 C, and the absolute maximum and minimum temperatures are 26.2 C and 17.8 C, respectively. Annual precipitation ranges from 801 to 825 mm depending on the elevation; most of which falls from May to August. Forest soil is dark brown soil and brown soil (Yang et al., 2005). The freezeethaw season often begins with snow falling in the last days of October, freezing time > 120 d, and freezing depth > 40 cm. Two typical forests dominated by fir (Abies faxoniana) (FF, 104 050 E, 32 580 N, 2600 m a.s.l.) and birch (Betula platyphylla) (BF, 104 070 E, 32 570 N, 2540 m a.s.l.) were selected as sample sites in the Nature Reserve. The slopes of the sample sites were NW10 and NW60 , and the forest soils were classified as dark brown soil (Cambosol) and brown soil (Cambosol) (Yang et al., 2005), respectively. 2.2. Experimental design Litter decomposition was studied using the mesh bag procedure, a technique that is widely used even though it has been noted that it may modify decomposition rates (O'Connell, 1997; Guo et al., 2006). In September 2006, fresh fir and birch senesced leaves were collected from the floor of sampled fir and birch forests, respectively. Decayed, weak and black leaves were excluded. Samples of air-dried litter (30 g for fir needles, 20 g for birch broad-leaves, corresponding of dry mass) were placed in nylon bags (20 20 cm) of 0.50 mm mesh with the edges sealed. Ten replicate samples were taken for each species for a total of 20 bags. The characteristics of the substrate litter are given in Table 1. Fir and birch litterbags were placed on the floor under their respective forests on October 25, 2006 at the beginning of the freezeethaw season. At the end of the freezeethaw season (April 9) and at the end of the next growing season (October 12), 2007,
FF
Temperature ( OC)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
1.5
BF
Temperature ( OC)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
2006-10-25
2007-4-9
Fig. 1. Hourly dynamics of floor temperature in the sample fir forest (FF) and birch forest (BF) during the freezeethaw season (from October 25, 2006 to April 9, 2007).
F. Wu et al. / Acta Oecologica 36 (2010) 135e140
litterbags were sampled randomly with five replicates, respectively from each species. Meanwhile, soil temperature was measured every hour using a buttony DS1923-F5 recorder (Maxim Com. USA) that was also placed on the floor (above the soil) in each of the two forests. The hourly soil temperature during the freezeethaw season (from October 25, 2006 to April 9, 2006) is described in Fig. 1.
2.3. Analyses and calculations The remaining litter was taken from the litterbags, and foreign materials were removed. The samples were oven-dried at 70 C for at least 48 h to constant weight, and weighed to the nearest 0.01 g. The oven-dried samples were finely ground to pass through the 1-mm stainless steel sieve for C and nutrient analysis. C and nutrients in samples were determined as described by Lu (1999). C content was determined by using the dichromate oxidation-sulphateferrous titration method. Sub-samples of 0.2500 g were acid digested with 8 ml H2SO4 (r ¼ 1.84 g cm3) and 3 ml H2O2 solution at 190 C for 10 min. The digested solution was then
600
FF
a x
transferred to a 100-ml volumetric flask, rationed, and stored for N, P and K contents measurements. N, P and K were determined by indophenol-blue colorimetry, phosphomolybdenum-yellow colorimetry and flame photometry, respectively. Lignin and cellulose were measured using the Acid Detergent Lignin method (Vanderbilt et al., 2008). All the analyses were carried out in triplicate. Litter dry mass loss (L), the release (R) rates of C, nutrients, lignin and cellulose, and their percentages (P) in the freezeethaw season for 1 year of decomposition were calculated as follows:
Lð%Þ ¼ 100 ðM0 Mt Þ=M0 Rð%Þ ¼ 100 ðM0 C0 Mt Ct Þ=M0 C0 Pð%Þ ¼ 100 ðM0 C0 M4 C4 Þ=ðM0 C0 M10 C10 Þ where M0, the dry mass of initial litter; Mt, the dry mass of the remaining litter in the bag when sampled; M4 and M10, the dry mass of the remaining litter in the bag when collected in April and October, respectively; C0, the concentration (g kg1) of C, nutrients,
14
BF
a
a x
500
12 x
400 300 200
N content (g k g-1 )
C content (g k g-1 )
700
100
a
ab
10
x
x b
8 6 4
0 April
2.1
x
1.2 0.9 0.6
-1
a
0.3
April
October
ab a
6
a
a
7
x
x
1.8
Initial
October
K content (g k g )
Initial
P content (g k g -1 )
x
2
0
1.5
137
x
x
Initial
April
5
b
x
4 3 2 1
0.0 Initial
April
0
October
October
600 500
a
x
ab
xy
b y
350
a x
Cellulose (g kg-1 )
Lignin (g kg-1)
300 400 300 200 100
ab xy
b y
250 200 150 100 50 0
0 Initial
April October Sampling time
Initial
April October Sampling time
Fig. 2. Variations of C, N, P, K, lignin and cellulose contents in fir (Abies faxoniana) and birch (Betula platyphylla) litter among substrate (Initial), after the freezeethaw season (April), and after one-year (October) decomposition under their respective (FF and BF) forests. Bars indicate SD, n ¼ 5; different letters denote the significant differences among different decomposition stages under the same forest, P < 0.05.
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F. Wu et al. / Acta Oecologica 36 (2010) 135e140
90
FF
BF *
80
Percentage (%)
70
* *
60 * 50 40 30 20 10 0
Mass
C
N
P
K
Lignin
Cellulose
Fig. 3. Percentages of mass, C, N, P, K, lignin and cellulose net loss in the freezeethaw season to one-year fir (Abies faxoniana) and birch (Betula platyphylla) litter decomposition under their respective (FF and BF) forests. Bars indicate SD, n ¼ 5; asterisks denote the significant differences with different forests, P < 0.05.
lignin and cellulose in the initial litter; Ct, the concentration (g kg1) of C, nutrients, lignin and cellulose in the remaining litter when sampled; C4 and C10, the concentration (g kg1) of C, nutrients, lignin and cellulose in the remaining litter when collected in April and October, respectively. 2.4. Statistical analysis All of the variables from measurements and calculations were analyzed using a t-test with substrate species as a factor. Relationships between initial litter characteristics and the rates of litter decomposition (mass, lignin, cellulose, and elements loss) after the freezeethaw season were determined using the Pearson's correlation coefficient test at the 0.05 level. All of the statistical analyses were performed using SPSS (Standard released version 11.5 for Windows, SPSS Inc., IL, USA) software package.
Table 3 Pearson's correlations between the initial characteristics in substrate litter and net loss rates of mass, C, N, P, K, lignin and cellulose in the freezeethaw season. Initial characteristics
Net loss rate in freezeethaw season Mass
C
C N P K Lignin Cellulose C/N N/P L/N
0.82** 0.83** 0.04 0.33 0.81** 0.71* 0.72* 0.73* 0.83**
0.86** 0.67* 0.46 0.68* 0.85** 0.66* 0.45 0.67* 0.58 0.84** 0.71* 0.85** 0.86** 0.67* 0.46 0.68* 0.85** 0.69* 0.48 0.70* 0.87** 0.68* 0.38 0.73* 0.85** 0.67* 0.46 0.68* 0.89** 0.55 0.33 0.57 0.85** 0.65* 0.44 0.66*
N
P
K
Lignin Cellulose 0.24 0.65* 0.72* 0.25 0.23 0.13 0.25 0.36 0.65*
0.68* 0.66* 0.85** 0.67* 0.69* 0.66* 0.67* 0.56 0.66*
**P < 0.01, *P < 0.05, n ¼ 10.
cellulose, 49% of C, 56% of N, 62% of P and 38% of K in 1 year net loss rates, respectively (Fig. 3).Comparatively, birch litter lost more than 20% of mass, 14% of lignin, 26% of cellulose, 21% of C, 27% of N, 15% of P and 13% of K during the freezeethaw season. The lost mass and components accounted for 65% of mass, 69% of lignin, 60% of cellulose, 59% of C, 71% of N, 37% of P and 39% of K in one-year net loss rates, respectively. 3.3. Correlations Net mass loss rate and C release rate positively correlated with N and cellulose content and N/P in substrate during the freezeethaw season, but negatively correlated with lignin content, and C/N and L/N (Table 3). N and K release rates positively correlated with K and cellulose contents, and C/N and L/N in substrate during the freezeethaw season. P release rate correlated only with P content in substrate. Lignin and cellulose loss rates positively correlated with N and P content in substrate during the freezeethaw season, but negatively correlated with L/N. 4. Discussion
3. Results 3.1. Elements, lignin and cellulose content C and P contents of both fir and birch litter showed few changes as decomposition proceeded (Fig. 2). N and K contents of fir litter decreased after 1 year of decomposition, whereas those of birch litter showed non-significant differences. In addition, lignin content of both fir and birch litter increased slightly as decomposition proceeded, but cellulose content decreased. 3.2. Mass loss and elements release Over the freezeethaw season, more than 18% of mass, 13% of lignin, 30% of cellulose, more than 14% of C, 30% of N, 17% of P and 17% of K were lost from fir litter (Table 2). The lost mass and components accounted for 64% of mass, 72% of lignin, 75% of
The results in this study supported our hypothesis that litter decomposition during the freezeethaw season accounted for a large proportion of the decomposition in the first year in these subalpine forests. The lost mass and released nutrients could be favorable to plant nutrition and growth in the following spring, which is closely tied to nutrient cycling, and is essential for the regeneration of organically bound nutrients (Freppaz et al., 2007; Berg and McClaugherty, 2008). The results are consistent with the findings of many previous studies in the other cold ecosystems (Gosz et al., 1973; Melillo et al., 1982; Moore, 1983; Hobbie and Chapin, 1996; Uchida et al., 2005), suggesting that more attention should be paid to ecological processes in winter and early spring. Changes in litter quality were obvious as decomposition proceeded. This could indicate the release processes of each component (Chapin et al., 2002; Berg and McClaugherty, 2008). Generally, elements are released along with the loss of litter mass as
Table 2 Mass, lignin, cellulose, C, N, P and K release rates (means SD, n ¼ 5) of (Abies faxoniana) and birch (Betula platyphylla) litter after the freezeethaw season (April), and after oneyear (October) decomposition under their respective (FF and BF) forests. Forest
Sampling time
Residual mass (g)
Net loss rate (%) Mass
Lignin
Cellulose
C
N
P
K
FF
April October
24.32 1.61 21.22 3.49
18.92 0.54a 29.27 1.17x
13.01 3.46a 18.11 1.35x
30.29 2.77a 39.88 0.99x
14.55 3.40a 29.35 1.16x
30.62 2.76a 53.80 0.76x
17.84 3.27a 28.80 1.17x
17.56 3.28a 45.74 0.89x
BF
April October
15.96 0.62 13.83 1.18
20.21 1.61b 30.83 0.59x
14.02 3.35a 20.26 4.11x
26.77 2.85b 43.94 2.89y
21.02 3.07b 35.18 3.34y
27.18 2.83a 38.18 3.18y
15.53 3.29a 41.83 3.00y
13.33 3.37a 33.68 3.41y
Different letters among variables with same sampling time under different forests denote significantly different by LSD within a column; n ¼ 5; P < 0.05.
F. Wu et al. / Acta Oecologica 36 (2010) 135e140
decomposition proceeds. However, C, N, P, K content of fir and birch litter showed few changes in the freezeethaw season. Cellulose was readily decomposable due to microorganism decomposition (Sinsabaugh and Linkins, 1989; Berg and McClaugherty, 2008), it showed a more rapid loss rate than mass, and then decreased content was observed as decomposition proceeded. In contrast, lignin content increased due to its recalcitrant character. Based on the changes in lignin, cellulose and element content, the high decomposition rates of litter in the freezeethaw season could be related to the following explanations. First, the decomposable carbohydrates in soluble litter material are relatively richer in the early stage of litter decomposition (Norden and Berg, 1990; Don and Kalbitz, 2005). The increased content of recalcitrant material such as lignin might inhibit decomposition in the later stage (Fig. 2; Kalbitz et al., 2004). Second, microbial activity does not completely cease even under frozen soil conditions (Weintraub et al., 2007). Freezing is often disruptive to litter physical structure (Melick and Seppelt, 1992; Harris and Safford, 1996), and increases substrate availability to microorganisms, and then to microbial activity. In addition, the freezeethaw cycle is the combined process of temperature change (Fig. 1) and the dryewet cycle. Hydraulic leaching in the thawing phase could be of sufficient power to promote nutrient release. Increased moisture and temperature are also beneficial to microbial activity in the early spring (O'Connell, 1988; Lemma et al., 2007). Consequently, the integrated action of physical, chemical and biological processes could contribute to the high decomposition rates of fir and birch litter during the freezeethaw season. Even so, mass loss and nutrient release during the freezeethaw season is tightly correlated with their respective traits (Meentemeyer, 1978; Aerts, 1997). In this study, the loss rates of cellulose and N were higher than the other elements in the freezeethaw season. This might be related to the decomposable and mobilizable character of cellulose and N. Although P and K are also welldocumented as highly mobile, K is often released with hydraulic leaching; whereas leaching occurs more largely in the growing season compared to that in the freezeethaw season. P could be immobilized by microorganisms (Ribeiro et al., 2002; Tian et al., 2003) since P is the limiting nutrient in these subalpine forests (Wu et al., 2009). Positive correlations between substrate P content and the loss rates of lignin and cellulose also support this opinion. In addition, initial litter quality also contributed to the changes of component loss rates during the freezeethaw season (Table 3). Litter with relative higher N/P, N and cellulose content have higher net mass loss rates, while the lower decomposition rate was observed in the litter with higher lignin content, C/N and L/Na finding consistent with other studies of litter decomposition during the growing season (Melillo et al., 1982; Zhang et al., 2008; Papa et al., 2008). This suggests that microbial activity still dominated the primary processes during litter decomposition in the freezeethaw season. Thus, varying litter decomposition rates between fir and birch were the result of differences in their individual traits. Moreover, the results in this study indicated that about half of the litter decomposition rate in the first year was attributed to the decomposition in the freezeethaw season, and the percentages of each component varied with their respective traits. Comparatively, lignin loss rates in the freezeethaw season showed the highest percentages (more than 70%) of loss rate in the first year, which could be closely attributed to the disruptive actions on litter physical structure by freezing (Melick and Seppelt, 1992; Harris and Safford, 1996). Meanwhile, the results supported the theory that freezing in the freezeethaw season could be beneficial to ecological processes such as the decomposition of recalcitrant components (Schimel and Clein, 1996; Groffman et al., 2001). In contrast, lower
139
percentages of 1 year P release rates for birch litter and K release rate for both fir and birch litter were observed in the freezeethaw season, which was related to the relatively less hydraulic leaching for K release in the freezeethaw season, and microorganism immobilization for P (Ribeiro et al., 2002; Tian et al., 2003) as discussed above. However, less N was released in the growing season compared to P, suggesting that N might be immobilized by microorganisms and could be the limiting factor in the growing season. The results were in agreement with the theory that different microorganisms dominated the decomposition in different decomposition stages (Schadt et al., 2003; Moorhead and Sinsabaugh, 2006). Similar with the net loss rate, the percentages of decomposition rates in the freezeethaw season to 1 year varied between birch and fir litter. In conclusion, both fir and birch litter showed high decomposition rates in the freezeethaw season, accounting for large percentages of the 1 year decomposition rate in the first year of litter decomposition. Net mass loss and nutrient release during litter decomposition were attributed to physical destruction, microbial activity, hydraulic leaching and their integrated effects in the freezeethaw season. However, it should be noted that our results only determined that the decomposition rate in the freezeethaw season and its percentages of the decomposition in the first year were significantly related to litter quality and the traits of each component. Because of the complex effects of biotic and abiotic factors, understanding of litter decomposition in the freezeethaw season should be strengthened through examination of the detailed processes in the future.
Acknowledgements We are very grateful to Prof. Bo Zeng for nice suggestions on the manuscript. The project was financially supported by National Natural Science Foundation of China (No. 30771702, 30471378), Program for New Century Excellent Talents in University (NCET-070592), National Key Technologies R & D Program of China (No. 2006BAC01A11), and Sichuan Excellent Youth Science and Technology Foundation (07ZQ026-022). We gratefully acknowledge the staff of the Wanglang National Nature Reserve for their help in the field work. References Aerts, R., 1997. Climate, leaf litter chemistry and leaf litter decomposition in terrestrial ecosystems: a triangular relationship. Oikos 79, 439e449. Berg, B., Berg, M.P., Bottner, P., Box, E., Breymeyer, A., de Anta, R.C., Couteaux, M., Escudero, A., Gallardo, A., Kratz, W., Madeira, M., Malkonen, E., McClaugherty, C., Meenteneyer, V., Munoz, F., Piussi, P., Remacle, J., de Santo, A.V., 1993. Litter mass loss rates in pine forests of Europe and Eastern United States e some relationships with climate and litter quality. Biogeochemistry 20, 127e159. Berg, B., McClaugherty, C., 2008. Plant Litter: Decomposition, Humus Formation, Carbon Sequestration, second ed. Springer, New York. Bird, J.A., Kleber, M., Torn, M.S., 2008. 13C and 15N stabilization dynamics in soil organic matter fractions during needle and fine root decomposition. Org. Geochem. 39, 465e477. Brooks, P.D., Schmidt, S., Williams, M.W., 1997. Winter production of CO2 and N2O from alpine tundra: environmental controls and relationship to inter-system C and N fluxes. Oecologia 110, 403e413. Chapin III, F.S., Matson, P.M., Money, H.A., 2002. Principles of Terrestrial Ecosystem Ecology. Springer-Verlag, New York. Clein, J.S., Schimel, J.P., 1995. Microbial activity of tundra and taiga soils at sub-zero temperatures. Soil Biol. Biochem. 27, 1231e1234. Coxson, D.S., Parkinson, D., 1987. Winter respiratory activity in aspen woodland forest floor litter and soils. Soil Biol. Biochem. 19, 49e59. Don, A., Kalbitz, K., 2005. Amounts and degradability of dissolved organic carbon from foliar litter at different decomposition stages. Soil Biol. Biochem. 37, 2171e2179. Edwards, A.C., Scalenghe, R., Freppaz, M., 2007. Changes in the seasonal snow cover of alpine regions and its effect on soil processes: a review. Quatern. Int. 162e163, 172e181.
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O'Connell, A.M., 1997. Decomposition of slash residues in thinned regrowth eucalypt forest in Western Australia. J. Appl. Ecol. 34, 111e122. Papa, S., Pellegrino, A., Fioretto, A., 2008. Microbial activity and quality changes during decomposition of Quercus ilex leaf litter in three Mediterranean woods. Appl. Soil Ecol. 40, 401e410. Ribeiro, C., Madeira, M., Araujo, M.C., 2002. Decomposition and nutrient release from leaf litter of Eucalyptus globulus grown under different water and nutrient regimes. For. Ecol. Manag. 171, 31e41. Robinson, C.H., 2001. Cold adaptation in Arctic and Antarctic fungi. New Phytol. 151, 341e353. Schadt, C.W., Martin, A.P., Lipson, D.A., Schmidt, S.K., 2003. Seasonal dynamics of previously unknown fungal lineages in tundra soils. Science 301, 1359e1361. Schimel, J.P., Clein, J.S., 1996. Microbial response to freezeethaw cycles in tundra and taiga soils. Soil Biol. Biochem. 28, 1061e1066. Schimel, J.P., Mikan, C., 2005. Changing microbial substrate use in Arctic tundra soils through a freezeethaw cycle. Soil Biol. Biochem. 37, 1411e1418. Sharma, E., Ambasht, R.S., 1987. Litterfall, decomposition and nutrient release in an age sequence Alnus nepalensis plantation stands in the eastern Himalaya. J. Ecol. 75, 997e1010. Sinsabaugh, R.L., Linkins, A.E., 1989. Cellulase mobility in decomposing leaf litter. Soil Biol. Biochem. 21, 205e209. Taylor, B.R., Parkinson, D., 1988. Does repeated freezing and thawing accelerate decay of leaf litter? Soil Biol. Biochem. 20, 657e665. Taylor, B.R., Parkinson, D., Parsons, W.F.J., 1989. Nitrogen and lignin content as predictors of litter decay-rates e a microcosm test. Ecology 70, 97e104. Tian, X.J., Sun, S., Ma, K.P., An, S.Q., 2003. Behavior of carbon and nutrients within two types of leaf litter during 3.5-year decomposition. Acta Bot. Sin. 45, 1413e1420. Uchida, M., Mo, W., Nakatsubo, T., Tsuchiya, Y., Horikoshi, T., Koizumi, H., 2005. Microbial activity and litter decomposition under snow cover in a cooltemperate broad-leaved deciduous forest. Agric. For. Meteorol. 134, 102e109. Vanderbilt, K.L., White, C.S., Hopkins, O., Craig, J.A., 2008. Aboveground decomposition in arid environments: results of a long-term study in central New Mexico. J. Arid Environ. 72, 696e709. Wang, K.Y., Yang, W.Q., Song, G.Y., Hu, T.X., 2004. Processes of Subalpine Forest Ecosystem in Western Sichuan. Sichuan Science and Technology Press, Chengdu, pp. 16e28 (in Chinese). Weintraub, M.N., Scott-Denton, L.E., Schmidt, S.K., Monson, R.K., 2007. The effects of tree rhizodecomposition on soil exoenzyme activity, dissolved organic carbon, and nutrient availability in a subalpine forest ecosystem. Oecologia 154, 327e338. Wu, F.Z., Wang, K.Y., Yang, W.Q., Lu, Y.J., Qiao, Y.Z., 2005. Effects of Fargesia denudate density on seasonal changes in litter nutrient concentrations and their potential retranslocation. Acta Phytoecol. Sin. 29, 537e542 (in Chinese with English abstract). Wu, F.Z., Yang, W.Q., Wang, K.Y., Wu, N., Lu, Y.J., 2009. Effects of stem density on leaf nutrient dynamics and nutrient use efficiency of dwarf bamboo. Pedosphere 19, 496e504. Yang, W.Q., Wang, K.Y., Kellomäki, S., Gong, H.D., 2005. Litter dynamics of three subalpine forests in western Sichuan. Pedosphere 15, 653e659. Yang, W.Q., Wang, K.Y., Kellomäki, S., Zhang, J., 2006. Annual and monthly variation in litter macronutrients of three subalpine forests in western China. Pedosphere 16, 788e798. Yang, Y.S., Chen, G.S., Guo, J.F., Lin, P., 2004. Decomposition dynamic of fine roots in a mixed forest of Cunninghamia lanceolata and Tsoongiodendron odorum in midsubtropics. Ann. For. Sci. 61, 65e72. Zhang, P., Tiana, X., He, X., Song, F., Ren, L., Jiang, P., 2008. Effect of litter quality on its decomposition in broadleaf and coniferous forest. Eur. J. Soil Biol. 44, 392e399.
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Original article
Litter decomposition in two subalpine forests during the freezeethaw season Fuzhong Wu, Wanqin Yang*, Jian Zhang, Renju Deng Faculty of Forestry, Sichuan Agricultural University, 625014 Ya'an, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 August 2009 Accepted 3 November 2009 Published online 25 November 2009
Mass loss and nutrient release of forest litter during the freezeethaw season could play an essential role in C and nutrient cycling in cold regions, but few studies in some key ecosystems have been available. In order to characterize litter decomposition during the freezeethaw season in a subalpine forest region of western China, a field experiment using the litterbag method was conducted on the decomposition of foliar litter of two dominant species, fir (Abies faxoniana) and birch (Betula platyphylla) under their respective forests. Over the freezeethaw season following leaf-fall, about 18% and 20% of mass, 13% and 14% of lignin, 30% and 26% of cellulose, 14% and 21% of C, 30% and 27% of N, 17% and 15% of P, and 17% and 13% of K were lost from fir and birch litters, respectively. The lost mass and components accounted for more than 64% and 65% of mass, 72% and 69% of lignin, 75% and 60% of cellulose, 49% and 59% of C, 56% and 71% of N, 62% and 37% of P, and 38% and 37% of K in 1 year net loss rate of fir and birch litter, respectively. In addition, the loss of mass, lignin, cellulose and component bio-elements during the freezeethaw season correlated closely with the initial substrate type and the levels of the individual bioelements. The results demonstrated that litter decomposition during the freezeethaw season contributes significantly to the first year decomposition in these subalpine forests. Ó 2009 Elsevier Masson SAS. All rights reserved.
Keywords: Freezeethaw Mass loss Nutrient release Litter quality Subalpine forest
1. Introduction Foliar litter decomposition is one of the most important ecological processes dominating C and nutrient cycling in the forests, which is the main input of organic C in forest soil (Melillo et al., 1989; Berg et al., 1993; Kalbitz et al., 2000; Herman et al., 2008). Measurements of litter decomposition in cold ecosystems during winter and early spring have been limited because of methodological limitations and the low activity of soil organisms (Uchida et al., 2005). In the past, it has been assumed that litter decomposition in winter could be ignored. However, more and more researchers have noted that microbial activity does not exactly cease in winter (Coxson and Parkinson, 1987; Clein and Schimel, 1995; Robinson, 2001; Schimel and Mikan, 2005). Some studies have observed significant mass loss and nutrient release from litter during winter in cool-temperate forests and other northern ecosystems (Gosz et al., 1973; Melillo et al., 1982; Hobbie and Chapin, 1996). Moreover, simulated laboratory experiments have also shown that repeated freezeethaw cycles exhibit significant effects on litter physical structure, and subsequently on decomposition rates (Hurst et al., 1985; Taylor and Parkinson, 1988; Freppaz et al., 2007; Henry, 2007). Consequently, the factors
* Corresponding author. Tel.: þ86 835 2882132; fax: þ86 835 2882016. E-mail address: [email protected] (W. Yang). 1146-609X/$ e see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.actao.2009.11.002
associated with freezing and thawing have an important role in litter decomposition during the cold winter (Edwards et al., 2007) in many alpine and subalpine regions. In general, litterfall occurs mainly just before the freezeethaw season. Nutrient release from litter in the cold winter can be favorable to plant nutrition and growth in the following growing season (Freppaz et al., 2007; Berg and McClaugherty, 2008), and the changed litter quality caused by freezing and thawing events might influence the later processes of litter decomposition (Weintraub et al., 2007). There are a variety of mechanisms that can induce mass loss and nutrient release from litter during the freezeethaw season. First, microbial activity does not completely cease when soil is frozen (Brooks et al., 1997). Biological decomposition might be responsible for much of the mass loss and nutrient release during the cold season (Uchida et al., 2005). Second, freezing events during winter have a destructive effect on the physical structure of litter (Schimel and Clein, 1996; Groffman et al., 2001), breaking down the recalcitrant components as lignin or resin canal in the litter. This increases the substrate availability to microorganisms, and consequently increases mass loss and nutrient release. Third, thawing events, associated with the increase of temperature and moisture during the early spring, can directly increase microbial activity, and concomitant leaching can induce large releases of nutrients (Sharma and Ambasht, 1987; O'Connell, 1988; Lemma et al., 2007). Fourth, the fresh litter that occurs in autumn (Norden and Berg, 1990; Don and Kalbitz, 2005) is often characterized with relative
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Table 1 Characteristics of substrate fir (Abies faxoniana) and birch (Betula platyphylla) litter under their respective (FF and BF) forests. Forest
C (g kg1)
N (g kg1)
P (g kg1)
K (g kg1)
Lignin (L) (g kg1)
Cellulose (g kg1)
C/N
N/P
L/N
FF BF
515.63 472.22
9.19 11.13
1.335 1.394
5.52 4.22
418.31 432.03
294.5 275.9
56.10 42.42
6.88 7.94
45.52 38.84
richer labile C in the early stage (Yang et al., 2004; Lemma et al., 2007), resulting in a subsequent rapid rate of litter decomposition during winter. Nevertheless, litter decomposition is tightly related to the substrate quality (Martínez-Yrízar et al., 2007; Zhang et al., 2008; Papa et al., 2008) and many other biotic and abiotic factors (Melillo et al., 1982; Taylor et al., 1989; Bird et al., 2008). The details of the processes and mechanisms of litter decomposition during the freezeethaw season have not been sufficiently documented. Fir and birch forests are two representative forests in many subalpine zones as them in Western China, and play important roles in regulating regional climate and conserving water and soil (Yang et al., 2005). The dynamics surrounding freezing development and subsequent thawing over about half of a year in winter and early spring may have significant effects on the belowground processes in these forests (Wang et al., 2004; Yang et al., 2006). But most studies have tended to focus on the litter decomposition in the plant growing season (Wu et al., 2005; Yang et al., 2006). Far less information has been available on litter decomposition during the freezeethaw period between two growing seasons in these subalpine forests. Therefore, based on the hypothesis that litter decomposition in the freezeethaw season could significantly contribute to the decomposition over the entire year, the changes in litter quality during the freezeethaw season and the entire first year were studied by a field experiment using the litterbag method under fir and birch forests. The objectives were to 1) characterize the fir and birch litter decomposition rate during the freezeethaw season, 2) quantify the percentages of litter decomposition in the freezeethaw season in relation to the rest of the first year, and further explore the contributions of seasonal freezeethaw cycles to litter decomposition. 2. Materials and methods 2.1. Study site This study was conducted in the Wanglang National Nature Reserve (103 550 e104100 E, 32 490 e33 020 N, 2300e4980 m a.s.l.), 1.5
which is located in Pingwu county, western Sichuan, China. The climatic and vegetation features were described by Wang et al. (2004). The mean annual temperature is 2.9 C, the annual cumulative temperature is (10 C) 1056.5 C, and the absolute maximum and minimum temperatures are 26.2 C and 17.8 C, respectively. Annual precipitation ranges from 801 to 825 mm depending on the elevation; most of which falls from May to August. Forest soil is dark brown soil and brown soil (Yang et al., 2005). The freezeethaw season often begins with snow falling in the last days of October, freezing time > 120 d, and freezing depth > 40 cm. Two typical forests dominated by fir (Abies faxoniana) (FF, 104 050 E, 32 580 N, 2600 m a.s.l.) and birch (Betula platyphylla) (BF, 104 070 E, 32 570 N, 2540 m a.s.l.) were selected as sample sites in the Nature Reserve. The slopes of the sample sites were NW10 and NW60 , and the forest soils were classified as dark brown soil (Cambosol) and brown soil (Cambosol) (Yang et al., 2005), respectively. 2.2. Experimental design Litter decomposition was studied using the mesh bag procedure, a technique that is widely used even though it has been noted that it may modify decomposition rates (O'Connell, 1997; Guo et al., 2006). In September 2006, fresh fir and birch senesced leaves were collected from the floor of sampled fir and birch forests, respectively. Decayed, weak and black leaves were excluded. Samples of air-dried litter (30 g for fir needles, 20 g for birch broad-leaves, corresponding of dry mass) were placed in nylon bags (20 20 cm) of 0.50 mm mesh with the edges sealed. Ten replicate samples were taken for each species for a total of 20 bags. The characteristics of the substrate litter are given in Table 1. Fir and birch litterbags were placed on the floor under their respective forests on October 25, 2006 at the beginning of the freezeethaw season. At the end of the freezeethaw season (April 9) and at the end of the next growing season (October 12), 2007,
FF
Temperature ( OC)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
1.5
BF
Temperature ( OC)
1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0
2006-10-25
2007-4-9
Fig. 1. Hourly dynamics of floor temperature in the sample fir forest (FF) and birch forest (BF) during the freezeethaw season (from October 25, 2006 to April 9, 2007).
F. Wu et al. / Acta Oecologica 36 (2010) 135e140
litterbags were sampled randomly with five replicates, respectively from each species. Meanwhile, soil temperature was measured every hour using a buttony DS1923-F5 recorder (Maxim Com. USA) that was also placed on the floor (above the soil) in each of the two forests. The hourly soil temperature during the freezeethaw season (from October 25, 2006 to April 9, 2006) is described in Fig. 1.
2.3. Analyses and calculations The remaining litter was taken from the litterbags, and foreign materials were removed. The samples were oven-dried at 70 C for at least 48 h to constant weight, and weighed to the nearest 0.01 g. The oven-dried samples were finely ground to pass through the 1-mm stainless steel sieve for C and nutrient analysis. C and nutrients in samples were determined as described by Lu (1999). C content was determined by using the dichromate oxidation-sulphateferrous titration method. Sub-samples of 0.2500 g were acid digested with 8 ml H2SO4 (r ¼ 1.84 g cm3) and 3 ml H2O2 solution at 190 C for 10 min. The digested solution was then
600
FF
a x
transferred to a 100-ml volumetric flask, rationed, and stored for N, P and K contents measurements. N, P and K were determined by indophenol-blue colorimetry, phosphomolybdenum-yellow colorimetry and flame photometry, respectively. Lignin and cellulose were measured using the Acid Detergent Lignin method (Vanderbilt et al., 2008). All the analyses were carried out in triplicate. Litter dry mass loss (L), the release (R) rates of C, nutrients, lignin and cellulose, and their percentages (P) in the freezeethaw season for 1 year of decomposition were calculated as follows:
Lð%Þ ¼ 100 ðM0 Mt Þ=M0 Rð%Þ ¼ 100 ðM0 C0 Mt Ct Þ=M0 C0 Pð%Þ ¼ 100 ðM0 C0 M4 C4 Þ=ðM0 C0 M10 C10 Þ where M0, the dry mass of initial litter; Mt, the dry mass of the remaining litter in the bag when sampled; M4 and M10, the dry mass of the remaining litter in the bag when collected in April and October, respectively; C0, the concentration (g kg1) of C, nutrients,
14
BF
a
a x
500
12 x
400 300 200
N content (g k g-1 )
C content (g k g-1 )
700
100
a
ab
10
x
x b
8 6 4
0 April
2.1
x
1.2 0.9 0.6
-1
a
0.3
April
October
ab a
6
a
a
7
x
x
1.8
Initial
October
K content (g k g )
Initial
P content (g k g -1 )
x
2
0
1.5
137
x
x
Initial
April
5
b
x
4 3 2 1
0.0 Initial
April
0
October
October
600 500
a
x
ab
xy
b y
350
a x
Cellulose (g kg-1 )
Lignin (g kg-1)
300 400 300 200 100
ab xy
b y
250 200 150 100 50 0
0 Initial
April October Sampling time
Initial
April October Sampling time
Fig. 2. Variations of C, N, P, K, lignin and cellulose contents in fir (Abies faxoniana) and birch (Betula platyphylla) litter among substrate (Initial), after the freezeethaw season (April), and after one-year (October) decomposition under their respective (FF and BF) forests. Bars indicate SD, n ¼ 5; different letters denote the significant differences among different decomposition stages under the same forest, P < 0.05.
138
F. Wu et al. / Acta Oecologica 36 (2010) 135e140
90
FF
BF *
80
Percentage (%)
70
* *
60 * 50 40 30 20 10 0
Mass
C
N
P
K
Lignin
Cellulose
Fig. 3. Percentages of mass, C, N, P, K, lignin and cellulose net loss in the freezeethaw season to one-year fir (Abies faxoniana) and birch (Betula platyphylla) litter decomposition under their respective (FF and BF) forests. Bars indicate SD, n ¼ 5; asterisks denote the significant differences with different forests, P < 0.05.
lignin and cellulose in the initial litter; Ct, the concentration (g kg1) of C, nutrients, lignin and cellulose in the remaining litter when sampled; C4 and C10, the concentration (g kg1) of C, nutrients, lignin and cellulose in the remaining litter when collected in April and October, respectively. 2.4. Statistical analysis All of the variables from measurements and calculations were analyzed using a t-test with substrate species as a factor. Relationships between initial litter characteristics and the rates of litter decomposition (mass, lignin, cellulose, and elements loss) after the freezeethaw season were determined using the Pearson's correlation coefficient test at the 0.05 level. All of the statistical analyses were performed using SPSS (Standard released version 11.5 for Windows, SPSS Inc., IL, USA) software package.
Table 3 Pearson's correlations between the initial characteristics in substrate litter and net loss rates of mass, C, N, P, K, lignin and cellulose in the freezeethaw season. Initial characteristics
Net loss rate in freezeethaw season Mass
C
C N P K Lignin Cellulose C/N N/P L/N
0.82** 0.83** 0.04 0.33 0.81** 0.71* 0.72* 0.73* 0.83**
0.86** 0.67* 0.46 0.68* 0.85** 0.66* 0.45 0.67* 0.58 0.84** 0.71* 0.85** 0.86** 0.67* 0.46 0.68* 0.85** 0.69* 0.48 0.70* 0.87** 0.68* 0.38 0.73* 0.85** 0.67* 0.46 0.68* 0.89** 0.55 0.33 0.57 0.85** 0.65* 0.44 0.66*
N
P
K
Lignin Cellulose 0.24 0.65* 0.72* 0.25 0.23 0.13 0.25 0.36 0.65*
0.68* 0.66* 0.85** 0.67* 0.69* 0.66* 0.67* 0.56 0.66*
**P < 0.01, *P < 0.05, n ¼ 10.
cellulose, 49% of C, 56% of N, 62% of P and 38% of K in 1 year net loss rates, respectively (Fig. 3).Comparatively, birch litter lost more than 20% of mass, 14% of lignin, 26% of cellulose, 21% of C, 27% of N, 15% of P and 13% of K during the freezeethaw season. The lost mass and components accounted for 65% of mass, 69% of lignin, 60% of cellulose, 59% of C, 71% of N, 37% of P and 39% of K in one-year net loss rates, respectively. 3.3. Correlations Net mass loss rate and C release rate positively correlated with N and cellulose content and N/P in substrate during the freezeethaw season, but negatively correlated with lignin content, and C/N and L/N (Table 3). N and K release rates positively correlated with K and cellulose contents, and C/N and L/N in substrate during the freezeethaw season. P release rate correlated only with P content in substrate. Lignin and cellulose loss rates positively correlated with N and P content in substrate during the freezeethaw season, but negatively correlated with L/N. 4. Discussion
3. Results 3.1. Elements, lignin and cellulose content C and P contents of both fir and birch litter showed few changes as decomposition proceeded (Fig. 2). N and K contents of fir litter decreased after 1 year of decomposition, whereas those of birch litter showed non-significant differences. In addition, lignin content of both fir and birch litter increased slightly as decomposition proceeded, but cellulose content decreased. 3.2. Mass loss and elements release Over the freezeethaw season, more than 18% of mass, 13% of lignin, 30% of cellulose, more than 14% of C, 30% of N, 17% of P and 17% of K were lost from fir litter (Table 2). The lost mass and components accounted for 64% of mass, 72% of lignin, 75% of
The results in this study supported our hypothesis that litter decomposition during the freezeethaw season accounted for a large proportion of the decomposition in the first year in these subalpine forests. The lost mass and released nutrients could be favorable to plant nutrition and growth in the following spring, which is closely tied to nutrient cycling, and is essential for the regeneration of organically bound nutrients (Freppaz et al., 2007; Berg and McClaugherty, 2008). The results are consistent with the findings of many previous studies in the other cold ecosystems (Gosz et al., 1973; Melillo et al., 1982; Moore, 1983; Hobbie and Chapin, 1996; Uchida et al., 2005), suggesting that more attention should be paid to ecological processes in winter and early spring. Changes in litter quality were obvious as decomposition proceeded. This could indicate the release processes of each component (Chapin et al., 2002; Berg and McClaugherty, 2008). Generally, elements are released along with the loss of litter mass as
Table 2 Mass, lignin, cellulose, C, N, P and K release rates (means SD, n ¼ 5) of (Abies faxoniana) and birch (Betula platyphylla) litter after the freezeethaw season (April), and after oneyear (October) decomposition under their respective (FF and BF) forests. Forest
Sampling time
Residual mass (g)
Net loss rate (%) Mass
Lignin
Cellulose
C
N
P
K
FF
April October
24.32 1.61 21.22 3.49
18.92 0.54a 29.27 1.17x
13.01 3.46a 18.11 1.35x
30.29 2.77a 39.88 0.99x
14.55 3.40a 29.35 1.16x
30.62 2.76a 53.80 0.76x
17.84 3.27a 28.80 1.17x
17.56 3.28a 45.74 0.89x
BF
April October
15.96 0.62 13.83 1.18
20.21 1.61b 30.83 0.59x
14.02 3.35a 20.26 4.11x
26.77 2.85b 43.94 2.89y
21.02 3.07b 35.18 3.34y
27.18 2.83a 38.18 3.18y
15.53 3.29a 41.83 3.00y
13.33 3.37a 33.68 3.41y
Different letters among variables with same sampling time under different forests denote significantly different by LSD within a column; n ¼ 5; P < 0.05.
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decomposition proceeds. However, C, N, P, K content of fir and birch litter showed few changes in the freezeethaw season. Cellulose was readily decomposable due to microorganism decomposition (Sinsabaugh and Linkins, 1989; Berg and McClaugherty, 2008), it showed a more rapid loss rate than mass, and then decreased content was observed as decomposition proceeded. In contrast, lignin content increased due to its recalcitrant character. Based on the changes in lignin, cellulose and element content, the high decomposition rates of litter in the freezeethaw season could be related to the following explanations. First, the decomposable carbohydrates in soluble litter material are relatively richer in the early stage of litter decomposition (Norden and Berg, 1990; Don and Kalbitz, 2005). The increased content of recalcitrant material such as lignin might inhibit decomposition in the later stage (Fig. 2; Kalbitz et al., 2004). Second, microbial activity does not completely cease even under frozen soil conditions (Weintraub et al., 2007). Freezing is often disruptive to litter physical structure (Melick and Seppelt, 1992; Harris and Safford, 1996), and increases substrate availability to microorganisms, and then to microbial activity. In addition, the freezeethaw cycle is the combined process of temperature change (Fig. 1) and the dryewet cycle. Hydraulic leaching in the thawing phase could be of sufficient power to promote nutrient release. Increased moisture and temperature are also beneficial to microbial activity in the early spring (O'Connell, 1988; Lemma et al., 2007). Consequently, the integrated action of physical, chemical and biological processes could contribute to the high decomposition rates of fir and birch litter during the freezeethaw season. Even so, mass loss and nutrient release during the freezeethaw season is tightly correlated with their respective traits (Meentemeyer, 1978; Aerts, 1997). In this study, the loss rates of cellulose and N were higher than the other elements in the freezeethaw season. This might be related to the decomposable and mobilizable character of cellulose and N. Although P and K are also welldocumented as highly mobile, K is often released with hydraulic leaching; whereas leaching occurs more largely in the growing season compared to that in the freezeethaw season. P could be immobilized by microorganisms (Ribeiro et al., 2002; Tian et al., 2003) since P is the limiting nutrient in these subalpine forests (Wu et al., 2009). Positive correlations between substrate P content and the loss rates of lignin and cellulose also support this opinion. In addition, initial litter quality also contributed to the changes of component loss rates during the freezeethaw season (Table 3). Litter with relative higher N/P, N and cellulose content have higher net mass loss rates, while the lower decomposition rate was observed in the litter with higher lignin content, C/N and L/Na finding consistent with other studies of litter decomposition during the growing season (Melillo et al., 1982; Zhang et al., 2008; Papa et al., 2008). This suggests that microbial activity still dominated the primary processes during litter decomposition in the freezeethaw season. Thus, varying litter decomposition rates between fir and birch were the result of differences in their individual traits. Moreover, the results in this study indicated that about half of the litter decomposition rate in the first year was attributed to the decomposition in the freezeethaw season, and the percentages of each component varied with their respective traits. Comparatively, lignin loss rates in the freezeethaw season showed the highest percentages (more than 70%) of loss rate in the first year, which could be closely attributed to the disruptive actions on litter physical structure by freezing (Melick and Seppelt, 1992; Harris and Safford, 1996). Meanwhile, the results supported the theory that freezing in the freezeethaw season could be beneficial to ecological processes such as the decomposition of recalcitrant components (Schimel and Clein, 1996; Groffman et al., 2001). In contrast, lower
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percentages of 1 year P release rates for birch litter and K release rate for both fir and birch litter were observed in the freezeethaw season, which was related to the relatively less hydraulic leaching for K release in the freezeethaw season, and microorganism immobilization for P (Ribeiro et al., 2002; Tian et al., 2003) as discussed above. However, less N was released in the growing season compared to P, suggesting that N might be immobilized by microorganisms and could be the limiting factor in the growing season. The results were in agreement with the theory that different microorganisms dominated the decomposition in different decomposition stages (Schadt et al., 2003; Moorhead and Sinsabaugh, 2006). Similar with the net loss rate, the percentages of decomposition rates in the freezeethaw season to 1 year varied between birch and fir litter. In conclusion, both fir and birch litter showed high decomposition rates in the freezeethaw season, accounting for large percentages of the 1 year decomposition rate in the first year of litter decomposition. Net mass loss and nutrient release during litter decomposition were attributed to physical destruction, microbial activity, hydraulic leaching and their integrated effects in the freezeethaw season. However, it should be noted that our results only determined that the decomposition rate in the freezeethaw season and its percentages of the decomposition in the first year were significantly related to litter quality and the traits of each component. Because of the complex effects of biotic and abiotic factors, understanding of litter decomposition in the freezeethaw season should be strengthened through examination of the detailed processes in the future.
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