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Response of labile soil organic matter to changes in forest vegetation in subtropical regions
Applied Soil Ecology 47 (2011) 210–216
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Response of labile soil organic matter to changes in forest vegetation in subtropical regions Qingkui Wang a,∗ , Silong Wang a,b a b
Huitong Experimental Station of Forest Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China Huitong National Research Station of Forest Ecosystem, Huitong 418307, China
a r t i c l e
i n f o
Article history: Received 6 September 2010 Received in revised form 10 December 2010 Accepted 14 December 2010 Keywords: Soil microbial biomass Water-soluble soil organic matter Density fractionation Tree species
a b s t r a c t Labile soil organic matter (SOM) can sensitively respond to changes in land use and management practices, and has been suggested as an early and sensitive indicator of SOM. However, knowledge of effects of forest vegetation type on labile SOM is still scarce, particularly in subtropical regions. Soil microbial biomass C and N, water-soluble soil organic C and N, and light SOM fraction in four subtropical forests were studied in subtropical China. Forest vegetation type significantly affected labile SOM. Secondary broadleaved forest (SBF) had the highest soil microbial biomass, basal respiration and water-soluble SOM, and the pure Cunninghamia lanceolata plantation (PC) the lowest. Soil microbial biomass C and N and respiration were on average 100%, 104% and 75%, respectively higher in the SBF than in the PC. The influence of vegetation on water-soluble SOM was generally larger in the 0–10 cm soil layer than in the 10–20 cm. Cold- and hot-water-soluble organic C and N were on average 33–70% higher in the SBF than in the PC. Cold- and hot-soluble soil organic C concentrations in the coniferous-broadleaved mixed plantations were on average 38.1 and 25.0% higher than in the pure coniferous plantation, and cold- and hot-soluble soil total N were 51.4 and 14.1% higher, respectively. Therefore, introducing native broadleaved trees into pure coniferous plantations increased water-soluble SOM. The light SOM fraction (free and occluded) in the 0–10 cm soil layer, which ranged from 11.7 to 29.2 g kg−1 dry weight of soil, was strongly affected by vegetation. The light fraction soil organic C, expressed as percent of total soil organic C, ranged from 18.3% in the mixed plantations of C. lanceolata and Kalopanax septemlobus to 26.3% in the SBF. In addition, there were strong correlations among soil organic C and labile fractions, suggesting that they were in close association and partly represented similar C pools in soils. Our results indicated that hot-water-soluble method could be a suitable measure for labile SOM in subtropical forest soils. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Soil organic matter (SOM), a key component of the soil-plant ecosystem, is closely associated with soil properties and processes (Chen et al., 2004). SOM can be divided into labile, slow and recalcitrant organic matter according to turnover rate (Six et al., 2002). Labile SOM with smaller turnover time can sensitively respond to changes in plant vegetation and land use, compared to total SOM in forest ecosystems (Gregorich et al., 1994; Laik et al., 2009). However, concerns about the impacts of different vegetation on forest soils have mainly been focused on total SOM and soil fertility such as soil nutrient status over the past century (Finzi et al., 1998; HagenThorn et al., 2004; Christenson et al., 2009). So, information about effects of plant vegetation on labile SOM in forests is quite limited.
∗ Corresponding author. Tel.: +86 24 83970344. E-mail address: [email protected] (Q. Wang). 0929-1393/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2010.12.004
Water-soluble SOM contributes to the soil nutrient cycle and is the main energy and substrate source of soil microorganisms, although it accounts for a small part of SOM (Qualls et al., 1991). Some studies in temperate forest ecosystems showed that watersoluble SOM was different under different vegetations (Smolander and Kitunen, 2002; Kiikkilä et al., 2005). Soil microbial biomass has been suggested to be a sensitive indicator of changes in total SOM given that it more readily responds to alterations in plant vegetation or land use (Saffigna et al., 1989; Warembourg et al., 2003). Tree species established under coal mining ecosystems affected soil microbial biomass C and microbial quotients in India (Sinha et al., 2009). Density fractionation physically separates SOM into light and heavy fractions (Golchin et al., 1994). The light SOM fraction, representing a transitory pool between undecomposed residues and humified SOM, responds much quickly to changes in land use and management practices (Gregorich and Janzen, 1996; Six et al., 2002; O’Hara et al., 2006), and can be further subdivided into a free light fraction located between aggregates and an occluded light fraction
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located within soil aggregates according to their location within the soil matrix (Golchin et al., 1994; Six et al., 1998). The occluded light fraction contains more alkyl carbon and less O-alkyl carbon compared with the free light fraction (Golchin et al., 1994). Since both fractions originate from plant material, forest vegetation would affect their composition and nature. However, information about the effect of plant species on the light SOM fraction in subtropical forests is limited. Soil respiration is a key component of the C cycle in terrestrial ecosystems and small changes may strongly affect soil C sequestration (Raich and Tufekcioglu, 2000). Therefore, studying soil respiration is crucial to understanding global C dynamics. Vegetation type is a critical biotic factor influencing soil respiration in forest ecosystems (Hibbard et al., 2005; Li et al., 2006). Many studies on soil respiration have been conducted in boreal forests (Xu and Qi, 2001; Menyailo et al., 2002; Søe and Buchmann, 2005), and tropical forests (Li et al., 2006). There is still limited information on the influence of vegetation type on soil respiration in subtropical forests. Native broadleaved forest in subtropical China has almost been extirpated by human activities, and replaced by coniferous plantation with short-rotation. Cunninghamia lanceolata, an important coniferous timber species with fast growth and good timber quality, has reached over 12 million ha, accounting for about 24% of all plantation forests in China (Chen and Wang, 2004). However, the system of successive planting widely used had resulted in a decline in soil fertility and timber productivity (Chen et al., 1990). Some studies showed that introduction of broadleaved trees, such as Alnus cremastogyne (a N-fixing species) and Kalopanax septemlobus, into pure C. lanceolata plantation forests enhanced litter production and carbon storage (Wang et al., 2007, 2009), and improved soil fertility (Chen et al., 1990; Chen and Wang, 2004). In this study therefore a secondary evergreen broadleaved forest, a pure C. lanceolata plantation and two mixed plantations of C. lanceolata with either A. cremastogyne or K. septemlobus were chosen to assess effects of plant vegetation on labile SOM. In this study, the effects of forest vegetation on labile SOM (soil microbial biomass, water-soluble SOM, free and occluded light SOM fraction) and soil respiration were examined in four forest ecosystems in subtropical China. The main objectives of this study were to compare the differences in labile SOM and soil respiration between the four subtropical forest ecosystems, and to examine how forest vegetation type affects labile SOM and soil respiration in subtropical regions.
2. Materials and methods 2.1. Site description The research was conducted at the Huitong National Research Station of Forest Ecosystem (26◦ 40 –27◦ 09 N, 109◦ 26 –110◦ 08 E). The station lies at the transition zone from the Yunnan–Guizhou plateau to the lower mountains and hills on the southern side of the Yangtze River at an altitude of 300–1000 m above sea level. The climate of this region is humid mid-subtropical monsoon with a mean annual temperature of 16.5 ◦ C from 1990 to 2005. The mean annual precipitation was about 1200 mm with about 67% occurring between April and August. In this study, the soil from the four forests investigated was originally similar and derived from shale and classified as Oxisol according to the second edition of U.S. Soil Taxonomy. The secondary broadleaved forest (SBF) was dominated by native evergreen broadleaved tree species including Castanopsis hystrix, Cyclobalanopsis glauca, Machilus Pauhoi, Liquidambar formosana, and Juglans cathayensis. The tree density was 937 stems ha−1 with
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14.0 m for tree height and 22.2 cm for tree diameter at breast height (DBH) in 2005. The three C. lanceolata plantations were approximately 300 m away from the SBF stand. The C. lanceolata plantations were established in the early spring of 1990 after clear-cutting of the first generation C. lanceolata plantation in the autumn of 1989. Planting density was 2000 stems ha−1 in the pure and mixed plantations. In the mixed plantations, the ratio of C. lanceolata to A. cremastogyne or K. septemlobus was 4:1. Detailed information on the pure plantation (PC) and the mixed plantations of C. lanceolata with A. cremastogyne (MCA) or K. septemlobus (MCK) was given by Wang et al. (2007).
2.2. Soil sampling and analysis The litter layer on the mineral soil was carefully removed by hand before soil sampling. The soil samples were taken using a 45mm-diameter hand auger at 0–10 cm and 10–20 cm depth from three different plots representing three replicates in each forest stand in May, 2005. The size of plots was about 15 m × 20 m for the SBF stand and 12 m × 15 m for the three C. lanceolata stands. In each plot, ten soil cores were randomly collected, and then mixed into one compositive sample. Visible roots and organic residues were immediately removed after sampling and then samples were sieved through a 2-mm mesh. Each sample was divided into two parts. One part was stored at 4 ◦ C for analysis of soil microbial biomass, respiration and cold-water-soluble SOM within 7 days. The other part was air-dried and ground for determination of hot-water-soluble SOM, light fraction soil organic C, pH, and soil organic C, and total N, P and K. Soil organic C was determined by an element analyzer (VarioMax C/N, Elementar, Germany) and total N by the semimicroKjeldahl method. Total P was measured colorimetrically and K by flame emission spectrometry (AA240, Varian, U.S.A.). Soil pH was determined with pH meter in a 1:2.5 (weight:volume) mixture of soil and deionized H2 O. Soil microbial biomass was determined by the fumigation extraction method. Microbial biomass C and N were calculated according Wu et al. (1990) and Brookes et al. (1985), respectively: microbial C or N = KEC or KEN × 2.2, where KE and KEN are the difference between C and N extracted from fumigated and unfumigated soils. Microbial quotient was calculated as the proportion of soil microbial biomass C in soil organic C (Mendham et al., 2002). Soil basal respiration was measured using the methods described by Chen et al. (2000). The moist field soils (50 g oven-dry equivalent) were aerobically incubated at 28 ◦ C in a 500 ml sealed glass jar for 24 h and carbon dioxide evolved from soil was trapped in 0.1 mol l−1 NaOH and measured by titration with 0.05 mol l−1 HCl. The carbon dioxide evolved was calculated from the difference in normality between NaOH blanks and samples. Metabolic quotient was calculated by dividing the hourly basal respiration rate by the corresponding soil microbial biomass C. Cold-water-soluble soil organic C and total N were determined following the method of Liang et al. (1998). In brief, 20-g fresh soil was extracted with 50 ml ultra-pure water in a centrifuge tube by shaking the mixture for 30 min on a reciprocal shaker, and then centrifuging it at 6000 rpm for 15 min. The supernatant was filtered through a 0.45-m glass fiber filter. Hot-water-soluble soil organic C and total N were extracted by the method of Sparling et al. (1998). In brief, 4 g air-dry soil was extracted with 20 ml ultra-pure water in a capped test-tube at 70 ◦ C for 18 h. The tubes were shaken by hand to resuspend the soil at the end of the incubation and then filtered through a 0.45-m glass fiber filter. The organic C and total N contents in the filtrate were measured using an element analyzer (High TOCII + N, Elementar, Germany).
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Table 1 Soil chemical properties of the different Chinese forest soils analyzed in this study. SOC (g kg−1 )
SBF PC MCA MCK
Total N (g kg−1 )
Total P (g kg−1 )
Total K (g kg−1 )
pH (H2 O)
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
21.5a 13.4b 11.7b 12.3b
13.5a 9.32b 9.69b 12.0b
2.09a 1.33b 1.34b 1.31b
1.35a 1.03a 1.16a 1.27a
0.197a 0.157a 0.169a 0.181a
0.138bc 0.124c 0.159a 0.154ab
24.5a 15.1c 19.0b 17.5bc
16.6a 11.5c 14.9b 14.2b
4.76a 4.27ab 4.39b 4.41b
4.68a 4.24ac 4.44b 4.39bc
SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively. Data within a column followed by the same letter are not significantly different at the 5% level.
The density fraction was measured according to Golchin et al. (1994), with three degrees of physical protection for soil organic C: non-protected (free light, extractable without sonication), occluded (extractable by sonication) and protected (remained in the residue after sonication). The procedure was as follows: (1) 20 g of sample was placed in a 200-ml centrifuge tube with 100 ml of NaI solution ( = 1.8 g ml−1 ), gently shaken by hand, and left standing at room temperature overnight. After centrifugation at 3500 rpm for 15 min, the supernatant was filtered through a membrane filter (0.45 m) into a millipore vacuum unit. The fraction recovered on the filter was washed with 100 ml of 0.01 mol l−1 CaCl2 solution and 200 ml of distilled water, and then the fraction was transferred to a pre-weighted 50 ml beaker. The sediment was resuspended in 100 ml NaI, centrifuged, and filtered as described above. The obtained fractions were added to the previous ones. After left standing for 24 h, it was dried at 60 ◦ C for about 72 h, weighted, and then taken as free light fraction. (2) The sediment was resuspended in 100 ml NaI, shaken and sonicated using an ultrasonic disintegrator for 15 min, at 100 W, and left standing 4 h. The centrifugation and filtration procedure was repeated two times as described above. The fraction recovered from the supernatant was referred to as occluded light fraction. (3) The sediment was resuspended in 100 ml distilled water, shaken for 20 min and centrifuged for 20 min at 4000 rpm. The sediment was washed with distilled water at least 3 times, then transferred to a pre-weighted beaker, dried at 60 ◦ C to constant weight and weighted. It was taken as heavy fraction. All fractions were ground and then analyzed for organic C content with an element analyzer (Vario-Max C/N, Elementar, Germany).
2.3. Statistical analyses Data for the same soil layer (0–10 cm and 10–20 cm) were subjected to one-way analysis of variance (ANOVA) using SPSS version 13.0 for Windows (SPSS Inc., Chicago, IL, USA), and all comparisons among the different forest types of soil chemical and labile SOM fractions were conducted by least significant difference tests (LSD). Significant differences in labile SOM fractions among different forests were reported at P < 0.05. Pearson linear correlations were used to assess the relationships among labile fractions of SOM.
3. Results 3.1. Soil chemical properties Vegetation significantly affected soil organic C in the 0–10 cm and 10–20 cm depth (Table 1). The soil organic C concentration was higher in SBF than in the C. lanceolata plantations, but did not differ between pure and mixed plantations. Soil total N was significantly influenced by vegetation cover in the 0–10 cm layer, with the SBF having higher concentrations than the C. lanceolata plantations. Vegetation did not affect soil total P levels in the 0–10 cm soil layer. Total P in the 10–20 cm soil layer was significantly different among the forests, with MCA and MCK having higher concentrations than SBF and PC. Soil total K was significantly affected by vegetation type, and was higher in the SBF. The PC had the lowest concentration of total K among the three plantations. Soil pH significantly differed among the forests and was higher in SBF than in the C. lanceolata plantations. 3.2. Labile soil organic matter Soil microbial biomass was significantly affected by vegetation cover (Table 2). Soil microbial biomass C was approximately 50% higher in the SBF than in the C. lanceolata plantations. Soil microbial biomass N was also significantly higher in the SBF than in the PC and MCK. The PC had the lowest soil microbial biomass N in both soil layers. Soil respiration in the 0–10 cm and 10–20 cm soil layers was also significantly influenced by vegetation type. CO2 release was significantly higher from SBF soils than from C. lanceolata plantation soils. Soil microbial quotient in the 0–10 cm soil layer was significantly influenced by forest vegetations. Soil metabolic quotient in the 0–10 cm layer was similar among the forests, but C. lanceolata plantations had a higher soil metabolic than SBF in the 10–20 cm layer. Water-soluble soil organic C and total N concentrations were significantly different among the different forests (Fig. 1). Compared to the C. lanceolata plantations, the SBF had higher concentrations of cold- and hot-water-soluble SOM. Generally, the effects of vegetation on water-soluble soil organic C and total N were larger in the 0–10 cm soil layer than in the 10–20 cm. Within the C. lanceolata plantations, the PC had the lowest concentrations of water-soluble soil organic C and total N. A. cremastogyne and K.
Table 2 Soil microbial biomass C and N, basal respiration, and metabolic quotient in different Chinese forest soils. MBC (mg kg−1 )
SBF PC MCA MCK
MBN (mg kg−1 )
Basal respiration (mg CO2 -C kg−1 d−1 )
Microbial C quotient
Metabolic quotient
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
442a 224b 292b 272b
323a 190b 228b 222b
44.0a 20.9c 37.6ab 33.2b
30.9a 18.7c 26.1ab 24.5b
23.9a 11.8b 12.2b 11.5b
11.4a 8.3b 8.6b 9.5b
2.52a 1.89c 2.49ab 2.20b
2.60a 2.13a 2.42a 1.87a
1.84a 1.99a 1.76a 1.78a
1.38b 1.93a 1.51ab 1.79ab
SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively. Data within a column followed by the same letter are not significantly different at the 5% level.
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Fig. 1. Cold- and hot-water-soluble soil organic C and total N concentrations in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
septemlobus introduced into pure C. lanceolata plantation increased water-soluble organic C and total N contents in soils. The dry weight of free and occluded light SOM fractions was strongly affected by vegetation (Fig. 2). In the surface soil layer (0–10 cm), the range of free light fraction was between 6.17 g kg−1 dry weight of soil in the MCK and 10.0 g kg−1 dry weight of soil
in the SBF, and in the 10–20 cm soil layer it varied from 4.13 to 8.50 g kg−1 dry weight of soil in the four forests. In the 0–10 cm soil layer, the amount of the occluded light SOM fraction was the highest in the SBF and lowest in the mixed plantation of C. lanceolata and K. septemlobus. Vegetation type did not significantly affect the occluded light fraction in the 10–20 cm soil layer, but mixed plan-
Fig. 2. Mean weight of free and occluded light SOM fractions at two depths (0–10 cm and 10–20 cm) in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
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Fig. 3. Mean C concentration in the free and occluded light SOM fractions at two depths (0–10 cm and 10–20 cm) in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
tations tended to have lower amount of the occluded light SOM fraction. The mean C concentration of the free light SOM fraction was significantly influenced by vegetation (Fig. 3). The mean C concentration of the free light SOM fraction extracted from different stands ranged from 298 to 389 g kg−1 in the upper 10 cm soil layer, and from 265 to 335 g kg−1 in the 10–20 cm soil layer. In both soil layers, the mean C concentrations of the free light SOM fraction were highest in the SBF and lowest in the PC. However, the mean C concentration of the occluded light SOM fraction was not significantly affected by vegetation and ranged from 75.0 to 93.9 g kg−1 . Mean values of the free and occluded light fraction C, determined as a percentage of soil organic C, were significantly different among the forests (Fig. 4). The SBF had the highest mean values and the PC had the lowest in both soil layers. The mean free light fraction soil organic C, as a percentage of soil organic C ranged from 15.6 to 18.1% and from 12.2 to 21.2% in the 0–10 and 10–20 cm soil layers, respectively. In the 0–10 cm soil layer, the mean occluded light fraction soil organic C as a percentage of soil organic C ranged from 4.17% in the MCK to 8.28% in the SBF. In the 10–20 cm soil layer, the PC had the highest mean value with 7.28%, the MCK lowest (4.01%).
Table 3 Correlations between soil organic carbon, labile soil organic matter pools and soil respiration in different Chinese forest soils.
MBC CWSOC HWSOC fLFC oLFC SBR
SOC
MBC
CWSOC
HWSOC
fLFC
oLFC
0.893** 0.713** 0.846** 0.856** 0.915** 0.883**
0.824** 0.948** 0.933** 0.871** 0.930**
0.874** 0.810** 0.615** 0.816**
0.922** 0.770** 0.866**
0.815** 0.852**
0.884**
SOC, soil organic carbon; MBC, soil microbial biomass carbon; CWSOC, cold-watersoluble soil organic carbon; HWSOC, hot-water-soluble soil organic carbon; fLFC, free light fraction carbon; oLFC, occluded light fraction carbon; SBR, soil basal respiration. **Significant at P < 0.01.
3.3. Relationship among labile fractions of SOM Relationships among soil organic C, labile fractions of soil organic C and soil respiration were examined using linear correlation techniques (Table 3). Soil microbial biomass C was strongly correlated with the free and the occluded light fraction C and hot-water-soluble organic C with r values of 0.933, 0.871 and 0.948, respectively (n = 24, P < 0.01). A stronger relationship was
Fig. 4. Mean free and occluded light fraction C as percent of soil organic carbon at two depths (0–10 cm and 10–20 cm) in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
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also observed between microbial biomass C and soil respiration (r = 0.930, n = 24, P < 0.01). The lowest r value (0.713) was found for the correlation between soil organic C and cold-water-soluble soil organic C. Free and occluded light fractions C were significantly correlated with other variables.
4. Discussion 4.1. Response of labile SOM to vegetation types Vegetation type affected water-soluble SOM levels. Higher concentrations of hot- and cold-water-soluble soil organic C and total N were found in the SBF than in the C. lanceolata plantations. This was consistent with other findings (Smolander and Kitunen, 2002; Kiikkilä et al., 2005; Wang and Wang, 2007). The average concentrations of water-extractable soil organic C and total N in a Betula pendula stand were 63 and 102% higher than that in a Pinus sylvestris forest in Kivalo, Northern Finland (Smolander and Kitunen, 2002). Among the afforested plots, the highest concentration of waterextractable soil organic C was observed under Terminalia arjuna and the lowest in an Albizia procera stand (Laik et al., 2009). Some explanations for the differences could be given according to the origin of water-soluble SOM. Plant litter and root exudates are considered to be the primary sources of water-soluble SOM (Qualls et al., 1991). The SBF had significantly higher leaf litter and root production and turnover than the C. lanceolata plantations (Wang et al., 2010). In addition, the nutrient contents of leaf litter and root material from native broadleaved tree species were also higher in the SBF than in C. lanceolata (Wang et al., 2010). Therefore, the differences in quantity and quality of leaf litter and root could explain, at least in part, the effects of vegetation type on water-soluble SOM levels. Soil microbial biomass and activities varied considerably among different forests. The SBF had higher soil microbial biomass C and N than the C. lanceolata plantations, which also confirms that tree species affected soil microbial processes (Menyailo et al., 2002; Sinha et al., 2009). B. pendula forests had higher soil microbial biomass C and N than Picea abies and P. sylvestris stands (Smolander and Kitunen, 2002). Sinha et al. (2009) observed that tree species established under a coal mining ecosystem significantly affected soil microbial biomass C, microbial quotient and basal respiration. Wang and Wang (2007) also reported that the soil microbial biomass C and N in a pure C. lanceolata plantation were 29 and 33% lower than in a broadleaved forest in San Menjiang Forest, Guangxi province. Microbial metabolic quotient is considered an index for evaluating the efficiency of soil microbes for C utilization measured during a short-term incubation (Wardle and Ghani, 1995). In this study, there was no significant difference in the microbial metabolic quotient among the forest stands. Vegetation type as a critical biotic factor affects soil microbial processes in forest ecosystems through its impact on production, quality and decomposition rates of litter and root material, substrate availability, microbial communities and soil microclimate (Hibbard et al., 2005; Li et al., 2006). Positive correlations between soil respiration and aboveground litter production were also found in tropical forests (Raich, 1998). Literature on the influence of vegetation type on the light SOM fraction was quite scarce. The C. lanceolata plantations had a significantly lower light fraction soil organic carbon than the SBF, which is consistent with the findings of Luan et al. (2010) at Longmen Mountain, Sichuan, China. This demonstrates that conversion from evergreen broadleaved forest into a C. lanceolata plantation reduces the light fraction soil organic C. Laik et al. (2009) observed that the light fraction soil organic C in a Pongamia pinnata stand was 79.8% greater than that under T. arjuna after 18 years of establishment. The effect of vegetation type on the light fraction soil organic C could be explained according to its source. The light frac-
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tion SOM obtained by density fractionation is mainly composed of plant residues, roots, and fungal hypha at different decomposition stages (Janzen et al., 1992; Six et al., 2002). In our study, unfortunately, the solid-state 13 C NMR spectrum of free and occluded light fractions could not be determined. Therefore, the effects of vegetation on the chemical composition of the free and occluded light fractions are unknown. Using 13 C NMR spectra, Golchin et al. (1994) found that the organic structure and chemical composition of the free light fraction was similar in four of the five soils (three forest soils and two grass soils), which was similar to the finding of Kögel et al. (1988) for forest litter. Due to the dominant influence of plant litter, the difference in the chemical composition of the fractions between soils can mainly be explained by the differences in vegetation type (Baldock et al., 1992). In addition, turnover of the free light fraction was generally faster than that of the occluded light fraction (von Lützow et al., 2007). The occurrence of charcoal in free and occluded light fractions is common under sites where frequent vegetation burning is done (Cadisch et al., 1996). For grassland soils, Brodowski et al. (2006) found that the highest amount of black carbon was accumulated in the occluded light fraction. In Chinese fir stands, slash burning after clear-cutting is usually used for site preparation. Thereby, black carbon was possibly present in the light SOM fraction. 4.2. Relationship among labile fractions of SOM The labile soil organic C fractions measured (cold-water, hotwater, chloroform released and density fraction) were significantly correlated with soil organic C, suggesting that SOM was a major determinant of the amount of labile SOM present (Gregorich et al., 1994; Carter, 2002). Laik et al. (2009) also found a significant relationship between soil microbial biomass C and the light fraction soil organic C in a calciorthent after 18 years of afforestation. The significant correlations among the four labile soil organic C fractions measured in this study show that they partly represent similar C pool in soil. Some studies showed that soil respiration was correlated with the light fraction soil organic C content, suggesting that light fraction soil organic C was the driving factor in soil respiration (Gregorich et al., 1994; Gregorich and Janzen, 1996; Alvarez and Alvarez, 2000). Water-soluble soil organic C was strongly correlated with soil microbial biomass and activities, which was supported by some studies showing close relationships between water-soluble SOM and CO2 fluxes from soils (Jandl and Sollins, 1997; Neff et al., 2000) and soil microbial biomass C (Sparling et al., 1998; Ghani et al., 2003; Wang and Wang, 2007) in forest and pasture ecosystems. The reason of this close correlation is that the hot-water-soluble methods may extract not only simple organic compounds and carbohydrates, but also some parts of the soil microbial components (Sparling et al., 1998; Chen et al., 2004; Rovira and Vallejo, 2007). For example, Ghani et al. (2003) reported that carbohydrates were rich in the hot-water extract of forest soils. Sparling et al. (1998) also observed that the hot-water-extractable organic C of the mineral soils was about 45% of the microbial biomass C. This study highlights that forest vegetation type can significantly affect labile SOM in subtropical soils. The SBF had highest soil microbial biomass, soil respiration and water-soluble SOM followed by the mixed plantations, and the PC had the lowest amounts. The light SOM fraction in the 0–10 cm soil layer was strongly affected by vegetation. Mean values of free and occluded light fraction soil organic C determined as a percentage of soil organic C were significantly different among the different forests, with the highest values found in the SBF and the lowest in the PC. The strong correlations among labile fractions suggest that they have a close association with one another and partly represented a similar C pool in soils. Soil microbial biomass and hot-water-soluble
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Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil
Response of labile soil organic matter to changes in forest vegetation in subtropical regions Qingkui Wang a,∗ , Silong Wang a,b a b
Huitong Experimental Station of Forest Ecology, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China Huitong National Research Station of Forest Ecosystem, Huitong 418307, China
a r t i c l e
i n f o
Article history: Received 6 September 2010 Received in revised form 10 December 2010 Accepted 14 December 2010 Keywords: Soil microbial biomass Water-soluble soil organic matter Density fractionation Tree species
a b s t r a c t Labile soil organic matter (SOM) can sensitively respond to changes in land use and management practices, and has been suggested as an early and sensitive indicator of SOM. However, knowledge of effects of forest vegetation type on labile SOM is still scarce, particularly in subtropical regions. Soil microbial biomass C and N, water-soluble soil organic C and N, and light SOM fraction in four subtropical forests were studied in subtropical China. Forest vegetation type significantly affected labile SOM. Secondary broadleaved forest (SBF) had the highest soil microbial biomass, basal respiration and water-soluble SOM, and the pure Cunninghamia lanceolata plantation (PC) the lowest. Soil microbial biomass C and N and respiration were on average 100%, 104% and 75%, respectively higher in the SBF than in the PC. The influence of vegetation on water-soluble SOM was generally larger in the 0–10 cm soil layer than in the 10–20 cm. Cold- and hot-water-soluble organic C and N were on average 33–70% higher in the SBF than in the PC. Cold- and hot-soluble soil organic C concentrations in the coniferous-broadleaved mixed plantations were on average 38.1 and 25.0% higher than in the pure coniferous plantation, and cold- and hot-soluble soil total N were 51.4 and 14.1% higher, respectively. Therefore, introducing native broadleaved trees into pure coniferous plantations increased water-soluble SOM. The light SOM fraction (free and occluded) in the 0–10 cm soil layer, which ranged from 11.7 to 29.2 g kg−1 dry weight of soil, was strongly affected by vegetation. The light fraction soil organic C, expressed as percent of total soil organic C, ranged from 18.3% in the mixed plantations of C. lanceolata and Kalopanax septemlobus to 26.3% in the SBF. In addition, there were strong correlations among soil organic C and labile fractions, suggesting that they were in close association and partly represented similar C pools in soils. Our results indicated that hot-water-soluble method could be a suitable measure for labile SOM in subtropical forest soils. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Soil organic matter (SOM), a key component of the soil-plant ecosystem, is closely associated with soil properties and processes (Chen et al., 2004). SOM can be divided into labile, slow and recalcitrant organic matter according to turnover rate (Six et al., 2002). Labile SOM with smaller turnover time can sensitively respond to changes in plant vegetation and land use, compared to total SOM in forest ecosystems (Gregorich et al., 1994; Laik et al., 2009). However, concerns about the impacts of different vegetation on forest soils have mainly been focused on total SOM and soil fertility such as soil nutrient status over the past century (Finzi et al., 1998; HagenThorn et al., 2004; Christenson et al., 2009). So, information about effects of plant vegetation on labile SOM in forests is quite limited.
∗ Corresponding author. Tel.: +86 24 83970344. E-mail address: [email protected] (Q. Wang). 0929-1393/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2010.12.004
Water-soluble SOM contributes to the soil nutrient cycle and is the main energy and substrate source of soil microorganisms, although it accounts for a small part of SOM (Qualls et al., 1991). Some studies in temperate forest ecosystems showed that watersoluble SOM was different under different vegetations (Smolander and Kitunen, 2002; Kiikkilä et al., 2005). Soil microbial biomass has been suggested to be a sensitive indicator of changes in total SOM given that it more readily responds to alterations in plant vegetation or land use (Saffigna et al., 1989; Warembourg et al., 2003). Tree species established under coal mining ecosystems affected soil microbial biomass C and microbial quotients in India (Sinha et al., 2009). Density fractionation physically separates SOM into light and heavy fractions (Golchin et al., 1994). The light SOM fraction, representing a transitory pool between undecomposed residues and humified SOM, responds much quickly to changes in land use and management practices (Gregorich and Janzen, 1996; Six et al., 2002; O’Hara et al., 2006), and can be further subdivided into a free light fraction located between aggregates and an occluded light fraction
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located within soil aggregates according to their location within the soil matrix (Golchin et al., 1994; Six et al., 1998). The occluded light fraction contains more alkyl carbon and less O-alkyl carbon compared with the free light fraction (Golchin et al., 1994). Since both fractions originate from plant material, forest vegetation would affect their composition and nature. However, information about the effect of plant species on the light SOM fraction in subtropical forests is limited. Soil respiration is a key component of the C cycle in terrestrial ecosystems and small changes may strongly affect soil C sequestration (Raich and Tufekcioglu, 2000). Therefore, studying soil respiration is crucial to understanding global C dynamics. Vegetation type is a critical biotic factor influencing soil respiration in forest ecosystems (Hibbard et al., 2005; Li et al., 2006). Many studies on soil respiration have been conducted in boreal forests (Xu and Qi, 2001; Menyailo et al., 2002; Søe and Buchmann, 2005), and tropical forests (Li et al., 2006). There is still limited information on the influence of vegetation type on soil respiration in subtropical forests. Native broadleaved forest in subtropical China has almost been extirpated by human activities, and replaced by coniferous plantation with short-rotation. Cunninghamia lanceolata, an important coniferous timber species with fast growth and good timber quality, has reached over 12 million ha, accounting for about 24% of all plantation forests in China (Chen and Wang, 2004). However, the system of successive planting widely used had resulted in a decline in soil fertility and timber productivity (Chen et al., 1990). Some studies showed that introduction of broadleaved trees, such as Alnus cremastogyne (a N-fixing species) and Kalopanax septemlobus, into pure C. lanceolata plantation forests enhanced litter production and carbon storage (Wang et al., 2007, 2009), and improved soil fertility (Chen et al., 1990; Chen and Wang, 2004). In this study therefore a secondary evergreen broadleaved forest, a pure C. lanceolata plantation and two mixed plantations of C. lanceolata with either A. cremastogyne or K. septemlobus were chosen to assess effects of plant vegetation on labile SOM. In this study, the effects of forest vegetation on labile SOM (soil microbial biomass, water-soluble SOM, free and occluded light SOM fraction) and soil respiration were examined in four forest ecosystems in subtropical China. The main objectives of this study were to compare the differences in labile SOM and soil respiration between the four subtropical forest ecosystems, and to examine how forest vegetation type affects labile SOM and soil respiration in subtropical regions.
2. Materials and methods 2.1. Site description The research was conducted at the Huitong National Research Station of Forest Ecosystem (26◦ 40 –27◦ 09 N, 109◦ 26 –110◦ 08 E). The station lies at the transition zone from the Yunnan–Guizhou plateau to the lower mountains and hills on the southern side of the Yangtze River at an altitude of 300–1000 m above sea level. The climate of this region is humid mid-subtropical monsoon with a mean annual temperature of 16.5 ◦ C from 1990 to 2005. The mean annual precipitation was about 1200 mm with about 67% occurring between April and August. In this study, the soil from the four forests investigated was originally similar and derived from shale and classified as Oxisol according to the second edition of U.S. Soil Taxonomy. The secondary broadleaved forest (SBF) was dominated by native evergreen broadleaved tree species including Castanopsis hystrix, Cyclobalanopsis glauca, Machilus Pauhoi, Liquidambar formosana, and Juglans cathayensis. The tree density was 937 stems ha−1 with
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14.0 m for tree height and 22.2 cm for tree diameter at breast height (DBH) in 2005. The three C. lanceolata plantations were approximately 300 m away from the SBF stand. The C. lanceolata plantations were established in the early spring of 1990 after clear-cutting of the first generation C. lanceolata plantation in the autumn of 1989. Planting density was 2000 stems ha−1 in the pure and mixed plantations. In the mixed plantations, the ratio of C. lanceolata to A. cremastogyne or K. septemlobus was 4:1. Detailed information on the pure plantation (PC) and the mixed plantations of C. lanceolata with A. cremastogyne (MCA) or K. septemlobus (MCK) was given by Wang et al. (2007).
2.2. Soil sampling and analysis The litter layer on the mineral soil was carefully removed by hand before soil sampling. The soil samples were taken using a 45mm-diameter hand auger at 0–10 cm and 10–20 cm depth from three different plots representing three replicates in each forest stand in May, 2005. The size of plots was about 15 m × 20 m for the SBF stand and 12 m × 15 m for the three C. lanceolata stands. In each plot, ten soil cores were randomly collected, and then mixed into one compositive sample. Visible roots and organic residues were immediately removed after sampling and then samples were sieved through a 2-mm mesh. Each sample was divided into two parts. One part was stored at 4 ◦ C for analysis of soil microbial biomass, respiration and cold-water-soluble SOM within 7 days. The other part was air-dried and ground for determination of hot-water-soluble SOM, light fraction soil organic C, pH, and soil organic C, and total N, P and K. Soil organic C was determined by an element analyzer (VarioMax C/N, Elementar, Germany) and total N by the semimicroKjeldahl method. Total P was measured colorimetrically and K by flame emission spectrometry (AA240, Varian, U.S.A.). Soil pH was determined with pH meter in a 1:2.5 (weight:volume) mixture of soil and deionized H2 O. Soil microbial biomass was determined by the fumigation extraction method. Microbial biomass C and N were calculated according Wu et al. (1990) and Brookes et al. (1985), respectively: microbial C or N = KEC or KEN × 2.2, where KE and KEN are the difference between C and N extracted from fumigated and unfumigated soils. Microbial quotient was calculated as the proportion of soil microbial biomass C in soil organic C (Mendham et al., 2002). Soil basal respiration was measured using the methods described by Chen et al. (2000). The moist field soils (50 g oven-dry equivalent) were aerobically incubated at 28 ◦ C in a 500 ml sealed glass jar for 24 h and carbon dioxide evolved from soil was trapped in 0.1 mol l−1 NaOH and measured by titration with 0.05 mol l−1 HCl. The carbon dioxide evolved was calculated from the difference in normality between NaOH blanks and samples. Metabolic quotient was calculated by dividing the hourly basal respiration rate by the corresponding soil microbial biomass C. Cold-water-soluble soil organic C and total N were determined following the method of Liang et al. (1998). In brief, 20-g fresh soil was extracted with 50 ml ultra-pure water in a centrifuge tube by shaking the mixture for 30 min on a reciprocal shaker, and then centrifuging it at 6000 rpm for 15 min. The supernatant was filtered through a 0.45-m glass fiber filter. Hot-water-soluble soil organic C and total N were extracted by the method of Sparling et al. (1998). In brief, 4 g air-dry soil was extracted with 20 ml ultra-pure water in a capped test-tube at 70 ◦ C for 18 h. The tubes were shaken by hand to resuspend the soil at the end of the incubation and then filtered through a 0.45-m glass fiber filter. The organic C and total N contents in the filtrate were measured using an element analyzer (High TOCII + N, Elementar, Germany).
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Table 1 Soil chemical properties of the different Chinese forest soils analyzed in this study. SOC (g kg−1 )
SBF PC MCA MCK
Total N (g kg−1 )
Total P (g kg−1 )
Total K (g kg−1 )
pH (H2 O)
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
21.5a 13.4b 11.7b 12.3b
13.5a 9.32b 9.69b 12.0b
2.09a 1.33b 1.34b 1.31b
1.35a 1.03a 1.16a 1.27a
0.197a 0.157a 0.169a 0.181a
0.138bc 0.124c 0.159a 0.154ab
24.5a 15.1c 19.0b 17.5bc
16.6a 11.5c 14.9b 14.2b
4.76a 4.27ab 4.39b 4.41b
4.68a 4.24ac 4.44b 4.39bc
SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively. Data within a column followed by the same letter are not significantly different at the 5% level.
The density fraction was measured according to Golchin et al. (1994), with three degrees of physical protection for soil organic C: non-protected (free light, extractable without sonication), occluded (extractable by sonication) and protected (remained in the residue after sonication). The procedure was as follows: (1) 20 g of sample was placed in a 200-ml centrifuge tube with 100 ml of NaI solution ( = 1.8 g ml−1 ), gently shaken by hand, and left standing at room temperature overnight. After centrifugation at 3500 rpm for 15 min, the supernatant was filtered through a membrane filter (0.45 m) into a millipore vacuum unit. The fraction recovered on the filter was washed with 100 ml of 0.01 mol l−1 CaCl2 solution and 200 ml of distilled water, and then the fraction was transferred to a pre-weighted 50 ml beaker. The sediment was resuspended in 100 ml NaI, centrifuged, and filtered as described above. The obtained fractions were added to the previous ones. After left standing for 24 h, it was dried at 60 ◦ C for about 72 h, weighted, and then taken as free light fraction. (2) The sediment was resuspended in 100 ml NaI, shaken and sonicated using an ultrasonic disintegrator for 15 min, at 100 W, and left standing 4 h. The centrifugation and filtration procedure was repeated two times as described above. The fraction recovered from the supernatant was referred to as occluded light fraction. (3) The sediment was resuspended in 100 ml distilled water, shaken for 20 min and centrifuged for 20 min at 4000 rpm. The sediment was washed with distilled water at least 3 times, then transferred to a pre-weighted beaker, dried at 60 ◦ C to constant weight and weighted. It was taken as heavy fraction. All fractions were ground and then analyzed for organic C content with an element analyzer (Vario-Max C/N, Elementar, Germany).
2.3. Statistical analyses Data for the same soil layer (0–10 cm and 10–20 cm) were subjected to one-way analysis of variance (ANOVA) using SPSS version 13.0 for Windows (SPSS Inc., Chicago, IL, USA), and all comparisons among the different forest types of soil chemical and labile SOM fractions were conducted by least significant difference tests (LSD). Significant differences in labile SOM fractions among different forests were reported at P < 0.05. Pearson linear correlations were used to assess the relationships among labile fractions of SOM.
3. Results 3.1. Soil chemical properties Vegetation significantly affected soil organic C in the 0–10 cm and 10–20 cm depth (Table 1). The soil organic C concentration was higher in SBF than in the C. lanceolata plantations, but did not differ between pure and mixed plantations. Soil total N was significantly influenced by vegetation cover in the 0–10 cm layer, with the SBF having higher concentrations than the C. lanceolata plantations. Vegetation did not affect soil total P levels in the 0–10 cm soil layer. Total P in the 10–20 cm soil layer was significantly different among the forests, with MCA and MCK having higher concentrations than SBF and PC. Soil total K was significantly affected by vegetation type, and was higher in the SBF. The PC had the lowest concentration of total K among the three plantations. Soil pH significantly differed among the forests and was higher in SBF than in the C. lanceolata plantations. 3.2. Labile soil organic matter Soil microbial biomass was significantly affected by vegetation cover (Table 2). Soil microbial biomass C was approximately 50% higher in the SBF than in the C. lanceolata plantations. Soil microbial biomass N was also significantly higher in the SBF than in the PC and MCK. The PC had the lowest soil microbial biomass N in both soil layers. Soil respiration in the 0–10 cm and 10–20 cm soil layers was also significantly influenced by vegetation type. CO2 release was significantly higher from SBF soils than from C. lanceolata plantation soils. Soil microbial quotient in the 0–10 cm soil layer was significantly influenced by forest vegetations. Soil metabolic quotient in the 0–10 cm layer was similar among the forests, but C. lanceolata plantations had a higher soil metabolic than SBF in the 10–20 cm layer. Water-soluble soil organic C and total N concentrations were significantly different among the different forests (Fig. 1). Compared to the C. lanceolata plantations, the SBF had higher concentrations of cold- and hot-water-soluble SOM. Generally, the effects of vegetation on water-soluble soil organic C and total N were larger in the 0–10 cm soil layer than in the 10–20 cm. Within the C. lanceolata plantations, the PC had the lowest concentrations of water-soluble soil organic C and total N. A. cremastogyne and K.
Table 2 Soil microbial biomass C and N, basal respiration, and metabolic quotient in different Chinese forest soils. MBC (mg kg−1 )
SBF PC MCA MCK
MBN (mg kg−1 )
Basal respiration (mg CO2 -C kg−1 d−1 )
Microbial C quotient
Metabolic quotient
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
0–10 cm
10–20 cm
442a 224b 292b 272b
323a 190b 228b 222b
44.0a 20.9c 37.6ab 33.2b
30.9a 18.7c 26.1ab 24.5b
23.9a 11.8b 12.2b 11.5b
11.4a 8.3b 8.6b 9.5b
2.52a 1.89c 2.49ab 2.20b
2.60a 2.13a 2.42a 1.87a
1.84a 1.99a 1.76a 1.78a
1.38b 1.93a 1.51ab 1.79ab
SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively. Data within a column followed by the same letter are not significantly different at the 5% level.
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Fig. 1. Cold- and hot-water-soluble soil organic C and total N concentrations in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
septemlobus introduced into pure C. lanceolata plantation increased water-soluble organic C and total N contents in soils. The dry weight of free and occluded light SOM fractions was strongly affected by vegetation (Fig. 2). In the surface soil layer (0–10 cm), the range of free light fraction was between 6.17 g kg−1 dry weight of soil in the MCK and 10.0 g kg−1 dry weight of soil
in the SBF, and in the 10–20 cm soil layer it varied from 4.13 to 8.50 g kg−1 dry weight of soil in the four forests. In the 0–10 cm soil layer, the amount of the occluded light SOM fraction was the highest in the SBF and lowest in the mixed plantation of C. lanceolata and K. septemlobus. Vegetation type did not significantly affect the occluded light fraction in the 10–20 cm soil layer, but mixed plan-
Fig. 2. Mean weight of free and occluded light SOM fractions at two depths (0–10 cm and 10–20 cm) in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
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Fig. 3. Mean C concentration in the free and occluded light SOM fractions at two depths (0–10 cm and 10–20 cm) in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
tations tended to have lower amount of the occluded light SOM fraction. The mean C concentration of the free light SOM fraction was significantly influenced by vegetation (Fig. 3). The mean C concentration of the free light SOM fraction extracted from different stands ranged from 298 to 389 g kg−1 in the upper 10 cm soil layer, and from 265 to 335 g kg−1 in the 10–20 cm soil layer. In both soil layers, the mean C concentrations of the free light SOM fraction were highest in the SBF and lowest in the PC. However, the mean C concentration of the occluded light SOM fraction was not significantly affected by vegetation and ranged from 75.0 to 93.9 g kg−1 . Mean values of the free and occluded light fraction C, determined as a percentage of soil organic C, were significantly different among the forests (Fig. 4). The SBF had the highest mean values and the PC had the lowest in both soil layers. The mean free light fraction soil organic C, as a percentage of soil organic C ranged from 15.6 to 18.1% and from 12.2 to 21.2% in the 0–10 and 10–20 cm soil layers, respectively. In the 0–10 cm soil layer, the mean occluded light fraction soil organic C as a percentage of soil organic C ranged from 4.17% in the MCK to 8.28% in the SBF. In the 10–20 cm soil layer, the PC had the highest mean value with 7.28%, the MCK lowest (4.01%).
Table 3 Correlations between soil organic carbon, labile soil organic matter pools and soil respiration in different Chinese forest soils.
MBC CWSOC HWSOC fLFC oLFC SBR
SOC
MBC
CWSOC
HWSOC
fLFC
oLFC
0.893** 0.713** 0.846** 0.856** 0.915** 0.883**
0.824** 0.948** 0.933** 0.871** 0.930**
0.874** 0.810** 0.615** 0.816**
0.922** 0.770** 0.866**
0.815** 0.852**
0.884**
SOC, soil organic carbon; MBC, soil microbial biomass carbon; CWSOC, cold-watersoluble soil organic carbon; HWSOC, hot-water-soluble soil organic carbon; fLFC, free light fraction carbon; oLFC, occluded light fraction carbon; SBR, soil basal respiration. **Significant at P < 0.01.
3.3. Relationship among labile fractions of SOM Relationships among soil organic C, labile fractions of soil organic C and soil respiration were examined using linear correlation techniques (Table 3). Soil microbial biomass C was strongly correlated with the free and the occluded light fraction C and hot-water-soluble organic C with r values of 0.933, 0.871 and 0.948, respectively (n = 24, P < 0.01). A stronger relationship was
Fig. 4. Mean free and occluded light fraction C as percent of soil organic carbon at two depths (0–10 cm and 10–20 cm) in different Chinese forest soils. In the same soil layer, bars labeled with the same letter are not significantly different between forests according to the LSD test (P < 0.05). SBF, PC, MCA and MCK present secondary broadleaved forest, pure Cunninghamia lanceolata plantation, mixed plantation of C. lanceolata and Alnus cremastogyne, and mixed plantation of C. lanceolata and Kalopanax septemlobus, respectively.
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also observed between microbial biomass C and soil respiration (r = 0.930, n = 24, P < 0.01). The lowest r value (0.713) was found for the correlation between soil organic C and cold-water-soluble soil organic C. Free and occluded light fractions C were significantly correlated with other variables.
4. Discussion 4.1. Response of labile SOM to vegetation types Vegetation type affected water-soluble SOM levels. Higher concentrations of hot- and cold-water-soluble soil organic C and total N were found in the SBF than in the C. lanceolata plantations. This was consistent with other findings (Smolander and Kitunen, 2002; Kiikkilä et al., 2005; Wang and Wang, 2007). The average concentrations of water-extractable soil organic C and total N in a Betula pendula stand were 63 and 102% higher than that in a Pinus sylvestris forest in Kivalo, Northern Finland (Smolander and Kitunen, 2002). Among the afforested plots, the highest concentration of waterextractable soil organic C was observed under Terminalia arjuna and the lowest in an Albizia procera stand (Laik et al., 2009). Some explanations for the differences could be given according to the origin of water-soluble SOM. Plant litter and root exudates are considered to be the primary sources of water-soluble SOM (Qualls et al., 1991). The SBF had significantly higher leaf litter and root production and turnover than the C. lanceolata plantations (Wang et al., 2010). In addition, the nutrient contents of leaf litter and root material from native broadleaved tree species were also higher in the SBF than in C. lanceolata (Wang et al., 2010). Therefore, the differences in quantity and quality of leaf litter and root could explain, at least in part, the effects of vegetation type on water-soluble SOM levels. Soil microbial biomass and activities varied considerably among different forests. The SBF had higher soil microbial biomass C and N than the C. lanceolata plantations, which also confirms that tree species affected soil microbial processes (Menyailo et al., 2002; Sinha et al., 2009). B. pendula forests had higher soil microbial biomass C and N than Picea abies and P. sylvestris stands (Smolander and Kitunen, 2002). Sinha et al. (2009) observed that tree species established under a coal mining ecosystem significantly affected soil microbial biomass C, microbial quotient and basal respiration. Wang and Wang (2007) also reported that the soil microbial biomass C and N in a pure C. lanceolata plantation were 29 and 33% lower than in a broadleaved forest in San Menjiang Forest, Guangxi province. Microbial metabolic quotient is considered an index for evaluating the efficiency of soil microbes for C utilization measured during a short-term incubation (Wardle and Ghani, 1995). In this study, there was no significant difference in the microbial metabolic quotient among the forest stands. Vegetation type as a critical biotic factor affects soil microbial processes in forest ecosystems through its impact on production, quality and decomposition rates of litter and root material, substrate availability, microbial communities and soil microclimate (Hibbard et al., 2005; Li et al., 2006). Positive correlations between soil respiration and aboveground litter production were also found in tropical forests (Raich, 1998). Literature on the influence of vegetation type on the light SOM fraction was quite scarce. The C. lanceolata plantations had a significantly lower light fraction soil organic carbon than the SBF, which is consistent with the findings of Luan et al. (2010) at Longmen Mountain, Sichuan, China. This demonstrates that conversion from evergreen broadleaved forest into a C. lanceolata plantation reduces the light fraction soil organic C. Laik et al. (2009) observed that the light fraction soil organic C in a Pongamia pinnata stand was 79.8% greater than that under T. arjuna after 18 years of establishment. The effect of vegetation type on the light fraction soil organic C could be explained according to its source. The light frac-
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tion SOM obtained by density fractionation is mainly composed of plant residues, roots, and fungal hypha at different decomposition stages (Janzen et al., 1992; Six et al., 2002). In our study, unfortunately, the solid-state 13 C NMR spectrum of free and occluded light fractions could not be determined. Therefore, the effects of vegetation on the chemical composition of the free and occluded light fractions are unknown. Using 13 C NMR spectra, Golchin et al. (1994) found that the organic structure and chemical composition of the free light fraction was similar in four of the five soils (three forest soils and two grass soils), which was similar to the finding of Kögel et al. (1988) for forest litter. Due to the dominant influence of plant litter, the difference in the chemical composition of the fractions between soils can mainly be explained by the differences in vegetation type (Baldock et al., 1992). In addition, turnover of the free light fraction was generally faster than that of the occluded light fraction (von Lützow et al., 2007). The occurrence of charcoal in free and occluded light fractions is common under sites where frequent vegetation burning is done (Cadisch et al., 1996). For grassland soils, Brodowski et al. (2006) found that the highest amount of black carbon was accumulated in the occluded light fraction. In Chinese fir stands, slash burning after clear-cutting is usually used for site preparation. Thereby, black carbon was possibly present in the light SOM fraction. 4.2. Relationship among labile fractions of SOM The labile soil organic C fractions measured (cold-water, hotwater, chloroform released and density fraction) were significantly correlated with soil organic C, suggesting that SOM was a major determinant of the amount of labile SOM present (Gregorich et al., 1994; Carter, 2002). Laik et al. (2009) also found a significant relationship between soil microbial biomass C and the light fraction soil organic C in a calciorthent after 18 years of afforestation. The significant correlations among the four labile soil organic C fractions measured in this study show that they partly represent similar C pool in soil. Some studies showed that soil respiration was correlated with the light fraction soil organic C content, suggesting that light fraction soil organic C was the driving factor in soil respiration (Gregorich et al., 1994; Gregorich and Janzen, 1996; Alvarez and Alvarez, 2000). Water-soluble soil organic C was strongly correlated with soil microbial biomass and activities, which was supported by some studies showing close relationships between water-soluble SOM and CO2 fluxes from soils (Jandl and Sollins, 1997; Neff et al., 2000) and soil microbial biomass C (Sparling et al., 1998; Ghani et al., 2003; Wang and Wang, 2007) in forest and pasture ecosystems. The reason of this close correlation is that the hot-water-soluble methods may extract not only simple organic compounds and carbohydrates, but also some parts of the soil microbial components (Sparling et al., 1998; Chen et al., 2004; Rovira and Vallejo, 2007). For example, Ghani et al. (2003) reported that carbohydrates were rich in the hot-water extract of forest soils. Sparling et al. (1998) also observed that the hot-water-extractable organic C of the mineral soils was about 45% of the microbial biomass C. This study highlights that forest vegetation type can significantly affect labile SOM in subtropical soils. The SBF had highest soil microbial biomass, soil respiration and water-soluble SOM followed by the mixed plantations, and the PC had the lowest amounts. The light SOM fraction in the 0–10 cm soil layer was strongly affected by vegetation. Mean values of free and occluded light fraction soil organic C determined as a percentage of soil organic C were significantly different among the different forests, with the highest values found in the SBF and the lowest in the PC. The strong correlations among labile fractions suggest that they have a close association with one another and partly represented a similar C pool in soils. Soil microbial biomass and hot-water-soluble
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organic matter were strongly correlated with the other variables. It is known that measurement of soil microbial biomass individually is costly and time consuming. In contrast, assessment of hot-watersoluble matter is simple and quick, and can also be determined in bulk samples at the same time. The hot-water-soluble method therefore is recommended as the best measure for labile SOM determination in subtropical forest soils. Acknowledgements This research was supported by the National Natural Science Foundation of China (31070436 and 41030533). We are grateful to two anonymous reviewers for critical evaluation of the manuscript and editor C.A.M. van Gestel for improving the manuscript. Thanks are also extended to Xiuyong Zhang and Zhenqi Shen for help during sampling and to Hong Gao and Xiaojun Yu for their many hours of total N analysis. References Alvarez, R., Alvarez, C.R., 2000. Soil organic mater pools and their associations with carbon mineralization kinetics. Soil Sci. Soc. Am. J. 64, 184–189. Baldock, J.A., Oades, J.M., Waters, A.G., Peng, X., Vassallo, A.M., Wilson, M.A., 1992. Aspects of the chemical structure of soil organic materials as revealed by solidstate 13 C NMR spectroscopy. Biogeochemistry 16, 1–42. Brodowski, S., John, B., Flessa, H., Amelung, W., 2006. Aggregate-occluded black carbon in soil. Eur. J. Soil Sci. 57, 539–546. Brookes, P.C., Landman, A., Pruden, G., Jenkinson, D.S., 1985. Chloroform fumigation and the release of soil N: a rapid direct extraction method to measure microbial biomass nitrogen in soil. Soil Biol. Biochem. 17, 837–842. Cadisch, G., Imhof, H., Urquiaga, S., Boddey, R.M., Giller, K.E., 1996. Carbon turnover (␦13 C) and nitrogen mineralization potential of particulate light soil organic matter after rainforest clearing. Soil Biol. Biochem. 28, 1555–1567. Carter, M.R., 2002. Soil quality for sustainable land management: organic matter and aggregation interactions that maintain soil functions. Agron. J. 94, 38–47. 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, 97–106. Chen, C.R., Condron, L.M., Davis, M.R., Sherlock, R.R., 2000. Effects of afforestation on phosphorus dynamics and biological properties in a New Zealand grassland soil. Plant Soil 220, 151–163. Chen, C.Y., Wang, S.L., 2004. Ecology of Mixed Plantation Forest. Science Press, Beijing, p. 3. Chen, C.R., Xu, Z.H., Mathers, N.J., 2004. Soil carbon pools in adjacent natural and plantation forests of subtropical Australia. Soil Sci. Soc. Am. J. 68, 282–291. Christenson, L.M., Lovett, G.M., Weathers, K.C., Arthur, M.A., 2009. The influence of tree species, nitrogen fertilization, and soil C to N ratio on gross soil nitrogen transformations. Soil Sci. Soc. Am. J. 73, 638–646. Finzi, A.C., van Breemen, N., Canham, C.D., 1998. Canopy tree–soil interactions within temperate forests: species effects on soil carbon and nitrogen. Ecol. Appl. 8, 440–446. Golchin, A., Oades, J.M., Skjemstad, J.O., Clarke, P., 1994. Study of free and occluded particulate organic matter in soils by solid-state 13 C NMR spectroscopy and scanning electron microscopy. Aust. J. Soil Res. 32, 285–309. Gregorich, E.G., Carter, M.R., Angers, D.A., Monreal, C.M., Ellert, B.H., 1994. Towards a minimum data set to assess soil organic matter quality in agricultural soils. Can. J. Soil Sci. 74, 367–385. Gregorich, E.G., Janzen, H.H., 1996. Storage of soil carbon in the light fraction and macro organic matter. In: Carter, M.R., Stewart, B.A. (Eds.), Advances in Soil Science. Structure and Organic Matter Storage in Agricultural Soils. CRC Lewis Publishers, Boca Raton, pp. 167–190. Hagen-Thorn, A., Callesen, I., Armolaitis, K., Nihlgard, B., 2004. The impact of six European tree species on the chemistry of mineral topsoil in forest plantations on former agricultural land. Forest Ecol. Manage. 195, 373–384. Hibbard, K.A., Law, B.E., Reichstein, M., 2005. An analysis of soil respiration across northern hemisphere temperate ecosystems. Biogeochemistry 73, 29–70. Ghani, A., Dexter, M., Perrott, K.W., 2003. Hot-water extractable carbon in soils: a sensitive measurement for determining impacts of fertilisation, grazing and cultivation. Soil Biol. Biochem. 35, 1231–1243. Jandl, R., Sollins, P., 1997. Water-extractable soil carbon in relation to the belowground carbon cycle. Biol. Fertil. Soil 25, 196–201. Janzen, H.H., Campbell, C.A., Brandt, S.A., Laford, G.P., Townley-Smith, L., 1992. Lightfraction organic matter in soils from long-term crop rotations. Soil Sci. Soc. Am. J. 56, 1799–1806. Kiikkilä, O., Kitunen, V., Smolander, A., 2005. Degradability of dissolved soil organic carbon and nitrogen in relation to tree species. FEMS Microbiol. Ecol. 53, 33–40.
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