Assessments of the impacts of Chinese fir plantation and natural regenerated forest on soil organic matter quality at Longmen mountain, Sichuan, China

Geoderma 156 (2010) 228–236

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Geoderma j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o d e r m a

Assessments of the impacts of Chinese fir plantation and natural regenerated forest on soil organic matter quality at Longmen mountain, Sichuan, China Junwei Luan a,b, Chenghua Xiang b,c,⁎, Shirong Liu a,⁎, Zongshi Luo c, Yuanbo Gong b, Xueling Zhu d a

The Research Institute of Forest Ecology, Environment and Protection, Chinese Academy of Forestry, Beijing 100091, PR China Sichuan Provincial Key Laboratory of Ecological Forestry Engineering, Sichuan Agricultural University, Ya’an 625014, PR China Sichuan Academy of Forestry, Chengdu 610081, PR China d Baotianman Natural Reserve Administration Bureau, Neixiang County, Henan Province 474350, PR China b c

a r t i c l e

i n f o

Article history: Received 16 June 2009 Received in revised form 20 December 2009 Accepted 15 February 2010 Available online 11 March 2010 Keywords: Land use change Labile organic carbon Permanganate oxidation Particulate organic matter Light fraction Chinese fir plantation Regenerated forest

a b s t r a c t Three soil labile organic carbon indicators, i.e., particulate organic carbon (POC) by size fractionation, light fraction organic carbon (LFOC) by density fractionation and permanganate-oxidizable carbon (MnoxC) oxidized by 333 mM KMnO4, were employed to identify the soil organic carbon (SOM) pools of a Chinese fir plantation (CF) and a regenerated forest (RF) that were converted originally from the natural evergreen broad-leaved forest (NF) 18 years ago after clear cutting on an upland yellow soil in Longmen mountain, southwest China. Bulk soil organic carbon and nitrogen concentration at the soil depth of 0–20 cm were significantly lower in CF and RF than in NF, while no significant C/N ratio change was found. Top soil MnoxC concentration of CF showed a significant increase while RF demonstrated a slight decline compared to NF, whereas, non-MnoxC of CF and RF both presented a significant decline compare to NF. A different indication between chemical oxidization and physical fractionation method was found. POC, particulate organic nitrogen (PON), mineral-associated organic carbon (MOC) and mineral-associated organic nitrogen (MON) were significantly reduced in CF and RF compared to NF. CF, POC and PON showed significant reductions than that in RF. The losses of TOC and N in the particulate separates were more than that in the mineral-associated separates for both CF and RF. A significant decline of LFOC at 0–10 cm, 10–20 cm and 20–30 cm soil depths were found in CF and RF compared to NF, and a significant larger loss of LFOC in CF than in RF was also found. The heavy fraction organic carbon (HFOC) at the 0–10 cm soil depth was also significantly reduced in CF and RF, while there was no significant difference between CF and RF. The absence of vegetation cover in CF and RF, especially at the early stage of restoration, was contributed to the reductions in both labile and non-labile carbon. Timely thinning after canopy closure or establishing CF mixing with native broad-leaved species should be encouraged to mitigate the labile carbon loss. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

1. Introduction Maintenance and improvement of soil organic matter (SOM) quantity and quality are generally accepted as one of the most important criteria for sustainable forest management, because it is a major reservoir of nutrients, water availability and other important chemical, physical and biological properties of soil (Haynes and Beare, 1996). Reforestation will inevitably impact the quality, quantity and spatiotemporal distribution of SOM (Ashagrie et al., 2005; Brown and Lugo, 1990; He et al., 2009; Zinn et al., 2002), which is made up of different fractions with turnover rates varying from hours to hundreds of years (Chan, 1997). Due to the high background soil organic C (TOC) pool

⁎ Corresponding authors. Sichuan Provincial Key Laboratory of Ecological Forestry Engineering, Sichuan Agricultural University, Ya’an 625014, PR China. Xiang is to be contacted at Tel.: + 86 28 82901395; fax: + 86 28 83229472. Liu, Tel.: +86 10 62889311; fax: +86 10 62884229. E-mail addresses: [email protected] (C. Xiang), [email protected] (S. Liu).

(Haynes, 1999), direct measurement of short-term total SOM losses resulting from land use change may not clearly show the effect of land use change. Hence, previous studies proposed some more sensitive indicators of SOM pool such as particulate organic carbon (POC) (Cambardella and Elliott, 1992; Christensen, 2001), light fraction organic carbon (LFOC) (Janzen et al., 1992; Six et al., 2002a; Six et al., 1998), readily oxidized fractions by 333 mM KMnO4 (Blair et al., 1995; Blair and Crocker, 2000). These indicators often respond more rapidly to management-induced changes in the SOC pool than bulk SOM, and could serve as early indicators for the overall stock change. On the premise that microbiological decomposition of organic matter in the soil is largely associated with an oxidation process of enzymatic character, the measurement of the labile carbon fractions by oxidation with 333 mM KMnO4 developed by Blair et al. (1995) has successfully been used by numerous researchers in SOM studies (Blair et al., 1998; Blair and Crocker, 2000; Whitbread et al., 1998), however, the appropriate concentration of KMnO4 has been widely debated (Vieira et al., 2007; Weil et al., 2003). The light fraction obtained through density

0016-7061/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2010.02.021

J. Luan et al. / Geoderma 156 (2010) 228–236

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Table 1 Soil organic C, and N concentrations (g kg− 1) and element ratios under the different forest types, results refer to the 0–10 cm, 10–20 cm, 20–30 cm, 30–50 cm soil depths. Soil depth

Treatment

Soil organic carbon (g C kg− 1 soil)

Total nitrogen (g N kg− 1 soil)

C:N

PH

0–10 cm

CF RF NF CF RF NF CF RF NF CF RF NF

69.43a 77.72a 112.59b 41.36ab 33.45a 49.76b 27.47a 26.87a 32.65a 18.48a 19.80a 24.64a

2.73a 2.86a 3.93b 1.97a 1.63b 2.34c 1.39a 1.33a 1.68b 1.17a 1.09a 1.36a

26.02a (2.9) 27.66a (2.9) 28.15a (2.0) 21.09a (1.9) 20.96a (2.2) 21.14a (0.84) 20.01a (1.7) 20.39a (1.4) 19.26a (0.85) 16.40a (1. 6) 17.59a (1.4) 17.57a (1.3)

5.3a 4.9b 4.8b 5.3a 5.1a 4.8b 5.3a 5.1a 4.8b 5.3a 5.2a 4.9b

10–20 cm

20–30 cm

30–50 cm

(7.5) (7.8) (12.1) (3.9) (2.7) (4.2) (2.6) (1.7) (3.1) (1.5) (2.8) (2.8)

(0.25) (0.24) (0.20) (0.12) (0.08) (0.13) (0.10) (0.05) (0.13) (0.11) (0.09) (0.11)

Bulk density (g cm− 3) (0.1) (0.1) (0.09) (0.08) (0.1) (0.09) (0.08) (0.09) (0.05) (0.05) (0.07) (0.07)

0.78a 0.90b 1.00b 1.02a 1.10ab 1.21b 1.16a 1.21a 1.23a 1.21a 1.21a 1.20a

(0.04) (0.04) (0.04) (0.06) (0.04) (0.05) (0.07) (0.05) (0.03) (0.07) (0.06) (0.05)

Means followed by different lower case letters in a column are significantly different from each other according to LSD test (P b 0.05). Numbers in parentheses are standard errors (n = 9), CF: 18-year old Chinese fir plantation; RF: regenerated forest after 18 years clear cutting; NF: natural forest.

fractionation is composed mainly of plant residues, roots, and fungal hypha at different decomposition stages (Janzen et al., 1992; Six et al., 2002a), it played an important role in determining the structure and function of the soil ecosystem by acting as an energy source for heterotrophic organisms and as a reservoir of labile C and plant nutrients (Laik et al., 2009). The particulate fraction obtained through particle-size fractionation has similar characteristics and is also considered to contain labile C (Cambardella and Elliott, 1994; Christensen, 1996). Several authors (Ashagrie et al., 2005; Guggenberger et al., 1994; Solomon et al., 2002; Zinn et al., 2002) found differences in the quality and amount of SOM associated with mineral particles of different sizes, and also reported relatively greater losses of OC in the coarser than in the finer particle size separates as a result of changes in land use. All closely relate to the decomposition process, thus, the three labile organic carbon indicators seem to have a close association with one another and have an important impact on soil quality. However, there seems still no consensus as to which method or combination of methods is most suitable to evaluate the land use change impacts on soil quality (Skjemstad et al., 2006). In upland area, clear cutting was commonly performed in southwest China during the last century. After clear cutting, monoculture Chinese fir plantation (CF) was widely planted in subtropical area of China to further meet the increasing timber demand. Up to 2004, 1239.1 × 104 ha of Chinese fir plantation had been planted, accounting for ca. 26.55% of total plantation area in China (Fang, 2004). Still, little is known about the impacts of Chinese fir plantation on SOC quality and quantity, although it provides rapid growth and a higher economic return in contrast to native tree species. In this study, therefore, physical size

fractionation (Cambardella and Elliott, 1992), density fractionation (Six et al., 1998) and chemical permanganate oxidation (Blair et al., 1995) were used 1) to identify SOC of a monoculture Chinese fir plantation (CF) and a regenerated forest (RF), which were converted from a natural evergreen broad-leaved forest (NF) 18 years ago on an upland yellow soil at Longmen mountain, southwest China; and 2) to determine whether these three methods can obtain similar labile organic carbon fraction when evaluating land use change impacts on soil quality. 2. Materials and methods 2.1. Site description Our study was conducted at the Kuanba forest region in Longmen mountain (32°12′N and 104°19′E) located at Pingwu county, Sichuan province at an altitude of 1400 m.a.s.l. north margin of subtropical, with mean annual precipitation of 1187 mm and mean annual temperature is 11 °C, and relative humidity at 88%. The principal parent materials are of phyllite origin from slope deposit and pluvial sediments. A Machilus pingii dominated mixed evergreen broad-leaved natural forest (shrub coverage 41.6 ±2.3%, grass coverage 24.3 ±2.7%), and an adjacent 18-year old Chinese fir plantation as well as a natural regenerated forest after 18-year clear cutting were selected for this study. All the plots have a similar slope of ca. 20–25°. Chinese fir plantation in the study area was established after a clear cutting of the natural forest, and the secondary forest was regenerated from natural forest clear cutting site. Clear cutting was done

Fig. 1. Soil C (a) and N (b) storage in 0–50 cm depth. CF: 18-year old Chinese fir plantation; RF: regenerated forest after 18 years clear cutting; NF: natural forest. Bars with different letters are significantly different at P b 0.05.

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Table 2 MnoxC and non-MnoxC and carbon pool lability (LMnoxC) for three forest type soils for 0–10 cm, 10–20 cm, 20–30 cm and 30–50 cm soil depths. Soil depth

Treatment

MnoxC (g kg− 1 soil)

Non-MnoxC (g kg− 1 soil)

MnoxC/TOC (%)

LMnoxC (g/g)

0–10 cm

CF RF NF CF RF NF CF RF NF CF RF NF

27.34a 20.36a 22.21a 15.67a 6.71b 9.87b 9.14a 4.95b 6.68ab 7.63a 3.79a 5.92a

42.09a 57.36a 90.39b 25.69a 26.74a 39.89b 18.33a 21.92ab 25.97b 10.85a 16.01ab 18.72b

39.6a 27.1ab 21.1b 38.8a 20.6b 21.9b 33.8a 19.2b 23.6ab 41.8a 19.9b 27.9ab

0.97a 0.40ab 0.28b 0.98a 0.28b 0.31ab 0.71a 0.25a 0.36a 1.84a 0.26a 0.52a

10–20 cm

20–30 cm

30–50 cm

(6.2) (3.3) (2.2) (2.9) (1.0) (1.1) (1.9) (0.57) (0.59) (2.1) (0.75) (1.2)

(5.5) (7.6) (11.1) (3.8) (2.8) (4.9) (2.2) (1.8) (3.5) (2.3) (2.4) (3.1)

Means followed by different lower case letters in a column are significantly different from each other at P b 0.05. Numbers in parentheses are standard errors (n = 9), CF: 18year old Chinese fir plantation; RF: regenerated forest after 18 years clear cutting; NF: natural forest.

and tree diameter at breast height (DBH) ranged from 4.5 to 23.3 cm with a height of 3 to16 m, for the dense canopy few shrub and grass were found in the understory (shrub coverage 2.3±0.8%, grass coverage 1± 0.2%). Except a few uncut trees, the diameter at breast height ranged from 3.2 to 6.9 cm, springwood dominated the natural regenerated forest (shrub coverage 67.6±7.6%, grass coverage 48.3±4.5%). 2.2. Sampling In each forest type, three 0.04 ha plots ca. 10 m apart from each other were located randomly and each plot was separated into four 0.01 ha subplots. Five pits were excavated to the depth of 50 cm randomly at three subplots in August 2006. Soil samples ca. 500 g were taken from the three sides of the pit by a shovel from 4 layers at depths of 0–10 cm, 10–20 cm, 20–30 cm and 30–50 cm, five pits samples in the 0.1 ha subplot of each soil layer were mixed and the final number of samples were reduced to nine per land use at each soil layer. After air drying, samples were sieved through 2 mm sieve size for physical fractionation, chemical oxidation and for bulk soil C, and N analysis. 2.3. Chemical oxidation with KMnO4

manually and the aboveground biomass was removed. Few management practices were performed on both of the forest types after clear cut. Tree density in the studied plantation compartment was about 2200 tree ha− 1

Soil samples containing 15 mg of organic C were placed in 100 ml snap-caps and 25 ml of 333 mM KMnO4 were added. The suspensions

Fig. 2. Size (a), density (b) and oxidizable (c) fraction C storage in 0–50 cm depth of soils. CF: 18-year old Chinese fir plantation; RF: regenerated forest after 18 years clear cutting; NF: natural forest. POC: particulate organic carbon; MOC: mineral-associated organic carbon; LFOC: light fraction organic carbon; HFOC: heavy fraction organic carbon; MnoxC: permanganate-oxidizable carbon; Non-MnoxC: non-permanganate-oxidizable carbon.

Fig. 3. Regression between (a) Lability of the MnoxC (LMnoxC) and POC (LPOC) fractions, (b) Lability of the MnoxC (LMnoxC) and LFOC (LLFOC) fractions, and (c) Lability of the LFOC (LLFOC) and POC (LPOC) fractions.

J. Luan et al. / Geoderma 156 (2010) 228–236

were horizontally shaken for 1 h at 12 rpm and centrifuged at 2000 rpm for 5 min. The supernatant solution was separated, diluted with distilled water at the proportions of 1:250, and then its absorbance at 565 nm was measured with a colorimeter. The depletion of KMnO4 concentration was directly related to the concentration of oxidizable C, namely of permanganate-oxidizable C (MnoxC), assuming that 1 mM MnO− 4 is consumed to oxidize 0.75 mol or 9 mg of C. In order to prevent KMnO4 photo-oxidation, care was taken to avoid the incidence of light on the solution (Blair et al., 1995). 2.4. Physical fractionation For the density fractionation, based on Six et al. (1998), a suspension of 10 g of soil sample and 25 ml NaI solution (density of 1.8 g cm− 3) were horizontally shaken for 18 h at 60 rpm, after the suspension was shaken at 60 rpm for 18 h, the supernatant solution containing the light fraction was vacuum filtered through a 0.45 μm fiberglass filter (previously dried at 50 °C and weighed). The filter and the retained light fraction were rinsed with distilled water to remove NaI salt, dried at 65 °C, grounded in an agate mortar and analyzed for organic C, namely of light fraction organic carbon (LFOC). For particle-size fractionation, 20 g of soil sample and 100 ml of Na hexametaphosphate solution (5 g L− 1) were added to a 250 ml snap-cap and horizontally shaken for 18 h (90 rpm). The soil suspension was passed through a 53 μm mesh and the retained coarse fraction was rinsed with a weak stream of distilled water, dried at 65 °C, ground and analyzed for organic C and N, namely of particulate organic carbon (POC) and particulate organic nitrogen (PON) (Cambardella and Elliott, 1992).

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among soil carbon pool lability. All statistical analyses were performed using the statistical package SPSS 13.0 for Windows. 3. Results and discussion 3.1. Soil organic C, and N in bulk soil samples Total soil organic C (TOC), and N (TN) concentrations in bulk soil samples decreased with soil depth at all study sites (Table 1). Significant decrease in TOC concentration at 0–10 cm and 10–20 cm soil depths were found in CF and RF compared to NF (Table 1). No significant difference in bulk TOC concentration at 0–50 cm soil depth was found between CF and RF (Table 1). In contrast to TOC, bulk soil N concentration showed a significant loss deeper into the soil under CF and RF, and a difference was also found at 20–30 cm soil depth

2.5. Soil analysis Soil organic carbon concentrations in bulk soil, particulate fractions(N53 μm) and light (densityb1.8 g cm− 3) fractions were determined by the wet oxidation method with 133 mM K2Cr2O7 at 170–180 °C, and soil nitrogen concentration in bulk soil and particulate fractions were determined by the micro-Kjeldahl method. Soil pH was measured from soil–water suspensions (soil:water ratio 1:5). Soil samples for bulk density determination were taken from the wall of the pits by a 100 cm3 metal cylinder for each soil increments, totaling nine per land use. 2.6. Definition of fractions Blair et al. (1995) defined the fraction that was not oxidized by permanganate as non-labile. We maintained this definition for consistency in the POC and LFOC measurements in the current study, and the non-labile fractions non-permanganate-oxidizable C (non-MnoxC), heavy fraction organic carbon (HFOC), and mineralassociated organic carbon (MOC) were calculated as TOC–MnoxC, TOC–LFOC and TOC–POC respectively. In addition, mineral-associated organic matter nitrogen (MON) was calculated as TN–PON. As Blair et al. (1995) defined the term ‘Lability’ of the OC as the ratio of the oxidized to non-oxidized OC, we also maintained this definition for the other two fractionation methods, and calculated as follows: LMnoxC = MnoxC/Non-MnoxC; LPOC = POC/MOC; LLFOC = LFOC/HFOC. 2.7. Statistical analysis One way analysis of variance (ANOVA) was performed to assess the effect of change in land use on soil organic C and N, soil organic C and N associated with the different particle-size/density fractions as well as permanganate-oxidizable carbon. Significant differences among the different land uses were reported from each depth at P b 0.05 separately. Regression analyses were used to examine the relationships between TOC and N in the soil fractions, among physical fractions and chemical fractions, and between plot stem basal area and labile carbon as well as

Fig. 4. Relationships of total soil organic C, particulate organic carbon, light fraction organic carbon and permanganate-oxidizable carbon for 0–10 cm soil increment with stem basal area (n = 6), data of CF was excluded.

30–50 cm

20–30 cm

10–20 cm

Means followed by different lower case letters in a column are significantly different from each other at P b 0.05 under each land use. Different upper case letters in a row indicate significant differences between means of different sizes at P b 0.05. Numbers in parentheses are standard errors (n = 9), CF: 18-year old Chinese fir plantation; RF: regenerated forest after 18 years clear cutting; NF: natural forest. WS: whole soil.

0.18a 0.40b 0.40b 0.16a 0.16a 0.22a 0.16a 0.13a 0.15a 0.21a 0.12a 0.13a 25.19a 24.27a 23.83a 20.43a 20.72a 19.17a 18.90a 19.73a 18.14a 15.36a 17.18a 16.69a (0.23) (0.19) (0.21) (0.11) (0.06) (0.12) (0.09) (0.05) (0.12) (0.10) (0.08) (0.09) 2.41a 2.43a 3.33b 1.76a 1.43b 2.12c 1.27a 1.22a 1.55b 1.08a 1.01a 1.27a 3.75abB 2.93aA 4.33bB 2.77aB 1.72bB 2.78aB 2.00aB 1.48bB 1.99abB 1.68aB 1.23aB 1.62aB (6.9) (6.4) (9.1) (3.3) (2.6) (2.9) (2.2) (1.7) (2.6) (1.7) (2.7) (2.6) 59.43ab 56.38a 81.20b 35.51ab 29.11a 40.80b 23.71a 23.87a 28.26a 16.08a 17.82a 21.91a 92.65abB 67.93aB 105.97bA 56.13aB 35.04bA 53.47aA 37.40aB 28.87aB 36.51aB 24.50aB 21.67aB 28.15aB 68.30a 83.25b 78.21b 66.97a 83.09b 79.89b 67.19a 82.50b 78.86b 67.66a 81.88b 79.65b 10.12a 15.14b 13.93b 10.13a 9.28a 9.07a 9.50a 8.06a 7.88a 8.63a 8.06a 6.58a 14.98a 27.10b 27.92b 13.91a 13.30a 17.31a 13.58a 11.48a 13.27a 14.91a 10.63a 11.40a 39.54a 47.95ab 56.81b 30.35a 29.63a 40.32b 30.06a 28.04a 49.14a 28.29a 22.98a 30.70a (0.03) (0.05) (0.04) (0.03) (0.01) (0.03) (0.02) (0.01) (0.02) (0.01) (0.01) (0.01) 0.26a 0.44b 0.54b 0.20ab 0.14a 0.21b 0.13a 0.11a 0.13a 0.10a 0.09a 0.09a 0.90aA 2.63bA 2.53bA 0.65aA 0.86abA 0.95bA 0.43aA 0.62bA 0.62bA 0.32aA 0.49bA 0.44bA 10.00a (1.4) 21.34b (3.4) 31.39c (4.4) 5.84ab (0.85) 4.34a (0.46) 8.96b (1.82) 3.76a (0.63) 3.00a (0.29) 4.39a (0.58) 2.39a (0.39) 1.98a (0.24) 2.73a (0.35) 35.92aA 125.23bA 148.11bA 20.36aA 26.01aA 38.91bA 13.07aA 17.33abA 20.54bA 8.92aA 11.30aA 13.23aA 31.70a 16.75b 21.79b 33.03a 16.91b 22.62b 32.81a 17.50b 21.14b 32.34a 18.12b 20.35b

MON MON MOM (g N kg− 1 fraction) (g kg− 1 soil) C:N POC/TOC PON/TN MOM/WS MOC MOC (%) (%) (%) (g C kg− 1 fraction) (g kg− 1 soil) POM C:N PON PON (g N kg− 1 fraction) (g kg− 1 soil)

CF RF NF CF RF NF CF RF NF CF RF NF 0–10 cm

Particulate organic matter C/N ratio (POC-to-N ratio) is higher than mineral-associated organic matter C/N ratio (MOC-to-N ratio) in the soil under three forest types as described by other researchers (Ashagrie et al., 2005; Six et al., 2002b). This could be attributed mainly to the accumulation of newly added and less decomposed organic matter in

POM/WS POC POC (%) (g C kg− 1 fraction) (g kg− 1 soil)

3.3. Particulate organic C and N

MOM (b53 μm)

CF has resulted in a significant increase in soil MnoxC at the 0–10 cm and 10–20 cm soil layers compared to NF. RF resulted in a decline in soil MnoxC but no significant difference was found (Table 2). The two restoration modes after clear cutting (reforestation or natural regeneration) show an opposite impact on soil MnoxC. The lability of C (LMnoxC) in the surface soil of the CF was 3.46 and 2.43 times of that in the NF and RF soil, respectively (Table 2). On the other hand, a significant decline of non-MnoxC of CF occurred for 0–50 cm soil increments, and a lower decline of non-MnoxC of RF only occurred for 0–20 cm soil increments compared with NF. The proportion of non-MnoxC decline of CF is 16.9%, 2.6%, 13.8% and 27.6% higher than RF for 0–10 cm, 10–20 cm, 20–30 cm and 30–50 cm soil increments, respectively. On an area basis, no significant difference were found for MnoxC stock among three forests; only significantly higher non-MnoxC stock in 0–10 cm soil depth of NF was found compared to CF and RF (Fig. 2c). As Skjemstad et al. (2006) and Tirol-Padre and Ladha (2004) reported, permanganate-oxidizable C is particularly sensitive to the presence of lignin, and Yang et al. (2003) reported that needles of Chinese fir have a higher lignin content than leaves of C. kawakamii and other native broad-leaved tree species, thus, in our study, higher litter lignin content of needle leaf may lead to the overestimated MnoxC concentration of CF when compared to RF and NF. On the other hand, in agreement with Skjemstad et al. (2006) results, we found MnoxC show a weak correlation with POC and LFOC compared to the strong relationship between POC and LFOC (Table 5), and Tirol-Padre and Ladha (2004) also did not find a significant correlation of MnoxC with MBC and DOC (other two popular labile carbon indicators). Similarly, in contrast to a significant positive relationship between LPOC and LLFOC, no correlation of LMnoxC with LPOC and LLFOC was found (Fig. 3). These findings indicated the different indication results of labile carbon obtained through oxidation method with physical fractionation method. Significant correlation of stem basal area (a proxy of plot biomass) with TOC, POC and LFOC (Fig. 4a, c, and d) while no relationship with MnoxC (Fig. 4b) was found for RF and NF further shows the different characters of chemical oxidized with physical fraction carbon. All of these suggested that we should be cautious to use permanganate oxidation method, which was developed and first taken use of on agro-ecosystem (lower lignin content) (Blair et al., 1995; Blair and Crocker, 2000), to evaluate soil organic matter quality change, especially on forest ecosystem for its high lignin content.

POM (N53 μm)

3.2. Permanganate-oxidizable carbon

Soil depth

(Table 1). CF and RF showed a slight decrease of bulk soil C/N ratio, but no significant difference was found. Similarly, Wang et al. (2005) reported significantly lower TOC and N concentration under the first and second generation plantations of Chinese fir compared to the native broad-leaved forest. In addition, mix Chinese fir with broadleaved species (Kalopanax septemlobus (Thunb.) Koidz or Alnus cremastogyne Burk) would increase both biomass C and soil C storage compare to pure Chinese fir plantation (Wang et al., 2009). CF showed a significant decline of top soil (0–20 cm) bulk density compared to NF (Table 1), which reflected not only the absence of heavy machines when logging and subsequently forest management, but the change of soil physical character due to land cover change. On an area basis, both TOC (0–20 cm) and TN (0–30 cm) stocks were significantly reduced for CF and RF compared to NF, while no significant difference was found between CF and RF (Fig. 1).

Lpoc (g/g)

J. Luan et al. / Geoderma 156 (2010) 228–236 Table 3 Particulate (N53 μm) and mineral-associated (b 53 μm) distribution and soil organic C and N concentrations (g kg− 1 size fraction) in size fractions or in soil (g kg− 1 soil) under three forest type soils for 0–10 cm, 10–20 cm, 20–30 cm, 30– 50 cm soil depths.

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J. Luan et al. / Geoderma 156 (2010) 228–236

the coarser fractions (Gerzabek et al., 2001; Guggenberger et al., 1994), which have a less aliphatic and humified nature in comparison to clay and silt-sized OM (Buyanovsky et al., 1994; Mahieu et al., 1999). Majority of the whole soil OC, N in three forest types were associated with the finer particle size (b53 μm), POC and PON only accounted for 10.63–27.92% and 6.58–15.14% respectively (Table 3), these findings are in agreement with other observations (Ashagrie et al., 2005; Desjardins et al., 1994; Solomon et al., 2002) for tropical soils, being the highest in the clay fraction. In general, TOC, N concentrations were higher in the mineral-associated fraction than particulate fraction at the 10–50 cm soil depth (Table 3), indicating a preferential shift of the organic matter to the finer fractions during the decomposition process (Ashagrie et al., 2005). The redistribution of sand-sized OM to clay-complexed OM during decomposition has already been reported by Zinn et al. (2002). However, there is a higher OC concentration in particulate fraction for RF and NF at 0–10 cm soil depth, which indicated the accumulating rate of partly decomposed organic matter is faster than the decomposing and shifting process at the soil surface for RF and NF. In agreement with other reports (Amelung et al., 1998; Zinn et al., 2007), we also found a dilution effect of organic carbon distribution in particle size. The OC concentrations in particle-size fractions were inversely related to the content of the respective fraction in the soil (Fig. 5). The proportional distribution of the particles in the N53 μm class of CF is significantly higher than RF and NF, RF was slightly lower than NF

Fig. 5. Graphical view of the soil organic carbon dilution effect for the particulate (N 53 μm) and mineral (b53 μm) fractions (in the regression equations, y and x correspond to values on y and x axes).

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Fig. 6. Relationships of particulate and mineral-associated organic C pools with total organic C (n = 107).

but no significant difference was found (Table 3). Sparse understory grass and shrub of CF may lead to increased soil erosion and was hence contributed to the loss of fine soil particles, suggesting that the textural composition of the soils under the plantation sites were seriously changed after 18 years. Nevertheless, no significant higher POC and PON concentration was found in CF (Table 3), in contrast, POC, and PON concentrations and C/N ratio declined significantly after conversion of the NF to CF in 0–10 cm soil increment (Table 3). This may be partly explained by dilution effect. The particulate fraction showed a higher loss of C and N (Table 3), suggesting that organic matter associated with the coarser fractions is more labile and first to be affected by changes in land use and soil management (Christensen, 1996; Solomon et al., 2002; Zinn et al., 2002). In addition, the degree of OC losses was larger than the losses of N for particulate and mineral-associated fractions, suggesting a decline of soil quality. On an area basis, soil C stocks for the different particle sizes were shown in Fig. 2a, both POC and MOC stocks of CF and RF declined significantly at 0–10 cm soil depth compared to NF, and CF showed a significantly larger loss of POM than RF. A plot of POC and MOC vs. TOC suggests that land use change (set aside or plantation after clear cutting) will decrease both POC and MOC pools (Fig. 6). In contrast to Ashagrie et al.'s (2005) results, clear cutting of the natural forest and replacing it by the Eucalyptus plantation resulted in the depletion of OC and N from the sand-sized fractions while enrichment of OC and N in the clay-sized fraction, both

Fig. 7. Relationships of particulate and mineral-associated organic C-to-N ratio with total organic C (n = 107).

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Table 4 Light and heavy fraction organic matter carbon for three forest type soils for 0–10 cm, 10–20 cm, 20–30 cm and 30–50 cm soil depths. Light fraction (ρ b 1.8 g/cm3)

Soil depth

0–10 cm

10–20 cm

20–30 cm

30–50 cm

CF RF NF CF RF NF CF RF NF CF RF NF

Heavy fraction (ρ N 1.8 g/cm3)

LF/WS (%)

LFOC (g C kg− 1 fraction)

LFOC (g kg− 1 soil)

LFOC/TOC (%)

HF/WS (%)

HFOC (g C kg− 1 fraction)

HFOC (g kg− 1 soil)

HFOC/TOC (%)

LLFOC (g/g)

2.34a 4.57b 9.42c 1.31a 2.49ab 3.98b 0.49a 2.10b 2.64b 1.22a 1.94a 1.87a

314.43a 269.5a 270.6a 342.5a 205.5b 174.0b 335.3a 164.6b 169.7b 213.3a 119.3b 143.5ab

6.62a (1.1) 11.74b (1.0) 23.09c (2.9) 3.23a (0.56) 3.76a (0.40) 6.21b (0.69) 1.59a (0.17) 2.49b (0.25) 3.36c (0.34) 1.47a (0.36) 1.87a (0.23) 2.21a (0.32)

9.46a 15.52b 20.43c 7.69a 11.94b 12.70b 6.10a 9.43b 10.54b 8.54a 9.96a 9.06a

97.66a 95.43b 90.58c 98.69a 97.51ab 96.46b 99.51a 97.90b 97.36b 98.78a 98.06a 98.13a

64.5a 69.3a 100.2b 38.7ab 30.5a 45.5b 26.0a 24.9a 30.2a 17.2a 18.3a 22.9a

62.81a 65.98a 89.50b 38.13ab 29.69a 43.54b 25.88a 24.38a 29.30a 17.01a 17.93a 22.43a

90.54a 84.48b 79.57c 92.31a 88.06b 88.71b 93.90a 90.57b 89.46b 91.46a 90.04a 90.94a

0.11a 0.19b 0.26c 0.08a 0.14ab 0.15b 0.07a 0.11b 0.12b 0.10a 0.11a 0.10a

(18.7) (17.9) (24.1) (3.1) (36.8) (22.0) (25.3) (30.6) (30.9) (31.9) (21.7) (21.7)

(7.1) (7.6) (11.9) (3.7) (2.9) (3.9) (2.6) (1.7) (3.0) (1.5) (2.7) (2.7)

(6.67) (7.1) (9.5) (3.6) (2.8) (3.8) (2.6) (1.6) (2.9) (1.5) (2.7) (2.6)

Means followed by different lower case letters in a column are significantly different from each other at P b 0.05. Numbers in parentheses are standard errors (n = 9), CF: 18-year old Chinese fir plantation; RF: regenerated forest after 18 years clear cutting; NF: natural forest. WS: whole soil.

particulate and mineral-associated OC and N declined due to the land use change in our results (Table 3, Fig. 2a). After clear cutting, the absence of vegetation cover for CF and RF at the early stage of restoration may explain both POC and MOC loss in our study. In contrast to Franzluebbers and Stuedemann (2002) who reported a negative relationship between POC-to-N ratio and TOC, we found a positive relationship between POC-to-N ratio and TOC (Fig. 7), which indicated the quality of the particulate and mineral-associated organic matter pool depends upon the quantity of TOC in soil. Furthermore, a higher slope of TOC vs. POC-to-N than TOC vs. MOC-to-N indicates easier change of particle-size organic matter quality. 3.4. Light fraction OC The dry weight of light fraction was strongly influenced by different land use types, and it is mainly dependent upon the intermediate products of the decomposition of roots and leaf litter (Laik et al., 2009). The higher stem basal area of NF than RF explained the LFOC difference between these two forest types (Fig. 4d), the CF data were excluded due to other processes that have influenced the manmade system more seriously. The data (Table 4) demonstrate that there were significant reductions in LFOC after conversion of the NF into CF and RF at the soil depths of 0–10 cm, 10–20 cm and 20–30 cm, and CF showed a significant larger reduction than RF for 0–10 cm and 20–30 cm soil increments (Table 4). HFOC also declined significantly after conversion from NF to RF and CF for the surface soil, but no significant difference was found between CF and RF (Table 4). In our study, vegetation structure of Chinese fir plantation was changed seriously compared with natural forest, hence soil surface physical process may have been changed (such as increased soil erosion) due to the declined understory vegetation coverage. In addition, quantity and quality of litter input were also changed. Thus the decomposition process may also have been changed dramatically. Both of these may be contributed to the larger loss of LFOC in CF compared to RF. The proportional reductions of LFOC are 71% and 49% for CF and RF at 0– 10 cm soil depth compared to NF, much higher than bulk soil organic carbon loss (38% and 31% for CF and RF), suggesting that SOM quality is more prone to changes in land use and soil management strategies than the total amount of SOM (Ashagrie et al., 2005). The effect of changes in soil management on soil quality rather than on total SOM was also reported by Janzen et al. (1992). LFOC occupies a substantial share in the organic carbon content of the soil, mean value of LFOC determined as a percentage of TOC (LFOC/TOC) varied between 20.43% in NF and 6.1% in CF (Table 4). Light fraction plays an important role in determining the structure and function of the soil ecosystem by acting as an energy source for heterotrophic organisms and as a reservoir of relatively labile carbon and plant nutrients (Laik

et al., 2009). In the upper 10 cm soil layer the mean carbon content of light fractions extracted from the NF, RF and CF were 245.1, 256.9 and 282.9 g kg− 1 light fraction respectively (Table 4). Similar results were reported by previous studies. For example, Besnard et al. (1996) reported a 250 g OC kg− 1 light fraction for a French forest soil, and Laik et al. (2009) reported a mean carbon content extracted from the different afforested plots varied from 287 to 386 g kg− 1 light fractions in the upper 15 cm soil layer in India. Higher carbon content of light fraction for CF (Table 4) suggests a higher proportion of light fraction at the earlier stage of decomposition. On an area basis, soil C stocks for the different density fractions were shown in Fig. 2b, conversion from NF to CF and RF significantly reduced 0–10 cm, 10–20 cm and 20–30 cm soil LFOC stocks, and a significant difference in LFOC stock was found between CF than RF in 0–10 cm soil depth. 0–10 cm and 10–20 cm soil HFOC stocks were also significantly reduced in CF and RF, but no significant difference was found between CF and RF. Significant relationship between LFOC and HFOC with TOC showed that land use change decreased both LFOC and HFOC (HFOC = 0.8053 × TOC + 3.0284, R2 = 0.99, P = 0.000) (Table 5). The significant positive relationship between LFOC and bulk soil C-to-N ratio (Table 5) suggests that light fraction of soil organic matter is a key attribute of soil quality (Liang et al., 2003), which is consistent with other authors' (Swanston et al., 2004; Whalen et al., 2000) findings. 3.5. Relationships among labile SOM pools Strong correlation between POC and LFOC with R value of 0.92 (P b 0.001) (Table 5) suggested that there was the similar origin of light fraction and particulate organic matter. They are both considered to represent partly decomposed plant materials at an early stage of Table 5 Regression models from regression analysis of a number of soil organic carbon measurements. Variables (x/y)

Models

MnoxC/C:N POC/C:N LFOC/C:N MnoxC/SOC POC/SOC LFOC/SOC MnoxC/POC MnoxC/LFOC POC/LFOC

y = 0.3606x + 17.242, R2 = 0.3299, P = 0.000 y = 4.847Ln(x) + 13.497, R2 = 0.4657, P = 0.000 y = 4.9309Ln(x) + 15.215, R2 = 0.4943, P = 0.000 y = 2.0105x + 20.934, R2 = 0.4389, P = 0.000 y = 2.792x + 21.188, R2 = 0.8013, P = 0.000 y = 4.315x + 20.175, R2 = 0.8402, P = 0.000 y = 0.5792x + 1.5586, R2 = 0.3544, P = 0.000 y = 0.3442x + 1.6005, R2 = 0.2852, P = 0.000 y = 0.6098x + 0.5451, R2 = 0.8472, P = 0.000

C:N: bulk soil C-to-N ratio, SOC: soil organic carbon, MnoxC: permanganate-oxidizable carbon determined by 333 mM KMnO4, POC: particulate organic matter carbon, LFOC: light fraction organic carbon, n=107 for all regression analysis.

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decomposition, which is characterized by a transitional stage in the humification process (Leifeld and Kögel, 2005; Six et al., 2002b). Lower R values between MnoxC with POC and LFOC, however, mean the different indication of physical fractionation and chemical oxidation methods to obtain labile carbon. Three labile organic carbon indicators (MnoxC, POC, and LFOC) are significantly correlated with TOC and bulk soil C-to-N ratio, suggesting that they are sensitive indicators of change in total soil organic matter (Carter, 2002) and also for soil quality (Gregorich et al., 1994). However, the concentration of MnoxC is less dependent on TOC than POC and LFOC (Table 5). LPOC and LLFOC showed significant decline, LMnoxC showed a significant increase in CF compared to RF and NF (Tables 2, 3, and 4). As we discussed above in Section 3.2, the chemical oxidation may attack part of the mineral-associated organic matter, which is not recovered through physical fractionation (Vieira et al., 2007). There are also arguments that the stronger oxidizing capacity of 333 mM KMnO4 concentration and its corresponding overestimation of the labile C fraction (Shang and Tiessen, 1997; Weil et al., 2003). Hence, Vieira et al. (2007) suggested that a lower concentration of 60 mM KMnO4 seems to provide a better estimation of labile soil C. The significant correlation of stem basal area with TOC, POC and LFOC (Fig. 4a, c, and d) after excluded CF data suggests some other processes also controlled the manmade ecosystem soil carbon sequestration other than residue input. Among the physical methods, the lower recovery of light fraction compared to particulate organic matter (Tables 3 and 4) may be supposedly attributed to the fact that NaI solution employed in density separation was not so efficient to recover the light fraction organic matter, as already evidenced in a study by Shang and Tiessen (2001) and Vieira et al. (2007). 4. Conclusions The conversion from NF to CF and RF both reduced the soil labile (POC, and LFOC) and non-labile (MOC, and HFOC) carbon stocks for surface soil, whereas plantation showed more intensive loss of labile carbon (POC, and LFOC) than the regenerated forest. A different indication existed between chemical oxidization and physical fractionation method, and caution may be required in using permanganate oxidization method to evaluate the impacts of land use change on soil quality, especially forest ecosystem. Timely thinning or establishing CF mix with native broad-leaved species in a close to natural way should be encouraged to obtain timber production and mitigate labile carbon loss. Acknowledgments We thank Drs. Jingxin Wang and Zheke Zhong for their valuable comments on the earlier versions of this manuscript, we also gratefully acknowledge an anonymous reviewer whose helpful comments improved our manuscript profoundly. This study was funded by China's National Natural Science Foundation (No. 30590383) and the Ministry of Finance (No. 200804001) and the Ministry of Science and Technology (No. 2006BAD03A04). References Amelung, W., et al., 1998. Carbon, nitrogen, and sulfur pools in particle-size fractions as influenced by climate. Soil Sci. Soc. Am. J. 62, 172–181. Ashagrie, Y., Zech, W., Guggenberger, G., 2005. Transformation of a Podocarpus falcatus dominated natural forest into a monoculture Eucalyptus globulus plantation at Munesa, Ethiopia: soil organic C, N and S dynamics in primary particle and aggregate-size fractions. Agric. Ecosyst. Environ. 106, 89–98. Besnard, E., Chenu, C., Balesdent, J., Puget, P., Arrouays, D., 1996. Fate of particulate organic matter in soil aggregates during cultivation. Eur. J. Soil Sci 47, 495–503. Blair, G.J., et al., 1998. Soil carbon changes resulting from sugarcane trash management at two locations in Queensland, Australia, and in North-East Brazil. Aust. J. Soil Res. 36, 873–881.

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