Potential impacts of climate warming on active soil organic carbon contents along natural altitudinal forest transect of Changbai Mountain

Acta Ecologica Sinica 30 (2010) 113–117

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Potential impacts of climate warming on active soil organic carbon contents along natural altitudinal forest transect of Changbai Mountain Zhang Xin-Yu a, Meng Xian-Jing a,b, Gao Lu-Peng a,c, Sun Xiao-Min a,*, Fan Jin-Juan b,**, Xu Li-Jun a a

The Sub-Center for Water Monitoring and Research, Chinese Ecosystem Research Network, Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China b Shenyang Agricultural University, Shenyang 110161, China c National Science and Technology Infrastructure Center, Beijing 100862, China

a r t i c l e

i n f o

Keywords: Soil core relocation experiment Forest soil Soil total organic carbon (SOC) Soil labile organic carbon (LOC) Microbial biomass carbon (MBC)

a b s t r a c t Understanding soil carbon fractions and their responses to the global warming is important for improving soil carbon management of natural altitudinal forest ecosystem. In this study, the contents of soil total organic carbon (SOC), soil labile organic carbon (LOC), and microbial biomass carbon (MBC) in soil upper layers (0–20 cm) were measured along a natural altitudinal transect in the north slope of Changbai Mountain. The results showed that under natural conditions the contents of SOC and LOC were largest in Betula ermanii forest (altitude 1996 m), moderate in spruce-fir forest (altitude 1350 m), and smallest in Korean pine mixed broad-leaf tree forest (altitude 740 m). MBC contents in different forest ecosystems decreased in the order of Betula ermanii forest, Korean pine mixed broad-leaf tree forest, and dark coniferous forest. In addition, the responses of SOC, LOC, and MBC to soil warming were conducted by relocating intact soil cores from high- to low-elevation forests for one year. As expected, the soil core relocation caused significant increase in soil temperature but made no significant effect on soil moisture. After one year incubation, soil relocation significantly decreased SOC contents, whereas the contents of LOC, MBC, and the ratios of LOC to SOC and MBC to SOC increased. Ó 2010 Ecological Society of China. Published by Elsevier B.V.

1. Introduction Over the next 100 years, anthropogenically induced climate warming from the production of greenhouse gases is hypothesized to increase global mean annual temperature by 1.54–5.8 °C [1]. Many different experimental approaches have been used to study the potential impacts of climate change on terrestrial ecosystems, including controlled-climate laboratory studies, and experimental field manipulations using plastic enclosures, buried heating cables, and infrared radiators [2,3]. Using natural temperature gradients caused by changes in altitude or aspect coupled with direct manipulation (soil or plant-soil reciprocal transplant) also have been used as surrogate experimental approaches which may provide a powerful and cost-effective and convenient tool for assessing the potential impact of climate change on terrestrial ecosystems [2,3]. The forest soil carbon pool accounts for about 73% of the soil carbon pool [4]. Thus, relatively small change of forest soil carbon pool may have an important role in regulating the global carbon balance. Understanding soil organic carbon fractions and their re-

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X.-Y. Zhang), [email protected] (X.-M. Sun), [email protected] (J.-J. Fan).

sponses to the global warming will help to reveal the dynamics and mechanism of soil carbon pool [5,6]. Disagreement exists, however, regarding the effects of climate change on soil organic carbon dynamics. Some study suggests that the decomposition of soil labile carbon is sensitive to temperature variation whereas resistant components are insensitive [7]. Whereas some research shows that non-labile SOC is more sensitive to temperature than labile SOC [8]. And some study suggests the similar response of labile and resistant soil organic matter pools to changes in temperature [9]. Temperate forests occupy a large area in northeastern China where effects of projected climatic warming on terrestrial ecosystems are significant. Understanding the soil carbon responses to the temperature in this area is important for improving soil carbon management of forest ecosystem [10]. However, our knowledge is limited to the effect of global warming on active soil organic carbon dynamics in natural temperate forest soils of Changbai Mountain. We sampled soil cores from the three typical soils under broadleaved Korean pine forest (altitude 740 m), dark coniferous forest (altitude 1350 m) and erman’s birch forest (altitude 1996 m) along natural altitudinal forest transect of Changbai Mountain, then transferred them from the high altitude to the low altitude for one year incubation. The objectives of this study were to assess the soil organic carbon fraction contents and the potential impacts

1872-2032/$ - see front matter Ó 2010 Ecological Society of China. Published by Elsevier B.V. doi:10.1016/j.chnaes.2010.03.011

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of climate warming on active soil organic carbon contents along natural altitudinal forest transect of Changbai Mountain. The results can improve the understanding for the response of carbon dynamics in temperate soils to varying vegetations and temperature. 2. Methods 2.1. Study site The study site was located at altitudes from 740 to 1996 m on the northern slope of Changbai Mountain, Northeastern China (42°240 N, 128°280 E). The forest ecosystem of the Changbai Mountain is the most typical mountain temperate forest ecosystem in eastern Asia, with vertical zonal distribution of vegetation resulted in a varied topography, weather, soil and other natural conditions in Changbai Mountain [11]. Broad-leaved Korean pine forest (below 1100 m altitude) at the foot of Changbai Mountain is a typical natural coniferous-latifoliate mixed forest whose dominant tree species are Pinus koraiensis, A. mono Maxim, T. amurensis Rupr, Fraxinus mandshurica Rupa and Quecusmongolica. The dominant tree species in Dark coniferous forest (altitude 1100–1700 m) are P. koraiensis and P. jezoensis. Erman’s birch forest (altitude 1700– 2000 m), which constitute the peculiar forest landscape of subalpine belts, is the forest-line vegetation dominated by single arboreal tree species of Betula ermanii [12]. The climate is continental temperate climate with obvious vertical climate-changing features. The mean annual temperature is about 0.9–3.9 °C and the mean annual precipitation is about 700 mm at the broad-leaved Korean pine forest zone, about 2.3–0.9 °C and 800 mm at the dark coniferous forest zone, and about 3.2 to 2.3 °C and 1000–1400 mm at the erman’s birch forest zone [12]. There is obvious vertical zonality of soil on the northern slope of Changbai Mountain. With the order of from top to bottom: mountain soddy forest soil (altitude 1700–2000 m), mountain brown conifer forest soil (altitude 1100–1700 m) and mountain dark brown forest soil (below 1100 m altitude) [12]. 2.2. Experimental design and field incubation On June 8th 2007, 15 3 m  3 m plots were established randomly within three forest types, ie, broad-leaved Korean pine forest, dark coniferous forest and erman’s birch forest with similar geomorphic and hydrological conditions. Five established plots in each forest type were about 2 m distance apart, three of the plots were selected as control and two as soil core sampling or incubation site. Eight (in one dark coniferous forest plot) or 16 intact soil cores (in two erman’s birch forest plots) were removed using 30cm inner diameter  30-cm long, thin-walled polyvinyl chloride (PVC) pipe that had been sharpened at one end. The intact soil cores were relocated from the high- to the low-elevation forest plots for one-year field incubation. Sixteen soil cores from erman’s birch forest were placed in one dark coniferous forest plot and one broad-leaved Korean pine forest plot, respectively. Eight soil cores from dark coniferous forest plot were placed in one broad-leaved Korean pine forest.

2.3. Sampling Six soil samples, at each depth of 0–10 cm and 10–20 cm, were collected from each control plot by auger, then pooled for one soil sample in June 2007, June, July and August of 2008, a total of 72 soil samples from control plots were sampled. In June, July and August of 2008, incubation soil cores were sampled by auger at each depth of 0–10 cm and 10–20 cm, 3 soil cores of each forest type were selected each time, a total of 54 soil core samples were analyzed. The fresh soil samples were separated from fine seeds, sieved through 2 mm mesh, then put into sealed hop-pockets and kept at 4 °C. 2.4. Methods of measurements The soil temperatures of plots and soil cores were measured in situ by SN-digital thermometer at depth of 5 cm during 9:00– 10:00 am and 2:00–3:00 pm on June 9th, July 13th and 12th of 2008. Soil moisture was determined by mass loss on drying oven. Fresh soil pH (soil/water, 1:2.5) was measured using pH meter. The total organic carbon contents were determined by rapid dichromate oxidation of dried and sieved (0.28 mm) samples, and the total nitrogen contents were measured by Kjeldahl procedure followed by colorimetric analysis. Soil bulk density values in the field were evaluated by the core method, using oven-dried soil mass and field volume of sample. The soil microbial biomass carbon (MBC) concentration was measured using the chloroform fumigation-K2SO4 extraction method, and the extracted solution was analyzed by a total carbon analyzer (elementar Liqui TOC II) [13]. The soil labile organic carbon(LOB) were oxidated by 333 mmol L1 KMnO4, then measured by colorimetry at 565 nm wavelength by spectrophotometer [14]. Main soil properties were summarized in Table 1. 2.5. Statistical analyses In this study, the data were statistically analyzed by SPSS 11.0 software for windows. Parametric statistics of ANOVA analysis was carried out to test for significant differences of soil properties among forest types and warming experimental treatments at p < 0.05. If the effects of forest types and warming experimental treatments were significant, mean separations were achieved using a protected least significant difference (LSD) test at p < 0.05. Pearson’s correlation analysis was performed to explain relationships among different fractions of soil organic carbon, soil temperature and soil moisture. 3. Results 3.1. Soil temperatures and soil moistures in the three typical forest soils and the transferred soil cores The soil temperatures of the 5 cm depth were significantly different among the three forest type, varied as broad-leaved Korean pine forest (PB) > dark coniferous forest (DC) > erman’s birch forest (EB) (Fig. 1). In June 9th, the soil temperature of PB was 3.7 °C high-

Table 1 soil properties for the three soil types of 0–20 cm layers in June 2007. Forest type

pH

C/N

Bulk density (g cm3)

Soil organic carbon density (kg m2)

Total nitrogen density (kg m2)

PB DC EB

5.2 ± 0.1 a 5.5 ± 0.1 a 5.3 ± 0.1 a

15 ± 1 b 24 ± 1 a 15 ± 1 b

1.10 ± 0.08 a 0.57 ± 0.03 c 0.74 ± 0.08 b

12.8 ± 2.2 a 5.6 ± 1.3 b 11.5 ± 0.6 a

0.9 ± 0.2 a 0.2 ± 0.1 b 0.8 ± 0.1 a

The data indicate the mean ± standard error, n = 3. PB: broad-leaved Korean pine forest; DC: dark coniferous forest; EB: erman’s birch forest. Means in same column followed by the same letters are not significant at p < 0.05. The same below.

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18

a a

a a

b

c

12 10

a

a

a

b

b

8 6

c

4 2

a

60

c

14

soil moisture (%)

Soil temperature (°C)

16

70

a a

b b

b

50

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a

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40 b

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30

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6th June

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13th July

b

12th Aug.

20 10

0

0

PB

DC

EB

DC-PB EB-DC EB-PB

PB

DC

EB

DC-PB EB-DC EB-PB

Fig. 1. Soil temperatures in 5 cm depth and soil moistures in 0–20 cm layers in the three typical forest soils and the transferred soil cores in 2008. Note: The error bar represents the standard error (n = 3), the treatments with same letters are not significant at p < 0.05. DC–PB: the soil cores of dark coniferous forest transferred to broad-leaf Korean pine forest; EB–DC: the soil cores of erman’s birch forest transferred to dark coniferous forest; EB–PB: the soil cores of erman’s birch forest transferred to broad-leaved Korean pine forest. The same below.

er than that of DC and 7 °C higher than that of EB, the soil temperature of DC was 4 °C higher than that of EB. While in July and August, temperature variation among the three forest types was smaller than those in June, that is, the soil temperature of PB was 1.1–1.5 °C higher than that of DC, and the soil temperature of DC was 2.2–4.4 °C higher compared with that of EB. Soil temperatures in low-elevation incubated soil cores were statistically similar to those in ambient control plots during 2008. The soil moisture of the 0–20 cm layers in EB was significantly larger than that in DC and PB. The soil moisture was larger in broad-leaved Korean pine forest than that in dark coniferous but the difference was not significant (Fig. 1). The soil moisture of dark coniferous forest, erman’s birch forest and their relocated soil cores were higher but not significantly in July than in June and August. Over a year period, water moisture was also statistically similar among transferred soil cores and ambient control plots. 3.2. The SOC contents in the three typical forest soils and the transferred soil cores The SOC contents increased with elevation increase along natural elevation gradients, varied as EB > DC > PB. Surface mineral soil (0–10 cm) had significantly higher mean SOC contents than subsurface soil (10–20 cm). In 0–10 cm soil layers, the SOC contents were significantly larger in EB than those in DC and PB, but the SOC contents of DC and PB were not significantly different. In 10–20 cm soil layers, the SOC contents of the three forest types were significantly different (Fig. 2). After one year incubation, soil

120

0-10 cm 10-20 cm

SOC (g kg-1 )

100

ab ab

bc

80

bc a

60 bc

b b

c

40 20

a

d

d

0 PB

DC

EB

DC-PB EB-DC EB-PB

Fig. 2. The SOC contents in the three typical forest soils and the transferred soil cores in 2008 (n = 9).

relocation resulted in SOC content decrease in DC–PB, EB–DC and EB–PB soil cores in 0–10 cm and 10–20 cm layers, and the effect on 10–20 cm layers were significant (Fig. 2). Pearson correlation analysis indicated that SOC content in 0–10 cm soil layer was significantly negatively related to soil temperature in 5 cm soil layer (r = 0.549, p < 0.01) but SOC content positively related to soil moisture (r = 0.884, p < 0.01). 3.3. The soil LOC contents in the three typical forest soils and the transferred soil cores The soil under natural altitudinal forest transect from the highelevation sites generally had high LOC contents than soils from the low-elevation sites, i.e. EB > DC > PB. Furthermore, surface mineral soil (0–10 cm) had significantly higher LOC contents than subsurface soil (10–20 cm). The EB soil had larger LOC contents than PB soil, but had smaller ratio of LOC/SOC (about 8–17%) than PB and DC (11–36%) (Fig. 3). After one year incubation, the relocated soil cores had larger LOC contents than the soils of control plots both in 0–10 cm and 10–20 cm layers, and the effects on EB–PB and EB–DC were significant in 0–10 cm layers. In addition, soil relocation increased LOC/SOC by 2.8–18.5% in EB–DC and EB–PB transferred soil cores relative to the control plots, and increased by 0.6–10.7% in DC–PB transferred soil cores relative to the control plots (Fig. 3). Pearson correlation analysis demonstrated that soil LOC content was positively related to SOC content (r = 0.857, p < 0.01) and soil moisture (r = 0.571, p < 0.01). 3.4. The Soil MBC contents in the three typical forest soils and the transferred soil cores Fig. 4 showed that under natural conditions the contents of soil MBC were largest in EB, moderate in PB, and smallest in DC, but the differences of soil MBC content in the three forest soils were not statistically significant. While the surface soil layers (0–10 cm) had significantly larger MBC contents than the subsurface soil layers (10–20 cm). The PB soils had higher ratios (1.1–3.2%) of MBC/ SOC than DC and EB soils (0.2–1.1%). Under incubation experimental conditions, the MBC contents appeared to increase in EB–PB, EB–DC and DC–PB soil cores compared with the control plots, and the MBC contents in the EB–DC and EB–PB were significantly larger than those in the control plots. Soil relocation experimental conditions increased MBC/SOC by 0.3–1.7% in the EB–DC and EB–PB soil cores relative to the control plots, and increased by 0.1–1.5% in the DC–PB soil cores relative to the control plots. Pearson correlation analysis demonstrated that soil MBC content was

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30

12

a

a

a

25

LOC (g kg-1 )

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20 15

c c

10

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c b b b

b af

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10

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LOC (g kg-1 )

a b

8

a ab

ab

ab ab

ab

6

b

ab

ab

ab

June

ab ab

b

July

b

4

Aug.

b b

2

5

0

0 PB

DC

EB DC-PB EB-DC EB-PB 0-10 cm

PB

DC

EB DC-PB EB-DC EB-PB 10-20 cm

Fig. 3. The soil LOC contents in the three typical forest soils and the transferred soil cores in 2008.

a

1500

a

1000 a

a

a

a

b

750

b

b

1000

c cd

bc b

b d

500

MBC (mg kg-1 )

MBC (mg kg-1 )

1250 750 500

b

b b

b

ab

July

b b

b

b

Aug

250

250 0

0 PB

DC

EB DC-PB EB-DC EB-PB 0-10 cm

PB

DC

EB DC-PB EB-DC EB-PB 10-20 cm

Fig. 4. The soil MBC contents in the three typical forest soils and the transferred soil cores in 2008.

positively related to LOC and SOC contents (r = 0.872 and 0.816, respectively, p < 0.01) and soil moisture (r = 0.636, p < 0.01). 4. Discussion 4.1. Effects of natural altitudinal gradient forests on soil carbon SOC contents under the three typical forests increased significantly with the increasing altitude. Pearson correlation analysis demonstrates that SOC content was negatively related to soil temperature but positively related to soil moisture. Our study showed that the elevation increase, accompanied by soil temperature decrease and soil moisture increase, resulted in SOC content increase. This was consistent with the result from central Oregon Cascade Mountains, USA [2]. Hart and Perry [2] found that high-elevation old-growth forest soils had higher carbon and nitrogen storage than their low-elevation analogues primarily because low temperatures limit net carbon and nitrogen mineralization rates at higher elevation. Pearson correlation analysis indicated that LOC content was significantly positively related to SOC content and soil moisture. Xu et al. [14] found that there was significant positive relationship between soil LOC content and crop yield but no significant relationship between SOC content and crop yield. Their study suggested that LOC was a more sensitive indicator of soil quality and soil fertility than SOC. The ratio of LOC to SOC reflects the activity of soil organic carbon and the instability of carbon [14]. Our results of LOC and LOC/SOC showed that soil active carbon pool in EB soil was larger but the stability of carbon-storage was still larger in EB than those in PB and DC soils. Pearson correlation analysis indicated that MBC content was significantly positively related to SOC content and soil moisture.

Under natural conditions the content of MBC was largest in EB soil and smallest in DC soil. The research of temperate volcanic forest soils showed that under similar climatic conditions and soil types, the MBC concentrations varied with tree species and temperature, and that high soil C-to-N ratios can contribute to decreased soil MBC concentration [15]. Comparing different forest stands, it was indicated that the pine forest soil always showed the lowest MBC concentration relative to soil total carbon, suggesting that microbial communities in the soil were less efficient in carbon use than communities in the other forest soils [15,16]. Those results in Japanese volcanic soil agreed with the results in Changbai Mountain, i.e. there was higher soil moisture, lower soil C/N in EB soil, and lower soil moisture, higher soil C/N in DC soil (Table 1 and Fig. 1). Furthermore, these results indicated that soil MBC was affected by soil carbon content, soil moisture, soil C/N, litter quality, and many other factors. Besides, the soil MBC/SOC reflects conversion efficiency of organic carbon into microbial carbon, and could be an indicator of turnover rates of biological activity of soil organic carbon. The soil MBC/SOC was larger in PB soil than those in DC and EB soils. The results suggested that the carbon-storage capacity of PB was smaller, but the ratio of MBC/SOC was larger. That reflected that the turnover rates of microbial activity of soil organic carbon pool was higher in PB compared with the DC and EB. 4.2. Effects of simulated warming on soil organic carbon As expected, the soil core relocation experiment caused significant increase in soil temperature but made no significant effect on soil moisture (Fig. 1). After one year incubation, soil relocation significantly decreased SOC contents in 10–20 cm soil layers, whereas the contents of LOC and MBC increased significantly in EB soils but not significantly in DC soils. Hart [3] transferred intact soil cores

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from a spruce-fir to a ponderosa pine forest 730 m lower in elevation. His results showed that soil transfer significantly increased net CO2 efflux (120%) relative to fluxes of these gases from soil cores incubated in situ. Wang et al. [12] conducted soil incubation experiment in laboratory to delineate the effect of soil temperature on soil respiration in broad-leaved Korean pine forest, dark coniferous forest and erman’s birch forest in Changbai Mountain. Their results indicated that the soil respiration rate was positively correlated to soil temperature from 0 to 35 °C in broad-leaved Korean pine forest and erman’s birch forest, and from 0 to 25 °C in dark coniferous forest. Soil CO2 releasing was dependent on the number of microbes, biomass and its activities in soil [16]. The decrease trend of SOC content induced by soil core incubation experiment was in agreement with the pattern of SOC content under natural altitudinal transect. The results indicated that temperature increase may result increase of soil organic matter mineralization and microbial biomass decomposition, induced the increase of soil labile carbon and microbial biomass carbon. Since LOC and MBC are more available to plant absorption and water leaching than some other part of soil organic carbon, the temperature increase may induce SOC decrease. The increase of MBC/SOC and LOC/SOC affected by soil core relocation experiment indicated that future climate warming may increase the soil turnover rates of microbial biomass carbon pool and decrease the stability of soil carbon pool in erman’s birch soil and dark coniferous soil of Changbai Mountain. 5. Conclusions The results showed that under natural conditions the contents of SOC and LOC were largest in EB, moderate in DC, and smallest in PB. MBC contents in different forest ecosystems decreased in the order of EB, PB, and DC. Pearson correlation analysis demonstrates that SOC content was positively related to soil moisture, LOC and MBC contents increased with contents of SOC and soil moisture. After one year incubation, soil relocation significantly decreased SOC contents, whereas the contents of LOC, MBC, and the ratios of LOC/ SOC and MBC/SOC increased. Acknowledgements The project was financially supported by the National Natural Science Foundation of China (Nos. 40701186, 30600091), and

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knowledge innovation funds of Chinese Academy of Science (No. KZCX2-YW-433-01). We thank the Changbai Mountain Forest Ecosystem Research Station for the support of the field work. We are also grateful to Dr. Zhuang Jie in the University of Tennessee for English polishing.

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