Influence of vegetation types and soil properties on microbial biomass carbon and metabolic quotients in temperate volcanic and tropical forest soils

Abstract

There is limited knowledge about the differences in carbon availability and metabolic quotients in temperate volcanic and tropical forest soils, and associated key influencing factors. Forest soils at various depths were sampled under a tropical rainforest and adjacent tea garden after clear-cutting, and under three temperate forests developed on a volcanic soil (e.g. Betula ermanii and Picea jezoensis, and Pinus koraiensis mainly mixed with Tilia amurensis, Fraxinus mandshurica and Quercus mongolica), to study soil microbial biomass carbon (MBC) concentration and metabolic quotients (qCO₂, CO₂-C/biomass-C). Soil MBC concentration and CO₂ evolution were measured over 7-day and 21-day incubation periods, respectively, along with the main properties of the soils. On the basis of soil total C, both CO₂ evolution and MBC concentrations appeared to decrease with increasing soil depth. There was a maximal qCO₂ in the 0–2.5 cm soil under each forest stand. Neither incubation period affected the CO₂ evolution rates, but incubation period did induce a significant difference in MBC concentration and qCO₂ in tea soil and Picea jezoensis forest soil. The conversion of a tropical rainforest to a tea garden reduced the CO₂ evolution and increased the qCO₂ in soil. Comparing temperate and tropical forests, the results show that both Pinus koraiensis mixed with hardwoods and rainforest soil at less than 20 cm depth had a larger MBC concentration relative to soil total C and a lower qCO₂ during both incubation periods, suggesting that microbial communities in both soils were more efficient in carbon use than communities in the other soils. Factor and regression analysis indicated that the 85% variation of the qCO₂ in forest soils could be explained by soil properties such as the C:N ratio and the concentration of water soluble organic C and exchangeable Al (P < 0.001). The qCO₂ values in forest soils, particularly in temperate volcanic forest soils, decreased with an increasing Al/C ratio in water-soluble organic matter. Soil properties, such as exchangeable Ca, Mg and Al and water-soluble organic C:N ratio, were associated with the variation of MBC. Thus, MBC concentrations and qCO₂ of the soils are useful soil parameters for studying soil C availability and microbial utilization efficiency under temperate and tropical forests.

Key words:

• carbon availability
• metabolic quotient
• microbial biomass
• respiration
• temperate forest
• tropical forest
• volcanic

INTRODUCTION

Tree species can have significant impacts on soil fertility and microbial community structure (Dickinson and Pugh 1974; Swift et al. 1979), which can in turn affect soil microbial biomass and microbial efficiency in carbon (C) use (e.g. Bauhus et al. 1998; Blagodatskaya and Anderson 1998; Chen and Xu 2005; Pinzari et al. 1998). Microbial C, especially the microbial C : organic C ratio, can reveal the status of microorganisms in the C availability of soils (e.g. Insam and Domsch 1988), and is considered to be a quantitative indicator for carbon dynamics in soils (Anderson and Domsch 1985; Insam and Haselwandter 1989; Xu et al. 2006). In temperate volcanic forest soils, it has been shown that metal–humus complexes (e.g. Al) are resistant to microbial decomposition and can offer physical protection against microbial breakdown (Shoji et al. 1993). The Al/C ratio of dissolved organic matter can be considered to be an important parameter for its stability against microbial decomposition (Boudot et al. 1989; Schwesig et al. 2003; Shoji et al. 1993). The quality and amount of litter and changes in soil properties can affect soil carbon availability and microbial utilization efficiency (e.g. Anderson and Domsch 1993; Blagodatskaya and Anderson 1998; Chen and Xu 2005; Huygens et al. 2005; Six et al. 2002). In tropical forests, high temperatures and rainfall increase soil C mineralization and nutrient leaching and, thus, increasing acidity can induce an accumulation of Al in soil profiles and a reduction of soil Ca and Mg. These variations are associated with the stability of metal–humus complexes and aggregate-C in the soil and their microbial decomposition (Aran et al. 2001; Huygens et al. 2005; Schwesig et al. 2003; Six et al. 2002). There are usually lower concentrations of soil water-soluble organic C and total C under tropical forests than under temperate forests because of climatic conditions, which can affect soil C availability and microbial utilization. However, there is very limited knowledge about the differences in microbial biomass C and metabolic status in temperate volcanic and tropical forest soils.

On the basis of microbial C concentration, soil basal respiration (qCO₂) is considered to be an alternative measurement of changes in microbial biomass in response to disturbance and environmental limitations (Anderson and Domsch 1985; Insam and Haselwandter 1989; Odum 1985; Wardle and Ghani 1995). The qCO₂ described by Odum (1969) and by Anderson and Domsch (1985) indicates the relative efficiency of soil microorganisms in carbon use measured during a short-term incubation (Anderson and Domsch 1993; Wardle and Ghani 1995), and the intensity of carbon mineralization (Dilly and Munch 1998). Bardgett and Shine (1999) showed that the metabolic quotient was significantly affected by changes in litter diversity, being lower at higher levels of litter diversity. Anderson and Domsch (1993) reported that qCO₂ values decreased in a mixed forest ecosystem compared with a single plantation forest ecosystem. In earlier studies, Insam and Haselwandter (1989) showed that the soil qCO₂ in an ecosystem decreases during succession. However, to date, our knowledge is limited with regard to the differences in qCO₂ under tropical and temperate forests at advanced succession stages. Indeed, a thorough discussion of the relationships between microbial C, qCO₂ and soil properties in tropical and temperate forests is lacking. This is even true for the specific features of temperate volcanic and tropical forest soils, with a low microbial C : organic C ratio.

The objective of this work was to study the differences in soil microbial C and qCO₂ in temperate volcanic and tropical forests, and the effects of soil properties on these biological variables. These results will improve our understanding of C availability and microbial utilization efficiency in temperate volcanic and tropical forest soils.

MATERIALS AND METHODS

Soil and litter sampling and chemical analysis

To examine the differences in soil carbon availability and metabolic quotient under different climatic conditions and in response to tree species, temperate and tropical forest soils at various depths were sampled near the Research Station of Changbai Mountain Forestry Ecology (128°28′E, 42°24′N) and the Research Station of Xishuangbanna Tropical Rainforest Ecosystem (101°16′E, 21°56′N), respectively. The area around the Changbai Mountain is a temperate, continental climate, with a long-term cold winter and warm summer. Annual mean temperature varies from 2.8°C at the bottom of the mountain to −7.3°C near the volcanic lake at the top, and annual mean precipitation varies from 750 to 1340 mm. Recent small-scale volcano eruptions occurred in 1597, 1668, 1702 and a very large-scale eruption occurred during 1000–1410 (Liu et al. 1992; Zhao 1981). Three natural temperate forests in a natural vertical distribution along the northern slope were considered to be broadleaved Korean pine mixed forest (Pinus koraiensis mainly mixed with Tilia amurensis, Fraxinus mandshurica and Quercus mongolica, > 200 years old, altitude 740 m), spruce (Picea jezoensis, > 200 years old, altitude 1680 m) and birch (Betula ermanii, > 200 years old, altitude 1910 m). The mixed forest is now at advanced succession stages in this zone. Soil sampling was done after selecting three 4 m × 4 m plots in each forest stand. Samples of seven soil layers, 0–2.5, 2.5–5, 5–7.5, 7.5–10, 10–12.5, 12.5–15 and 15–20 cm, were collected separately within each plot in July 2005 using a thin stainless spade, when above-ground plant debris was removed. Thus, in total 63 temperate forest soil samples were taken simultaneously. In the tropical zone, the annual mean temperature is approximately 22°C, and annual mean precipitation is approximately 1400 mm, with an obvious dry and wet season over the year. Samples of the corresponding seven soil layers were collected in May 2005 under a seasonal tropical rainforest and adjacent tea garden, as previously described. A substantial proportion of the tree species in the seasonal rainforest (> 200 years old, altitude 720 m) is deciduous under the monsoon climate, although they do not shed leaves during the same seasons (Cao et al. 1996). The undisturbed rainforest has entered an advanced succession stage in this tropical zone. The tea garden was established approximately 60 years ago on the site after the existing rainforest was harvested. Fresh soils were kept separately in plastic bags and rapidly transported to the laboratory. Four 4 m × 4 m plots were selected under each stand, and one mixture of litter within each plot was collected. Fresh moist soils were sieved (2 mm, or 4 mm for 0–2.5 cm temperate soils) to remove small stones and roots, and stored in the dark at 4°C prior to incubation.

Temperate and tropical forest soils were classified as Andosols and Oxisols (Food and Agriculture Organization soil classification), respectively. Soil water-holding capacity was determined by saturating each soil in glass funnels with water and allowing drainage to field capacity under cover for 2–3 h at ambient temperature. Duplicate soils were dried at 105°C for 24 h to determine moisture content, and litter samples were dried at 70°C for 24 h to make powder for use. Total C and N concentrations in soil and litter samples were measured using a CN analyzer (MT-700 with an Auto Sampler MTA-600, Yanaco, Kyoto, Japan). Fresh soil pH (soil : water, 1:2.5) was measured using a portable pH meter. Soil water soluble organic C and total N concentrations were extracted with deionized water at a soil : water ratio of 1:5 (w/w) and measured using a TOC/TN-analyzer (Shimadzu TOC-VCPH/TN, Kyoto, Japan). Soil exchangeable metals such as Ca, Mg and Al were extracted by shaking 10-g fresh soil with a 50-mL 1 mol L⁻¹ KCl solution for 60 min on an end-over-end shaker, and then filtered into 50-mL plastic bottles. Concentrations of these metals in the filtrates were measured using inductively coupled plasma-mass spectrometry. The main soil properties under temperate and tropical forests are presented in Table 1. Total C and N concentrations in litter samples are presented in Table 2.

Soil respiration and microbial C during two incubation periods

All fresh soil samples taken from the forest floors were used to study soil respiration and microbial biomass carbon (MBC) concentration. Soil moisture contents were adjusted to 50–55% (w/w) (approximately 45% water-holding capacity) after 20-g samples of fresh soils (in triplicate) were added to 250-mL glass bottles and sealed with butyl rubber stoppers. This consistency of soil water content can reduce the uncertainty about the proportion of microbial biomass released by the fumigation–extraction (Davidson et al. 1989; Sparling et al. 1990). Control bottles containing no soil were used as blanks to derive CO₂ production from the soils. To compare the effects of incubation time on microbial C and qCO₂ in soil, the oxic incubation was done at 25°C in the dark for 7 and 21 days, respectively. Three milliliters of headspace gas was sampled from each bottle 4, 7, 11, 14, 18 and 21 days after initiating the incubation, and immediately injected into a gas chromatograph for CO₂ measurement; the rubber stoppers were removed from the bottles for 2 h to refresh the air. At the end of the 7-day and 21-day incubations, soil MBC concentrations were measured using the chloroform fumigation–extraction method (Vance et al. 1987). Soil salt-extractable organic C was extracted by shaking 5.0-g fresh soil with a 25-mL 0.5 mol L⁻¹ K₂SO₄ solution for 60 min on an end-over-end shaker. The mixtures were centrifuged at 6400 g for 5 min and then filtered into 50-mL plastic bottles. The filtrate was analyzed for organic C using a TOC/TN-analyzer (Shimadzu TOC-VCPH/TN).

Carbon dioxide concentrations in the headspace were quantified using a modified gas chromatograph (Agilent 5890) equipped with a flame ionization detector (FID). Carbon dioxide was separated using one stainless steel column (2 m length and 2.2 mm internal diameter) that was packed with 50–80 mesh porapack Q, hydrogen was then used to reduce CO₂ to CH₄ in a Nickel catalytic converter at 375°C, and the CH₄ was detected using the FID. The oven was operated at 55°C and the FID at 200°C, and N₂ was used as a carrier gas at a flow rate of 20 cm³ min⁻¹. The CO₂ concentration was calculated using a linear calibration using a certified CO₂ concentration that contains 0.400% CO₂ in N₂.

Calculation and statistical analysis

Soil respiration rates were obtained from the increase in cumulative CO₂ concentration (minus the blank) against incubation time, and were expressed on the basis of initial soil total C. Because the ratio of soil microbial C to total C concentration reflects the linkage and interaction between the two parameters (Insam and Domsch 1988), MBC levels were calculated as the proportion of initial soil total C using the equation: biomass C = 2.22 × E _c, where E _c is the difference in K₂SO₄-extractable organic C between fumigated and non-fumigated samples (Vance et al. 1987; Wu et al. 1990). The metabolic quotient (qCO₂) was calculated as the ratio of microbially respired C (µg g⁻¹ over the 7 and 21 days) to MBC and used as an indicator of microbial metabolism in soil. Means of three replicates and standard errors were calculated. Significant differences between means were analyzed using a t-test (STATISTICA software for Windows [release 4.5]), with a confidence interval of 95%. Principal component and regression analyses were carried out to explain some soil properties that affect microbial respiration, MBC and qCO₂ in forest soils. Least significant differences (LSD) were calculated at the 5% level to assess the differences in soil properties with depth.

RESULTS

Soil and litter properties under tropical and temperate forests

Some of the properties of temperate and tropical forest soils at different depths are presented in Table 1. The concentrations of total C and N and water soluble organic C in temperate forest soils were much larger than those in tropical forest soils (P ≤ 0.05), and were reduced at greater rates with increasing soil depth, compared to the reduction in the concentrations in tropical forest soils. Comparing selected temperate forest stands, the results showed that the broadleaved Korean pine mixed forest soil at less than 20 cm depth had a lower soil C:N ratio and lower water soluble organic C concentration (P ≤ 0.05) than the birch and spruce forest soils. Soil exchangeable Ca and Mg concentrations under temperate forests, especially under the broadleaved Korean pine mixed forest, were much larger than those in tropical forest soils (P ≤ 0.05), and there was a sharp reduction in these concentrations with increasing soil depth (P ≤ 0.05). This is contrary to the change in exchangeable Al concentration in the 0–20 cm soil. The accumulation of soil exchangeable Al became larger with increasing soil depth, and its maximal concentration occurred in sub-surface soils under spruce and birch (P ≤ 0.05). The exchangeable Al concentration in tropical forest soil was much larger than that in temperate forest soil (P ≤ 0.05), which was contrary to the variation in soil pH. There was a larger total N concentration in litter under birch forest and tea garden than under the other forests, and a similar total C concentration occurred in all the litters, with the exception of the litter under the broadleaved Korean pine mixed forest (P ≤ 0.05) (Table 2). There was a lower C:N ratio in the litter under birch forest and tea garden than under the other forests (P ≤ 0.05) (Table 2).

Microbial respiration, microbial carbon and metabolic quotients in forest soils

Soil microbial respiration and microbial C concentration were calculated according to initial soil total C. Figure 1 shows the variations of microbial respiration, microbial C and metabolic quotients in temperate and tropical forest soils during the 7-day and 21-day incubations. On the basis of soil total C, CO₂ evolution rates and MBC concentrations appeared to decrease with increasing soil depth (P ≤ 0.05). There was a maximal qCO₂ in the 0–2.5 cm soil under each forest stand. Both incubation periods had no impact on microbial respiration, but did affect microbial C and qCO₂ measurements in tea soil and spruce forest soil. The rainforest conversion to tea garden increased soil microbial respiration and qCO₂, resulting in an increase in microbial C utilization inefficiency (P ≤ 0.05). Comparing selected temperate forests, the results showed that the broadleaved Korean pine mixed forest soil at less than 20 cm depth was characterized by lower microbial respiration relative to soil total C and lower qCO₂, and with a larger MBC concentration relative to soil total C during both incubations (P ≤ 0.05). This indicated that microbial communities in the soil were more efficient in carbon use than communities in the other soils. With the exception of the 0–2.5 and 2.5–5.0-cm soil layers, there was no difference in qCO₂ values in the broadleaved Korean pine mixed forest and rainforest soils at depths.

Relationship between microbial carbon and metabolic quotients and soil properties

Variations in soil microbial C concentrations were associated with soil properties, such as exchangeable Ca, Mg and Al, and the concentration ratios of water soluble organic C to total N (Fig. 2). The metabolic quotients appeared to increase with increasing C:N ratio and water soluble organic C concentration in the soil (P < 0.0001) (Fig. 3a,b), but decreased with increasing soil exchangeable Al concentration (P = 0.002) (Fig. 3c). The qCO₂ values in forest soils, particularly in temperate volcanic forest soils, decreased with an increasing Al/C ratio of water soluble organic matter (P < 0.01) (Fig. 3d).

DISCUSSION

Two eigenvectors from a principal component analysis reveal that both microbial biomass C and metabolic quotients were mainly associated with soil properties such as C:N ratio, exchangeable Ca, Mg and Al, and water soluble organic C; neither the 7-day nor the 21-day incubation affected the correlation (Table 3). Hence, the 7-day incubation can reflect the change in microbial biomass C and metabolic quotients of the soils (Fig. 1b,c). Considering that there are small variations in the soil properties after a 7-day incubation (data not shown), soil properties such as C:N ratio, exchangeable Ca, Mg and Al, and soluble organic C can, thus, affect soil microbial biomass C and metabolic quotients under temperate and tropical forests.

The broadleaved Koran pine mixed forest soil had a larger microbial C relative to soil total C and a lower

Table 1 Main properties of the temperate and tropical soils at various depths

Soil depths (cm)	Total C	Total N	C:N ratios	WOC	WTN	pH (water)	Exchangeable metals
							Ca	Mg	Al
	(mg g^−1 dry soil)			(µg g^−1 dry soil)			(µg g^−1 dry soil)
Temperate soils under vegetation
Broadleaved Korean pine mixed forest
0–2.5	151.0(24.4)	11.3(1.8)	13.4(0.1)	192(33)	153(24)	5.9(0.1)	4517(964)	579(136)	0(0)
2.5–5	81.9(8.6)	7.3(0.8)	11.3(0.1)	94(14)	56(10)	5.4(0.1)	2571(652)	360(92)	3(2)
5.0–7.5	50.1(5.8)	4.7(0.5)	10.5(0.2)	75(8)	36(5)	5.3(0.1)	2095(381)	301(55)	10(4)
7.5–10	41.2(8.5)	3.8(0.8)	10.7(0.3)	105(11)	37(6)	5.4(0.1)	1556(327)	235(42)	15(6)
10–12.5	20.4(1.8)	2.0(0.1)	10.3(0.5)	83(4)	20(2)	5.4(0.1)	917(80)	161(16)	24(5)
12.5–15	12.2(2.7)	1.3(0.2)	9.2(0.9)	86(33)	19(5)	5.6(0.1)	701(34)	133(11)	26(4)
15–20	7.7(1.2)	1.0(0.1)	7.6(0.8)	62(11)	15(3)	5.6(0.1)	533(22)	106(3)	28(5)
LSD0.05	18.6	1.4	0.9	34	19	0.2	842	119	8
Spruce coniferous forest
0–2.5	203.5(14.9)	12.4(1.0)	16.5(0.3)	431(103)	117(14)	5.7(0.1)	3542(676)	288(38)	0(0)
2.5–5	126.8(18.5)	8.3(1.2)	15.3(0.3)	270(56)	62(12)	5.3(0.1)	1713(146)	163(29)	5(3)
5.0–7.5	75.5(9.9)	5.3(0.9)	14.3(0.6)	187(23)	32(3)	5.1(0.1)	929(41)	102(18)	27(10)
7.5–10	48.7(2.9)	3.5(0.3)	13.8(0.3)	146(13)	21(1)	5.1(0.2)	716(29)	86(14)	36(20)
10–12.5	14.6(3.7)	1.2(0.2)	11.6(0.8)	113(27)	16(5)	5.1(0.1)	320(146)	40(8)	11(3)
12.5–15	7.6(1.4)	0.8(0.1)	8.9(0.9)	85(4)	12(1)	5.0(0.1)	188(24)	29(8)	9(4)
15–20	4.8(0.9)	0.7(0.1)	6.7(0.8)	64(13)	8(1)	4.9(0.3)	166(39)	27(6)	7(3)
LSD0.05	17.4	1.2	1.1	82	13	0.3	471	36	16
Birch broadleaf forest
0–2.5	137.1(18.7)	7.8(0.7)	17.4(0.9)	530(57)	68(9)	5.1(0.1)	1173(204)	146(21)	11(2)
2.5–5	84.5(13.7)	4.8(0.3)	17.3(1.7)	214(40)	24(2)	5.1(0.2)	581(82)	76(12)	31(10)
5.0–7.5	69.6(13.6)	4.1(0.4)	16.9(1.6)	153(18)	17(1)	5.2(0.3)	367(88)	52(4)	32(13)
7.5–10	54.5(5.5)	3.3(0.3)	16.4(1.3)	145(22)	17(0)	5.3(0.1)	385(87)	51(5)	29(12)
10–12.5	46.3(8.1)	2.9(0.3)	15.8(1.4)	125(24)	16(1)	5.3(0.2)	367(142)	46(12)	29(15)
12.5–15	32.9(3.6)	2.5(0.3)	13.4(0.4)	93(9)	13(1)	5.6(0.1)	245(82)	31(6)	12(3)
15–20	34.1(3.2)	2.5(0.2)	13.4(0.7)	89(4)	13(1)	5.6(0.0)	193(54)	29(3)	11(1)
LSD0.05	19.2	0.7	2.1	52	6	0.3	202	19	17
Tropical soils under vegetation
Tea garden
0–2.5	14.7(0.8)	1.2(0.0)	11.9(0.3)	79(4)	16(3)	4.9(0.2)	236(11)	38(8)	30(8)
2.5–5	11.7(0.8)	1.1(0.1)	10.6(0.2)	59(3)	11(2)	4.6(0.2)	105(25)	15(4)	47(5)
5.0–7.5	10.8(0.7)	1.0(0.0)	10.5(0.5)	58(3)	9(1)	4.6(0.2)	85(12)	12(3)	61(14)
7.5–10	10.9(0.9)	1.0(0.0)	10.9(0.9)	61(4)	9(1)	4.6(0.2)	61(2)	8(1)	65(11)
10–12.5	10.0(0.8)	1.0(0.0)	10.0(0.8)	62(7)	7(2)	4.5(0.2)	40(3)	7(1)	76(8)
12.5–15	9.4(1.0)	1.0(0.0)	9.6(0.7)	57(5)	7(1)	4.5(0.1)	42(11)	8(1)	89(12)
15–20	8.4(0.8)	0.9(0.0)	9.3(0.9)	56(4)	6(0)	4.5(0.1)	43(15)	8(2)	93(12)
LSD0.05	1.5	0.0	1.2	8	3	0.3	23	7	19
Seasonal rainforest
0–2.5	36.5(6.3)	3.2(0.5)	11.5(0.3)	75(10)	32(9)	4.4(0.1)	216(62)	50(1)	24(7)
2.5–5	17.3(2.3)	1.6(0.3)	10.8(0.6)	50(8)	26(3)	4.0(0.1)	55(24)	20(7)	41(10)
5.0–7.5	14.1(1.0)	1.3(0.1)	10.9(0.4)	48(3)	20(2)	4.1(0.1)	41(5)	20(6)	53(8)
7.5–10	11.8(0.3)	1.1(0.1)	10.7(0.3)	43(7)	17(1)	4.1(0.1)	26(8)	13(5)	48(8)
10–12.5	10.4(0.6)	0.9(0.1)	11.2(0.6)	24(1)	12(0)	4.1(0.0)	38(2)	12(3)	51(3)
12.5–15	9.8(0.7)	0.9(0.1)	10.6(0.6)	32(6)	12(2)	4.3(0.1)	31(5)	12(5)	54(7)
15–20	8.6(0.2)	0.8(0.1)	10.5(0.8)	35(8)	11(1)	4.2(0.1)	35(1)	11(4)	53(5)
LSD0.05	4.5	0.5	0.9	12	7	0.2	44	8	13
Values are the means of three replicates (standard errors are shown in parentheses). WOC, water soluble organic C; WTN, water soluble total N. Least significant differences (LSD) (5%) were used to assess differences in the soil properties at the various depths.

Table 2 Total C and N concentrations of litter under different forest stands

Forest types	Total C	Total N	C:N ratios
	(mg g^−1 dry weight)
Broadleaved Korean pine mixed forest	432.8(1.3)	12.6(0.1)	34.4(0.1)
Spruce forest	458.2(4.4)	12.8(0.6)	36.2(1.0)
Birch forest	450.2(7.1)	16.5(0.8)	27.5(0.8)
Tea garden	447.7(2.7)	17.6(0.3)	25.5(0.2)
Rainforest	456.8(4.4)	13.4(0.8)	34.5(1.0)
LSD0.05	6.7	0.9	2.2
Values are the means of four replicates (standard errors are shown in parentheses). Least significant differences (LSD) (5%) were used to assess differences in litter properties under the forest stands.

Figure 1  Effects of tree species and soil depths on (a) microbial respiration, (b) microbial biomass C and (c) metabolic quotient in temperate and tropical soils during the 7-day (▪) and 21-day incubations (□). Error bars are the standard error of three replicates.

Figure 2  Relationship between soil microbial biomass C concentrations in the 7-day incubation to the concentrations of exchangeable (a) Ca, (b) Mg and (d) Al, and to (c) the ratios of water soluble organic C to total N concentration in temperate and tropical soils at various depths. Error bars are the standard error of three replicates. The linear or non-linear regression indicates the microbial C, y, against (the concentrations of soil exchangeable Ca, Mg and Al, or the ratios of soil water soluble organic C to total N concentration), x.

Figure 3  Relationships between microbial metabolic quotients in the 7-day incubation to (a) the total C:N ratios, (b) the concentrations of water soluble organic C and (c) exchangeable Al, and to (d) the ratios of exchangeable Al to water soluble organic C concentration in temperate and tropical soils at various depths. Error bars are the standard error of three replicates. The linear or non-linear regression indicates the metabolic quotients, y, against (the total C:N ratios, the concentrations of water soluble organic C and exchangeable Al, or the ratios of exchangeable Al to water soluble organic C concentration in the soil), x.

qCO₂ than birch and spruce forest soils (Fig. 1b,c). This was attributable to the larger soil substrate availability and lower C:N ratio under the mixed forest than under birch and spruce (Table 1). Vance and Chapin (2001) showed that a lower microbial C : total C ratio implies that there is lower substrate availability in forest soils corresponding with their higher C:N ratio. Litter diversity in a forest ecosystem increases soil microbial biomass and the activity of the associated enzymes, and a mixture of leaf litter can improve the quality of forest soils (Hu et al. 2006). Anderson and Domsch (1993) reported that the qCO₂ value decreased in a mixed (Fagus–Quercus) forest ecosystem compared with a single plantation (Fagus, Picea) forest ecosystem. A substantial proportion of tree species in the rainforest is deciduous under the monsoon climate, although they do not shed leaves during the same seasons (Cao et al. 1996). As a mixed forest ecosystem, the rainforest and broadleaved Korean pine forest are more than 200 years old, and both forests have entered an advanced succession stage in respective districts. Thus, this advanced succession stage can induce lower qCO₂ values in the soils at depths (Fig. 1c). This was in good agreement with the results reported by Insam and Haselwanter (1989),

Table 3 Two eigenvectors from a principal component analysis of the standardized data of different soil chemical and biological properties under temperate and tropical forests

	Centre
	1	2
Total C content	0.962	−0.054
Total N content	0.944	−0.190
Ratio of total C:N content	0.685	0.313
Soil pH (soil : water, 1:2.5)	0.596	0.192
Water soluble organic C content	0.913	0.256
Water soluble N content	0.865	−0.388
Water soluble organic C:N ratio	−0.076	0.890
Exchangeable Mg content	0.697	−0.538
Exchangeable Ca content	0.800	−0.436
Exchangeable Al content	−0.651	0.047
Metabolic quotient after 7 days	0.829	0.479
Soil respiration rate after 7 days	0.866	0.089
Microbial C content after 7 days	0.110	−0.750
Metabolic quotient after 21 days	0.795	0.496
Soil respiration rate after 21 days	0.839	0.129
Microbial C content after 21 days	0.084	−0.823

who showed that soil qCO₂ decreases with plant succession. The rainforest conversion to tea garden increased CO₂ evolution rates and qCO₂ values of the 0–20 cm soil because of changes in the properties of the litter and soil (Tables 1,2). Stepwise multiple regression analysis showed that the variations in qCO₂ in temperate and tropical forest soils can be explained by soil properties such as C:N ratio, soluble organic C : total N ratio, and the concentration of water soluble organic C and exchangeable Al. The multiple linear regressions were expressed as follows:

where y is the microbial metabolic quotient (qCO₂, mg CO₂-C g⁻¹ biomass-C day⁻¹) and a, b and c represent C:N ratio, the concentration of water soluble organic C and exchangeable Al (µg g⁻¹ dry soil), respectively, in the soil.

where y is the microbial metabolic quotient (qCO₂, mg CO₂-C g⁻¹ biomass-C day⁻¹) and a, b and c represent C:N ratio, water soluble organic C : total N ratio and exchangeable Al concentration (µg g⁻¹ dry soil), respectively, in the soil.

The regression models were both significant and explained 85% and 65% of the variability of qCO₂ in the soils, respectively. It is clear that soil C:N ratio had a larger contribution to the qCO₂ than soil water soluble organic C and exchangeable Al (Eqs 1,2). It is noteworthy to mention that under experimental conditions, qCO₂ values of forest soils appeared to increase with increasing soil C:N ratio (Fig. 3a) and with increasing water soluble organic C (Fig. 3b). This is to say that great microbial inefficiency in carbon use can occur in forest soils associated with large soil C:N ratios and with large soluble organic C concentrations (Eqs 1,2). This was in good agreement with our previous results using Japanese temperate volcanic forest soils (Xu et al. 2006). A lower concentration of water soluble organic C and lower C:N ratio through the soil profile under the broadleaved Koran pine mixed forest than under birch and spruce (Table 1) can, thus, explain a lower qCO₂ value in the soil (Fig. 1c).

There was a significantly positive relationship between qCO₂ and soil pH (r = 0.47, P < 0.01), and between qCO₂ and exchangeable Ca concentration (r = 0.38, P ≤ 0.05), in the soils at various depths. This relationship did not change when temperate and tropical forest soils were separated (data not shown). This mainly resulted from a large qCO₂, soil pH and exchangeable Ca concentration in the 0–2.5-cm soil under each forest stand (Table 1, Fig. 1c). However, in the soil profiles under temperate and tropical forests, qCO₂ was negatively correlated with exchangeable Al concentration (r = −0.51, P = 0.002). Hence, high specific microbial respiration may be indicative of acid stress rather than nutrient stress in the soils. This will, however, be difficult to distinguish under field conditions because acid soils usually have a poorer nutrient status than soils with higher pH.

Microbial metabolic quotient is considered to be an index for the evaluation of the efficiency of soil microbial communities for substrate utilization (Insam 1990). The more efficiently the microorganisms function, the greater the fraction of substrate C that is incorporated into biomass and the lower the C per unit biomass that is lost through respiration. This can result in a low metabolic quotient. Hence, the relatively lower qCO₂ under the mixed temperate forest and the rainforest compared with the simple plantation (Picea, Betula) forest (P ≤ 0.05) (Fig. 1c) can reflect an increase in the efficiency of substance utilization by the soil microbial community. The large microbial C utilization in the mixed temperate forest soil was in accordance with the results reported by Anderson and Domsch (1993), who observed that qCO₂ decreased in a mixed forest ecosystem compared with a simple plantation forest ecosystem. Litter under tropical and temperate forests contained different total C and N concentrations, and different C:N ratios (Table 3). The quality and decomposition of litter can affect metal ions, soluble organic C and C:N ratios in surface soils (Table 1), thus affecting the qCO₂.

There is general consensus that the interaction of soil organic matter with metal ions such as Al³⁺ and Fe³⁺ is the main reason for the stability of soil organic matter in soils against microbial biodegradation (Lundström et al. 2000; Nierop et al. 2002; Zysset and Berggren 2001). Microbial C utilization in the soils can be influenced not only by Al concentration, but also by its combination with soluble organic C. Figure 3d shows the relationship between qCO₂ and the concentration ratios of soil exchangeable Al to water soluble organic C. The qCO₂ values in forest soils, particularly in temperate volcanic forest soils, decreased with an increasing Al/C ratio of water soluble organic matter. The Al/C ratio of dissolved organic matter can be considered to be an important parameter for its stability against microbial decomposition (Boudot et al. 1989; Schwesig et al. 2003). In long-term incubation studies, Schwesig et al. (2003) showed that for natural dissolved organic matter, Al/C ratios > 0.1 increased the half-life of the stable dissolved organic matter fraction up to fourfold. Hence, Al concentration in combination with dissolved organic C can affect soil microbial C utilization and the stability of dissolved organic matter under temperate and tropical forests.

The parameter soil microbial C : organic C ratios might provide additional information on the differences in carbon availability because this parameter is less affected by storage conditions. The ratios respond readily to disturbance effects and can provide an effective warning on the deterioration of soil quality (Bauhus et al. 1998; Insam and Domsch 1988; Pinzari et al. 1998; Priha and Smolander 1997; Valsecchi et al. 1995). Both the 7-day and 21-day incubations could induce a significant difference in MBC concentration and qCO₂ in tea soil and spruce forest soil (Fig. 1b,c). Hence, the same incubation conditions are needed to examine differences in soil carbon availability and microbial utilization status under different climatic conditions and forests. Microbial biomass C concentrations under temperate and tropical forests appeared to increase with increasing exchangeable Ca and Mg concentration in soil, and at high concentrations, the increase in soil C availability slowed under temperate forests (Fig. 2a,b). This indicates that the addition of Ca and Mg (e.g. gypsum) can improve microbial C availability in soils, particularly in tropical forest soils. The comparatively large microbial C relative to soil total C under the broadleaved Koran pine mixed forest indicated good microbial growth in the soil (Fig. 1b), which was, in part, attributed to larger concentrations of exchangeable Ca and Mg under the mixed forest than under the birch and spruce forests (P ≤ 0.05) (Table 1).

Stepwise multiple regression analysis indicated that variations in microbial biomass C concentrations in temperate and tropical forest soils can be explained by soil properties such as the water soluble organic C : total N ratio, and the concentration of exchangeable Mg and Al. The multiple linear regressions were expressed as follows:

where y is microbial biomass C (µg g⁻¹ C) and a, b and c represent the water soluble organic C : total N ratio and the concentration of exchangeable Mg and Al (µg g⁻¹ dry soil), respectively, in the soil.

The regression model was significant and explained 49% of the variability in microbial biomass C in the soils. It is clear that the soil water soluble organic C : total N ratio had a larger contribution to the microbial C concentration than the concentration of soil exchangeable Mg and Al (Eqn 3). As shown in Fig. 2d, there was a significantly negative relationship between microbial biomass C concentration and exchangeable Al concentration in tropical forest soils (r = −0.62, P = 0.002). However, no obvious relationship between either variable was observed when temperate and tropical soils were combined (Fig. 2d). This is different from the relationship indicated by Eq. 3. Although there was a negative relationship between exchangeable Al concentration and soil respiration (r = −0.38, P ≤ 0.05), the Al toxic effects on soil microorganisms varied with the experimental conditions and with soil properties such as soluble carbon and pH. Many studies have shown that soil microbial communities and growth and root biomass can affect soil respiration and carbon availability (e.g. Li et al. 2000). Further understanding of the interactions between soil microbial activities, microbial biomass and soil properties is necessary to improve the prediction of C cycling processes in soil. Neill and Gignoux (2006) have showed that microbial growth should be considered when modeling carbon processes in soil. Considering that the Al effects on soil carbon availability and microbial biomass are complicated, it is vital that we further examine the effects of Al ions on carbon availability and microbial utilization in temperate volcanic and tropical forest soils.

CONCLUSIONS

Comparing selected temperate and tropical forests, our results showed that broadleaved Korean pine mixed forest and rainforest soils at less than 20 cm depth had a larger MBC concentration relative to soil total C during short-term incubations, and a lower qCO₂, suggesting that microbial communities in both forest soils were more efficient in carbon use than the communities in the other soils. The conversion of a tropical rainforest to a tea garden reduced CO₂ evolution and increased the qCO₂ in the soil. The results indicated that the variations of qCO₂ and microbial C in temperate and tropical forest soils were mainly attributable to soil properties such as C:N ratio, soluble organic C, exchangeable Ca, Mg and Al ions.

ACKNOWLEDGMENTS

This work was funded by the National Natural Sciences Foundation of China (Grant No. 20477044) and by the Hundred Talents Project from the Chinese Academy of Sciences. The authors thank Dr Sha L.Q. from the Xishuangbanna Tropical Botanical Garden and Professor Han S.J. from the Research Station of Changbai Mountain Forest Ecology for their assistance and support with soil sampling.