Soil microorganisms play important roles in soil carbon (C) and nitrogen (N) cycling and mineralization, and they are highly sensitive to changes in soil conditions (
Journal of Resources and Ecology, Volume. 11, Issue 5, 454(2020)
Effects of Forest Types and Environmental Factors on Soil Microbial Biomass in a Coastal Sand Dune of Subtropical China
Coastal sand dune ecosystems generally have infertile soil with low water-holding capacity and high salinity. However, many plant species have adapted to the harsh sand environment along the southeastern coast of China. Studying the microbial biomass in such an ecosystem can improve our understanding of the roles that microbes play in soil fertility and nutrient cycling. We investigated the differences in soil microbial biomass carbon (MBC) and nitrogen (MBN) contents and their seasonal dynamics in five forest types (a secondary forest and plantations of Casuarinas, Pine, Acacia, and Eucalyptus). The results indicated that the seasonal variations of soil MBC and MBN contents in all five forest stands were higher in spring and winter, but lower in summer and autumn. The MBC content was lower in the Casuarinas plantation than in the other plantations in the same soil layer. However, no significant differences were observed in MBN contents among the different forest types. The MBC and MBN concentrations were positively correlated with soil moisture, but negatively correlated with soil temperature. The MBC and MBN contents also decreased with increasing soil depth. Across all soil layers, secondary forest had the highest MBC and MBN concentrations. Our study also showed that the MBC and MBN contents were positively affected by total soil carbon (TC), pH, and litter N content, but were negatively impacted by soil bulk density and litter C content. Moreover, the MBN content was positively correlated with root N content. In summary, environmental factors and the differences in litter and fine roots, soil nutrient contents, as well as the soil physical and chemical properties caused by different tree species collectively affected the concentrations of the soil MBC and MBN.
1 Introduction
Soil microorganisms play important roles in soil carbon (C) and nitrogen (N) cycling and mineralization, and they are highly sensitive to changes in soil conditions (
The metabolic processes of soil microorganisms are affected by the comprehensive characteristics of plant community, soil temperature, soil moisture, soil pH, soil foundation fertility, and substrate (
Sand dunes are common on the southeastern coast of China, where the infertile soil has low water-holding capacity and high salinity. Casuarina equisetifolia L. is an actinorhizal N-fixing species with good wind and salt resistance, and it is often used for the establishment of protection forests in coastal sandy areas. C. equisetifolia plantations in China cover approximately 3×105 ha and play an important role in local environment improvement and ecological security maintenance efforts (
In coastal sand dunes, the knowledge of environmental controls and variations in their soil microbial biomass across forest types in coastal sandy areas will help to guide afforestation species selection and ecosystem management. Therefore, it is important to understand how the differences in species composition and the seasonal fluctuations of soil temperature and moisture can affect the dynamics of soil microbial biomass, and what roles soil microbial biomass play in soil C and N cycling. The goals of this study were to: 1) Investigate the soil MBC and MBN contents and their seasonal dynamics across five forest types (a secondary forest of L. glutinosa and plantations of Casuarinas, Pine, Acacia, and Eucalyptus) in the southeastern coast of Fujian province; 2) Identify factors that correlate with soil MBC and MBN contents, including litterfall, litter C and N contents, fine root biomass, and root C and N contents; and 3) Measure the physical and chemical properties of the soils, including soil temperature and soil moisture; and evaluate their effects on the soil C and N cycling across the different forest types.
2 Materials and methods
2.1 Study site
The study was conducted at Chishan Forestry Center of Dongshan County (23°38°N, 117°24°E) in southeastern Fujian Province, China (
Figure 1.Fig. 1
Figure 2.Fig. 2
Five forest stands, including a secondary forest and plantations of Casuarina, Pine, Acacia, and Eucalyptus, were selected as experimental groups in December 2014. These stands have similar elevations and soil types, each with an average slope of less than 10°. The detailed site characteristics and topsoil properties of the five forest stands are shown in
Detailed site characteristics and topsoil properties of the five forest stands on the southeastern coast of China
Detailed site characteristics and topsoil properties of the five forest stands on the southeastern coast of China
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The secondary forest is composed of native vegetation resulting from secondary succession with distinct tree, shrub, and herbaceous layers, which regenerates naturally and has been afforded long term protection by local people. The age of the secondary forest is more than 50 years, and the dominant species in the canopy are L. glutinosa and Celtis sinensis Willd. The shrub layer is predominantly Glochidion eriocarpum, Litsea rotundifolia, Bridelia tomentosa, and Anodendron affine (Hook. et Arn.) Druce, and the main herb species are Ophiopogon bodinieri and Arthraxon hispidus var. hispidus. Both Casuarinas and Acacia plantations were established in 1992 with main tree species of C. equisetifolia and Acacia crassicarpa, respectively. The pine plantation was planted with Pinus elliottii Engelm. and slash pine in 1993. The Eucalyptus plantation was established in 2004 with Eucalyptus urophylla × E.grandis crosses; the previous species on this site was P. elliottii, which was planted in 1976. The canopy densities of the different plantations are greater than 0.7 with few shrubs and grasses in the understory (
2.2 Experimental design and measurement
In each of the five forest stands, four 20 m × 20 m plots were established with a 10 m buffer between each plot. In July 2015, ten soil cores (1.0 m length × 5 cm diameter) were collected along the diagonal of each plot from the soil surface to a 100-cm depth. Fine roots (< 2 mm diameter) were washed out and the remaining fine-root biomass was estimated following
Five litter traps (1 m × 1 m) with 1 mm nylon mesh were randomly arranged 0.3 m above the forest floor in each plot for litter collection. From March 2015 to February 2016, litter was collected monthly for determining annual litterfall.
Ten soil pits along the diagonal of each plot were used to collect soil at both 0-10 cm (topsoil) and 10-20 cm (subsoil) depths in April (spring), July (summer), September (autumn), and November (winter) 2015. A 500 g sample was collected from each layer of every plot, and 300 g portions of each were air-dried, ground, and sieved through a 2-mm sieve for microbial biomass analysis. The remaining soil was ground and sieved through a 0.149-mm sieve for total soil carbon (TC) and total N (TN) analysis. Soil bulk density in each layer was calculated using intact soil cores (100 cm3). Soil temperature was obtained using an instantaneous digital thermometer (AM-11T, Avalon company, USA), and soil moisture was determined using an oven-drying method.
Soil microbial biomass C (MBC) and N (MBN) were analyzed following the chloroform fumigation-extraction method (
2.3 Data analysis
Data from each of the four plots were averaged for further analysis. To test the significances of differences in soil C and N contents, soil pH, soil bulk density, as well as the soil MBC and MBN contents between different forest types, we used one-way analysis of variance (ANOVA) with Duncan’s multiple comparison method at a significance level of 0.05. We also used Pearson correlation analysis to determine how annual litterfall, litter C and N content, fine root biomass, root C and N content, soil temperature, and soil moisture correlated with the soil MBC and MBN contents. All statistical analyses were performed using the PASW Statistics 18.0 for Windows.
3 Results
3.1 Environmental variables
The soil pH, soil C and N content, and soil C: N ratio under different forests all decreased with the increase of soil depth (
Soil pH, soil bulk density, soil C and N content, and soil C: N ratio for the different forests (N=4)
Soil pH, soil bulk density, soil C and N content, and soil C: N ratio for the different forests (N=4)
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In the subsoil, the soil pH and soil C and N concentrations in the secondary forest were all higher than those of the plantations, whereas the soil bulk density was lower than those of the plantations. No significant differences were found in soil pH, soil C content, or soil bulk density among the different plantations. In terms of soil N content, there was no significant difference among Eucalyptus, Pine, and Acacia plantations, which were each significantly higher than that of Casuarinas, and no significant difference was observed in the soil C: N ratio among the different forests.
The temperatures of topsoil and subsoil were both higher in summer and autumn and lower in spring and winter, which is consistent with the general rule (
Figure 3.Fig. 3
3.2 Seasonal variations of MBC and MBN
No significant difference was found in the seasonal variations of MBC content in the topsoil of Pine and Casuarinas plantations or in the subsoil of Acacia plantation, except that the MBC contents under different forests varied greatly among the different seasons (
The MBN content of different forests showed obvious seasonal variations, except for Eucalyptus (
Figure 4.Fig. 4
3.3 MBC and MBN under different forests
The annual mean MBC and MBN contents under different forests decreased with the increase of soil depth (
Figure 5.Fig. 5
The soil MBC: MBN ratio in the topsoil of Eucalyptus was significantly higher than that of the subsoil. However, no significant difference was observed between topsoil and subsoil in the other forests. In the topsoil, the soil MBC: MBN ratio of Eucalyptus was significantly higher than those of the other forests, and no significant difference was observed among other forests. In the subsoil, the corresponding ratio was highest in Pine, followed by Eucalyptus and Acacia plantations, and lowest in secondary forest and Casuarinas (
Figure 6.Fig. 6
The percentages of soil MBC and MBN within TC and TN, respectively, in topsoil were all lower than those in subsoil under each of the different forests. The percentages of soil MBC within TC under Eucalyptus, Pine, Casuarinas, and secondary forest differed significantly between topsoil and subsoil. The percentages of soil MBN within TN under Eucalyptus, Acacia and secondary forest differed significantly between topsoil and subsoil. Specifically, the percentage of soil MBC within TC in the subsoil of Eucalyptus was significantly higher than that of Casuarinas. In addition, no significant differences were found among different forests either in topsoil and subsoil. The percentage of soil MBN within TN in the topsoil of Casuarinas was significantly higher than those of Eucalyptus, Acacia and Pine, and the corresponding value in secondary forest was also higher than that of Pine. However, no significant differences were found among different forests either in topsoil and subsoil (
Figure 7.Fig. 7
3.4 Relationship between soil MBC and MBN contents and environmental factors
The effects of soil temperature and water content on the seasonal changes of soil MBC and MBN vary with forest types. However, the seasonal variations of soil MBC and MBN under different forests were all negatively correlated with soil temperature and positively correlated with soil water content (
Pearson correlations between seasonal variations of soil MBC and MBN and soil temperature as well as water content
Pearson correlations between seasonal variations of soil MBC and MBN and soil temperature as well as water content
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The MBC and MBN contents under different forests were significantly and positively correlated with TC, pH and litter N, and negatively correlated with soil bulk density and litter C. In addition, the MBN content was also significantly and positively correlated with root N both in topsoil and subsoil, and the MBC content in the subsoil was significantly and negatively correlated with root C (
Pearson correlations between soil MBC and MBN and properties of soil, litter and fine roots
Pearson correlations between soil MBC and MBN and properties of soil, litter and fine roots
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4 Discussion
4.1 Soil C and N contents under different forests
The qualities and quantities of soil C and soil N can be affected by forest types due to the variations in tree species composition, litter quantity and quality, root turnover rate, as well as root exudates in different forests (
4.2 MBC and MBN contents, and MBC/MBN ratios under different forests
The soil microbial biomass varies greatly among different forests, which is the combined result of tree species and environmental factors (
The mean MBC and MBN contents decreased with the increase of soil depth under the different forests, which is consistent with previous findings (
The MBC: MBN ratio reflects the relative ratio between fungi and bacteria in the soil (
The percentages of soil MBC and MBN within TC and TN in topsoil, respectively, were lower than those in subsoil under the same forests, which indicate that the carbon and nitrogen accumulations in the subsoil are higher than in the topsoil, and the soil organic matter is transferred from topsoil to subsoil (
4.3 Seasonal variation of MBC and MBN and its influencing factors
In this study, the MBC and MBN contents of different forests showed similar seasonal variations, which were single peak curves with higher values in spring or winter and lower values in summer or autumn. This agrees with the results from the coastal sandy dunes of Taiwan (
The soil pH is considered to be the main factor driving the spatial distribution of a soil microbial community, and pH is closely related to the microbial activity; when the pH value is lower than 7, the soil microbial activity increases gradually with an increase of soil pH (
5 Conclusions
The differences in soil microbial biomass C and N contents and their seasonal dynamics under five forest types (a secondary forest of L. glutinosa, and Casuarinas, Pine, Acacia, and Eucalyptus plantations) in the southeastern coast of Fujian province were investigated. The seasonal variations showed MBC and MBN contents which were all higher in spring and winter but lower in summer and autumn, and the seasonal variations of MBC and MBN were negatively correlated with soil temperature and positively correlated with soil water content, which indicated that the amount of microbial biomass in the sand dune forest ecosystem was largely controlled by soil moisture and soil temperature.
The mean MBC and MBN contents decreased with an increase of soil depth across the five forest types. Specifically, the MBC and MBN contents under secondary forest were significantly higher than those of plantations among the different soil layers; the MBC content under Casuarinas was the lowest in both topsoil and subsoil. These results imply that the long-term management of Casuarinas plantations led to soil decline, and the introduction of native tree species can modify soil parameters and consequently influence the belowground microbial community.
The MBC and MBN contents were significantly positively correlated with TC, pH, and litter N, and negatively correlated with soil bulk density and litter C. In addition, the MBN content was also significantly positively correlated with root N in both topsoil and subsoil, and the MBC content in the subsoil was significantly negatively correlated with root C. We found that several factors—environmental factors and the differences in litter and fine roots, soil nutrient contents, as well as the soil physical and chemical properties caused by different tree species—acted together to affect the concentrations of the soil MBC and MBN.
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Wei GAO, Meimei LIN, Yongrong HUANG, Shide HUANG, Gongfu YE, Zhiqun HUANG. Effects of Forest Types and Environmental Factors on Soil Microbial Biomass in a Coastal Sand Dune of Subtropical China[J]. Journal of Resources and Ecology, 2020, 11(5): 454
Category: Forest Ecosystem
Received: Mar. 15, 2020
Accepted: May. 21, 2020
Published Online: Oct. 17, 2020
The Author Email: HUANG Zhiqun (zhiqunhuang@hotmail.com)