Tree species composition and nutrient availability affect soil microbial diversity and composition across forest types in subtropical China
Introduction
Soil microorganisms comprise a major proportion of terrestrial biodiversity and are essential to many ecological processes, including primary production, nutrient cycling, and the decomposition of organic matter (Bardgett and van der Putten, 2014, Chen et al., 2019). Global forests, particularly those in China, continue to change and suffer the loss of tree species diversity due to the conversion of natural forests to fast-growing tree plantations to meet increasing anthropogenic demands for timber and other wood products (Chen et al., 2020, Newbold et al., 2015). Despite the importance of soil microbial communities for ecosystem functionality (Chen et al., 2019, Li et al., 2019), the effects of forest type conversion on soil microbial diversity and composition remain poorly understood.
Under similar climates and localized site conditions, the conversion of forest types may affect soil microbial communities through changes in tree species composition and diversity. Tree species influence soil microbial communities through their litter, mycorrhizal fungal associates, and exudates (Prescott and Grayston, 2013). The identities of tree species have been observed to influence the diversity of soil bacteria and fungi (Liu et al., 2018, Xiao et al., 2019) and community composition (Guo et al., 2016). Meanwhile, microbial diversity is expected to increase with tree species diversity, due to the strong links between the identities of tree species and soil microorganisms (Wardle et al., 2004), and the dominant effects of trees in forest ecosystems (Grime, 1998). In species-rich forests, enhanced aboveground and belowground litter productivity (Ma and Chen, 2018, Zheng et al., 2019) and litter diversity can increase the quantity and variety of available food resources, as well as expand niches for soil microorganisms (Hooper et al., 2000, Lange et al., 2015, Xiao et al., 2020). However, microbial alpha diversity is not always positively associated with tree species richness (Cai et al., 2018, Rodrigues et al., 2013, Xu et al., 2015). Moreover, fungi may benefit more from tree species richness than bacteria (Rousk et al., 2010).
Changes in the soil environment induced by forest type conversion can also impact microbial diversity and composition (Meng et al., 2020). The soil environment is a strong determinant for microbial diversity and composition (Nakayama et al., 2019, Prescott and Grayston, 2013). For example, soil microorganisms can benefit from mineral fertilization (Geisseler and Scow, 2014), whereas the excessive addition of nitrogen increases soil acidity and adversely affects soil microbial communities (Zhang et al., 2018). Microbial diversity and composition may vary both within individual soil profiles and across a wide range of ecosystem types (Eilers et al., 2012). Furthermore, soil residing bacteria and fungi may respond differently to the soil environment. For instance, bacteria are more sensitive to soil C:N ratios than are fungi (Guo et al., 2019). Since soil nutrients are limited in terrestrial ecosystems (Yuan and Chen, 2012) for which plants and soil microorganisms compete (Kuzyakov and Xu, 2013), we anticipated that the addition of soil nutrients would promote soil microbial diversity and influence the composition of soil microbes.
We examined soil microbial diversity and composition across forest types and soil depths in subtropical China. Specifically, we address two overarching questions: (i) How are the diversity and composition of bacteria and fungi altered across a wide range of forest types? (ii) How are changes in the diversity and composition of soil bacteria and fungi correlated with those of vegetation and the soil environment induced by forest type conversion?
Section snippets
Study area
This study was conducted in the TianBaoYan Nature Reserve (25°50′51″ to 26°01′20″ N and 117°28′03″ to 117°35′28″E), which is a transition zone that lies between the middle and lower reaches of the Daiyun Mountains, located in Yongan County, Fujian Province, China. The study area is characterized by a subtropical marine monsoon climate, with a mean annual temperature of 15 °C, annual precipitation of 2039 mm, and an annual mean relative humidity of 80%. The study plots ranged in elevation from
Results
Tree species richness was highest in the broadleaf dominated forests, followed by native broadleaf and bamboo dominated forests, and lowest in the pure bamboo forests (Table 1). Both the mean tree height and stand basal area were the highest in broadleaf dominated and bamboo dominated forests, followed by the native broadleaf forests, with the lowest in the pure bamboo forests. The mean tree D.B.H. and stand density did not differ significantly between forest types (P > 0.05, Table 1). The soil
Discussion
Our study revealed evidence for the impacts of forest type conversions on soil bacterial and fungal diversity, and the composition in subtropical forests. We found that the soil fungi:bacteria ratio, as well as the bacterial and fungal diversity of all three sampled soil depths increased following the conversion of native broadleaf forests to managed mixed and pure bamboo forests. Both bacterial and fungal community compositions were significantly altered following forest type conversion. We
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Tiangui Yang, Yegui Wu, Jialiang Chen, and Chun Feng for assisting with fieldwork and laboratory analysis, and Neil Hillis, Eric Searle, Chen Chen and two anonymous reviewers for their constructive comments. Funding from the Special Fund for Basic Scientific Research of International Centre for Bamboo and Rattan (2018YFD0600103) and the National Natural Science Foundation of China [31971548] supported this research.
Data availability:
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