Rare Taxa Drive the Response of Soil Fungal Guilds to Soil Salinization in the Taklamakan Desert

Abstract

Salinization poses great threats to soil fungal communities that would cause the losses of ecosystems services. Soil fungal communities are composed of different functional guilds such as saprotrophic, symbiotrophic, and pathotrophic fungi, and each guild includes many rare taxa and a few abundant taxa. Despite of low abundance, rare taxa may be crucial in determining the responses of entire soil fungal communities to salinization. However, it remains poorly understood how rare taxa mediate the impacts of soil salinization on soil fungal community structure. Here, we took advantage of a salinity gradient in a desert ecosystem ranging from 0.60 to 31.09 g kg^-1 that was created by a 12-year saline-water irrigation and assessed how the rare vs. abundant taxa of soil saprotrophic, symbiotrophic, and pathotrophic fungi respond to soil salinization through changes in the community biodiversity and composition. We found that the rare taxa of soil saprotrophic, symbiotrophic, and pathographic fungi were more sensitive to changes in soil salinity compared to the abundant taxa. In addition, the community composition of rare taxa of the saprotrophic and pathotrophic fungi not the symbiotrophic fungi was positively associated with soil salinity change. However, the symbiotrophic fungi showed greater variations in the species richness along the salinity gradient. These findings highlight the importance to differentiate rare taxa in predicting how the biodiversity and functional groups of soil fungal communities respond to soil salinization.

Keywords: biodiversity; desert; functional guilds; rare biosphere; saprotrophic fungi; soil salinization; symbiotrophic fungi.

Figure 1

The conceptual framework outlines changes in community structure of three functional guilds of soil fungi along a soil salinity gradient. (A) Alpha diversity. We expect that the alpha-diversity of symbiotrophic fungi would increase with soil salinity because symbiotrophic fungi depend highly on host plant at high-salinity sites (Ren et al., 2016), while that of soil saprotrophic and pathotrophic fungi would decrease with soil salinity because high salinity can hinder the activity of soil fungi that mainly use soil organic matter (Rath et al., 2016). (B) Community composition. Since soil symbiotrophic, sapraotrophic, and pathotrophic fungi have different niches (Frac et al., 2018), we therefore expect that symbiotrophic fungi occur at higher salinity levels, then do pathotrophic and sapraotrophic fungi.

Figure 2

Relationships between the relative abundance of functional guilds of soil fungi and soil salinity and properties. (A) Lines are derived from the linear mixed effects models, and solid and dashed lines denote significant and insignificant regression coefficients, respectively. (B) Points denote regression coefficients and error bars denote 95% confidence intervals. ^*p < 0.05. Abbreviations: Salinity, soil salinity; STN, soil total nitrogen; Clay, soil clay content; STP, soil total phosphorus; avaP, soil available phosphorus.

Figure 3

Relationships between the community structure of functional guilds of soil fungi and soil salinity. (A) Alpha diversity; lines are derived from the linear mixed effects models and solid and dashed lines denote significant and insignificant regression coefficients, respectively. (B) Community composition; solid lines denote salinity gradients and dashed lines denote 0.95 confidence ellipses.

Figure 4

Impacts of soil salinity and properties on the alpha diversity of functional guilds of soil fungi. (A) Saprotrophic fungi; (B) pathotrophic fungi; (C) symbiotrophic fungi. Points denote regression coefficients and error bars denote 95% confidence intervals. ^*p < 0.05; ^**p < 0.01. Abbreviations: Salinity, soil salinity; STN, soil total nitrogen; Clay, soil clay content; STP, soil total phosphorus; avaP, soil available phosphorus.

Figure 5

Impacts of soil salinity, geographic distance, and soil properties on the community composition of functional guilds of soil fungi. (A) Saprotrophic fungi; (B) pathotrophic fungi; (C) symbiotrophic fungi. Points denote partial Mantel coefficients and error bars denote 95% confidence intervals. ^*p < 0.05; ^**p < 0.01. Abbreviations: GD, geographic distance; Salinity, soil salinity; STN, soil total nitrogen; Clay, soil clay content; STP, soil total phosphorus; avaP, soil available phosphorus; Depth, soil depth.