Shifts of understory vegetation induced by thinning drive the expansion of soil rare fungi

Effects of thinning on the structure of soil microbial communities in a subtropical secondary evergreen broad-leaved forest

Introduction

Thinning is a common practice to enhance tree growth, but its effect on rhizosphere soil microorganisms in subtropical secondary evergreen broadleaved forests remains unclear.

Methods

This study used 16S rDNA amplicon sequencing to explore soil microflora of five shrubs and five tree species.

Results

The results showed that thinning altered nutrient distribution and pH in rhizosphere soil, impacting microbial richness, which varied by tree species. The dominant bacterial phyla were Acidobacteria, Proteobacteria, Actinobacteria, and Firmicutes. Although the dominant microbial species remained largely unchanged, thinning increased the relative abundance of Firmicutes. Thinning intensity between 10-15% significantly altered the structure of soil microbial communities, demonstrating species-specific responses.

Discussion

These changes in microbial structure may influence tree growth. This study proposed the potential effects of thinning on rhizosphere soil microorganisms and suggests future research to investigate the specific microbial mechanisms affected by thinning.

Soil Fungal Community Diversity, Co-Occurrence Networks, and Assembly Processes under Diverse Forest Ecosystems

Fungal communities are critical players in the biogeochemical soil processes of forest ecosystems. However, the factors driving their diversity and community assembly are still unclear. In the present study, five typical vegetation types of soil fungal communities in Liziping Nature Reserve, China, were investigated using fungal ITS sequences. The results show that the topsoil fungal community is mainly dominated by the phyla Ascomycota, Basidiomycota, and Mortierellomycota. Although there was no significant difference in α diversity (Shannon, Simpson, and Pielou evenness indices) among different forest types, there was a significant difference in β diversity (community composition). This study found that soil pH, soil organic carbon, total nitrogen (TN), total phosphorus (TP), and the total nitrogen/total phosphorus (N/P) ratio are the main environmental factors that affect soil fungal communities. Each forest type has a specific co-occurrence network, indicating that these community structures have significant specificities and complexities. Deciduous evergreen broad-leaved forests as well as deciduous broad-leaved and evergreen broad-leaved mixed forests showed high modularity and average path lengths, indicating their highly modular nature without distinct small-scale characteristics. Furthermore, our findings indicate that the structures of topsoil fungal communities are mainly shaped by stochastic processes, with the diffusion limitation mechanism playing a particularly significant role.

Effects of stand density on the structure of soil microbial functional groups in Robinia pseudoacacia plantations in the hilly and gully region of the Loess Plateau, China

Elucidating the responses of soil microbial functional groups to changes in stand density is crucial for understanding the sustainability of forest development. In this study, we obtained soil samples from Robinia pseudoacacia plantations of three different stand densities (low, middle, and high densities of 750, 1125, and 1550 trees ha-1, respectively) in the hilly and gully region of the Loess Plateau, China. We sought to determine the effects of stand density on the structure of soil microbial functional groups. Stand density had no significant effects on species diversity indices of fungal trophic modes or bacterial functional groups involved in carbon (C) cycling and nitrogen (N) cycling. However, differences in stand density substantially altered the composition of fungal functional groups. In low-density plantations, saprophytic fungi were the main trophic mode, with a high relative abundance of ∼62 %, whereas the fungal communities associated with middle- and high-density plantations were dominated by other fungi with a combined trophic mode, which accounted for ∼43 % and ∼41 % of the fungal trophic modes, respectively. Furthermore, we detected increases in the relative abundance of plant pathogens, nitrifiers, and nitrous oxide-denitrifying bacteria with increasing stand density. Results of the Monte Carlo test showed that soil pH influenced the composition of soil fungal (but not bacterial) groups. These findings suggested that a high density of trees might inhibit the decomposition of recalcitrant organic material and stimulate nitrous oxide emission, consequently decreasing soil nutrient availability and stimulating soil N loss. Moreover, high-density stands might increase the potential risk for plant disease. Overall, the present study suggested that reducing stand density to coverage between 750 and 1125 trees ha-1 would increase soil nutrient availability and prevent N loss from the soil. To verify these suppositions, further research is needed to determine the links between microbial functional groups composition and soil biogeochemistry.

Spatially-explicit effects of small-scale clear-cutting on soil fungal communities in Pinus sylvestris stands

Clear-cutting is a common silvicultural practice. Although temporal changes in the soil fungal community after clear-cutting have been widely investigated, little is known about stand-level variations in the spatial distribution of soil fungi, particularly at the clear-cut edge. We performed spatial soil sampling in three clear-cuts (0.5 ha), edge habitats, and surrounding forests 8 years after clear-cutting to examine the impact of clear-cutting on the soil fungal community (diversity, composition, guilds, and biomass) and soil properties in a managed Pinus sylvestris forest in northern Spain. Our analyses showed small differences in the composition of the soil fungal community between edge, forest, and clear-cut zones, with <4 % of the species strictly associated with one or two zones. The richness, diversity, and evenness of the fungal community in the edge zone was not significantly different to that in the forest or clear-cut zones, although the clear-cut core had approximately a third fewer ectomycorrhizal species than the edge or the forest. Saprotrophic fungi were widespread across the clear-cut-forest gradient. Soil fungal biomass varied significantly between zones, ranging from 4 to 5 mg g-1 dry soil in the forest and at the forest edge to 1.7 mg g-1 dry soil in the clear-cut area. Soil organic matter, pH, nitrogen, and phosphorus did not differ significantly between edge, forest, and clear-cutting zones and were not significantly related to the fungal community composition. Overall, our study showed that small-scale clear-cut treatments are optimal to guarantee, in the medium-term, soil fungal communities within harvested areas and at the forest edge that are comparable to soil fungal communities in the forest, even though the amount of fungal biomass in the clear-cut zone is lower than at the forest edge or in the forest.