Decaying logs contribute greatly to the overall compositions of soil fungal communities. Logs increased the diversity of the fungal communities, thereby altering their compositions, consistent with our hypotheses. The diversity of the soil fungal community increased as the decay class increased. The decaying logs, however, did not affect the richness of the soil fungal community and the relative abundance of dominant species. The proportion of functional groups varied with decay classes of logs, indicating that decaying logs favor different functional groups of soil fungi, rather than dominant species thus enriching soil fungal communities. The composition of fungal communities of the soil under decaying logs was closely related to biochemical properties (e.g. pH, concentrations of microbial biomass nitrogen and total phosphorus) and environmental factors (e.g. soil water content). Our results indicated the conservation of logs with different decay classes in the forest ecosystem is of great significance for improving the diversity of soil fungi.
4.1 Variation of biochemical properties in soil under decaying logs
The soil concentrations of MBN, MBP, OC and TN were higher under the decaying logs than in CK. Previous studies have reported the same trend. Kappes et al. (2007) have found that the presence of coarse woody debris improved the chemical and biotic parameters of soil in acidified broadleaved forests. Adamczyk et al. (2016) have reported that the removal of coarse woody residue negatively affected the soil microbial biomass and net N mineralization, since the decomposition and use of decaying logs by soil microorganisms break organic matter down into inorganic substances and participates in soil processes through the debris food chain. The concentrations of MBN, MBP, OC, TN and TP in the soil under decaying logs also increased significantly with the decay class, indicating that the heterogeneity among the decay classes significantly affected the biochemical properties of the soil (Wojciech et al. 2019). MBC concentration, however, did not differ significantly among the classes, implying that the effect of decaying logs was probably more important for the acquisition by soil microbes of N than C (Adamczyk et al. 2016).
4.2 The diversity of soil fungal communities increased along the log decomposition
The Shannon-Wiener and Simpson indices for the soil fungal communities differed significantly among the decay classes and were the highest in the late stage of decay, suggesting that the fungal communities in the soil under the decaying logs increased in species richness during the decay of the logs. The results for fungi were consistent with those reported by Makipaa et al. (2017), who found that the fungal community in the wood was more similar to that in the soil in the late stage of decay than early stage of decomposition and that some microbial species occurred in the soil only when decaying logs were present on the soil surface. Decaying logs serve as an additional source of nutrients but also provide a more stable moisture content of the soil surface in the late stage of decay (Zalamea et al. 2016), offering beneficial habitat conditions for some mycorrhizal species. The Chao1 and ACE richness indices for the soil fungal community in our study did not differ significantly among the decay classes. The presence of the decaying logs, especially highly decayed logs, thus affected the composition of soil microbial communities.
4.3 Effect of decaying logs on taxonomic composition of soil fungi
Basidiomycota and Ascomycota were the two dominant phyla across CK and all decay classes, consistent with a previous study (Chen et al. 2017). Other studies have reported that exogenous materials such as plant litter and wood were the main sources of C for Basidiomycota, which can degrade lignin and humic acids (Purahong et al. 2016), but the relative abundance of Basidiomycota in our study did not differ significantly between the soil under the decaying logs and CK, indicating that the abundance of dominant fungal species is determined by the regional climate and soil type. The relative abundance of Basidiomycetes and Ascomycetes was generally lower in the soil under the decaying logs than in CK, perhaps owing to some wood-decaying fungi can colonize the soil, thereby increasing fungal diversity under decaying logs and decreasing the relative abundances of these two groups of fungi. The relative abundances of some Basidiomycota genera, such as Piloderma, Inocybe, Sebacina, Russula and Phlebia, and that of some Ascomycete genera, such as Ilyonectria, Fusarium, Humicola and Pleotrichocladium (Fig. S3), however, differed significantly among the decay classes, indicating that, in addition to mycorrhizal capacity, these soil fungi also have the capacity to decompose wood. Previous studies have demonstrated that some mycorrhizal groups have retained mechanisms of decomposition similar to those of brown- and white-rot fungi and actively participate in the decomposition of organic matter (Lindahl and Tunlid 2015). Mortierellomycota, a typical species found in soil (Lindahl et al. 2007), was as abundant in heavily decayed wood as in soil (Makipaa et al. 2017). Some phyla with lower relative abundances in our study were also significantly differentially represented in the decay classes. Decaying logs are thus an important source of nutrients for some soil fungal communities.
4.4 Response of soil fungal functional group to log decomposition
According to the classification results of FUNGuild, the relative abundance of fungal functional groups varied with the trophic mode in the decomposing process of logs. Pathogenic, saprotrophic, and symbiotic fungi can drive plant-soil feedbacks in complex ways (Semchenko et al. 2018). Pathogenic fungi obtained nutrients by damaging host cells. The results implied that pathogenic fungi colonized on decaying logs at the initial stage of decomposition, and began to damage plant cells in a large amount in the intermediate stage of decomposition. Wojciech et al. (2019) have demonstrated that the logs in the first and second stages of decay, without clear signs of decomposition. Besides, the soil under Minjiang fir decaying logs was dominated by saprotrophic fungi. Wood saprotroph and saprotroph were heterotrophic organisms that obtain a major fraction of their metabolic carbon from dead organic matter (Lindahl and Tundia 2015). Previous studies have shown that soft-rot fungi were common in the early stages but white- and brown-rot fungi became dominant as decay progressed (Rajala et al. 2011). In this study, the relative abundance of wood saprotroph and saprotroph increased drastically at decay class IV, indicating that more saprophytes were needed to decompose refractory organics such as lignocellulose at the highly decayed logs.
Arbuscular mycorrhizal fungi acquired carbon from host plants and obtained substantial amounts of nitrogen from decomposing organic materials (Hodge and Fitter 2010), and deliver mineral nutrients to the host, in particular phosphate (Ezawa and Saito 2018). Coline et al. (2018) have shown that water availability played a key role in affecting soil arbuscular mycorrhizal fungi. Therefore, with the increase of nutrients and moisture content, the relative abundance of Arbuscular mycorrhizal fungi increased with the decomposing process of logs. Our findings are contrary to previous studies that have demonstrated that ectomycorrhizal fungi became the most abundant group in the late stages of succession (Rajala et al. 2011). Lindahl and Tunlid (2015) have proposed that ectomycorrhizal fungi do not regularly use organic matter as a source of metabolic carbon, but rather that their access to host sugars facilitates co-metabolic degradation of complex, recalcitrant organic complexes, thereby releasing N from organic pools, which may explain why ectomycorrhizal fungi are less affected by the decay classes of logs. In addition to the changes in soil nutrients and environmental conditions, the interaction between fungi with different trophic modes also affected the proportion of fungal functional groups (Lindahl et al. 1999). The results indicated that logs with different decay classes conserved different soil fungi groups.
4.5 Influencing factors of soil fungal community composition
Organic matter is an important component of soil, and its concentration strongly affects the structure and function of soil microbial communities (Grayston et al. 2004; Franklin and Mills 2009). The composition of soil OC affects the abundance and structure of microbial communities (Zhang et al. 2020). N addition can also alter the composition of soil fungal communities, such as symbiotic mycorrhizal fungi (Morrison et al. 2016; Corrales et al. 2017). The concentrations of OC and TN in our study were poorly correlated with the relative abundances of the soil microorganisms, but the concentrations of MBC and MBN significantly affected the relative abundances of fungi, perhaps due to the role of highly heterotrophic metabolic microorganisms such as saprophytes. Changes to the concentrations of MBC and MBN could then better represent the dynamic changes of microorganisms in soil under decaying logs than could the concentrations of OC and TN. The concentration of TP is among the main factors influencing microbial activity at regional spatial scales (Cao et al. 2016). The concentration of TP in our study was significantly correlated with the relative abundances of some of the fungal communities. Similar observations were also reported for soil planted with Chinese kale (Pongsilp and Nimnoi 2020). The significant differences in MBC, OC and TP concentrations among the decay classes may to some extent account for the separation of Glomeromycota.
Soil pH is also a critical factor for the diversification of fungi and strongly influences the structure of soil microbial communities (Geisseler and Scow 2014). pH can directly affect soil microbial communities and indirectly affect the structure of some microbial communities through interactions among soil elements (e.g. precipitation of ions) (Lammel et al. 2018). The dominance of some phyla in our study, such as Basidiomycetes and Ascomycetes, depended strongly on soil pH. Water content was also strongly correlated with the characteristics of the microbial communities (Brockett et al. 2012). These results indicated that the dominant fungi in the soil under decaying logs might respond to the changes in water content by the allocation of limited resources.