4.1 Soil characteristics
Due to different root exudates of different stand types and different chemical components and decomposition rates of litter, the organic nutrients input into the soil are different [16, 17, 18], lead to significant differences in understory soil characteristics. This study showed that there were significant differences in soil properties under 5 different stand types. Previous studies have shown that the litter of deciduous coniferous forest contains some undecomposable organics, such as lignin, tannin and wax, which cause the litter decomposition is relatively slow [19] and the amount of the accumulated litters is relatively large, the massive litters covered the soil surface, thus declined the utilized rate of soil nutrients and sped up the accumulation of soil nutrients. This conclusion confirmed that the soil nutrient content under Pinus tabuliformis forest was higher than other forest stand. The highest soil acidity of Quercus mongolica forest undergrowth was 5.87. Compared with other broad-leaved forests, the litter of Quercus mongolica forest was relatively low, and in this litters, the N content is relatively few, the content of C/N and lignin is relatively high, its nutrient content is relatively less, which cause the pH value of undergrowth is relatively little. Under the same climatic conditions, there are significant differences in soil characteristics under different stand types, which has a certain impact on soil microbial community.
4.2 Diversity of soil microbial communities
Different stand types are key factors determining of forest soil microbial community [20, 21, 22, 23]. Our results showed that the soil bacterial diversity under Juglans mandshurica forest was the highest, and the bacterial diversity under broad-leaved forest were significantly higher than soil under Pinus tabuliformis forest and Quercus mongolica forest. Generally, the quality and the decomposition rate of litter produced by broad-leaved forest is more than that of coniferous forest [24], which has provided enough substance conditions for rapid growth and breed of bacteria. The decomposition of broad-leaved forest litter is mainly based on catalase and polyphenol oxidase, the activity of polyphenol oxidase is positively correlated with bacteria diversity, and catalase activity is positively correlated with bacteria amount [25]. In conclusion, the differences of soil bacteria diversity of different forest stand of the same broad-leaved forests are not clear, the soil bacteria diversity of broad-leaved forests were greater than coniferous forests. In the present study, the diversity of soil fungus is that the soil under Pinus tabuliformis forests was more than the rest stand types, the differences of soil fungus diversity of the rest broad-leaved forests were not so obvious. The significant differences in soil microbial communities between coniferous and broad-leaved trees has been previously reported [8]. Compared with broad-leaved forests, the litter layers of coniferous trees are thicker, the lignin content more higher [26], but most complex organics in soil need to be decomposed by fungus [27], and some soil fungus could coexist with plants or form mycorrhiza fungi, this interspecies cooperation approach improved the competitiveness of cooperative soil fungus, thus it can improve the diversity of fungus community with limited resources and limited space. Our result showed the undergrowth soil nutrient content of Quercus mongolica forest is relatively low, and its diversity indexes of undergrowth soil microbial communities are also relatively low, which indicates that soil microbial diversity has prominent relations with soil nutrient content, the rich soil nutrient content will promote the richness of soil microbial communities.
4.3 Soil microbial community structure
Soil microbial community structure play an important role in soil nutrient cycling, and the dominant species of soil microbial play an important role in soil genesis [28]. Soil microbial community has obvious stand type specificity [8]. The dominant bacteria of soil are Proteobacteria, Acidobacteria and Actinobacteria, which is similar to the results of dominant bacteria detected in other forest ecosystem environments, it explained that the ecological amplitude of the three kinds of microflora is relatively wide [11, 29, 30], and they have a good adaptability to forest environment. However, the relative abundance of different bacteria phylum were significantly different in the soil of different stand types. The average abundance of Actinobacteria (19.75%) in soil under Quercus mongolica forest was significantly lower than other, and the average abundance of Acidobacteria (26.83%) was significantly higher than other. Actinobacteria belongs to G+ bacteria, and it is able to decompose cellulose and lignin [31]. Acidobacteria are acidophilic and oligotrophic [32]. This study showed that the soil pH value under Quercus mongolica is the lowest and the soil nutrient content is significantly lower than that of other stand types. This condition is more suitable for Acidobacteria to exist and breed, which is consistent with the results of Peng’s and Naether’s study [33, 34]. The composition of bacterial dominant genera under the 5 stand types were similar, but the content of individual dominant genera is different, Subgroup-2 only existed in the soil under Quercus mongolica forest, which may have a great relationship with environment.
In this study, the dominant microflora of soil fungus were Basidiomycota and Ascomycota, the contents of them have difference with significance. The results showed that the dominant phyla of fungus were different under different stand types, which is consistent with the results of Sheng's study [35]. The dominant soil fungus under the soil of Betula dahurica, Pinus tabuliformis and Juglans mandshurica forest are Ascomycota, followed by Basidiomycota, the same situation appeared in tropical forest soil, subtropical forest soil in Australia, Guandi mountain forest soil and aeolian sand area in Northwest Liaoning. However, Basidiomycota was the dominant fungus in the soil under Betula platyphylla and Quercus mongolica forest, followed by Ascomycota, the same results also appeared in European forest soil, temperate forest soil and subtropical forest soil in China. Other research results showed that Ascomycota was the most abundant fungi in broad-leaved forest. However, the results of this study didn’t confirm this conclusion, which showed the complexity and difference of soil environment under different stand types. Other research founded that there was usually one phylum with the highest fungal abundance in different stand types in the same area, but this study founded that the phylum with the highest fungal abundance were difference in different stand types. This result showed that stand types and soil properties have significant impact on soil community, while other conditions such as climate have little impact on soil community. Another possibility was that the results were related to the scale or the methods used to access soil microbial community. In this study, only 3 groups of repetitions were set up, which may have a small number of repetitions, which had an impact on the research results. In this study, the species of soil fungus of different forest stand were rich and significant different at the genus level. Many fungi are specific root symbionts and pathogens, their growth and reproduction are more directly dependent on the biological nutritional interaction between litter and trees [36/37]. The dominant plants in each sample plots were different, which will also leaded to great differences in fungal communities [38,,39, 40].
4.4 Relationship between soil and microbial community
We found that soil respectively explained 49.1% of the variance in bacterial community composition and explained 70.6% of the variance in fungal community. These results indicated that soil played considerable roles in changing the composition of microbial community, especially in fungal community. However, some variances in bacterial and fungal community compositions have not been explained, therefore, other factors such as climate and season may also drive the composition of soil microbial communities, which needs to be further studied. Soil pH can change bacteria towards its nutrient utilization rate, the physiological metabolism activity, and the competition among populations, which can directly or indirectly affect soil bacteria diversity, the appropriate soil pH value will promote microbial growth [41]. RDA analysis showed that TN, AN and SOM were positively correlated with the abundance of Actinobacteria, which proved that C and N in soil provided material basis for the growth of Actinobacterial [42], growth and development of Actinobacterial are closely related to soil carbon and nitrogen cycle [43]. Some studies showed that soil pH and soil available nutrients have strong impact on fungal community, but this study founded fungi have no significant correlation with soil pH, which may be related to the small range of soil pH under 5 stand types. RDA analysis showed that Basidiomycota negatively correlated with AP, which conforms to the research results of Deng J [44], but contrary to some previous studies [45, 46]. Because soil microorganisms contribute to different soil nutrient element cycles and play different roles in different nutrient element cycles, the chord diagrams could prove that soil microorganisms are closely related to soil nutrients.