Clarifying soil microbial community attributes and assembly processes in monoculture plantations are critical in understanding the effects of forest-to-plantation conversions on soil microorganisms and their functional potentials. Here, we found that soil bacterial taxonomic α-diversity (Shannon Wiener index) in EP-2-year was significantly lower than in the other three plantations. A previous study showed no difference in soil bacterial Shannon diversity between Rubber and Eucalypts plantations either in dry season or in wet seasons [
37], which was partly in disagreement with our results. Perhaps the difference in plantation age between the previous and this study contribute to this controversial result, since the diversity of soil microbial communities varied substantially with the succession of Eucalypts plantations [
36]. The fungal Shannon diversity, however, was significantly lower in EP than in RP, regardless of plantation age. This suggested that the taxonomic α-diversity of soil bacterial community was more sensitive to dominant tree species than tree age in monoculture plantations. Regarding the phylogenetic α-diversity (Faith’s Phylogenetic index), both bacterial and fungal communities had a lower value in EP than in RP. In contrast to the Shannon diversity, phylogenetic α-diversity of fungal community was more responsive to plantation age than bacterial community, as indicated by a greater value in EP-4-year than in EP-2-year. The overall higher microbial α-diversity in RP might be associated with the greater soil nutrients content in RP than those in EP (
Figure S2). Although declines in soil nutrients in monoculture plantations has been widely recognized, Eucalypt and Rubber trees generally exert different effects on the soil fertility and may provide distinct niches for soil microbes. The quality and quantity of litterfall from RP, especially in older plantations, is comparable to that of undisturbed natural forests [
48,
49]. A recent study further demonstrated that litter decomposition rates increase with RP age, potentially leading to an improvement of soil quality in older plantations [
32]. In contrast, soil nutrients levels decline dramatically after the establishment of Eucalypt plantations, which has been proposed to be related to their poor-quality and highly recalcitrant litterfall [
50,
51]. In congruence with these previous results, we found greater soil C, N and P content in RP than in EP (
Figure S2). Higher levels of soil nutrients and faster litter decomposition and turnover dynamics may provide a broader range of environmental niches, and hence support more diverse microbial communities.
Bacterial and fungal structure based on both taxonomic and phylogenetic matrices was strongly affected by the plantation type and age but not soil depth (
Figure 3). The constrained ordination analyses showed that microbial structure was mainly affected by soil total K in EP while by soil available nutrients (i.e., AN, AP, and AK) in RP. The similar community structure along the 0–50 cm soil profile in RP possibly resulted from the similar available nutrient content along soil depth (
Figure S2). Although in EP soil total K varied markedly with soil depth (0–50 cm), its variation is insufficient to cause microbial community shifts in comparison to the amount of variation of soil total K among the plantations (
Figure S2). Moreover, the similar soil microbial community along 0–50 cm could be partly attributed to the homogeneity of the vertical soil environments caused by the root exudates, as the roots of the dominant trees can reach a depth of more than 50 cm. Although we tried to avoid the root areas during sample collection, some fine roots were still present in the samples. Thus, the effects of root systems could not be totally removed. Microbial community attributes in deeper soil environments need to be investigated to explore the vertical variation in RP and EP in future studies. The constrained ordination analyses illustrate how microbial community structure in RP varies in relation to soil available nutrients content, whereas the microbial community structural variation in EP is probably driven by soil nutrient limitations. Looking at the soil microbial community composition at phylum level, the EP had a higher relative abundance of Chloroflexi and Basidiomycota, and their relative abundance was negatively correlated with soil AN, AP and AK content. In contrast, the phyla that positively related to soil available nutrients and were more abundant in RP, including Actinobacteria, Gemmatimonadetes, Ascomycota and Zygomycota (
Figure 1,
Figures S4 and S5). It was surprising that soil pH had little effect on soil microbial structure despite a significant difference in soil pH among the four plantations. This result was inconsistent with recent studies demonstrating that pH played a decisive role in shaping soil microbial communities in both Rubber and Eucalypts plantations [
34,
35,
52]. One reason for this might be that the soil pH value in this study was less likely to impose stress on soil microorganisms, as the average pH (5.0) in our study site is relatively higher than that in the recent studies (4.0–4.6). Interestingly, we found a significant positive correlation between microbial structure and soil total K content in EP. This has not been evidenced previously, even though K is required for many plants and microorganisms to maintain essential metabolisms, such as photosynthesis, ATP production, sugars translocation, starch production, nitrogen fixation and protein synthesis [
53]. The effects of K deficiency on plants are mainly indirect and limit the plant use of other nutrients. For example, it has been proved that increased K allows for rapid assimilation of N and P in the plants [
54]. Similarly, a continuous K supply in the degraded soils in EP may alleviate nutrient limitations by accelerating plant uptake of other available nutrients or by improving the efficiency of plant nutrients’ use. In this case, Eucalypt trees may shift soil microbial composition towards a community that has a high ability to mobilize soil K. For instance,
Burkholderia spp., which is able to release K from K-bearing minerals by excreting organic acids [
53], was found in a much higher relative abundance in the EP sites (0.71%) than in the RP sites (0.09%). Furthermore, we found a positive correlation between soil total K content and microbial structure in EP. Another explanation for the different fungal composition between RP and EP might be the distinct mycorrhizal status of their dominant tree species. It has been suggested that Eucalypt is mainly an ectomycorrhizal species [
55,
56,
57]. Accordingly, the fungi belonging to Basidiomycota phylum, which can form mycorrhizal symbiosis with plants, dominated in EP (
Figure 1B). The Rubber trees, however, largely form arbuscular mycorrhiza with fungi belonging to Zygomycota [
58], resulting in a higher relative abundance of Zygomycota in RP than in EP (
Figure 1B). However, together with poor litter quality, the slow rates of litter decomposition, the low availability of nutrients in EP, competition between soil microorganisms and plants for nutrients is very high, which potentially depletes soil nutrient pools over time. This may be one mechanism underling the observed severe soil degradation in established EP. For RP, however, greater soil nutrients availability can increase microbial nutrients assimilation and enlarge soil microbial nutrients pools, which can potentially serve as plant-available nutrients after being released from microbial cells [
59].