Updates on Plants, Soil, Microorganisms, and Their Interactions in Forest Ecosystems

Forests, covering one-third of the global landmass, are the world’s most vital terrestrial ecosystem [1]. They offer multiple ecosystem services [2], such as food production, biodiversity conservation, sandstorm prevention, water and soil conservation, nutrient cycling, carbon (C) sequestration, climate mitigation, culture and recreation, etc. [3,4,5,6,7,8]. Generally, plants, soil, and microbes do not exist alone in an ecosystem; rather, they are tightly linked to each other [9,10,11,12]. Their complex coupling interrelationships are crucial for driving ecological processes and ecosystem functions in forest ecosystems [13].

Aboveground plants provide essential resources, such as organic carbon and nutrition, mainly through litter and root exudates, maintaining soil microbial activity and functional performance [12]. In turn, soil microorganisms (e.g., bacteria and fungi) decompose dead plant material, thereby influencing soil nutrient availability, plant growth, and the composition of aboveground communities [10]. Therefore, these interdependencies among plants, soil, and microorganisms play a very important role in determining the key ecosystem processes. Any disruption in these linkages caused by human-driven environmental changes, including climate warming, increased atmospheric nitrogen (N) deposition, and land use changes, can result in imbalances in nutrient cycling [14,15], which could ultimately affect the overall ecological functioning and stability of the forest ecosystems [16]. In these circumstances, delving deeper into the relationships among plants, soil, microorganisms, and their responses to environmental changes can enhance our comprehension of the internal mechanisms in forest ecosystems. This Special Issue selected 15 papers to discuss the new knowledge and different perspectives on plant–soil microbe interactions, aiming to promote forest management.

Forest succession is recognized as a key biotic factor that plays a significant role in shaping above-ground ecological processes and material cycling. For example, Yuan et al. [17] discovered that forest aging can lead to divergent changes in the stoichiometric characterizations of C, N, and P among plants, litter, soil, and microorganisms. This finding contributes to a better comprehension of the nutrient utilization strategies and regulatory mechanisms in Pinus taiwanensis plantation forest ecosystems. Qiu et al. [18] demonstrated that variations in fungal functional structure intensify during the succession process of Pinus tabulaeformis plantations in the North Warm Temperate Zone. This variation is primarily influenced by soil pH, dry matter content, C/N ratio, and N content. Ren et al. [19] investigated the shifts in soil microbial communities and assembly processes during vegetation succession in a subtropical forest. Their study revealed that the community assembly of soil bacteria transitions from a deterministic process to a stochastic process and back to a deterministic process as forest succession progresses from abandoned land to evergreen broad-leaved forests. In contrast, stochastic processes largely govern the community assembly of soil fungal communities during all succession stages. Moreover, soil organic carbon (SOC) is responsible for the change in bacterial communities, whereas SOC, total N (TN), C:N ratio, and pH collectively regulate the changes in fungal community structure. Li et al. [20] found that the introduction of broad-leaved tree species to a Chinese fir plantation not only increased the soil nutrient content, but also enhanced the diversity of soil fungal communities, resulting in the microbial communities of mixed forests being more diverse. Therefore, for the sustainable development of Chinese fir plantations, it is crucial to direct more focus on the middle and late stages of their growth rather than just the early stages. These results imply that mixed forest models should be prioritized as the primary forestry management approach.

Climate warming, atmospheric N deposition, and land use changes are three important human-driven abiotic factors that may impact soil nutrient cycling and soil microorganisms in forest ecosystems. For instance, Shu et al. [21] observed that soil N mineralization was more influenced by warming in the subtropical Castanopsis hystrix plantation than in the temperate Quercus aliena natural forest, which may be attributed to differences in the soil nutrient availability, fine root biomass, and microbial biomass of forests in the two climatic zones. Hou et al. [22] showed that after four years of the continuous addition of N, soil microbial communities in subtropical planted coniferous forests displayed diverse responses that were dependent on the afforestation tree species. Long-term high-level N additions consistently lead to detrimental effects on soil microbial communities (especially fungi), highlighting that soil pH and N availability are crucial factors influencing the diversity of soil bacterial and fungal communities in these forest ecosystems. Li et al. [23] studied soil microbial communities in Pseudotsuga sinensis forests with different degrees of rocky desertification in the Karst Region of Southwest China. They found that the richness and diversity of microbial communities decreased with an increase in the degree of rocky desertification, primarily influenced by soil bulk density, pH, SOC, available N, and available P. Xiao et al. [24] found that relatively heavy thinning can accelerate soil P-bioavailable turnover by stimulating multiple nutrient cycles, highlighting the essential roles of microbial biomass turnover and soil nutrient supply in the plant-available P mechanism in Eucalyptus coppice plantations.

Elevation is a significant natural factor that plays a crucial role in influencing plant functional traits, soil characteristics, soil microbes, and their relationships within forest ecosystems. For example, Zhang et al. [25] discovered a notable increase in aboveground vegetation productivity across diverse altitudinal vegetation belts in the Chinese Tianshan Mountains since the early 21st century. Huang et al. [26] found that the differences in soil water availability and soil development due to different altitude habitats could significantly impact leaf-scale water use efficiency and nutrient status on the western slope of Wuyi Mountain in southern China. In the Fanjing Mountain Forest ecosystem, Wang et al. [27] found that the soil properties and enzyme activities were the main factors that drove the elevational distribution of soil microbial communities. Jin et al. [28] explored the differences in soil properties and root zone bacteria communities at different altitudes within the distribution range Davidia involucrata in the Sichuan Province of China. They also found a significant link between soil parameters and soil bacterial communities in root zones of mature trees across varying altitude gradients.

Moreover, some research was conducted on the application of soil microbial inoculants in agroforest ecosystems. For example, Lai et al. [29] found that ribosome-inactivating proteins, such as curcin protein, exhibit broad-spectrum antifungal properties. Ribosome proteins could significantly impact soil fungi and thereby affect the soil ecosystem in Jatropha plantations. Deng et al. [30] isolated an endophytic bacterium (Pseudomonas sp. En3) from the leaf of Populus tomentosa. These plant growth-promoting endophytic bacteria have a notable growth-promoting impact on poplar seedlings. de Oliveira et al. [31] found that, when inoculated with plant growth-promoting bacteria (Exiguobacterium sibiricum), the plant of Corymbia citriodora exhibited a taller height, higher chlorophyll b content, larger shoot and total dry mass, and greater ability to colonize the roots, leading to the production of higher-quality seedlings.

In summary, the 15 publications in this Special Issue represent a small sample of current scientific research, offering new insights into plants, soil, microorganisms, and their interactions in various forest ecosystems. These findings may have implications for forest conservation, restoration, and management. Moving forward, there is a need to delve deeper into the relationships among plants, soil, and microorganisms, particularly within the framework of constructing a global ecological civilization.