Response of Soil Physicochemical Properties and Microbial Community Composition in Larix olgensis Plantations to Disturbance by a Large Outbreak of Bark Beetle

Forests are affected by a wide range of disturbances globally, resulting in the decline or death of large areas of them. There is a lack of comparative studies on how soil properties change in forests that die under the influence of disturbances, especially considering different levels of disturbance. For this study, we took Larix olgensis—a major plantation forest species in northeast China—as the research object, one in which a large outbreak of bark beetle led to large-scale forest death, and set up fixed sample plots characterized by different disturbance intensities. We investigated the responses of soil physicochemical properties and microbial community compositions to different disturbance intensities through the determination of soil nutrient indices and high-throughput sequencing. The results show that there were significant differences (p < 0.05) in the effects of different disturbance intensities on soil physicochemical properties, where the soil moisture content, total nitrogen, total carbon, and total phosphorus in the control group were significantly higher than those in the disturbed groups. The soil pH was highest under low-intensity disturbance and the soil total potassium content was highest under high-intensity disturbance. At different disturbance intensities, the highest soil moisture content was found in the high-intensity group. Proteobacteria, Actinobacteria, Verrucomicrobia, Acidobacteria, Candidatus_Rokubacteria, Chloroflexi, Gemmatimonadetes, and Thaumarchaeota were the dominant populations with higher abundances; meanwhile, the relative abundance of Bacteroidetes, Tenericutes, and a tentatively unclassified fungus differed significantly (p < 0.05) across disturbance intensities. Among the dominant microbial populations, Acidobacteria showed a significant negative correlation with soil pH and a significant positive correlation with total potassium content, Thaumarchaeota showed significant positive correlations with soil moisture content and total nitrogen content, and Firmicutes and Gemmatimonadetes showed significant negative correlations with total carbon content in the soil. Furthermore, soil total nitrogen content was the key factor driving changes in microbial communities. The results of this study provide a scientific basis for the study of the long-term effects of tree mortality caused by insect pests on soil microbial communities and their response mechanisms, which is of great theoretical value for the establishment of scientific and effective methods for woodland restoration.