This study quantified the horizontal influence range of larch stumps and coarse roots on the phosphorus (P) fraction and availability of microsite soils and explored whether this influence range changes with different plantation types. The total P, available P and P fractions were measured in microsite soils at 0–75 cm horizontal distances from stumps and coarse roots at soil depths of 0–40 cm in a pure larch (Larix olgensis Henry) plantation and a mixed larch–ash (Fraxinus mandshurica Rupr.) plantation. Soils at horizontal distances of 85–95 cm from the stumps and coarse roots were used as the controls. Larch stumps and coarse roots affected the total P concentration at depths of 0–40 cm in the mixed plantations, and the maximum horizontal influence range reached 75 cm. However, in the pure plantation, only the total P at 0–10 cm depths were affected, and the maximum influence range was 35 cm. The NaOH-P_i and NaOH-P_o changes in the pure plantation were similar to those of total P, while those of HCl-P_i, HCl-P_o and NaHCO₃-P_o in the mixed plantation were similar to those of total P. Larch stumps and coarse roots could affect the total P and P fraction concentrations in microsite soils. The horizontal distance of soil total P and P fractions concentrations affected by larch stumps and coarse roots in the mixed plantation was greater than that in the pure plantation. These results suggest that the position of stumps and coarse roots should be considered when reforestation sites are selected.

Thinning is an important management practice for reducing plant competition and improving wood production in forests. The residues from thinning can contain large amounts of carbon (C) and nitrogen (N), and the management methods applied directly after thinning can affect the input of nutrients to soil, change the availability of substrates to soil bacterial communities, and thus affect soil bacterial community structure. Our objective was to determine the effects of different thinning residue treatments on soil bacterial community structure and diversity. Illumina high-throughput sequencing technology was used to sequence the bacterial 16SrRNA V3–V4 variable region of the soil (0–10 cm) of a Larix olgensis plantation to compare the composition and diversity of soil bacterial communities following removal of thinning residues (tree stems plus tree crowns) (RM) and retention of thinning residues (crowns retained with stem removal) (RT) treatments. Total soil carbon (TC) and nitrogen (TN) content in the residue retention treatment were significantly greater than in residue removal treatments (p < 0.05). The relative abundance of the dominant soil bacteria phyla were, in descending order: Proteobacteria, Verrucomicrobia, Acidobacteria, Chloroflexi, Actinobacteria, Nitrospirae, Planctomycetes, Gemmatimonadetes, and Bacteroidetes, with a total relative abundance of more than 80%. Acidobacteria were enriched in the RM treatment, while Proteobateria, Actinobacteria and Bacteroidetes were greater in the RT treatment. Rhizobiales and Rhodospirillales (belonging to the α-Proteobacteria) were enriched in the RM treatment. Soil bacteria α diversity was not significantly different among different treatments. Spearman correlation analysis showed that the α diversity index was significantly negatively correlated with TC and TN. Lefse analysis revealed that 42 significant soil bacteria from phylum to genus were found in the two different thinning residue treatments. Redundancy analysis showed that soil TC and TN were the major drivers of variation in soil bacterial community structure. Overall, thinning residue retention increased the availability of resources to the soil bacterial community, thus changing bacterial community structure. This research provides a theoretical basis for the regulation of plantation forest soil fertility and quality.

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.

Stumps are a significant component of coarse woody debris in plantations, but their effect on microsite soil organic carbon (C) and enzyme activities remains understudied. Soil (Alfisol) samples were collected at varying distances from larch (Larix olgensis Henry) stumps and at different soil depths (0–20 cm and 20–40 cm) to analyze soil total organic C (TOC), particulate organic C (POC), easily oxidizable C (EOC), microbial biomass C (MBC), and enzyme activities. Results indicated that stumps significantly affected TOC and POC contents, with the greatest horizontal range of impact reaching up to 15 cm in both the topsoil and subsoil layers. Stumps also significantly affected MBC content, with the greatest horizontal range of impact reaching up to 55 cm in the subsoil layer. EOC content was the most affected, with the stumps’ impact extending to 55 cm in both soil layers. Additionally, the study showed that stumps had a significant impact on the activities of β-glucosidase and β-cellobiohydrolase, with the greatest horizontal range of impact reaching up to 15 cm for glucosidase and 35 cm for cellobiohydrolase in the topsoil layer. Stumps also significantly affected the activities of phenol oxidase and peroxidase, with the maximum horizontal range of stump impact extending up to 35 cm for phenol oxidase and 55 cm for peroxidase in the topsoil layer. This study enhances our understanding of the role of stumps in plantation ecosystems and offers valuable insights for future management strategies to maintain soil fertility and improve site productivity.

The stump and coarse root biomass remaining after tree harvesting are often overlooked by researchers, which may lead to underestimation of their role in carbon cycling, so we constructed two sets of additive models for larch (Larix olgensis Henry) plantations in Northeast China. Due to the absence of tree diameter at breast height data after harvesting, only the sole predictor variable stump disc diameter could be used to predict stump and coarse root biomass, and the results showed that stump disc diameter predicted stump biomass with higher accuracy than coarse root biomass predictions. In addition, to investigate the effect of the site class of complex stands on the predictive capability of the model, the generic model in this study was employed with all site class data and a specific model was developed and employed with all the site class data. We found that the generic model had different degrees of error compared to the predicted results for each site class, overestimating the total biomass by 15% and underestimating it by 10%, especially for site class IV. In conclusion, to obtain a biomass prediction model with reliable results, the impact of more complex site class effects should be considered.

Picea koraiensis Nakai is a precious tree species in Northeast China with excellent traits, but research on thinning effects on its growth remains limited, especially regarding soil-thinning–growth interactions. This study focused on a 50-year-old Picea koraiensis plantation in the Mengjiagang Forest Farm, Jiamusi. Four thinning intensities were set: CK (no thinning), T1 (10%–20%), T2 (20%–30%), and T3 (40%–50%). Short-term (1–3 years) stand growth, soil properties, microbial biomass, and extracellular enzyme activities were measured, with stand volume and large-diameter timber yield estimated via self-established equations. Results showed that T3 significantly promoted average DBH (1.98 × CK) and tree height growth (1.60 × CK). T2 achieved the highest increases in stand volume (38.07 m³/ha) and large-diameter timber yield (56.02 m³/ha), exceeding other treatments by 1.20–7.12 m³/ha and 5.60–11.64 m³/ha, respectively. Stand growth indices were positively correlated with thinning intensity, soil microbial biomass carbon, and soil C/P ratio; DBH and height also correlated with soil catalase activity. Thinning intensity has a direct effect on stand growth. Meanwhile, observational data show that it is significantly correlated with changes in soil organic carbon fractions and soil extracellular enzyme activity, and these correlations may constitute potential pathways that indirectly affect stand growth. Moderate-intensity thinning (20%–30%) is recommended for scientific tending and large-diameter timber cultivation of middle-aged Picea koraiensis plantations in this region.