Elucidating the influence mechanisms of seed germination and seedling growth is important for revealing the natural regeneration of forest plantations. We collected the seeds from 58-year-old Quercus acutissima Carruth. forest, and the seeds were further divided into three classes: large, medium, and small, and sown under the forest gaps (I, 197.82 m²; II, 91.85 m², III, understory) to observe seed germination and early seedling growth. Precipitation in the study area and soil moisture content in the forest gaps were also observed during the trial period. The results showed that the precipitation in 2019 was similar to that in 2020; both were significantly lower than the precipitation in 2021. The difference in soil water content between gaps I and II was not significant, and both were significantly lower than III. The order of seedling emergence rate in gaps was II > III > I, but the minimum was almost close to zero in I. Large and medium seeds showed significantly greater emergence rate than small seeds. The seedlings of II had higher seedling height, ground diameter, ground diameter relative growth rate, seedling biomass, root surface area, and root volume than those of III. Large seeds had the highest ground diameter, ground diameter relative growth rate, biomass, root mass ratio, root shoot ratio, and root surface area. Correlation analysis showed that seedling biomass was significantly and positively correlated with root surface area and root volume, and significantly and negatively correlated with specific root length and specific root surface area. The regulation of soil moisture in the gap and the adaptability related to seed size were two key factors influencing the seed germination and early seedling growth of Q. acutissima.

The mixing of poplar and robinia in coastal saline land is a useful attempt at difficult site afforestation. Investigating the long–term mixing effects of nitrogen–fixing and non–nitrogen–fixing tree species on the spatial heterogeneity of N and P nutrients and their ecological stoichiometric characteristics in the coastal saline–alkali soil can provide a scientific basis for soil improvement and plantation management in the coastal saline–alkali soil. By replacing time with space, poplar and robinia mixed forests and corresponding pure forests with the ages of 3, 7 and 18 years were selected, and soil profiles of 0–20 cm, 20–40 cm and 40–60 cm were dug up to determine the contents of total nitrogen, hydrolyzed nitrogen, total phosphorus and available phosphorus, the activities of soil urease and phosphatase and the number of soil bacteria, fungi and actinomycetes in rhizosphere soil. The mixture of poplar and robinia and the increase in planting years led to the heterogeneity of soil N and P in a coastal saline–alkali forest, which could significantly increase the contents of soil total nitrogen, hydrolyzed nitrogen, total phosphorus and available phosphorus between soil layers. Compared with the pure forest of poplar and robinia at the same age, the soil urease activity in the 0–20 cm soil layer of an 18a poplar and robinia mixed forest increased by 94.75% and 73.36%, and the soil phosphatase activity increased by 30.36% and 70.27%. The mix of poplar and robinia significantly increased the abundance of soil microorganisms in saline–alkali soil. The number of bacteria, fungi and actinomycetes in the 0–20 cm soil layer of the 18a poplar and robinia mixed forest was the highest, which were 703,200, 31,297 and 1903, respectively. Redundancy analysis showed that there was a significant positive correlation between soil N and P nutrient contents, soil enzyme activities and microbial abundance. The soil depth of N and P nutrient decomposition and transformation in the mixed poplar and robinia plantation was expanded. The soil N and P nutrient contents, enzyme activities and microbial abundance in the 40–60 cm soil layer of the mixed forest were higher than those of the pure forest. With the increase in plantation years, the depth of soil that can be used in the forest land is increasing. The mixture of poplar and robinia plantation is an excellent choice for the construction of coastal saline–alkali land plantation, which has a significant mixed gain for the decomposition and transformation of N and P nutrients and increases the depth of the available soil layer in the forest land in coastal saline–alkali land. However, the coastal saline–alkali land soil N/P is < 14 and is still restricted by nitrogen, so the application of nitrogen fertilizer can be increased during the afforestation process.

Third body plant fibers have an important influence on the friction and wear properties of metal friction pairs. In this study, wheat straw powder with different particle sizes was prepared, and a pin-on-disk friction tester was employed to evaluate the friction and wear properties of the wheat straw powder third body on steel friction pairs under various conditions. The results show that the straw powder played an isolation role in the steel–steel friction pair, which reduced the contact area between the pin and the disc and thus reduced the friction coefficient. Compared with the large particle size powder, the small particle size powder became embedded in the friction interface or in the groove caused by wear, buffered the friction stress, and reduced the friction coefficient and wear. When lubricants such as lubricating oil or lubricating grease were added, the friction coefficient was significantly reduced. Under the influence of lubricant, the particle size of straw powder had no significant effect on the wear. The results of this study can provide a reference for the third body friction and wear properties of straw powder and the design of friction pairs for metal parts.