Pinus sylvestris var. mongolica is widely planted in China as a windbreak and sand fixation tree. To improve the current situation of large-scale declines of forested areas planted as P. sylvestris var. mongolica monocultures, we investigated the biological and microbial effects of stand establishment using mixed tree species. The interactions during the mixed decomposition of the litter and leaves of different tree species are an important indicator in determining the relationships among species. In this experiment, a method of simulating the mixed decomposition of P. sylvestris var. mongolica and Morus alba litter under P. sylvestris var. mongolica forest was used to determine the total C, total N, and total P contents in the leaf litter, and the microbial structures were determined by using Illumina MiSeq high-throughput sequencing. It was found that with samples with different proportions of P. sylvestris var. mongolica and M. alba litters, the decomposition rate of P. sylvestris var. mongolica × M. alba litter was significantly higher than that of the pure P. sylvestris var. mongolica forest, and the microbial community and composition diversity of litter in a pure P. sylvestris var. mongolica forest could be significantly improved. The possibility of using M. alba as a mixed tree species to address the declines of pure P. sylvestris var. mongolica forest was verified to provide guidance for pure P. sylvestris var. mongolica forests by introducing tree species with coordinated interspecific relationships and creating a mixed forest.

The intercropping of nitrogen-fixing and non-nitrogen-fixing tree species changed the availability of soil nitrogen and soil microbial community structure and then affected the regulation process of soil carbon and nitrogen cycle by microorganisms in an artificial forest. However, there is no consensus on the effect of soil nitrogen on soil microorganisms. In this study, the intercropping of mulberry and twigs was completed through pot experiments. Total carbon, total nitrogen, and total phosphorus in the rhizosphere soil were determined, and the composition and structure of the soil microbial community were visualized by PCR amplification and 16S rRNA ITS sequencing. The analysis found that the intercropping of Morus alba L. and Lespedeza bicolor Turcz. had no significant effect on soil pH but significantly increased the contents of total carbon, total nitrogen, and total phosphorus in the soil. The effect on the alpha diversity of the bacterial community was not significant, but the effect on the evenness and diversity of the fungal community was significant (p < 0.05). It was also found that soil nutrients had no significant effect on bacterial community composition but had a significant effect on the diversity within the fungal community. This study added theoretical support for the effects of intercropping between non-nitrogen-fixing tree species and nitrogen-fixing tree species on soil nutrients and microbial community diversity.