*Article* **Do Mixed** *Pinus yunnanensis* **Plantations Improve Soil's Physicochemical Properties and Enzyme Activities?**

**Chen Liang 1, Ling Liu 2, Zhixiao Zhang 2, Sangzi Ze 3, Mei Ji 2, Zongbo Li 4, Jinde Yu 1, Bin Yang 4,\* and Ning Zhao 1,4,\***


**Abstract:** Many survival and ecological problems have emerged in *Pinus yunnanensis* pure pine forest plantations that are usually assumed to be solved by creating mixed plantations. On this basis, we determined the physicochemical properties and enzyme activities of three soil layers in pure and three types of mixed *P. yunnanensis* plantation stands (admixed species: *Alnus nepalensis*, *Celtis tetrandra*, and *Quercus acutissima*) in Southwest China. We used one-way ANOVA with Tukey's test to analyze the effects of plantation type and depth on the soil's properties and variations among different depths. Principal component analysis combined with cluster analysis was used to evaluate the soil quality of different forest types comprehensively. The results showed that the stand with a mixing proportion of 2:1 of *P. yunnanensis* and *A. nepalensis*, *C. tetrandra*, and *Q. acutissima* had higher total porosity, moisture content, total nitrogen, available phosphorus, total phosphorus, sucrase, urease, and catalase enzyme activities than other proportions of mixed forest and *P. yunnanensis* pine pure forest. In general, the mixed *P. yunnanensis* plantation could improve the soil quality, especially its chemical properties and enzymes. This study provides a basis for creating a mixed-mode of *P. yunnanensis* and other tree species that can not only improve the economy of forest land but also enhance the ecological value.

**Keywords:** *Pinus yunnanensis* pure plantation; *Pinus yunnanensis* mixed plantation; soil; physicochemical properties; enzyme activities; principal component analysis and cluster analysis
