**1. Introduction**

*Pinus yunnanensis* is an ecologically and commercially important tree species distributed in subtropical Southwest China, with over 4 million hectares in plantations and natural forested areas [1]. The *P. yunnanensis* pine forests not only provide a large amount of high-quality timber and industrial raw materials but also play an important role in maintaining species diversity, conserving water resources, and retaining soil [2]. In recent years, because the composition of artificially constructed *P. yunnanensis* pure pine forests has become very simple and the planting area too large, a large number of survival and ecological problems in *P. yunnanensis* pine plantations have emerged. For example, diseases and insect pests are becoming more and more serious, the mountain ecosystem security has been seriously threatened, soil fertility and water retention have declined sharply, and biodiversity has been severely reduced [3,4]. Therefore, the outlook for pure coniferous forests is not optimistic. A change in the species composition and structure of forest stands, the biological characteristics, and the ecology of forest trees is needed to establish a more

**Citation:** Liang, C.; Liu, L.; Zhang, Z.; Ze, S.; Ji, M.; Li, Z.; Yu, J.; Yang, B.; Zhao, N. Do Mixed *Pinus yunnanensis* Plantations Improve Soil's Physicochemical Properties and Enzyme Activities? *Diversity* **2022**, *14*, 214. https://doi.org/10.3390/ d14030214

Academic Editors: Michael Wink, Lin Zhang and Jinniu Wang

Received: 12 January 2022 Accepted: 11 March 2022 Published: 14 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

complex and stable forest ecosystem than pure forests provide in order to increase the ecological and economic benefits of the forest. A large number of studies have proved that mixed forests play an important role in enhancing soil fertility, improving forest nutrient status, increasing forest species diversity, improving the stability of stand structure and productivity, and improving the ecological environment of forest plants [5–7]. Therefore, planting mixed forests is particularly important. Changing the species structure of forest stands leads to more complex and stable mixed forests [8]. Mixed forest is an important form of forest species and tree species allocation in the shelter forest system, and it is also a good way to form a reasonable structure of tree species. By selecting appropriate tree species to mix with *P. yunnanensis* pine, we can make full use of aboveground and underground energy and space so as to increase the biomass of mixed forests and improve the soil' nutrient status of forest land. This provides an important theoretical basis for the growth of forest vegetation and the construction of *P. yunnanensis* pine mixed forests.

Changes in plant diversity are known to affect aboveground and belowground ecosystem functioning, including the diversity of belowground communities of organisms [9–13]. Additionally, higher plant diversity may produce a higher biochemical diversity of root exudates, which further increases the soil's organic matter content [14]. Soil is an important part of the forest ecosystem and plays an important role in maintaining the stability of the terrestrial ecosystem, material circulation, and energy conversion [15]. Soil is also the foundation of plant growth and development. Soil fertility and quality directly determine the biological output and function of the forest. Foresters have always relied on knowledge of the soil's physiochemical properties to assess the value of the forests [16]. Soil fertility is a comprehensive reaction of the soil's physiochemical and biological properties [17]. In terms of chemical properties, the stoichiometric characteristics of soil N and P can reflect not only soil fertility but also the composition of soil's organic matter, soil quality, and the capacity of nutrient supplies [18]. Therefore, the content and distribution pattern of N and P available in the soil is important for plant growth. Soil enzymes can promote chemical reactions in organisms and are the total embodiment of soil's biological activity [19]. In addition, soil enzyme activities regulate the function of ecosystems and play a key role in nutrient cycling, which has been used by scholars as a parameter to evaluate soil quality [20]. The quality of the soil largely depends on the function of the soil, which indicates a combination of its physical, chemical, and biological characteristics [21]. The quality of soil nutrients is the result of the combined effects of soil physics, chemistry, biology, and other factors [22]. It is common to evaluate soil quality and nutrients by measuring the soil's physicochemical and biological indexes.

In recent years, a large number of experimental studies have focused on the innovation of *P. yunnanensis* cultivation technology, but they lack an understanding of the physical and chemical properties of *P. yunnanensis* plantation soil. Therefore, it is important to evaluate the advantages of different mixed forests by studying soil quality. In this study, the *P. yunnanensis* pine pure forest was used as the control group, and different mixed forest groups were set up to analyze and compare the physicochemical properties and enzyme activities of soil. This work provides a theoretical reference for establishing a mixed pattern of *P. yunnanensis* pine and other tree species, which could not only improve the forest economy but also enhance its ecological value.
