**5. Conclusions**

We found that, compared with the Chinese fir plantation, the mixed Chinese fir plantation with a broadleaved species increased nutrient availability in both rhizosphere and bulk soils. Meanwhile, the introduction of broadleaf species to pure Chinese fir plantation generally decreased the tree nutrient availability of the transportive root and two-year old needles of Chinese fir, possibly due to the lower competition of Chinese fir than broadleaved species for nutrient uptake. Therefore, the effect of mixture may bring about a negative nutrient feedback between P availability in rhizosphere soil and P concentration in plant tissues attributed to root exudates and plant competition between Chinese fir and broadleaf species. In conclusion, replacing monoculture plantations of Chinese fir into mixed-species forest promotes soil nutrient availability, and it might be an option for multi-purpose forest managemen<sup>t</sup> in China, from where the largest areas of Chinese fir plantations alter the functioning of the soil ecosystem. However, the mixed forest may a ffect P and N stocks of Chinese fir tissues. Thus, the mixture e ffect may di ffer in soil and plant nutrients, and when soil nutrient availability is expected to be improved, converting a pure conifer plantation into a mixed-species forest can be considered.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1999-4907/11/4/461/s1, Figure S1: The study sites in Huitong Forest Ecological Station of Hunan Province and Datian Experimental Forest Farm of Fujian Province, Table S1: Summary of ANOVA about e ffects of forest type, soil type, site location and their interaction on phosphorus fractions under pure Chinese fir plantation (PP) and its mixed plantation with broadleaved tree species (MP) at Hunan and Fujian provinces, China, Table S2: ANOVA of e ffect of forest type, root functional order, site location and their interaction on root nitrogen, phosphorus concentrations and N/P between pure Chinese fir plantation (PP) and mixed plantation with broadleaved tree species (MP) at Hunan and Fujian province, China.. Table S3: ANOVA of e ffect of forest type, leaf age, site location and their interaction on leaf nitrogen, phosphorus concentrations and N/P ratio between pure Chinese fir plantation (PP) and mixed plantation with broadleaved tree species (MP) at Hunan and Fujian province, China. Table S4: ANOVA of effect of forest type, twig age, site location and their interaction on twig nitrogen, phosphorus concentrations and N/P ratio between pure Chinese fir plantation (PP) and mixed plantation with broadleaved tree species (MP) at Hunan and Fujian province, China.

**Author Contributions:** F.-S.C. contributed to the study conception and design. Material preparation, data collection and analysis were performed by W.-S.B., H.-J.G., C.-c.Z., Y.Z., A.N.S., X.-M.F., J.F., H.-M.W. and F.-S.C. The first draft of the manuscript was written by W.-S.B. and all authors commented on previous versions of the manuscript. All authors have read and agreed to the published version of the manuscript

**Funding:** This study was supported by grants from the National Natural Science Foundation of China (31730014, 31,870,427, and 31760134), Jiangxi Provincial Department of Science and Technology (20181ACH80006), the China Scholarship Council (201908360227), and the Jiangxi Province Science Foundation for Youths (20181BAB214014). We greatly appreciate Xiaoli Fu and Shebao Yu for their help in field sampling.

**Conflicts of Interest:** The authors declare no conflict of interest.
