**Mixed Broadleaved Tree Species Increases Soil Phosphorus Availability but Decreases the Coniferous Tree Nutrient Concentration in Subtropical China**

**Wen-Sheng Bu 1,2, Han-Jiao Gu 1,2, Can-can Zhang 1,2, Yang Zhang 1, Anand Narain Singh 3, Xiang-Min Fang 1,2, Jing Fan 1,2, Hui-Min Wang 4 and Fu-Sheng Chen 1,2,\*,**†


Received: 10 March 2020; Accepted: 17 April 2020; Published: 19 April 2020

**Abstract:** Phosphorus (P) is a key limiting nutrient in subtropical forests and mixed forests with broadleaved species have been expected to stimulate P cycling, compared to pure conifer plantations. However, the mixture e ffect of Chinese fir (*Cunninghamia lanceolata* (Lamb.) Hook.) and broadleaved species on rhizosphere soil and coniferous tree P dynamics is unclear. In our study, eight plots of a single species of a Chinese fir plantation (pure plantation, PP) and eight mixed plantations (mixed plantation, MP) with broadleaved tree species (*Michelia macclurei* Dandy in Hunan Province or *Schima superba* Gardn. et Champ. in Fujian Province) were selected in subtropical China. Six P fractions in the rhizosphere and bulk soils were analyzed by a modified Hedley P fractionation method. Phosphorus fractions and nitrogen (N) concentrations in di fferent root orders, di fferent age fresh needles and twigs, and needle and twig litter of Chinese fir were measured. Our results showed that available P, slowly released P, occluded P, and the total extractable P in rhizosphere soil were significantly higher in MP than PP (*p* < 0.05). In contrast, P and N concentrations in the transportive roots and two-year old needles were generally higher in PP than MP. Meanwhile, the slowly released P, occluded P, total extractable P, and residual P in rhizosphere soil were negatively correlated with P concentrations in young (absorptive and transportive roots, one- and two-year old needles) but not old tissues (storative roots, three-year old needles and litters). In conclusion, mixture may increase soil P availability through the rhizosphere e ffect, but can decrease P and N concentration of Chinese fir tissues by competition between Chinese fir and broadleaved species. Clearly, the mixture e ffect may di ffer in soil and plant nutrients, and this issue needs be taken into consideration when converting a pure conifer plantation into a mixed-species forest.

**Keywords:** *Cunninghamia lanceolata*; mixture effect; nutrient cycling; rhizosphere effect; species competition
