**1. Introduction**

Forest plantations in China have an area of 69 million hectares, occupy 31.8% of the total area of Chinese forests, and play important social, economic, and environmental roles [1]. However, most of these plantations were planted as monoculture conifers, such as Chinese fir (*Cunninghamia lanceolate* (Lamb.) Hook.) and *Pinus massoniana* Lamb., which might lead to nutrient imbalance, soil degradation, and the reduction of ecosystem stability [2]. In Southern China, Chinese fir has been widely planted and provides excellent commercial woods with easy processing because it is a native, fast-growing woody species with a 20–25 year rotation period and more than 1000 years of managemen<sup>t</sup> practice history [3]. The removal of harvested wood and accumulated litter could alter the biogeochemistry in Chinese fir plantations, which may result in the decline of forest productivity due to multiple rotations [4]. Therefore, converting Chinese fir monoculture into mixed with broadleaved trees has become a common trend of forest management, since the mixed plantations seemed to have many advantages over pure stands, such as higher rates of litter decomposition and maintaining soil nutrient cycling [5], i.e., there were hardly any, or even negative e ffects of mixture on ecosystem functions, such as tree biomass accumulation and useful wood yield [6].

Phosphorus (P) is a key limiting nutrient in terrestrial ecosystems especially in the tropics and subtropics, where P is limited in the highly weathered soil and is found in pools with low amount available for plant [7]. Moreover, soil P utilization percentage is only 10%–25% in subtropical China [8] in comparison to P-rich soil from other tropics of the world. Shenoy et al. found that suboptimal levels of P could decrease crop growth and resulted in 5%–15% yield losses [9]. Many Chinese fir plantations have been established in these P-deficient sites, therefore, the availability of soil P is one of the most important limiting factors causing a decline in forest productivity because Chinese fir has a high use efficiency of which P is required for its optimal growth [8]. As mentioned above, the mixed-species forests have many advantages over pure plantations, and can improve root distribution patterns, plant residue compositions, and soil microbial diversity [10]. Theoretically, mixed forest plantations with Chinese fir and broadleaved trees would stimulate P cycling by mobilizing soil fixed P and increasing plant uptake, compared to pure stands of Chinese fir. However, the e ffect of mixed species on soil–tree P cycling in the Chinese fir forest ecosystem is little studied [8] and future P scenario cannot be e ffectively forecasted with the increasing conversion from pure plantations to mixed forests.

The nutrient availability is usually a major constraint to tree growth in most terrestrial ecosystems, while trees take up most of the mineral nutrients through the rhizosphere. The rhizosphere is a vital region of ecosystem biogeochemistry and governs the nutrient availability through plant roots to stimulate soil microbial activity [11]. Nutrients in the rhizosphere can rapidly transform from unavailable to available forms through the interaction of root exudates and microorganisms [12]. Plants generally secrete 10%–30% root exudates into rhizosphere soil through the root system. In order to increase the carbon availability in rhizosphere, microorganisms utilized those root exudates as food and energy [13]. Therefore, the microorganism population density might be much larger in the rhizosphere than in bulk soil. Plants absorb most mineral nutrients with e ffective nutrient transformation and mineralization dominated by rich microorganisms in rhizosphere soil [11]. Unfortunately, the di fference of nutrient transformation and availability between the rhizosphere and bulk soil is less understood especially in forest ecosystems [14]. The modified P functional fractionation method in acidic soils provides an e ffective tool to explore the P supply process in the rhizosphere soil of subtropical forests [15].

On the other hand, the e ffects of species interactions on ecological processes such as nutrient cycling and tree growth in mixed stands have raised significant interest in recent years [2,16]. However, little is known about the influence of mixture on tree internal nutrient cycling [17], for example, the nutrient concentrations in various components of fresh tissues found in trees and their litters [18]. This result suggests that there is a change in plant physiology based on what is around the plant, or is it simply a function of competition? The understanding of internal nutrient cycling is of grea<sup>t</sup> importance because it not only depends on site nutrient availability but also reflects the competition among the di fferent tree species in the mixed forest [19]. By now, mixed-species plantations containing a leguminous species have shown the potential to improve nutrient cycling [20]. However, there is a lack of evidence to validate the potential of mixed-species plantations without leguminous species, particularly in the tropics and subtopics [16]. There is no significant di fference between mixtures and monocultures, when all of the empirical studies are combined [21]. The lack of a general trend, and the limited number of studies on the impact of mixed stands on tree nutrients, switches further studies. In addition, nutrient dynamics generally vary with the age of plant tissue. For example, more nutrients were resorbed to young leaves from old leaves when the acquisition of nutrients from the soil became more di fficult [22]. Thus, the di fferentiation among the di fferent ages of plant tissues might be helpful to explore nutrient internal cycles in response to the changing environment.

In the present study, eight plots of single-species Chinese fir plantation (pure plantation, PP) and eight plots of mixed plantation with a broadleaved tree species (mixed plantation, MP) with similar standing density in mature forest were selected to assess the e ffect of mixed species on soil and tree P cycling including belowground rhizosphere processes and nutrient concentrations of di fferent functional root orders, and nutrient concentrations in aboveground components (fresh twigs, needles, and their litters) of di fferent ages in Hunan and Fujian provinces of China. Both are the center distribution zones of Chinese fir production in China [23]. Our main hypotheses are: (1) the mixture with broadleaved trees improves soil P availability, and then increase tree nutrient levels; (2) the mixture e ffect (the di fference between PP and MP) on rhizosphere soil P supply varies with P fraction, and depends on its rhizosphere e ffect (the increasing percentage in rhizosphere soil compared to bulk soils); and (3) the response of Chinese fir nutrients to the mixed with broadleaved tree varies with root orders, needle and twig ages due to combined e ffects of soil nutrient supplies, tree species competition, and plant physiological requirement. In addition, we explored the relationships among the nutrients of belowground component (root–soil), and aboveground nutrients (fresh twigs, needles, and their litters) in these ecosystems. Our results will help assess the potential e ffect of stand transformation from the monoculture plantation to mixed forest, and will provide theoretical support for forest management.
