*3.1. Soil General Properties*

Organic C and total N concentrations in rhizosphere and bulk soils were generally higher in MP than PP in both sites. However, the C/N ratio of rhizosphere soil in PP at the Hunan site was higher than that in MP, while it showed the opposite trend at the Fujian site. The C/N ratio in bulk soil was not significantly different between PP and MP in both sites. Organic C and total N were generally higher in rhizosphere than bulk soil under all the treatments. Meanwhile, organic C and C/N in rhizosphere and bulk soils were generally higher in the Fujian than in the Hunan sites, but the total N was not significantly different between both sites. In addition, there is not a significant difference in pH between rhizosphere and bulk soils and among the four treatments (Table 2).

### *3.2. Phosphorus Fractions in Rhizosphere and Bulk Soils*

Average concentrations of soil available P, slowly released P, and occluded P (not soluble P), mineral P and residual P reflected significantly higher concentrations in MP than PP whether in rhizosphere or bulk soil (Figure 1 and Table S1). Average concentrations of soil soluble P, slowly released P, and residual P were higher in rhizosphere than bulk soil, and these three P fractions showed positive rhizosphere effects in all treatments. In contrast, the rhizosphere effects of available P, occluded P, and mineral P varied with forest type and site location (Figure 1 and Table S1). Soil P fractions including slowly released P, occluded P, mineral P, and residual P, but not available P, and soluble P was generally higher in the Fujian than the Hunan sites (Figure 1 and Table S1).

**Table 2.** General chemical properties of pure Chinese fir plantations (PP) and mixed plantations with a broadleaved tree species (MP) in Hunan and Fujian provinces of China. Values are means and standard errors of four replicates).


Note: Different letters indicate significant differences among four treatments at probability level of *p* < 0.05.

**Figure 1.** Variations in rhizosphere and bulk soil phosphorus fractions in pure Chinese fir plantations (PP) and mixed plantations with broadleaved tree species (MP) in Hunan and Fujian provinces of China. Note: The bar represents standard error and meaning of abbreviated words are as follow: PP-HN indicates the pure plantation in Hunan province, MP-HN indicates the mixed plantation in Hunan province, PP-FJ indicates the pure plantation in Fujian province, MP-FJ indicates the mixed plantation in Fujian province. Av-P indicates available P, So-P indicates soluble P, Sl-P indicates slowly released P, Oc-P indicates occluded P, Mi-P indicates mineral P, Re-P indicates residual P. The symbols located above and below the dotted line indicate the positive and negative rhizosphere effect, respectively. Among treatments, different small and capital letters indicate the significant differences in rhizosphere and bulk soils, respectively, probability level *p* < 0.05.

When the data from Hunan and Fujian were pooled, total P values were not significantly different between PP and MP either in rhizosphere or bulk soil, but the total extractable P was higher in MP than PP in both rhizosphere and bulk soils (Figure 2). The percentage of total extractable P to total P increased from 14.8% to 16.3% in bulk soil and from 13.8% to 16.8% in rhizosphere soil with the broadleaved tree species mixture. Among the five extractable fractions, slowly released P was the dominant form and its percentage to total extractable P ranged from 73.6% to 77.7%, followed by occluded P (11.2%~14.1%) in our study (Figure 2).

**Figure 2.** The components of soil phosphorus fractions in pure Chinese fir plantations (PP) and mixed plantations with broadleaved tree species (MP) in Hunan and Fujian provinces of China. Note: The bar represents standard error. Av-P indicates available P, So-P indicates soluble P, Sl-P indicates slowly released P, Oc-P indicates occluded P, Mi-P indicates mineral P, Re-P indicates residual P. Different small and capital letters indicate the significant differences of total extractable P (the sum of Av-P, So-P, Sl-P, Oc-P, and Mi-P) and total P, respectively, among the four soil sources (two forest types in rhizosphere or bulk soils, probability level *p* < 0.05.

### *3.3. Nutrient Levels in Di*ff*erent Root Orders*

Total P concentration in absorptive and transportive roots is higher in PP than MP in the Fujian site, whereas it was not significantly different in roots between PP and MP in the Hunan site. Total N concentrations in absorptive and transportive roots were higher in PP than MP in each site of Hunan and Fujian provinces. Thus, the average N concentration in roots was also higher in PP than MP, although there were not statistical differences between both forest types in storative roots. The N/P ratio in all orders of roots seemed to be higher in PP than MP in the Hunan site (Figure 3 and Table S2). Meanwhile, total P and N concentrations were generally higher in absorptive than transportive and storative roots, and the N/P ratio was highest in storative roots, followed by transportive roots, and lowest in absorptive roots. In addition, the average P concentration was significantly higher in the Fujian than the Hunan site, while the average N concentration and N/P ratio showed the opposite trends (Figure 3).

### *3.4. Nutrient Distributions in Aboveground Tissues*

Average P concentrations in the two-year old fresh needles were generally higher in PP than MP, although P concentrations in other aged needles were not significantly different between the two forest types (Figure 4a,b). In contrast, average N concentrations of all ages' needles and in each ages' needles were not significantly different between two forest types except higher N concentration in PP than MP in the two-year old fresh needles (Figure 4a,b and Table S3). Average P concentration in needles across all plots was highest in fresh needle of one-year old, followed by two- and three-year old needles and

lowest in needle litter, while average N concentration was higher in fresh needles of one- and two-year old than three-year old needles and needle litter (Figure 4a,b). The ratio of N/P varied slightly between the two forest types and between the two site locations, but was higher in needle litter than fresh needles of various ages (Figure 4c).

**Figure 3.** Phosphorus and nitrogen concentrations and N/P ratios of Chinese fir various functional roots in pure Chinese fir plantations (PP) and mixed plantations with broadleaved tree species (MP) in Hunan and Fujian provinces of China. (**a**) Total Phosphorus (g kg−1), (**b**) Total nitrogen (g kg−1), (**c**) N/P. Note: The bar represents standard error. Different small and capital letters indicate the significant differences among the four treatments within the root order, and among the three root orders, respectively, probability level *p* < 0.05.

Both P and N concentrations of twig were not significantly different between PP and MP, although N concentrations of the two- and three-year old twigs were significantly larger in PP than those in MP at the Hunan site. Additionally, those nutrient concentrations were highest in fresh twigs of one-year old, and lowest in twig litters, and significantly higher in Hunan than Fujian sites when just considered the effect of site location (Figure 5a,b and Table S4). The average N/P ratio was not significantly different between two forest types and between two site locations, but highest in twig litters and lowest in fresh twig of one-year old (Figure 5c and Table S4). In addition, the stem N and P concentrations and N/P ratio were not significantly different between both forest types except for a higher N/P ratio in PP than MP in the Fujian site (Figure 5).

**Figure 4.** Phosphorus and nitrogen concentrations and N/P ratios of Chinese fir different-aged fresh needles and needle litters between pure Chinese fir plantations (PP) and mixed plantations with broadleaved tree species (MP) in Hunan and Fujian provinces of China. (**a**) Total Phosphorus (g kg−1), (**b**) Total nitrogen (g kg−1), (**c**) N/P. Note: The bar represents standard error. Different small and capital letters indicate the significant differences among the four treatments within a same age needle, and among these four age needles, respectively, probability level *p* < 0.05.

**Figure 5.** Phosphorus and nitrogen concentrations and N/P ratio of Chinese fir different-aged fresh twigs, twig litters, and stems between pure Chinese fir plantations (PP) and mixed plantations with broadleaved tree species (MP) in Hunan and Fujian provinces of China. (**a**) Total Phosphorus (g kg−1), (**b**) Total nitrogen (g kg−1), (**c**) N/P. Note: The bar represents standard error. Different small and capital letters indicate the significant differences among the four treatments within a same-aged twig, and among the four ages of twigs and stems, respectively, probability level *p* < 0.05.

### *3.5. Linkages Among Soil, Root and Needle Nutrients*

Unlike our expectations, we did not find any positive relationships between all P fractions in rhizosphere soils and P concentrations in roots of different functions and needles of various ages. In contrast, slowly released P, occluded P, and residual P negatively correlated with the P concentrations in some functional roots and fresh needles of some ages, and the extractable P negatively correlated with P concentration in one-year old needles (Table 3). Meanwhile, the rhizosphere effect of some P fractions negatively correlated with P concentration in some roots, fresh needles, and needle litters except for a positive correlation between soluble P and P concentration in absorptive roots (Table 3).

Likewise, slowly released P, occluded P, mineral P, extractable P, and residual P in rhizosphere soils negatively correlated with N concentrations in transportive or storative roots, and in two- or three-year old needles. However, rhizosphere soil soluble P concentration was positively correlated

with absorptive root N concentration, and both mineral P and extractable P were positively correlated with N concentration in one-year old needle (Table 4). Meanwhile, the rhizosphere effects of slowly released P and residual P were positively correlated with N concentrations in two- and three-year old needles, but negatively correlated with N concentration in one-year old needle. In addition, N concentration in needle litter was not significantly correlated with any P fractions and their rhizosphere effects (Table 4).

**Table 3.** Correlation efficiencies (*n* = 16) between rhizosphere soil phosphorus supplies (rhizosphere soil P fractions and their rhizosphere effects) and tree tissue phosphorus concentrations (total P of various Chinese fir tissues) under all studied stands in Hunan and Fujian provinces, China.


Note: Av-P indicates available P, So-P indicates soluble P, Sl-P indicates slowly released P, Oc-P indicates occluded P, Mi-P indicates mineral P, Re-P indicates residual P. ns indicates not significant, \* *p* < 0.05, \*\* *p* < 0.01.

**Table 4.** Correlation efficiencies (*n* = 16) between rhizosphere soil phosphorus supplies (rhizosphere soil P fractions and their rhizosphere effects) and tree tissue nitrogen concentrations (total N of various Chinese fir tissues) under all studied stands in Hunan and Fujian provinces of China.


Note: Av−P indicates available P, So−P indicates soluble P, Sl−P indicates slowly released P, Oc−P indicates occluded P, Mi−P indicates mineral P, Re−P indicates residual P. ns indicates not significant, \* *p* < 0.05, \*\* *p* < 0.01.
