**3. Results**

### *3.1. Leaf and Soil C, N, and P Concentrations*

The concentrations of the C and N in green leaves significantly increased from RP10 to RP36; however, the concentrations were decreased from RP36 to RP45 (*p* < 0.05). The concentrations of C and N ranged from 380.81–475.38 g kg−<sup>1</sup> and 22.38–32.49 g kg−<sup>1</sup> in all stand ages, respectively (Table 2). The P concentration of green leaves showed no significant variation among the different stand ages, except P concentration in RP45 was significantly lower (*p* < 0.05).

**Table 2.** Leaf and soil C, N, and P concentrations in different stand aged black locust forests.


The data are shown as mean ± standard error.

The C and N concentrations of senesced leaves increased from RP20 to RP45 with stand age (*p* < 0.05). The concentrations of C and N ranged from 347.27–453.97 g kg−<sup>1</sup> and 11.64–19.40 g kg−<sup>1</sup> in all stand ages (Table 2). The P concentration of senesced leaves showed an increasing trend with stand age, with a range of 1.14–1.37 g kg−1. The C, N, and P concentrations were all higher in green leaves than that in senesced leaves in each stand age. The only exception was the C concentrations in RP45, which was higher in senesced leaves (*p* < 0.05).

The C and N concentrations of soil significantly increased with stand ages (*p* < 0.05), ranging from 4.56–14.41 g kg−<sup>1</sup> and 0.50–1.20 g kg−1, respectively. The soil P concentrations showed no significant changes with stand ages (Table 2).

### *3.2. Leaf and Soil C:N:P Stoichiometry Ratios*

The C:P and N:P in green leaves increased with stand age (*p* < 0.05), ranging from 184.22–245.60 and 10.83–16.72 with an average of 218.50 and 14.11, respectively (Figure 1). The C:N of green leaves declined as a whole with stand age (*p* < 0.05), ranging from 14.60–17.56 with an average of 15.70. The C:N, C:P, and N:P of senesced leaves all increased and then decreased with stand age (*p* < 0.05), with ranges of 22.62–29.95, 274.35–355.60, and 10.73–15.54 with averages of 23.69, 326.77, and 13.84, respectively (Figure 1). The highest C:N values in senesced leaves were measured in RP20; the highest C:P and N:P in senesced leaves were measured in RP30. Generally, the C:N and C:P of senesced leaves were higher than that of green leaves in all stand ages (*p* < 0.05), and the N:P of senesced leaves were higher from RP10 to RP30 and lower from RP36 to RP45 compared with that of green leaves (*p* < 0.05).

**Figure 1.** Leaf and soil C:P ratio ( **A**), N:P ratio (**B**) and C:N ratio ( **C**) in black locust forests with different stand ages. Different lowercase letters represent significant differences among stand ages at *p* < 0.05, and the same letters indicate no significance.

The soil C:P and N:P increased with stand age (*p* < 0.05), ranging from 8.66–25.47 and 0.93–2.11 with averages of 14.98 and 1.48, respectively. The soil C:N decreased by 11.8% from RP10 to RP20, and then increased by 47.8% from RP30 to RP45 (*p* < 0.05), with an average C:N of 9.83 (Figure 1).

### *3.3. NRE and PRE in Forests of Different Stand Ages and Their Relationship*

The NRE and PRE varied among stands of different ages, with the tendency to increase and then decrease with stand age (*p* < 0.05). The NRE and PRE values ranged from 46.8% to 57.4% and from 37.4% to 58.5%, with averages of 52.61 and 51.89, respectively. The highest values for NRE and PRE were obtained in RP20 (Figure 2A). The NRE was higher than the PRE in RP10 and RP20, and lower in other stand ages (*p* < 0.05). A significantly positive correlation was found between NRE and PRE ( *R*<sup>2</sup> = 0.316, *p* < 0.05) (Figure 2B). In addition, the PRE:NRE decreased with the age of the stand (*p* < 0.05) (Figure 2C).

**Figure 2.** (**A**) Changes of N and P resorption efficiency (NRE and PRE), (**B**) PRE:NRE along the chronosequence of black locust forests, and ( **C**) the relationship between NRE and PRE. Data are shown as the mean ± standard deviation. Different lowercase letters represent significant differences among stand ages at *p* < 0.05, and the same letters indication no significant difference.

### *3.4. Relationships between NuRE, Nutrient Concentrations, and Stoichiometry Ratios in Leaves and Soil*

Results from the Pearson correlation analysis showed that the NRE was significantly positively correlated with the C of green leaves and P of senesced leaves (*p* < 0.05); the NRE was significantly negatively correlated with the C:N of senesced leaves among the different stand ages (*p* < 0.05), whereas no significant correlation was found with other indicators (Table 3). The PRE was significantly correlated with most of the nutrient concentration and stoichiometry indicators in leaves and soil (*p* < 0.05). The exception for this was that the soil P; C, N, and C:N of green leaves; and C:N, C:P, and N:P of senesced leaves were not significant. The PRE was positively correlated with the P concentrations of green leaves, whereas it was negatively correlated with other significant related indicators (Table 3).


**Table 3.** Pearson correlations between nutrient resorption efficiency, nutrient concentrations, and stoichiometry ratios in leaf and soil samples.

NRE and PRE represent the N and P resorption efficiency, respectively. *R* is the Pearson correlation coefficient.
