**3. Results**

### *3.1. Plant Growth and Soil Nutrients*

After 4 years of N addition, the aboveground biomass of *D. dichotoma* and *L. gracile* significantly decreased by 82.1% and 67.2%, respectively (*p* < 0.05), while the biomass of *M. dodecandrum*, which was much lower than that of the other two species, did not respond to N addition (Table 1). Moreover, N addition did not alter the richness of understory plants (Table 1). In contrast, the average girth growth rates of overstory trees (Chinese fir) within four years after N addition treatment and litterfall productivity in the fifth year significantly increased by 18.28% and 36.71%, respectively (Figure 1, Table A2).


**Table 1.** The aboveground biomass and richness of the major understory plants in the Chinese fir forest plantation treated by nitrogen addition or in the control in 2015.

The values are the means ± SE (*n* = 4). ns = not significant at *p* > 0.05 level.

**Figure 1.** Girth growth from 2011 to 2015 (**a**) and litterfall production in 2016 (**b**) of the Chinese fir tree plantation treated by nitrogen addition (+N) or in the control (CK). Note: Values shown are the means ± SE (*n* = 4). Lowercase letters indicate significant differences at the *p* < 0.05 level between the control and N-treatment plots.

As expected, the rhizosphere soil NH4+-N and NO3−-N concentrations of all three species were generally higher in the N addition treatment than in the control, while the available P concentration and AP and NAG activities were unaffected by N addition in both rhizosphere and bulk soils (except for NAG activity, which significantly increased in the rhizosphere soil of *L. gracile* in response to the N addition treatment) (Table 2).


**Table 2.** Available nutrients and enzyme activities in rhizosphere and bulk soils of the three understory plant species in the Chinese fir plantation treated by nitrogen addition or in the control.


**Table 2.** *Cont.*

The values are the means ± SE (*n* = 4). Available P = Available phosphorus, NAG = N-acetyl-β-D-glucosaminidase, AP = acid phosphatase; ns = not significant at *p* > 0.05 level.

### *3.2. Nutrients and NSCs in Plant Tissues*

Nitrogen addition, plant species, and plant tissue had significant interactive effects on the nutrients and NSC concentrations of the three understory plants (Figure 2). The average tissue N concentration increased, P and ST concentrations did not change, and average SS and NSC concentrations decreased in response to the N addition treatment (Figure 2). When analyzed by species and tissue type, N addition significantly increased the N concentration in both tissue types of all three species (Figure 2a); significantly increased the P concentration in the aboveground tissue of *M. dodecandrum* (Figure 2b); significantly decreased the ST concentration in the aboveground tissue of *M. dodecandrum* and belowground tissue of *D. dichotoma* (Figure 2d); significantly decreased the SS in the aboveground and belowground tissues of *M. dodecandrum* and the aboveground tissue of *D. dichotoma* (Figure 2c); and significantly decreased the NSC concentrations of *M. dodecandrum* while significantly increasing the NSC concentration in the aboveground tissue of *L. gracile* (Figure 2e).

**Figure 2.** Total nitrogen (**a**), total phosphorus (**b**), soluble sugar (**c**), starch (**d**) and nonstructural ccarbohydrates (**e**) concentrations in aboveground and belowground tissues of the three understory plant species in the Chinese fir plantation treated by nitrogen addition (+N) or in the control (CK). Note: Values shown are the means ± SE (*n* = 4). The asterisks (\*) indicate significant differences at the *p* < 0.05 level between the control and N-treatment plots within a single species. Different capital letters indicate significant differences (*p* < 0.05) among the three species within the same tissue based on Duncan's multiple range test. Total N = total nitrogen, Total P = total phosphorus, SS = soluble sugar, ST = starch, NSC = nonstructural carbohydrates; N = nitrogen addition, S = species, T = tissues. NS, not significant; \* *p* < 0.05; \*\* *p* < 0.01; \*\*\* *p* < 0.001. DID: *D. dichotoma*, LOG: *L. gracile*, MED: *M. dodecandrum.*

### *3.3. The Ratios of N, P, and NSCs in Plant Tissues*

Similarly, N addition, plant species, and plant tissue had significant interactive effects on the ratios of N, P, and the NSCs of the major understory plants (Figure 3). Nitrogen addition did not affect the N/P ratio in the aboveground tissue of any species, but it significantly increased the belowground N/P ratio in all plant tissues (Figure 3a). For *M. dodecandrum*, both the NSC/N and NSC/P ratios significantly decreased in the aboveground and belowground tissues due to N addition; for *D. dichotoma*, only the NSC/N ratio in the aboveground tissue significantly deceased with N addition; and for *L. gracile*, both the NSC/N and NSC/P ratios were unaffected by N addition (Figure 3b,c).

**Figure 3.** N/P (**a**), NSC/N (**b**), NSC/P (**c**) in aboveground and belowground tissues of the three understory plants in the Chinese fir forest plantation treated by nitrogen addition (+N) or in the control (CK). Note: Values shown are the means ± SE (*n* = 4). The asterisks (\*) indicate significant differences at the *p* < 0.05 level between the control and N-treatment plots within a single species. Different capital letters indicate significant differences (*p* < 0.05) among the three species within the same tissue based on Duncan's multiple range test. N = nitrogen, P = phosphorus, NSC = nonstructural carbohydrates; N = nitrogen addition, S = species, T = tissues. NS, not significant; \* *p* < 0.05; \*\* *p* < 0.01; \*\*\* *p* < 0.001. DID: *D. dichotoma*, LOG: *L. gracile*, MED: *M. dodecandrum.*

### *3.4. Linkages between Plant Tissue Acquirement and Rhizosphere Soil Supply*

The tissue N concentration was generally and positively correlated with rhizosphere soil NH4+-N and/or NO3−-N for each of the three understory plants (Table 3). The belowground tissue P concentration was significantly and positively correlated with rhizosphere soil available P for *D. dichotoma* and *L. gracile* but not for *M. dodecandrum*, while the aboveground tissue P concentration was significantly and positively correlated with rhizosphere soil NO3−-N for *L. gracile* and *M. dodecandrum* but not for *D. dichotoma* (Table 3). The belowground tissue NSC concentration was negatively correlated with rhizosphere soil NH4+-N for *M. dodecandrum* and *D. dichotoma* and positively correlated with rhizosphere soil available P for *L. gracile*, while the aboveground tissue NSC concentration was positively correlated with rhizosphere soil NH4+-N and NO3−-N for *L. gracile* and negatively correlated with rhizosphere soil NH4+-N for *M. dodecandrum* (Table 3). Likewise, the correlations among the ratios of N/P, NSC/N, NSC/P and soil available nutrients varied with plant species and tissues (Table 3).


**Table 3.** The correlation coefficients (*n* = 8) between plant tissue and rhizosphere soil nutrients for each of the three understory plant species in the Chinese fir plantation.

NSCs = nonstructural carbohydrates; ns, not significant, *p* > 0.05; \* *p* < 0.05; \*\* *p* < 0.01.
