## 3 Results
### Plant Nutrient and Growth Characteristics  
Fertilizer addition increased foliar nutrient content to some extent, which depended on the nutrient element and leaf life stage (Figure 4). Green leaf uniformly exhibited a higher nutrient concentration ([C], [N], and [P]) than senescent leaf, but nutrients showed different patterns under the two fertilization scenarios. For green leaf, moderate fertilization increased leaf [N] and [C] in both N and N + P addition scenarios, while there was no significant difference among other treatments conditions (\(p>0.05\), Figure 4). For senescent leaf, the resorption proficiency ([C], [N] and [P]) trend paralleled that of the green leaf, although the magnitudes differed. Similarly, there was no significant difference in [C] and [N] among different treatments in the senescent leaf. While P addition increased [P] in both green and senescent leaf, [P] exponentially increased with the N + P fertilizer supply gradient (Figure 4). Green leaf [P] was significantly correlated with soil inorganic nitrogen concentration (indicated by soil hydrolyzable nitrogen, HN) rather than inorganic phosphorus (indicated by soil AP) (Table 2), while there was no correlation of leaf [N] with either soil P or N content.  
Figure 3: Soil total N:P and total C:N under different treatment. Error bars refer to \(\pm 1\) standard error (different capital letters indicate a significant difference among factors under N addition only; lowercase letters indicate a significant difference among factors under N + P addition; * indicate a significant difference between two fertilization scenarios under same addition intensity, \(p<0.05\)).  
The C:N ratio of both green (2.15 \(\pm\) 0.05 under N + P treatments; 2.12 \(\pm\) 0.08 under N treatments) and senescent (2.47 \(\pm\) 0.09 under N + P treatments; 2.15 \(\pm\) 0.25 under N treatments) leaves showed no significant difference among the treatments (Figure 4). The significant increase in [P] under N + P treatments led to big variations in foliar N:P (Figure 4), which declined with increasing fertilizer addition. There was no significant effect of fertilization treatments on nutrient utilization efficiency in carbon fixation indicated by C:P and C:N.  
The fresh seed yields of _T. grandis_ in these orchards averaged 203.2 \(\pm\) 15.0-230.2 \(\pm\) 8.4 kg/tree (Table 3), with a significant decrease under high fertilization (P\({}_{0}\)N\({}_{4}\) and P\({}_{1}\)N\({}_{4}\)), while moderate N + P fertilization increased seed yield. The number of annual shoots per 0.5 m-long branch ranged between 57.0 \(\pm\) 2.3 and 60.5 \(\pm\) 4.2 and showed no significant difference under fertilizer treatments (\(p>\) 0.1).  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & **Soil TN** & **Soil TP** & **Soil HN** & **Soil AP** & **SOC** \\ \hline Leaf P & 0.789 ** & 0.809 ** & 0.906 * & 0.541 & 0.691 ** \\ Leaf C:P & \(-\)0.564 * & \(-\)0.548 * & \(-\)0.917 * & \(-\)0.507 & \(-\)0.476 \\ Leaf N:P & \(-\)0.582 * & \(-\)0.549 * & \(-\)0.900 * & \(-\)0.488 & \(-\)0.512 \\ \hline \end{tabular}  
* indicate statistical significances at \(\alpha\) 0.01 and 0.05.  
\end{table}
Table 2: Pearson correlation coefficients of the nutrients in leaves and soil in the N + P addition scenario.  
Figure 4: Effect of different fertilizer treatments on the green (left) and senescent (right) leaf nutrient content ([C], [N] and [P]) and C:N, N:P ratios. Error bars refer to \(\pm\)1 standard error (different capital letters indicate a significant difference among factors under N addition only; lowercase letters indicate a significant difference among factors under N + P addition; * indicate a significant difference between two fertilization scenarios under same addition intensity, \(p<\) 0.05).