## 4 Discussion
### Effect of Fertilizer on Plant Nutrient Resorption  
Acquisition (root uptake) and conservation (resorption from senescent tissue) are two important biological strategies for plants to maintain balanced nutrition where soil nutrients are deficient. These processes are also important in cycling nutrients between the soil and plants [78]. Choosing between alternative strategies for plants depends on the cost (time and energy) of each process and the species characteristics. Short-term experiments have demonstrated that, generally, plants will reduce nutrient utilization efficiency when the surrounding nutrient availability increases [73], while the relationship between foliar resorption and soil nutrient concentration is not consistent [34; 79; 80].  
Nutrient resorption was another important index indicating environmental nutrient deficiency or not. Our results indicated a constant or slight decreased NRE, which further verified our supposition concerning N enrichment in these orchards (Figure 5). According to nutrient balance theory, N addition aggravated P-limitation that was illustrated by increasing PRE. However, increased P migration from senescent leaf to green leaf (PRE) in response to increasing N + P fertilizer supply conflicts with the negative correlation between fertilizer treatment and nutrient resorption [73]. This kind of counterintuitive result has been observed in a single-resource conservation standpoint in previous studies [81; 82]; yet multiple-element theory much more appropriately explains variability in foliar nutrient resorption [67]. Based on a Pearson correlation analysis, we found that green leaf [P] was not related to soil [AP], but it was highly correlated with soil N content (Table 3). It should be noted that the root system can directly uptake inorganic nutrients (AP and HN) and therefore plays a more important role in nutrient absorption than organic nutrients. This inorganic nutrient preference may help explain the absence of a common trend in studies comparing P resorption to soil P availability [34; 80]. In our study, although the soil and leaves were characterized by a higher [P] compared to other studies of evergreen trees [52], excess N addition was responsible for the amplified P-limitation (Figure 5), which subsequently elevated leaf P uptake directly [83; 84].  
The standardized major axis (SMA) analysis showed that leaf [P] is independent of soil [P], which supports the above hypothesis. Strong dependencies, indicated by a slope \(>\) 1, were found only between senescent and green leaf [C] (Table 4). For an evergreen species with a long leaf life span, _T. grandis_ exhibited a more conservative P use strategy as opposed to the external root uptake strategy, regardless of soil P availability. Similar results were also reported in some non-mycorrhizal species [85]. This mechanism allows plants to maintain a higher nutrient absorption efficiency with relatively less cost (energy and time) but lowers the utilization efficiency and productivity [86].  
On these bases, contrary to previous research [87], fertilizer treatment in this study also failed to increase the vegetative (indicated by the annual shoots) and reproductive growth (indicated by the yield of fresh seeds) of _T. grandis_. Although moderate P addition improved foliar nutrients content and seed yields slightly, while high intensity fertilization in both scenarios showed negative effects on seed yield (Table 3). Moreover, the fertilizer significantly reduced the C:P (indicating the P utilization efficiency in productivity) of _T. grandis_ in this study, suggesting that the P utilization efficiency of plants was reduced by the increased nutrient supply [88]. Our analysis suggested that both P and N fertilizer might be unnecessary for _T. grandis_.