## 1 Introduction
[28] reported N saturation in three subtropical sites and noted that P deficiency is becoming progressively more problematic. The anthropogenic alternation of regional P and N cycling has led to large areas of southern China forests shifting to human-induced P-limitation [27]. NPP/GPP has transformed from being N-limited to P-limited in many forest ecosystems [4; 26; 29; 30]. Soil N and P availability, especially when combined with N deposition and/or external addition, can influence forest productivity and ecosystem processes [3; 31]. Trees can keep leaf nutrient concentrations and their ratios stable by modulating the nutrients coming from branches, roots or senescent leaves [32; 33]. Recently, foliar N and P concentration stoichiometry ratios (i.e., N:P, C:N, C:P) have been used to indicate soil N- and/or P-limitations on plant growth [24; 34; 35]. In addition, nutrient resorption [nitrogen (NRE) and/or phosphorus (PRE)] from senescing plant tissues and the proficiency of nutrient conservation [36], are also widely used as indicators in studies of nutrient cycling between plants and soil in fluctuating environments. Generally, nutrients would transfer from senescent leaf to trunk before falling off, thereby maintaining the plant nutrients at a favorable level. The NRE and PRE might depend on the type of nutrient limitation [37] and vary in response to the plant's intrinsic genetic characteristics [38; 39]. NRE/PRE is commonly employed to determine the relative limitation between N and P; NRE/PRE values > 1 imply a stronger N-limitation at the ecosystem scale [26]. However, previous studies predominantly focused on the effect of single N or P addition on leaf nutrients or resorption; thus, the effect of multiple nutrient addition on NRE and/or PRE in subtropical forests is not well understood. Meanwhile, the relationship between plant P uptake and nutrient environmental supply is more comprehensive than N. Under intensive management, the long-term addition of balanced compound fertilizer may cause excess P in orchards, as plants generally require less P than N [40]. P is an essential element for nucleic acids and membrane lipids. Although P sensing and signaling are not fully understood, there appears to be a series of physiological processes in plants that are either stimulated or suppressed in response to P supply [41]. Unlike the immobile N in the plant cell wall, most leaf P is hydrolyzable and, therefore, more easily resorbed [42; 43]. In fact, due to its indiscriminate uptake, greater variability of foliar P has been reported even in growth stages in which it is not needed [44]. Species in P-poor environments, such as subtropical evergreen trees, are equipped with a corresponding adaptation mechanism that makes them more susceptible to toxic eutrophication under excessive P addition [45]. At present, there is not even a clear relationship between N and P fertilization with economic benefit (such as seed yield) in our research area; furthermore, an excessive or unreasonable rate of fertilizer consumption faces both increasing economic investments and the deterioration of the soil's physical and chemical properties, let alone its internal nutrient influence mechanism. Thus, it is critical to determine the optimal amount of fertilizer necessary to achieve ideal growth. A comprehensive understanding of fertilizer impact on crop quantity and soil quality is critical for improving fertilizer treatment strategies in economic consideration and maintaining a healthy soil environment. To solve the above problems, we examined the C, N, and P stoichiometry of soil, as well as the green and senescent leaf of mature _T. grandis_ trees, in nine orchards with varying N and N + P fertilizer treatments.