**1. Introduction**

Plant nutrient resorption is the process by which a plant transfers nutrients to its growing tissues before the branches and leaves wither, effectively lengthening the retention time of nutrients in plants [1,2]. This process reduces the reliance of plants on soil nutrients, helping to maintain the ecological stoichiometry balance in plants; it also reduces nutrient leaching after litter decomposition, minimizing the ecosystem nutrient loss [2,3]. Nutrient resorption from senescing organs is one of the

most important mechanisms for plants to adapt to nutrient limitation and improve nutrient utilization efficiency through conserving and optimizing the use of nutrients. This is also an important strategy to regulate nutrient cycling within plant–soil systems. Nutrient resorption can be defined as nutrient resorption efficiency (NuRE), i.e, the proportion of nutrients withdrawn from green leaves prior to senescence [4]. Therefore, research on the plant NuRE is helpful to reveal the response mechanism of plants to a nutrient-poor environment.

Nitrogen (N) and phosphorus (P), strong limiting factors of natural ecosystems, play an important role in plant growth and metabolism [5,6]. Previous studies have reported that natural forests in low latitudes areas (e.g., tropical forests) are usually limited by N [7] and those in high latitudes are limited by P [6]. In addition, chronosequence studies have shown that young forests are usually vulnerable to N limitation, while ageing forests are usually vulnerable to P limitation, especially in areas with nutrient deficiency or imbalanced N and P input [8,9]. Plant N and P resorption efficiency (NRE and PRE, respectively) attract ample attention because they play an important role in revealing the adaptability of plants to a nutrient limiting environment [10,11]. Vergutz estimated that the mean NRE and PRE for global terrestrial plants were 62.1% and 64.9%, respectively [12]. However, the NRE and PRE varies greatly depending on the species [13] and life-form [14]; NRE and PRE are also easily influenced by environmental factors (e.g., soil nutrient availability, elevation, temperature, precipitation, etc.) [13,15,16] or disturbances [17].

In forest plantations, plant photosynthetic characteristics, soil nutrient supply, and plant nutrition demands usually vary with stand age [18]. These changes may have substantial effects on plant nutrient conservation strategies, thus leading to the change of NuRE with stand age [10,19]. However, previous studies have shown inconsistent trends (e.g., increase [18], decrease [20], and no significant change [21]) of NuRE in response to stand age, which may be related to the tree species and nutrition status [10,21]. The measurement of C:N:P stoichiometry is an effective way to investigate plant and soil nutrient status. For instance, leaf N:P was used in a previous study to indicate the N (N:P < 14) or P (N:P > 16) limitation of the plants [22]. Therefore, studying NRE and PRE characteristics and their possible links with nutrition stoichiometry, together with forest age changes, may help to reveal the adaptability of forests to varying levels of nutrients [18,19]. This is of grea<sup>t</sup> significance to the sustainable managemen<sup>t</sup> of plantations that are subject to nutrient restriction [10].

Due to its nitrogen fixation, fast growth, and tolerance to drought and barren lands, black locust (*Robinia pseudoacacia* L.) has become one of the most widely introduced tree species for ecological restoration and timber production worldwide [23–25]. The Loess Plateau, China, is a very ecologically fragile region that suffers from severe soil erosion and low availability of nutrients [26]. To restore the region, black locust has been introduced as the dominant plantation species with the aim to aid in soil and water conservation [27,28]. Black locust can significantly improve the soil quality by accumulating soil C and N [25,29]. However, black locust plantations grown in the long-term (for approximately 35 years) suffer from withering branches and a decline in productivity, which may indicate forest degradation [30]. Soil nutrient limitation (usually P limitation in the arid areas) [29,31] and water deficits [32,33] also may become serious with stand development. However, few studies have been conducted on how the resorption of N and P vary with the age of the black locust plantation. Therefore, we studied the adaptability of black locust to the changing nutrient status, especially the variation of NuRE with stand age, which may reveal the possible mechanism of degradation of the black locust plantation.

In this study, we examined the changes of nutrients in soil and leaves (both mature leaves and withered leaves) of black locust plantations covering an age range from 10 to 45 years old in the Loess Plateau of China. The objectives of this study were to quantify the variations of N and P resorption across the black locust plantation chronosequence and to find out whether there are any relationships between NuRE and nutrient stoichiometry indexes in leaves and soil. These data were then used to reveal the responses of plant nutrient use strategy to the altered leaf and soil nutrient status in black locust plantations of different ages. Because nutrient resorption is an important nutrient conservation mechanism and the plant nutrient demand may increase with stand age, we hypothesized that the N and P resorption will: (1) increase across the black locust plantation chronosequence, and (2) show significant negative relationships with leaf and soil N and P contents. The results may provide novel insight into the understanding of the nutrient cycling of black locust forests in arid and semi-arid areas.
