Nutrient resorption, a physiological process by which plants reallocate nutrients from senescent structures to other living tissues for later use (Clark 1977; Turner 1977; Yuan and Chen 2015), can improve nutrient utilization (Chapin 1980; Vitousek 1984; Wu et al. 2020) and reduce plant nutrient uptake from the environment (Brant and Chen 2015; Lü et al. 2020; Yuan and Chen 2015). Nutrient resorption is most commonly quantified by nutrient resorption efficiency (RE) and resorption proficiency (RP) (Lü et al. 2020; Wu et al. 2020). Nutrient RE is the difference between the amount of a given nutrient in green versus fully senesced tissue relative to the amount in green tissue, and nutrient RP is the absolute level of nutrients found in senesced leaves (Chang et al. 2017; Killingbeck 1996). N and P resorption play an important role in affecting plant growth (Sterner and Elser 2002), contribute to leaf N and P, and determine plant photosynthesis, reproduction, and physiological processes (Kerkhoff et al. 2006; Koerselman and Meuleman 1996; Tian et al. 2018). Resorption is estimated to supply 31% and 40% of annual plant N and P demands on a global scale, respectively (Cleveland et al. 2013). Changes in nutrient (N and P) supply influence plant nutrient resorption (Chen et al. 2007). however, these findings remain highly controversial (Lü et al. 2020; Yuan and Chen 2015), especially for N addition.
The increase in atmospheric N deposition, mainly derived from burning fossil fuels and by using artificial fertilizers, is an important phenomenon in global climate change (Janssens et al. 2010). The latest research shows that the average annual N deposition in China reached 19.6 ± 2.5 kg N ha− 1 yr− 1, which far exceeded that of Europe and the United States; thus, N deposition rates in China are among the most serious in the world (Yu et al. 2019). High N deposition can change plant N and P resorption by changing the N and P content in plants and soils, which in turn influences the N and P cycles in ecosystems (Du et al. 2016; Farrer et al. 2013; Lü et al. 2020; McNeil et al. 2007; Yuan and Chen 2015; Zhao et al. 2020). A meta-analysis by Yuan and Chen (2015) showed that N enrichment inhibited plant N resorption; however, other studies observed neutral and positive effects on plant N resorption (Li et al. 2010; Lü et al. 2010, 2013, 2020; van Heerwaarden et al. 2003). N addition also promoted (Lü et al. 2013), inhibited (Sardans et al. 2016), and/or had no effect (Lü et al. 2010; Zhang et al. 2017b) on foliar P resorption in forests. However, these studies have only partially observed N or P resorption; few studies have simultaneously considered resorption of both nutrients (N and P) in forests.
Biochar is produced by the pyrolysis of organic matter in a high-temperature and oxygen-limited environment (Antal and Grønli 2003) and is widely applied in forestry ecosystems (Li et al. 2018) for soil amendment (Jeffery et al. 2015). It has a high surface area and high pH and contains various forms of N and P nutrients (e.g., NH4+ and ortho-P) (Gul and Whalen 2016). Over the past few decades, most studies have focused on the effects of biochar amendments on soil physical and chemical properties, the soil organic carbon pool, and soil greenhouse gas emissions (Li et al. 2018; Song et al. 2016a). For example, biochar application enhanced soil fertility by increasing soil pH and cation exchange capacity (CEC), thereby increasing soil N and P concentrations (Biederman and Harpole 2013; Chan et al. 2007; Nelson et al. 2011), which affected foliar N and P concentrations (Major et al. 2010; Zhang et al. 2019). However, there are relatively few studies addressing the potential effects of biochar application on plant N and P resorption. By understanding these mechanisms, we can predict potential long-term changes in plant productivity in biochar-amended forests, especially in subtropical plantations where soils are usually acidic.
Moso bamboo (Phyllostachys edulis), one of the most economically important bamboo species, is widely distributed in the tropical and subtropical regions of East and Southeast Asia (Song et al. 2011, 2020). In China, it covers an area of 4.68 million hectares, accounting for 73% of the total bamboo forest area in China (Li and Feng 2019). Due to its rapid growth and strong regenerative ability (Song et al. 2016b), Moso bamboo is the main source of non-timber forest products in China (Song et al. 2015) and has a high potential for C sequestration (Song et al. 2017a). The subtropics of China, the main growing region of Moso bamboo, is subjected to high N deposition, with an average rate of 30 kg N ha− 1 yr− 1 (Jia et al. 2014), which is expected to continue to increase in the next few decades (Galloway et al. 2008; Liu et al. 2013). Our previous study found that N addition increased foliar N and P concentrations and soil available N (AN) and available P (AP) in Moso bamboo plantations (Li et al. 2016; Song et al. 2016c). In addition, biochar applications significantly increased soil bacterial diversity and decreased soil urease and acid phosphatase activities (Li et al. 2018; Peng et al. 2019). However, the effects of N deposition and biochar amendment on leaf nutrient resorption in Moso bamboo forests are still unclear.
In this study, we applied N and biochar to a Moso bamboo plantation to investigate leaf nutrient resorption by Moso bamboo plants and their responses to N deposition and biochar amendment. The primary hypotheses of this study were the following: (1) N addition reduces foliar nutrient (N and P) resorption due to an increase in soil nutrient availability; (2) biochar amendment reduces foliar nutrient (N and P) resorption due to an increase in soil nutrient availability; and (3) biochar amendment enhances the negative priming effect of N addition on foliar nutrient (N and P) resorption.