C, N, P and K and their stoichiometric of Wuyi Rock tea cultivars
The leaf N and P had no significantly relationship for the three tea cultivars, indicating that the tea leaf N and P nutrients were relatively independent and has a weak interaction. There were also no correlations among other leaf nutrients of tea cultivars. This due to nutrients of three tea cultivars intercept atmospheric dust, soil fine particulate matter, and other organic debris. These results were different from previous studies of allometric relationship between leaf N and P (Reich and Oleksyn 2004; Zhu et al. 2020).
The allocation of contents C, N, P, and K directly affects plant growth and different tea cultivars had different nutrients in different components. In this study, with the exception of C, N, and P in the leaf component, nutrients of litter and soil components were significantly different in different tea cultivars. Nutrients in the three components for the same tea cultivar also had significantly differences, this was related to their biological characteristics. The leaf N and P nutrients of three tea cultivars were higher than those of the global plants (N: 20.1 g·kg− 1, P:1.99 g·kg− 1) (Elser et al. 2000) and China’s flora (N: 20.2 g·kg− 1, P:1.5 g·kg− 1) (Han et al. 2005; Ren 2012), while leaf C was lower than global and China’s plants, this was link to the weak acquisition of C resource and high N and P absorption and utilization efficiencies due to the distribution of shrubs and clusters of tea cultivars. Leaf N:P is an important indicator for determining nutrient limitation for the plants. When leaf N:P < 14, the plant growth is limited by N, if 14 < N:P < 16, the plant growth is restricted by N and P co-limitation, and if N:P > 16, P limitation was limited for the plant growth (Koerselman and Meuleman 1996). In this study, the ratio of N:P was lower than 14 of three tea cultivars, indicating that those tea cultivars growth were limited by N, this result was consistent with the lower N content in the soil, further reflecting that the study area provided less available N for the Wuyi Rock tea growth. The ratios of leaf C:N and C:P in the three tea cultivars were lower than that of global plants (Han et al. 2005; Ren 2012), suggesting that the Rock tea cultivars had strong adaptation and soil was infertile in this study. Leaf K content was lower than the soil but higher than the litter due to K in the litter was prone to return to the soil in the Rock tea cultivars.
Litter is an intermediary linking soil and plant, and quality and decomposition of litter directly affect the soil nutrient, regulate the nutrient cycling, and then impact on the plant growth (Bai et al. 2019; Peng et al. 2022). The contents of N and P in the litter for the three tea cultivars were higher than those of global plants (Kang et al. 2010), indicating that the tea cultivars had high nutrient utilization efficiencies. Litter C:N is an indicator of litter decomposition rate, and lower litter C:N had higher litter decomposition rate due to strong the microbial activity and invertebrate digestion (Shen et al. 2021). In this study, litter C:N in the JG was higher than that of the RG and SX, indicating that the JG had higher litter decomposition rate than the RG and SX, and the JG had high nutrient turnover and cycling. Related studies have reported that litter N and P are completely absorbed by leaves when litter N < 7g·kg− 1 and P < 0.5g·kg− 1, but the litter N and P are not completed absorption, when litter N > 10 g·kg− 1 and P > 0.8g·kg− 1 (Killingbeck 1996; Yang et al. 2018). In this study, three tea cultivars showed N and P were not fully absorbed, which were consistent with lower N and P resorption efficiencies of three tea cultivars.
Soil nutrients are main nutrient source for plant growth. The contents of C and N in the JG and RG were higher than the SX, demonstrating that the JG and RG had strong C storage capacity in the soil and water conservation site. With the exception of soil organic C, the understory soil nutrients of the Wuyi Rock tea showed nutrient deficiency, especially P contents. This was due to Wuyi Rock tea cultivars had strong adaptation to barrenness, and they were used for soil and water conservation in the mountainous and hilly areas (Mekonnen 2021). With the exception of the RG (416:10:1), the ratio C:N:P in the soil for the JG (111:3:1) and SX (124:3:1) were lower that global ecosystems (186:13:1) (Cleveland and Liptzin 2007) and Chinese soil (134:9:1)(Tian et al. 2009), especially C:P was 416:1 of the RG, indicating that lower P resource in the soil of the RG than that of the JG and SX. Overall, three tea cultivars showed P deficiency in the soil. Conversely, Wuyi Rock tea cultivars showed strong adaptation to environment and high ecological benefits (Liu et al. 2020).
Nutrient resorption efficiencies of different tea cultivars
Different Wuyi Rock tea cultivars had different nutrient absorption efficiencies. With the exception of KRE in the SX, the values of NRE, PRE and KRE of the Wuyi Rock tea were lower than those of global plants (Aerts 1996; Chen et al. 2021b; Vergutz et al. 2012) due to probably leaf mass loss was not considered for the estimation of NuRE and different plant growth forms. A relatively less nutrient resorption in Wuyi Rock tea cultivars under nutrient limited conditions in the soil and water conservation area was in agreement with leaf economic spectrum (Wright et al. 2004). NRE and PRE of this study were also lower than trees and shrubs at the global scale (Yuan and Chen 2009). Compare to shrub, the trees have stronger ability to absorb water and soil nutrients with more fine roots (Kulmatiski et al. 2010). Interesting, we found that PRE of three tea cultivars had significantly lower values than NRE and KRE, indicating that P cycling was slow in Wuyi Rock tea ecosystem, which could reduce dependence on P resource. Increased utilization of N and K, especially N, could alleviate the effect of P deficiency on the growth of tea tree. In this study, NRE was higher than PRE in three tea cultivars, indicating that Wuyi Rock tea plants resorbed more N than P in the soil and water conservation area, which did not support our first hypothesis. The information of NRE and PRE were helpful to understand nutrient-use strategies and nutrient limitation of plant (Zhang et al. 2022a; Zhang et al. 2020). KRE (> 65%) were higher in this study than NRE and PRE, as more K was resorbed from senesced leaves to mature leaves than retained in the senesced leaves, further confirming that K was resorbed preferentially during the process of regeneration of senesced and new leaves (Du et al. 2016; Sartori et al. 2022).
The scaling exponents of NRE VS PRE of the Wuyi Rock tea cultivars were smaller than 1, which was consistent with previous reports (Han et al. 2005; He et al. 2020). Leaf N is involved in the synthesis of protein and leaf photosynthesis (Ghimire et al. 2017)d plays an important role in the synthesis of nucleic acids. Compared to P, most of leaf N is invested in Rubisco, and could be re-mobilized from the senesced leaf (Poorter et al. 2006; Veneklaas et al. 2012). With the exception of NRE vs KRE of the JG, KRE increased with the increases of NRE and PRE, indicating that tea plants had a positive relationship between K and N and P. Foliar nutrient status is assessed to nutrient availability for understanding the nutrient resorption patterns (Streit et al. 2022; Xing et al. 2022). In this study, there is a positive relationship between the green leaf N and nutrient resorption efficiencies, suggesting that tea plants had selection favors with high N content in the green leaves tend to resorb more N nutrient in P deficient area, these results were not consistent with the previous studies (Kobe et al. 2005; Tong et al. 2021; Vergutz et al. 2012). Simultaneously, the nutrient resorption efficiencies had significantly negative relationship with the senesced leaf nutrients in this study, indicating that more nutrients in the senesced leaves were reabsorbed when NuRE were higher (Zhang et al. 2022b). These findings confirmed that environmental and biological factors drive the nutrient resorption plasticity (Drenovsky et al. 2019).
Different tea cultivars had different nutrient resorption efficiencies. All three tea cultivars had low PRE and high KRE in this study. For the JG and SX, the lower PRE and higher KRE were attributable to higher C:P and low C:K in the soil (low P and high K), indicating that the study site was P deficiency and K abundance, as confirmed by positive relationships between PRE and the soil C:P and between KRE and the soil C:K. These findings were in accordance with the results of Chen and Chen (2021). The low P and high K in soil were mainly caused by tea tree litter input, and could increase KUE and reduce PUE for tea cultivars in the soil and water conservation site which allowing the tea plants depend more on leaf nutrient and less on soil nutrients. Moreover, low PRE in Wuyi Rock tea enhanced P nutrient return to soil during the litter decomposition for increasing available P in the soil (Xu et al. 2020). This study site has low P nutrient in the soil, resulting in positive effects on P uptake and then lead to low PRE (Fig. 1). In addition, more N remain in the senesced leaves of the tea cultivars highlighted that the tea ecosystem accelerated N cycling and reduced dependence on P resource for the tea growth in P poor site. These findings were different from some previous studies that reported improved P cycling for plant growth (Brant and Chen 2015; Chen and Chen 2021). Further, high KRE in three tea cultivars was attributable to high K in plant and soil systems.
Response of nutrient homeostasis and nutrient utilization strategies
Homeostasis theory holds that organisms maintain the relative stability of its elements to adapt to the changes of the external environments (Sardans et al. 2012). The strength of homeostasis is evaluated by the ability of the plants response to environment (Yu et al. 2010). In this study, leaf N and P contents of the JG and SX had strong homeostasis, indicating that the leaf N and P nutrients were less vulnerable to soil nutrient status. Nevertheless, leaf N and P contents of the RG showed low homeostasis, indicating that leaf N and P contents of the RG change with soil P contents. These results partially supported the second hypothesis. Compared to leaf N of the RG, leaf P had stronger homeostasis, this finding was inconsistent with the results of Yu et al. (2010) due to N limitation in that study, but our study site was P limited. Our results, to a certain extent, also verified “the Stability of Limiting Elements Hypothesis”(Han et al. 2011; Sterner and Elser 2002), that is, limiting elements remained relatively stability in response to the changes of the external environments.
Nutrient resorption efficiencies of different tea cultivars had different relationships with soil nutrients and leaf traits. For the JG and RG, soil nutrients mainly affected PRE and KRE, but NRE was closely related to leaf traits. Our results indicated that inherent features of tea cultivars might be a good indicator of NRE. This finding seemed to be in accordance with earlier studies (Deng et al. 2019; Wang et al. 2022a). In addition, NRE was closely related to soil K for the RG, indicating that soil nutrients might be served as an indicator of nutrient resorption efficiencies in the soil and water conservation site. Overall, our results were essentially in agreement with the third hypothesis.
In this study, soil nutrition had no relationship with leaf N and P contents, but green leaf nutrition of tea cultivars directly regulated the nutrient use efficiencies. N, P, and K of the senesced green leaves of different tea cultivars had significantly effect on the nutrient resorption efficiencies, but nutrient resorption efficiencies were less affected by the nutrition of green leaves. Those result also confirmed the framework in this study. KRE and KUE of Wuyi Rougui and Wuyi Shuixian were higher than that Wuyi Jingui, indicating that the strong acquisition and resorption of phosphorus and potassium in soil erosion area. In the management of tea trees, Wuyi Rougui and Wuyi Shuixian is more suited to planting in the phosphorus and potassium deficiency area than other Wuyi Rock tea cultivars. Therefore, Wuyi Rougui and Wuyi Shuixian are suitable species for restoration in soil and water conservation, soil erosion area.