C, N, and P contents and stoichiometric ratio characteristics in the organs of different PFGs
Significant differences in C, N, P contents and stoichiometries were detected in multiple organs among the different PFGs (Fig. 2). Our results proved that the plants of the different PFGs allocated most of their N and P to leaves, followed by roots and branches (Fig. 2b, c). This is in general agreement with the results of previous studies conducted in forests, shrubs and grasslands systems (Hong et al. 2014; Yan et al. 2016; Zhang et al. 2018b). Leaves with relatively high metabolic activity are responsible for many physiological functions and require higher quantities of N and P for biochemical reactions than organs with lower metabolic activity (Wright et al. 2004). In addition, roots are responsible for nutrient and water absorption, while branches are responsible for bud initiation and nutrient exchange (Zhang et al. 2018a). Therefore, relatively higher N and P concentrations in roots and branches represent high rates of nutrient recycling and could allow the higher levels of transport loading (Zhang et al. 2018b). The significantly higher N and P concentrations in the leaves than in the other organs resulted in lowest leaf C:N and C:P ratios (Fig. 2d, e). Previous studies also showed that C:N and C:P ratio values were lower in leaves than in other organs (Cao and Chen 2017; Jiang et al. 2017). In comparison, the N:P ratio in leaves was much higher than that in other organs (Fig. 2f). This is because leaves with high metabolic rates (“metabolic” organs) can maintain a relatively constant N:P ratio to meet the diverse physiological needs of metabolic processes, while other organs (“structural” organs), with P concentrations rising faster than N concentrations, show a decrease in the N:P ratio (Kerkhoff et al. 2006; Zhang et al. 2018a). These results indicate that the functional differences among plant organs resulted in diverse nutrient concentrations; and that more metabolically active organs had higher N and P nutrient contents.
At the PFG level, tree organs had significantly higher C concentrations and C:N and C:P ratios than shrub and herb organs (Fig. 2a, d, e). A possible explanation for this result is that to support their large skeletal structures, trees synthesize more photosynthate and accumulate more organic matter than understory plants (Cleveland et al. 2011). Herbaceous plants had significantly higher N and P concentrations than the woody species (Fig. 2b, c). This result may be explained by the fact that herbs require high nutrient investment to support their rapid growth and reproduction during the relatively short growing season (Zheng and Shangguan 2007). Furthermore, plant nutrients exhibit a size scaling pattern in which N and P concentrations are diluted with increasing plant size; therefore, small plants (herbs) have high N and P concentrations (Elser et al. 2010). In the present study, herb leaves were observed to have lower N:P ratios than tree and shrub leaves (Fig. 2f). This is consistent with the growth rate hypothesis, which indicates that rapidly growing organisms have a low N:P ratio (Elser et al. 2000). However, the N:P ratio in herb roots was higher than that in the woody plant roots (Fig. 2f), which is inconsistent with previous research (He et al. 2008). This inconsistency might be caused by the roots not being classified into more fine classes, as fine roots (diameter < 2 mm) were not evaluated separately, because fine roots have higher N and P nutrient concentrations than coarse roots (Shen et al. 2017). Previous studies have proposed that a leaf N:P ratio < 14 indicates N limitation, while a N:P > 16 indicates P limitation (Koerselman and Meuleman 1996). In this study, the leaf N:P ratios of all PFGs were less than 14 (Fig. 2f), which suggests the occurrence of N limitation in the study area.
Contributions of soil and litter nutrient characteristics to the nutrient characteristics of different PFGs
In our study, the C concentration in the tree organs was negatively correlated with the SOC content (Fig. 3). First, SOC is mainly derived from the metabolism of amino acid, which results in a positive relationship between SOC and plant organ N and P (Sinsabaugh et al. 2010; Zhang et al. 2019). Simultaneously, previous study reported that the fixation of C during the process of plant metabolism requires the participation of a large number of proteases (which consumes a high amount of N) and that the assembly of proteases requires the replication of a large number of nucleic acids (which consumes a great deal of P), which results in negative relationships of C with N and P in plant organs (Elser et al. 2000). Thus, plant C has a negative correlation with SOC. The C concentration in the tree organs was positively correlated with the organic carbon in the litter (Fig. 4), because litter C is derived from that in plant organs (Zhang et al. 2017). In contrast, we found no clear relationship between the C concentration in shrub and herb organs and soil and litter organic carbon (Fig. 5–6). A possible explanation for this result may be that trees produce a higher amount of litter biomass annually and can recycle more C than shrubs and herbs by microorganisms (Liu et al. 2018), which may obscure the relationship between the C concentration in shrub and herb organs and soil and litter organic carbon.
In present studies, the N and P contents in the tree organs and P contents in herb organs were generally positively correlated with those in the soil and litters, and the stoichiometric ratios in the different organs of the trees and herbs were also correlated to some extent with those of the soil and litter (Fig. 3, 4, 6 and Tables 3, 4). These finding aligns with previous studies showed that organisms and their environments are closely connected by the exchange of chemical elements (Ladanai et al. 2010; Odum et al. 1972). Litter is the main form of nutrient restitution, and plants primarily absorb nutrients from the soil (Hatton et al. 2015; Townsend et al. 2007); consequently, the dynamic cycling of nutrients among plants, litter and soil supports their close relationship. Moreover, the inconsistent nutrient connections between plant organs and soil and litter indicated the different PFGs had different nutrient utilization strategies in the ecosystem (Zhang et al. 2019). However, our results revealed the decoupling of the relationship between nutrient elements in shrub organs and that in soil and litter (Fig. 5). One possible explanation for this might be that shrubs can obtain nutrients from different sources, e.g., atmospheric nutrient deposition and biological fixation (especially for N) (Gundale et al. 2010; Phoenix et al. 2010), which might lead to the observed decoupling phenomenon. Additionally, earlier studies had proved that the decoupling of nutrient cycling relationships among different components of the ecosystem can be observed when plants respond passively to external environmental conditions (Ladanai et al. 2010). In our study region, shrubs are often passively disturbed by herbivores; therefore, the state in which nutrient elements are retained in shrub organs may have changed, resulting in the decoupling of the cycling of nutrients among the shrub organs, soil and litter. In generally, these results confirm our hypothesis that C, N, P contents and stoichiometries in plant organs is correlated with litter and soil nutrient characteristics.
The aim of this study was to examine the potential contributions of soil and litter nutrient characteristics to the nutrient characteristics of different PFGs. Our VPA results show that soil nutrient characteristics were the main controlling factors of the nutrient characteristics of the different PFGs (Fig. 7). As mentioned in the Introduction, soil is the main nutrient source for plants; therefore, soil nutrients play a dominant role in the nutrient characteristics of plants (Han et al. 2011). In addition, the litter nutrient characteristics also had important contributions to the nutrient characteristics of the three PFGs, showing the following order: herbs > trees > shrubs (Fig. 7). Because nutrient release from litter is a continuous process, nutrients infiltrate the soil from the top to the bottom (Zeller et al. 2001). Furthermore, plant roots show some degree of below ground niche partitioning, and the root systems of different PFGs have different depth distributions (Carrera et al. 2000; Cremer et al. 2016). These two factors lead to a hierarchy order of plants absorbing nutrients released from the litter, which in turn leads to litter nutrients having different influences on the nutrient characteristics of different PFGs. Previous studies have also reported that herbaceous roots are mainly distributed in shallower soil than woody plant roots; hence, herbs can preferentially absorb nutrients returned from the litter (Ren et al. 2017; Zhang et al. 2019). Thus, litter nutrient characteristics contribute more to the nutrient characteristics of herbaceous plants than woody species. Overall, these VPA results imply that both soil and litter nutrients play the important roles in plant nutrient cycling.