4.1. The C, N, P contents and stoichiometry in the plant-litter-soil system
The C, N and P contents of plant tissues reflect the capacity of plants to absorb and store nutrients (Zhen 2014), as well reflect the distribution strategy of nutrients among organs when plants respond to various soil environment (Li 2019). As the activity center of photosynthesis, the nutrient content of leaves is always concerned. In this study, the average contents of C, N and P in needle were 649.59 g·kg-1, 10.31 g·kg-1, 0.77 g·kg-1, respectively, Which were the highest than other tissues. Needles are often considered a source organ (Chapin III et al., 1990), carbohydrates and proteins containing C, N, P elements produced by photosynthesis are temporarily stored in needles to maintain cellular metabolic activities (Piper, 2011). In addition, the branch C (616.60 g·kg-1) and N (5.95 g·kg-1) contents were higher than those in root, indicating that pines were more inclined to allocate nutrients to aboveground tissues. Root transport nutrient to needle along the trunk and branch after absorbing nutrient from the soil, providing sufficient nutrient for photosynthesis. This is in line with the principle of priority distribution of nutrient (Dietze et al., 2014), that is, plants prefer to allocate more nutrient to tissues in need. The litter C (551.41 g·kg-1) and N (4.28 g·kg-1) content were much lower than those in other plant tissues. This seems to be caused by the nutrient return strategy of plants. The litter transferred its own nutrient to other tissues before aging and falling (Würth et al., 2005). Simultaneously, the litter provided abundant carbon sources for soil microorganisms, which accelerated the decomposition of nutrient, and finally returned the decomposed nutrient to the soil (Cotrufo et al., 2015). The complex strategy of nutrient allocation among plant tissues is beneficial for plants to adapt the changing environment.
Plant ecological stoichiometry emphasizes the relationship among the main elements C, N and P of plants (Coile 1934), reveals the adjustment mechanism of nutrient proportion in the plant-litter-soil system. The C:N and C:P ratios of needle were 61.64-65.21 and 801.82-976.95, respectively, which were higher than the average level (40.40, 728.00) of coniferous forests in subtropical regions (Wang et al., 2011). This result indicated that Pinus elliottii in the study area has a stronger ability to assimilate C than other conifer species. Compared with the CK, P2 and P3 treatments reduced the C:N ratios of root and litter, which seems to be the P2 and P3 addition level accelerate the transport of C element from root to other tissues and expedite the decomposition of C element in litter. The N:P ratio of leaves was used as an indicator to measure the demand for N and P of ecosystem (He et al., 2008). In this study, the C:P and N:P ratios of needle were 801.82-976.95 and 13.13-15.25, respectively, which decreased gradually with the increase of P addition level. The plant growth rate hypothesis (Yu et al., 2012) suggests that plant adapt to their own growth rate by regulating the distribution of C:N:P during growth and development. Generally, the growth rate is negatively correlated with C:P and C:N ratios (Yu et al., 2012). The results of this study were consistent with this hypothesis, because the positive correlation between growth rate and P addition level has been confirmed in our unpublished studies. According to the N:P threshold hypothesis (Koerselman et al., 1996), it was judged that the P limitation of Pinus elliottii plantation was alleviated with the increase of P addition level. However, the factors affecting plant N:P ratio are complex (Sabine et al., 2004), and it is unreliable to evaluate the limiting elements in plant growth with a single index. The limiting elements of the ecosystem should be determined by a long-term experiment combined with soil environmental conditions and the ability of nutrient absorption of other associated tree species.
The mean values of soil C, N and P contents under different treatments were 12.88 g·kg-1, 0.99 g·kg-1 and 0.75 g·kg-1, respectively. The C and P contents were higher than the national average (11.12 g·kg-1, 0.65 g·kg-1), and the N content was lower than the national average (1.06 g·kg-1) (Tian et al., 2010). In addition, with the increase of P addition level, the contents of soil C and P increased gradually, indicating that P addition promoted the accumulation of C and P element in soil. The stoichiometric ratio of soil be used to evaluate the soil quality (Fan et al., 2015). Our results showed that the average value of C:N ratio was 13.37, which was higher than the national average level (11.09) (Tian et al., 2010), indicating that the soil N mineralization ability was strong in the study area. The soil C:P (17.74) was much lower than the national average (61.00) (Tian et al., 2010). We speculate that the results are related to our research object. We selected the slash pine in the middle-aged forest stage, which is in the stage of rapid growth. The activity of soil microorganisms at this stage is relatively complex and can release more P element from organic matter.
In addition, the stoichiometric ratio of soil is a very complex concept and is also affected by many factors (Wang et al., 2018). For example, the type of tree species, and the age of trees and the fungi:bacteria ratio were the important determinate of soil C:N:P stoichiometry (Bai et al., 2019; Jia et al., 2023; Zhao et al., 2018). Our results showed that the response of the soil C:N:P stoichiometry towards P addition did not change significantly with the P application rates, which indicated that the P addition effects on soil C:N:P stoichiometry show stability across various treatments and the same climatic conditions. This agrees well with the strategy of soil C:N:P stoichiometry toward drought (Su and Shangguan, 2022). Moreover, this result may be due to the short duration of the experiment, and the effect of P addition on soil environmental improvement is not obvious. In the future, long-term observation experiments should be carried out to understand the improvement effect of P addition on soil C:N:P stoichiometry.
4.2. Stoichiometric homeostasis of plant tissues
In response to changes in the soil nutrient environment, plant tissues maintain the stability of chemical composition in its body through homeostasis regulation (Yu et al., 2011). This process is thought to have complex regulatory mechanisms that change with tree species and tree ages (Bai et al., 2019). Our results showed that the stoichiometric homeostasis of elements and stoichiometric ratios are various among plant tissues, which means that there is a trade-off between nutrient uptake and distribution (Gu et al., 2017). This verifies our second hypothesis that stoichiometric homeostasis characteristics of plant and litter with soil differs among plants tissues and element types. In the study, the C, N and P contents of root were defined as “strictly homeostasis”, and the root were more stable than branch and needle, indicating that the root of slash pine had stronger C, N and P homeostasis in response to P addition, while the flexible homeostasis of the needle was easier to identify the absorption and limitation of nutrients. This seems to be the reason why scholars usually use leaf stoichiometry to determine nutrient limiting elements. However, different from our previous results (Jia et al., 2023), roots have worse homeostasis than branches and leaves in response to different age states. It is verified again that the homeostasis of plant tissues is not only related to the environment of soil nutrients, but also affected by the growth stage of trees.
As the main organ of photosynthesis in pines, the needle is important for the tree growth and the accumulation of biomass (Wang et al., 2018), thus its nutrient content is limited to a certain range to provide the best physiological traits for organisms (Aerts and Chapin, 2000). In this study, the order of C, N and P homeostasis in the needle was C > N > P, and the P homeostasis of needle was “weakly plastic”. This mechanism of homeostasis seems to be determined by the distribution characteristics of elements in organisms. The C content constitutes the plant skeleton in vivo with the most stable distribution. The P is a limiting element in the subtropical region of southern China, the P content in needle is more active when dealing with P addition. Meanwhile, it shows that the homeostasis of elements with more content in the organism is higher than that of the elements with less content.
Compared with C, N or P alone, the C:N:P stoichiometric homeostasis can better reflect the nutrient consumption and nutrient storage capacity during plant growth (Blouin et al., 2012). In the present study, except for the C:P of branch and the C:N of needle and root, the C:N:P stoichiometric homeostasis in other tissues were characterized as “strictly homeostatic”. Specially, the N:P homeostasis in all tissues were “strictly homeostatic”. This may be because the N:P is an important indicator of nutrient limitation in plant growth (Tian et al., 2021). Only when soil nutrient elements are scarce, the N:P ratio of plant tissues will fluctuate greatly and show poor homeostasis. In addition, the high N:P homeostasis in plant tissues also reflected that the nutrient limitation was relatively stable under the condition of P addition in this study area.