The abundance and internal connection of 13C and 15N in Zanthoxylum planispinum var. dintanensis plantation at different plantation age
The content of 13C in plant leaf is positively correlated to water use efficiency (Chen et al. 2011). The results of current study demonstrated that the water use efficiency did not significantly varied with plantation age, possibly resulting from the trade-off of the resource acquisition and usage in the plantation (Heberling et al. 2012). Next, the plants need to enhance their resource competition ability to survive in the vulnerable karst environment, which makes the water use efficiency critical and hardly showing differences. The leaf 15N in 10–12 years group was significantly higher than that of the other 3 age groups, probably due to the tremendous needs of N in the vigorous fruit bearing period, which stimulated the root system to transfer more N to leaves for synthesis of photosynthetic products, thus meeting the high metabolism requirement. The leaf 15N increased first and then decreased with plantation age, which was inconsistent with Wang et al.’s research (2019). The inconsistency was caused by the different N isotope fractionation speed due to different photosynthetic type of different species.
There were no significant differences in soil and litter 13C in the 4 plantation age groups, because the organic matter mainly came from litter (Peri et al. 2012). Balesdent et al. (1993) found that soil 13C was not significantly positively correlated to litter δ13C, which was consistent with our results, indicating that soil cannot fully inherit leaf 13C, even if we don’t consider the C isotope fractionation in the litter decomposition process. Soil 13C is a result of the mixing of new and old C, an effect called isotopic mixing (Liao et al. 2006). The results of Buchmann et al. (1997) and Farquhar et al. (1989) showed that the soil δ13C usually fall in the range of 1.0–3.0‰, a value higher than 3.0‰ indicates that the organic matter input into the soil may be a mixture of C3 and C4 plants. The average variation of 13C in current study was 7.46‰, indicating that the vegetation in this area may have changed greatly, soil organic matters were simultaneously affected by Zanthoxylum planispinum var. dintanensis and other plants containing high 13C, which is a reasonable result of agricultural transformation. The current study also showed that the δ15N in Zanthoxylum planispinum var. dintanensis plantation was not varied significantly with plantation age, which is inconsistent with the results of Zheng et al.’s research, saying that soil δ15N varied with Caragana intermedia plantation age in Fujian area (Zheng et al. 2015). The reason is that the structure of Zanthoxylum planispinum var. dintanensis plantation is simple, with few species of understory plants and small amount of surface litter. It is also related to the frequent interference of human activities such as chemical fertilizer, insecticide and herbicide application.
The 13C in Zanthoxylum planispinum var. dintanensis leaf was significantly negatively correlated to N content, which is consistent with Tsialtas et al.’s (2001) results, yet opposite to Zhang et al.’s (2015) research, suggesting that the soil nutrients supply in different environment is different, and so is the resource utilization strategy of plants. The reason is that the leaf N can regulate the stomatal density, higher leaf N content promotes the absorption of CO2, increases plant photosynthesis rate, and decreases the ration of intracellular and extracellular CO2 concentration (Ci/Ca), thus increasing 13C (Macfarlane et al. 2007; Diefendorf et al. 2010). The research area is a barren karst region, where needs supplementary fertilization for plant growth. Modern agriculture emphasizes the supplement of N and P, which leads to more uptake of N into leaves, increases stomatal density and Ci/Ca ratio, thus decreasing 13C. Our results also showed that leaf 13C was significantly positively and negatively correlated to litter 15N and soil 15N, respectively, indicating that there was a certain coupling relationship among leaves, litter, and soil. A possible reason could be that ecosystem C and N cycles are closely coupled, the potential of C fixing is greatly limited by the capability of soil providing N (Li et al. 2012; Zechmeister-Boltenstern et al. 2015); meanwhile, it may be related to the mechanism of nutrient reabsorption and the isotope fractionation happened in the process of plant nutrient cycling. However, due to the large number of influencing factors and limited measurement indicators, the reason for the weak inheritance cannot be clarified. Further research is needed in the future.
The driving mechanism of soil stoichiometry to plantation C and N isotopes fractionation
Soil stoichiometry links the chemical cycles in different ecosystem parts, reflecting the flowing of elements (Yang et al. 2018), indirectly regulating forest C and N isotopes fractionation via changing the coupling relationship between soil and microorganism stoichiometry. It is an important index for the evaluation of ecosystem element cycle and internal stability (Zhou et al. 2014; Mooshammer et al. 2014). The contents of soil elements can affect the testing results and restrict the application of stable isotope technology in soil C and N cycles. Stevenson et al.’s study (2010) indicated that the soil C/N was significantly negatively correlated to 15N. The reason was that the biological activity of microbes in soil with different C/N were different, which led to different fractionation speed and degree in the process of mineralization. Generally, the growth of microorganism is limited by N content in high C/N soil, thus weakening the 15N fractionation in the mineralization process; on the other hand, under low C/N condition, the growth of microorganism is limited by C content, thus strengthening the N decomposition in the process of mineralization (Collins et al. 2008). The current study showed that there was a weak correlation between soil C/N and 15N, which was not completely consistent with previous studies. A possible explanation was that the small amount of litter in plantation, the strong human interference, and the high concentration allelochemicals secreted by Zanthoxylum planispinum var. dintanensis partially inhibited the microbial activity. Our research also demonstrated the negative correlation between soil C/N and 13C. The reason is that the decomposition of organic matter by soil microoganisms is limited by the quality of substrate, the decomposition rate of C and the degree of C isotope fractionation are relatively low in soil with high C/N (Xu et al. 2012; Zhao et al. 2019), thus presenting a smaller 13C value. Wang et al. (2015) reached similar conclusion, nonetheless, Peri et al. (2012) found that soil C/N did not affect the soil 13C in their study of the primeval forests in southern Patagonia. A possible reason could be that the climate was different in each research area, and so was the litter type and amount, which urged the plants to adopt different resource utilization and adaptation strategies. Soil C and N are indispensable elements for plant survival, therefore it is scientifically feasible to use C/N to determine the composition characteristics of the soil 13C and 15N, though it is not the only criterion. In the future, coupling study with other factors should be carried out for comprehensive evaluation.
As the most active part of the soil organic matters (Arunachalam et al. 1999), biomass can establish good connections with 13C and 15N through the decomposition of organic matter and microbial activity. Our results showed that soil MBC was positively correlated to soil 13C, which is related to the isotope fractionation in the process of microorganism decomposition (Billings & Richter,2006). During the process, 12C enters the released CO2 preferentially, and the heavier 13C more likely enters the soil microorganism biomass (de Rouw et al. 2015) before its returning to the soil organic matter and enrichment in soil. Relevant research have shown that soil 13C is positively correlated with organic C (Wynn & Bird, 2008). When the decomposition of organic C speeds up, more 12CO2 will be released from the soil system, thus resulting the enrichment of 13C in soil (Wynn et al. 2007). Inconsistent with these results, the organic C did not show significant correlation with 13C in the current study, indicating that the soil organic C in our research area had no significant influence on C isotope fractionation. Possible reasons could be: in order to improve the economic value of the Zanthoxylum planispinum var. dintanensis plantation, pruning need to be carried out in winter and summer, thus reducing the litter return and nutrients; the unique soil structure, the nutrients loss pathway, and the soil barrenness are all related. In conclusion, litter and microorganisms are important sources of soil nutrients, which should be further protected for improvement of the soil quality.
Differences between artificial and natural ecosystems
In the artificial forest and secondary forest systems, the content of soil C decreased gradually from leaf, litter, to soil (Table 5). The reason is that leaf is the main place of photosynthesis and C accumulation, before its aging and withering, the old leaf needs to transfer a part of the nutrients to new leaves or perennial tissues (Brant & Chen, 2015; Lu et al. 2017). The leaf 13C content in artificial forest is significantly higher than that of secondary forest, indicating that Zanthoxylum planispinum var. dintanensis plantation has higher water use efficiency. This is because the application of pesticides and chemical fertilizers inhibits the soil microbial activity; in addition, the small amount of litter results in low soil nutrients compensation, leading to even lower N content and higher environment stress (Table 5). Plants reduce N use by improving water use in the environment of low N (Zhou et al. 2016).
Table 5
Differences between artificial ecosystem and natural ecosystem
Forest type/Index
|
Artificial forest
|
|
Secondary forest (Wu et al.,2021)
|
C (g/kg)
|
N (g/kg)
|
13C (‰)
|
δ15N (‰)
|
|
C (g/kc)
|
N (g/kg)
|
13C (‰)
|
15N (‰)
|
Leaf
|
407.1 ± 6.1a
|
29.2 ± 4.7a
|
-19.4 ± 1.4a
|
2.0 ± 1.0ab
|
|
438.2 ± 32.8a
|
20.9 ± 8.8a
|
-29.2 ± 1.7ab
|
-0.1 ± 2.3b
|
Litter
|
384.2 ± 31.5b
|
25.1 ± 6.9a
|
-28.0 ± 0.6ab
|
2.9 ± 1.3a
|
|
421.4 ± 36.8a
|
15.7 ± 4.0a
|
-26.8 ± 4.9a
|
0.8 ± 2.0-b
|
Soil
|
10.6 ± 3.9c
|
1.8 ± 0.6b
|
-26.9 ± 0.7ab
|
3.3 ± 0.7a
|
|
107.8 ± 56.9b
|
10.6 ± 8.2a
|
-24.9 ± 2.4a
|
7.3 ± 2.1a
|
Data represent means ± SD (n = 3). Different letters indicate significant differences at P < 0.05 using Pearson correlation analysis test. |
In artificial and natural ecosystems, 15N distribution increases with the increase of leaves, litter, and soil, which is consistent with the results of Perakis et al. (2015). The soil δ15N in secondary forest is significantly higher than that of artificial forest, indicating that isotope fractionation occurred in the processes of plant metabolism and the transformation and transportation of N in plants. The fractionation is more intense in secondary forest. Possible reasons could be that the soil N availability of plantation is affected by the amount of fertilizer, it decreases with the increase of fertilizer amount. By contrast, in natural ecosystem, the soil N availability is relatively high, the N cycle is more open, and more 14N gas is released; moreover, natural ecosystem is less suffered from human activities, the biodiversity is high, the fractionation of soil N isotope is more intense, and the δ15N is more enriched in it.
To sum up, affected by microoganisms, climates and many other factors, the 13C and 15N distribution law and nutrients allocation are different in different ecosystems. Generally, the growth rate of Zanthoxylum planispinum var. dintanensis in the artificial ecosystem is slightly higher because of the better management and fertilization measures applied for the purpose of maximizing the ecological and economic benefits of the plantation However, there are still some problems in plantation, such as single stand, low stability in the ecosystem and serious diseases and insect pests, which limit its sustainable development. Therefore, stable isotope technology can be used to explore the natural abundance characteristics of 13C and 15N and study the nutrients restriction status in different tree species, select suitable interplanting mode, enrich the plantation litter types, enhance the stability of ecosystem, thus providing scientific basis for delaying the growth decline of Zanthoxylum planispinum var. dintanensis plantation.