Yitong Li

In this study, the effect of different planting combinations on the amino acid concentration in the pericarp of Zanthoxylum planispinum ‘dintanensis’ (hereafter referred to as Z. planispinum) was studied, and the response of amino acid concentration to soil factors was clarified. The aim of this study was to screen optimal planting combinations and provide a theoretical basis for improving pericarp quality. Five planting combinations of Z. planispinum in a karst rocky desertification area were selected as the research objects, and the concentration and accumulation of free amino acids in the pericarp of Z. planispinum were analyzed. Then, combined with existing soil quality data, the pericarp quality of Z. planispinum was comprehensively evaluated by principal component analysis, and the effect of soil factors on amino acid concentrations was clarified by redundancy analysis. The results are as follows: (1) except for arginine, serine, proline, alanine, tyrosine and cystine, the concentrations of other free amino acids significantly differed among the five planting combinations. In general, the planting combination has a great influence on the concentration of free amino acids in the pericarp of Z. planispinum, especially essential amino acids; (2) free amino acid concentration in the pericarp of Z. planispinum mostly increased in combination with Sophora tonkinensis Gagnep. (hereafter referred to as S. tonkinensis) and decreased in combination with Prunus salicina Lindl; (3) principal component analysis showed that the concentration of free amino acid in the pericarp of Z. planispinum was generally at a high level when combined with S. tonkinensis or Lonicera japonica Thunb. (hereafter referred to as L. japonica). Among them, the amino acids in the pericarp of Z. planispinum with S. tonkinensis were closer to the ideal protein standard of FAO/WHO; (4) soil-available potassium, available phosphorus, microbial biomass nitrogen, available calcium and microbial biomass phosphorus in soil factors had significant effects on amino acid concentration after a redundancy analysis. It can be seen that the available nutrients and soil microbial biomass contribute greatly to the amino acid concentration of the pericarp. According to the soil quality and the amino acid quality of the pericarp, planting with L. japonica can improve the amino acid quality of the pericarp of Z. planispinum, as well as selecting Z. planispinum + L. japonica as the optimal planting combination.

Leaf structural and physiological traits, nutrients, and other functional properties reflect the ability of plants to self-regulate and adapt to the environment. Species diversity can positively affect plant growth by improving the habitat, and offers mutual interspecies benefits. Therefore, optimizing the types of plants grown in a specific area is conducive to achieving sustainable development goals for plant growth. In this study, companion planting of Zanthoxylum planispinum ‘dintanensis’ (hereafter Z. planispinum) with Prunus salicina Lindl., Sophora tonkinensis Gagnep., Arachis hypogaea L. and Lonicera japonica Thunb. was investigated, along with a monoculture Z. planispinum plantation. The effect of different planting combinations on the adaptive mechanisms of Z. planispinum and its response to the soil was explored. These results revealed that Z. planispinum preferred the slow growth strategy of small specific leaf area, high leaf water content, and high chlorophyll content after combination with P. salicina. Conversely, after combination with S. tonkinensis, Z. planispinum exhibited a fast growth strategy. Combination with A. hypogaea enabled Z. planispinum to adopt a transition from slow to fast growth. Z. planispinum regulated its economy of growth through multiple functional trait combinations, indicating that planting combinations impacted its adaptive strategies. The adaptability of Z. planispinum in combination with P. salicina, L. japonica, A. hypogaea and S. tonkinensis decreased in turn, with only the adaptability of Z. planispinum + S. tonkinensis lower than that of the pure forest. Leaf functional traits were jointly influenced by soil water content, microbial biomass carbon (MBC), MB nitrogen (N), MB phosphorus (P), available N, total P and available calcium (C:N:P). The main contributors were soil water content, the different component levels and stoichiometry of elements and the MB. The results demonstrated that companion planting can promote or inhibit the growth of Z. planispinum by adjusting its functional traits.

The exploration of the functional traits of Zanthoxylum planispinum ‘dintanensis’ and Zanthoxylum amatum ‘novemfolius’ at the seedling stage may provide a scientific basis for the zoning of germplasm resources and the introduction of plant varieties. The seedlings of the above mentioned two species with an age of about 8 months were selected as the study material, and the structure, nutrients, and physiological traits of leaves and roots were determined, respectively, to reveal their survival strategies. The results demonstrated the following four key elements. (1) The leaf thickness, specific leaf area, and leaf δ¹⁵N value of Z. planispinum ‘dintanensis’ were significantly higher than those of Z. amatum ‘novemfolius’ (p < 0.05). However, the root diameter and Ca content showed the opposite results, and the other traits were not significantly different. (2) Leaf functional traits except Ca, as well as specific root length, and root P, Ca, and physiological traits had strong dominant effects on the functional trait system. (3) Among the main functional traits, leaf K and root P preferred environmental selection, leaf C, N, and P favored stable inheritance, and specific leaf area, leaf δ¹⁵N, and root Ca were affected by varieties. (4) There were significant synergies (promotion) and trade-offs (inhibition) between the functional traits, and the leaves and the root system were closely correlated. It is speculated that Z. planispinum ‘dintanensis’ may have a stronger defense system. Its leaves are mainly related to growth and defense functions, and its roots are mainly related to the regulation of functions.

The soil quality of plantations with different planting patterns and the effect of soil quality on stoichiometry provide a theoretical basis for the selection of Zanthoxylum planispinum var. dintanensis (hereafter Z. planispinum) planting patterns and nutrient management. Four mixed plantations: Z. planispinum + Prunus salicina, Z. planispinum + Sophora tonkinensis, Z. planispinum + Arachis hypogaea, and Z. planispinum + Lonicera japonica, and a monoculture Z. planispinum plantation were selected to clarify the effect of soil quality on stoichiometry. The results showed that the soil quality index (SQI) of Z. planispinum + L. japonica (1.678) was the highest, indicating that it was the preferred planting combination and that it was significantly limited by soil water content (SWC). The nutrient forms, SWC, and pH all have significant effects on processes such as nutrient transformation and cycling. The contributions of total Ca and total Mg in soil nutrients to stoichiometry were relatively high, while the effect of SQI on stoichiometry was not significant. The microbial stoichiometry ratio was mainly influenced by microbial biomass phosphorus, reflecting that microorganisms have strong internal stability. Strong interactions among soil factors occur, affecting elemental geochemical processes. The regulatory effects of different soil factors on their stoichiometry should be emphasized.

To explore the changes of leaf functional traits of Zanthoxylum planispinum ‘dintanensis’ with growth and development and its relationship with soil properties, which can clarify the response of the plantation to soil properties and suitable strategy. The research results can provide a scientific basis for plantations management. We explored the response of leaf functional traits to soil by using redundancy analysis in 5–7-, 10–12-, 20–22-, and 28–32-year Z. planispinum ‘dintanensis’ plantations. The results showed that: (1) The coefficients of variation of leaf traits ranged from 0.41% to 39.51%, with mostly medium and low variation, with the lowest variability in leaf water content (0.51–0.85%); The 5–7, 10–12, 20–22-year-old plantations were laid at the “slow investment-return” end of the economic spectrum while 28–32-year plantations were close to “fast investment-return” end. (2) The Z. planispinum ‘dintanensis’ tended to suit the environment via making trade-off and coordination of leaf functional traits. Leaf dry matter content decreased with an increase in leaf carbon/leaf nitrogen ratio, which is the trade-off between nitrogen usage efficiency and nutrient fixation capacity in Z. planispinum ‘dintanensis’. (3) Redundancy analysis suggested that soil carbon/nitrogen ratio, soil total calcium, soil water content, soil available phosphorus, soil carbon/calcium ratio were highly correlated with leaf functional traits, while soil elemental stoichiometry had a greater reflection on leaf functional traits than their own content.

Plantations with different allocation patterns significantly affect soil elements, microorganisms, extracellular enzymes, and their stoichiometric characteristics. Rather than studying them as a continuum, this study used four common allocations of plantations: Zanthoxylum planispinum var. dintanensis (hereafter Z. planispinum) + Prunus salicina, Z. planispinum + Sophora tonkinensis, Z. planispinum + Arachis hypogaea, and Z. planispinum + Lonicera japonica plantations, as well as a single-stand Z. planispinum plantation as a control. Soil samples from depths of 0–10 and 10–20 cm at the five plantations were used to analyze the element stoichiometry, microorganisms and extracellular enzymes. (1) One-way analysis of variance (ANOVA) showed that the contents of soil organic carbon (C), nitrogen (N), phosphorus (P), and potassium (K) of Z. planispinum + L. japonica plantation were high, while those of calcium (Ca) and magnesium (Mg) were low compared to the Z. planispinum pure plantation; soil microbial and enzyme activities were also relatively high. Stoichiometric analysis showed that soil quality was good and nutrient contents were high compared to the other plantations, indicating that this was the optimal plantation. (2) Two-way ANOVA showed that stoichiometry was more influenced by plantation type than soil depth and their interaction, suggesting that plantation type significantly affected the ecosystem nutrient cycle; soil microbial biomass (MB) C:MBN:MBP was not sensitive to changes in planting, indicating that MBC:MBN:MBP was more stable than soil C:N:P, which can be used to diagnose ecosystem nutrient constraints. (3) Pearson’s correlation and standardized major axis analyses showed that there was no significant correlation between soil C:N:P and MBC:MBN:MBP ratios in this study; moreover, MBN:MBP had significant and extremely significant correlations with MBC:MBN and MBC:MBP. Fitting the internal stability model equation of soil nutrient elements and soil MBC, MBN, and MBP failed (p > 0.05), and the MBC, MBN, and MBP and their stoichiometric ratios showed an absolute steady state. This showed that, in karst areas with relative nutrient deficiency, soil microorganisms resisted environmental stress and showed a more stable stoichiometric ratio. Overall stoichiometric characteristics indicated that the Z. planispinum + L. japonica plantation performed best.

Understanding the relationships between carbon; nitrogen, their stable isotopes δ¹³C and δ¹⁵N, and soil stoichiometry may further our understanding of the regulatory mechanisms of the soil quality index on the equilibrium on isotopic fractionation. Four plantations of Zanthoxylum planispinum var. dintanensis (5–7, 10–12, 20–22 and 30–32 years) in the karst plateau gorge area, Guizhou Province, China, were selected to determine the variation characteristics and interactions between leaves, leaf litter, soil carbon (C), soil nitrogen (N) and their isotopes with plantation age, and to explore the relationship between soil stoichiometry and the isotopes δ¹³C and δ¹⁵N. The results were as follows: (1) the δ¹³C in leaves, litter, and soil were −28.04‰ ± 0.59‰, −26.85‰ ± 0.67‰, and −19.39‰ ± 1.37‰, respectively. The contents of δ¹⁵N were 2.01‰ ± 0.99‰, 2.91‰ ± 1.32‰, and 3.29‰ ± 0.69‰, respectively. The contents of δ¹³C and δ¹⁵N were ranked in the order, soil > litter > leaf. (2) With increasing plantation age, the soil ¹³C decreased; the leaf and the litter δ¹⁵N increased first then decreased, and the litter δ¹³C and the soil δ¹⁵N did not vary significantly. (3) The litter layer was positively correlated with soil δ¹³C and negatively correlated to δ¹⁵N. (4) Redundancy analysis showed that the soil microbial biomass carbon (MBC) and the bacteria/fungi (BAC/FUN) were the dominant factors affecting the natural abundance of C and N isotopes