4. Discussion

Coastal saline soil contains excess salinity, with Na+ and Cl- being the main base ions (Zhang et al., 2022). Soil organic matter (SOM) is dominant to global carbon cycle (Bhattacharyya, 2022), S-AP activity is an indicator of soil phosphorus cycle (Liberti et al., 2013), and S-UE plays a crucial role in influencing the content of soil nitrogen (Albiach et al., 2000). In this paper, salinity, contents of Na+ and Cl- in the rhizosphere soils were significantly lower than bulk soils, whereas SOM and activities of S-AP and S-UE showed the opposite trend (Figure 1). These results indicated that M. azedarach played a critical role in reducing soil salinity and promoting soil nutrient cycling.
Excess salinity in saline-alkali land can cause osmotic stress, ion toxicity, oxidative damage, nutrient deficiency and even cell death in plants (Elkelish et al., 2019; Ghazali, 2020). Plants can cope with salt stress by regulating biosynthesis of metabolites such as sugars, amino acids and flavonoids (Batista-Silva et al., 2019).
Sugars can be bound to inorganic solutes (e.g. K+) to maintain cell expansion and are active in osmotic regulation under salt stress (Arbelet-Bonnin et al., 2018). Similar results were reported in other studies, with sugars acting not only as osmoprotectants and cell membrane stabilizer, but also as scavengers of reactive oxygen species (Nishizawa et al., 2008). The contents of various sugars were up-regulated under medium and high salinity (Table 1), indicated that M. azedarach could produce more sugars to regulate osmotic pressure when exposed to a gradient of salt stress.
Besieds, amino acids are important osmoprotectants and antioxidants, and their accumulation can enhance the stability of cell membrane and improve salt tolerance of plants (Slama et al., 2015; Widodo et al., 2009). Hosseinifard et al. (2022) discovered that proline could inhibit the accumulation of toxic ions under salt stress. With the increase in the severity of salt stress, the amino acid DEMs were up-regulated, and more amino acids were synthesized under medium compared with low salinity (Figure 5a-b, Table 2). The result showed that M. azedarach increased its salt tolerance by up-regulating amino acid as the severity of salinity progressed from low to medium.
Flavonoids have antioxidant activity, protecting cell structures from oxidative damage and enhance the stress resistance of plants (Saeed et al., 2012; Sumner et al., 2003). More than 70% of the flavonoids were up-regulated in M. azedarach roots with soil salinity increased (Table 3). And DEMs involved in flavonoid biosynthesis were largely up-regulated with salinity increased (Figure 5). These results indicated thatM. azedarach could enhance the antioxidant activity and salt tolerance of plants by up-regulating the expression of flavonoids.
Among M. azedarachmetabolites, traumatic acid was shown to participate in the plant adaptation and tolerance to salinity by diminishing the negative effects of sodium and chloride ions and increasing the monosaccharide content (Pietryczuk et al., 2014). Alkaline phosphatase can release inorganic phosphorus for plants growing on phosphorus-deficient saline soils (Li et al., 2021a). In the present study, alkaline phosphatase activity was positively correlated with traumatic acid concentration (Figure 6), indicating that there should be a synergistic effect between soil alkaline phosphatase activity and traumatic acid in M. azedarach , facilitatingM. azedarach tolerance to salt stress.