3.2.3. KEGG pathway analysis
The number of DEMs enriched in the biosynthesis pathway of flavonoids and amino acids ranked the top five among KEGG pathways (Figure 4). The expressions of L-proline, L-lysine, L-isoleucine, S-adenosylmethionine, and L-saccharopine in the biosynthesis of amino acids were higher in RM than in RL (Figure 5a), while citrate and L-saccharopine were higher in RH than in RM (Figure 5c). With soil salinity increased (RH vs. RL), three of the four DEMs in the biosynthesis of amino acids (L-proline, aspartic acid, and citrate) were up-regulated (Figure 5b). Among the DEMs involved in flavonoid biosynthesis, the expressions of (-)-naringenin, vitexin, (+)-gallocatechin, pinocembrin, (-)-epicatechin and epigallocatechin were higher in RM vs. RL (Figure 5a) and RH vs. RL (Figure 5b).

3.3. Combined analysis of soil properties and M. azedarach DEMs

Six soil chemical properties (salinity, Na+, Cl-, K+, available phosphorus and available potassium) were correlated negatively with four DEMs ofM. azedarach : prunasin, androstendione,o -succinyl-L-homoserine and traumatic acid (Figure 6). By contrast, alkaline phosphatase activity was positively correlated with these four DEMs. Soil pH was negatively correlated with prunasin, and Ca2+ was positively correlated with prunasin and corticosterone.