1. Introduction

Soil salinization, as one of the major factors compromising crop growth and leading to soil degeneration in natural ecological environment, which have become a global concern (Haj-Amor et al., 2022; Singh, 2016). Currently, 20% of cultivated land and 50% of irrigated land globally are under the threat of salinization (Hopmans et al., 2021; Rozentsvet et al., 2017). With climate change, population increase, intensive food production and irrigation practices, soil salinization is expected to be exacerbated (Singh, 2021). The area of saline-alkali land in China is approximately 36 million ha, distributed mainly in the north-eastern and north-western parts and along the coast (Jiang et al., 2022; Wang et al., 2020).
Excessive salt in saline-alkali soils have always been regarded to be the greatest hazard to crop production (Zörb et al., 2019). Salt stress have profound influences on the regulation of plant morphology, physiology and metabolic process (Yang & Guo, 2018), which can lead to plant dehydration, ion toxicity, and even plant death (McDowell et al., 2022).
Plants possess a wide range of metabolic strategies to promote osmotic adaptation and antioxidant mechanisms under salt stress (de Freitas et al., 2019). Roots are the main organ for sensing salt stress signals and responding to excess salt (Redwan et al., 2017). The analysis of root metabolite differences under different salinity conditions is important in elucidating the salt tolerance mechanism of plants (Barding et al., 2013; Ben Hsouna et al., 2022).
M. azedarach has high adaptability and strong salt tolerance; hence, growing M. azedarach on saline-alkali land has been considered as a promising strategy for degraded land restoration (Dias et al., 2014; Husain & Anis, 2009). M. azedarach can effectively increase bacterial communities diversity and enhance environmental carrying capacity (Li et al., 2021b; Shahbaz & Ashraf, 2013).
Understanding of how M. azedarach metabolome responds to different soil salinities is poor. In this paper, soil properties andM. azedarach root metabolome were characterized under high, medium and low salinity to explore the ameliorative effects of M. azedarach on soils with different salt content and the metabolome changes of M. azedarach in response to different severities of salt stress.