Quinoa (Chenopodium quinoa), a herbaceous annual, has been widely cultivated in recent years because of its high nutritional value and strong tolerance to abiotic stresses. The study was conducted at two planting densities (LD, 10 plants/m2; HD, 65 plants/m2) on ameliorated coastal mudflats in Jiangsu Province, China (118° 46′ E, 32° 03′ N). The results showed soil salinity and organic matter were higher in the HD than LD treatment, and salinity of the rhizosphere soil was higher than that of the non-rhizosphere soil. Quinoa grown in HD was taller, with thicker stalks and lower yields per plant, but higher yield per unit area. Amplicon sequencing showed that Proteobacteria, Bacteroidota and Acidobacteria were the dominant bacterial phyla. Regarding the rhizosphere soil, the Shannon index was higher in the HD than LD, and Proteobacteria and Bacteroidota were more abundant in the HD treatment. Fifty-one differential metabolites were identified by metabolomic assays, belonging to 14 annotated metabolic pathways. S-adenosylmethionine was the most abundant and up-regulated metabolite (fold change >1.67), and was more abundant in the roots from the LD than HD treatment. Docosahexaenoic acid was more abundant in the HD than LD treatment, and was down-regulated metabolite. In conclusion, planting density was an important factor affecting quinoa yield; compared with unplanted soil, planting quinoa at low density increased the content of the important metabolite S-adenosylmethionine in the root system of quinoa, and high density cultivation of quinoa increased soil salinity and microbial abundance and diversity.

Saline-alkali soils are widely distributed in China, affecting plant growth and sustainable development of ecosystems. This study characterized the effects of planting Melia azedarach L. on chemical properties and microbial communities in saline-alkali soils [bare (CK), bulk (BS) and rhizosphere soil (RS)]. Compared with the bare soil, planting Melia azedarach L. lowered salt content and concentrations of extractable Na, K, Ca, Mg and Cl-, but significantly increased organic matter, total nitrogen, total phosphorus, available phosphorus, soil urease activity and alkaline phosphatase activity in the rhizosphere soil. High-throughput sequencing results indicated that bacterial richness and diversity decreased in the order RS>BS>CK. The richness of fungi was ranked RS>CK>BS, and their diversity decreased in the order CK>RS>BS. The three dominant bacterial phyla were Proteobacteria, Actinobacteria and Bacteroidetes, and the three dominant fungal phyla were Ascomycota, Basidiomycota and Glomeromycota. Redundancy analysis indicated that total phosphorus concentration and alkaline phosphatase activity significantly influenced bacterial diversity, whereas soil Ca and Mg concentrations were closely related to the fungal community diversity. In conclusion, planting Melia azedarach L. improved soil properties, increased the diversity and richness of soil microbial communities, and thus ameliorated the saline-alkali soil.