3.1 Soil salinity, organic matter and biomass of quinoa
Soil salinity ranged from 1.21 to 3.83 g/kg (Fig. 1a). Compared with unplanted soil, the salt content was significantly higher in the rhizosphere soil in both density treatments as well as in the non-rhizosphere soil of the high-density treatment. The soil organic matter content ranged from 10 to 13.4 (g/kg), being significantly lower in the unplanted soil compared with all other soil samples (Fig. 1b).
As the planting density increased, the stem diameter and total above-ground biomass and fresh panicle weight per plant decreased significantly, whereas total fresh panicle weight per unit area increased significantly (Table 1). There was a slight (non-significant) increase in plant height with an increase in planting density.
As shown in Fig. 2, the total above-ground biomass per plant showed a positive correlation with the fresh panicle weight and stem diameter. The fresh panicle weight was negatively correlated with the salt content in the rhizosphere soil.
3.2 Soil microorganisms
For the diversity of microbial communities detected in different samples at 97% consistency threshold, the following conclusions were drawn based on the analysis of Observed Taxonomic Units (OTUs) and Shannon and Chao1 indices (Table 2). The Shannon index showed the bacterial diversity was higher in high-density rhizosphere soils (HDR) than in low-density rhizosphere soils (LDR). However, there was no significant difference in bacterial diversity between rhizosphere and non-rhizosphere soils at the same planting density, and there was no variability in OTUs and Chao1 indices.
The bacterial OTU numbers in LDN, LDR, HDN, and HDR were 5018, 4789, 5168, and 5202, respectively (Fig. 3). The number of OTUs was higher in the high-density than low-density planting soil, but there was no obvious difference between the OTUs in the rhizosphere and non-rhizosphere soil.
Soil samples from 12 different treatments were annotated with the database Silva138. The top eight phyla in terms of relative abundance were Proteobacteria , unidentified_Bacteria ,Bacteroidota , Acidobacteriota , Crenarchaeota ,Firmicutes , Actinobacteriota , and Gemmatimonadota . Among them, Proteobacteria ,unidentified_Bacteria ,Bacteroidota,and Acidobacteriota had the highest relative abundance, accounting for more than 60% of all phyla. Proteobacteria was the most abundant phylum in soil; the relative abundance in LDN, LDR, HDN, and HDR was 26, 25, 27, and 26%, respectively. The relative abundance of Bacteroidota in LDN, LDR, HDN, and HDR was 9, 9, 10, and 13%, and that of Acidobacteriota was 8, 10, 9, and 7%, respectively (Fig. 4).
Fig 5 showed the LEfSe analysis of soil prokaryotic microorganisms (bacterial domain) as influenced by different treatments. The cladograms showed the taxa (highlighted by small circles) that played an important role in the structure of the microbial community. The LDA scores distribution histograms showed significant differences in abundance among the treatment soils, with the length of the bars representing the magnitude of the microbial influence.
The LDA-based LEfSe identified four bacterial taxa with LDA thresholds greater than 3.5 in the LDR, namely: Vicinamibacteraceae (f) > Vicinamibacteria (c) >Vicinamibacterales (o) > Latescibacterota(p). Among them, Vicinamibacteraceae , Vicinamibacteria andVicinamibacterales belong to the Acidobacteriota . The bacteria endemic in the LDN were Bacillales (o) >Bacillus (g) > Bacillaceae (f), whereasOceanospirillales (o) were abundant in the HDR.
The soil properties explained a large proportion of variability in the relative abundance of Bacteroidota , Actinobacteriota ,Acidobacteriota , and Firmicutes . The relative abundance ofBacteroidota was correlated positively with salinity and pH, and negatively with organic matter. The relative abundance ofActinobacteriota was correlated positively with salinity, negatively with organic matter content, and was largely independent of soil pH. The relative abundance of Acidobacteriota was correlated positively with organic matter, and negatively with pH and salinity.Firmicutes were mostly independent of soil chemical properties (Fig. 6).