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).