GC-MS analysis: herbicidal activity of major compounds in BIO
Total ion chromatograph of BIO aqueous extracts was obtained from the GC/MS chromatogram (Fig. 1). GC-MS revealed three main herbicidal components (hexadecanoic, isovaleric, and 2-methylbutyric acids) in crude extract. The IC50 values of hexadecanoic acid, isovaleric, and 2-methylbutyric acids were 22.81, 8.34, and 12.48 mg/L, respectively. The inhibitory effects of the three compounds decreased in the following order: isovaleric acid > 2-methylbutyric acid > hexadecanoic acid. With an increase in dosage, the inhibitory effects of isovaleric and 2-methylbutyric acids increased (Fig. 2).
Main bacteria in BIO
The top five genera in the BIO samples were Pseudomonas, Anaeromyxobacter, Haliangium, Candidatus Nitrosotalea, and Geobacter, whereas those in the CBF samples were Cercisgigantea, Halocella, Pseudomonas, Actinomadura, and Nonomuraea (Fig. 3a). The five phyla with the highest relative abundance in the BIO samples were Proteobacteria, Acidobacteria, Chloroflexi, Nitrospirae, and Verrucomicrobia, whereas those in the CBF samples were Firmicutes, Cyanobacteria, Proteobacteria, Actinobacteria, and Bacteroidetes (Fig. 3b).
Effect of BIO on the soil chemical properties
There was no significant difference in the extractable P content (values ranged from 0.21 to 0.45 mg/kg in all samples) between BIO treatments and untreated control (CK) samples, and between the HPS and CBFS treatments. Soil pH values under the BIO-50 and BIO-100 treatments were close to that of the untreated control, but an increased dosage of BIO (BIO-200 to BIO-800) increased these values by 3.36%-4.46% compared with the untreated soil. There were marginal differences in the recorded pH values among HPS, CBFS, and CK. Similar results were obtained for the total K, total N, total P, exchangeable K, hydrolytic N, and organic matter content. These results demonstrate that applying BIO in paddy fields will only result in minor changes in the soil chemical properties.
Effect of BIO on soil bacterial communities
Sequencing the full-length 16S rRNA genes revealed a diverse bacterial community composition and dynamics. The number of OTUs in all samples was 405–1633, and 47 OTUs were common in all samples, as shown by the flower plot (Fig. 4). The analysis results of alpha diversity indices (Chao1and Shannon) of the different soil treatments (BIO and CBF) are shown in Fig. 5. These indices displayed uniform species abundance among BIO-treated soil samples, and even the highest dose of BIO had negligible effect on the diversity index. Furthermore, there were no significant differences among the CBF, BIO, and HP soil samples. However, the alpha diversity of CBF samples was significantly lower (p < 0.05) than that of the other (BIO treatment, CBF, BIO, and HP) soil samples.
Beta diversity is commonly used to compare differences among a set of samples. Our PCA analysis of beta diversity indicated that all replicates of treated soils (CBF soil samples, HP soil samples, untreated soil samples, and BIO samples) clustered together (Fig. 6).The first and second axes showed 2.78% and 2.59% of the variance and 5.37% in total of the cumulative variance, which indicating that the soil bacterial community was no significantly differences among the CBF, BIO, and HP soil samples. Bacterial community structure at the phylum level in the BIO-treated soil samples, CBF soil samples, and HP soil samples is shown in Fig. 7a. The five most dominant phyla in both BIO samples and BIO-treated, CBF, and HP soil samples were Proteobacteria, Acidobacteria, Chloroflexi, Nitrospirae, and Verrucomicrobia. Bacterial community structure (at the genus level)in the BIO- treated, CBF, and HP soil samples is shown in Fig. 7b. The five most representative genera among the BIO-treated soil samples were Anaeromyxobacter, Candidatus Nitrosotalea, Clostridium sensustricto1, Haliangium, Candidatus, and Nitrotoga.
Soil chemical properties, bacterial composition, and weed control effect
The RDA analysis among microbial community, soil chemical properties, and weed control effect was illustrated in Fig. 8. We observed significant positive correlations among soil pH, total K, and bacterial abundance in our study. For instance, soil organic matter with Anaerolinea (p = 0.81) and Sorangium (p= -0.74). In addition, the total K content had a distinct relationship with Spirochaeta2 (p = 0.74), whereas the abundance of Sphingomonas (p= -0.74) had an obvious effect on the total K content. Interestingly, Candidatus Koribacter was the most correlated genus (p=-0.67) with the weed control effect, followed by Clostridium sebsustricto9 (p=-0.62) and Nonomuraea (p = 0.54). The rice yield had a distinct relationship with Nonomuraea (p = 0.83), Nitrospira (p = 0.79), and Candidatus Koribacter (p=-0.76).