[Effects of Foliar Application of Silicon Fertilizers on Phyllosphere Bacterial Community and Functional Genes of Paddy Irrigated with Reclaimed Water].

Agricultural utilization of reclaimed water is considered to be an effective way to solve water shortage and reduce water environmental pollution. Silicon fertilizer can improve crop yield and quality and enhance crop resistance. The effect of foliar spray with silicon fertilizer on phyllosphere microbial communities remains lacking. In this study, a pot experiment was conducted to explore the effects of different types of silicon fertilizer on the composition and diversity of a phyllosphere bacterial community and the abundances of related functional genes in rice irrigated with reclaimed water. The results showed that Firmicutes, Proteobacteria, Actinobacteriota, Bacteroidota, and Verrucomicrobiota dominated the phyllosphere bacteria of rice. The relative abundance of Bacillus was higher than that of other treatments in RIS3. Reclaimed water irrigation significantly increased the relative abundances of the potential pathogens Pantoea and Enterobacter. The unclassified bacteria were also an important part of the bacterial community in the rice phyllosphere. Bacillus, Exiguobacterium, Aeromonas, and Citrobacter were significantly enriched by silicon fertilizer treatments. Functional prediction analysis showed that indicator species were mainly involved in metabolism and degradation functions, and the predicted functional groups of phyllosphere bacteria were attributed to chemoheterotrophy, aerobic chemoheterotrophy, nitrate reduction, and fermentation. Quantitative PCR results showed that AOA, AOB, and nifH genes were at low abundance levels in all treatments, and nirK genes was not significantly different among treatments. These results contribute to the in-depth understanding of the effects of foliar spray silicon fertilizer on the bacterial community structure and diversity of rice phyllosphere and provide a theoretical basis for the application of silicon fertilizer in reclaimed water irrigation agriculture.

[Effects of Soil Amendments on the Bacterial Diversity and Abundances of Pathogens and Antibiotic Resistance Genes in Rhizosphere Soil Under Drip Irrigation with Reclaimed Water].

Due to reclaimed water, irrigation can cause human health and environmental risks. Soil amendments are applied to reveal the abundance of pathogens and antibiotic resistance genes in rhizosphere soil irrigated by reclaimed water and to better understand the effects of environmental factors on the rhizosphere soil bacterial composition, which has guiding significance for the reasonable use of soil amendments. In this study, the effects of biochar, bioorganic fertilizer, humic acid, loosening soil essence, and corn vinasse on bacterial community diversity and certain gene abundances in rhizosphere soil under drip irrigation with reclaimed water were studied using high-throughput assays and quantitative PCR. The results showed that biochar significantly increased pH, organic matter, and total nitrogen contents in the rhizosphere soil. The corn vinasse significantly decreased soil pH and increased the contents of total nitrogen and total phosphorus but significantly increased the soil EC value (P<0.05). The effects of the five soil amendments on the α-diversity of rhizosphere bacteria were not significantly different. The bacterial community structure and diversity of rhizosphere bacteria were similar at different taxonomic levels, but their relative abundance was different. α-Proteobacteria, γ-Proteobacteria, Bacteroidia, Actinobacteria, Acidimicrobiia, and Anaerolineae were the dominant bacteria in all treatments. The dominant genera consisted of Pseudomonas, Sphingobium, Sphingomonas, Cellvibrio, Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium, Flavobacterium, and Algoriphagus (relative abundance>1%). Correlation analysis of environmental factors showed that the composition of the rhizosphere bacterial community was strongly correlated with pH, EC, total nitrogen, and total phosphorus content. The abundances of pathogenic bacteria and antibiotic resistance genes were 103-107 copies·g-1 and 104-108 copies·g-1, respectively. There were significant differences in the detection levels of pathogens and antibiotic resistance genes. Bioorganic fertilizer, loosening soil essence, and corn vinasse significantly increased the abundances of some antibiotic resistance genes, whereas humic acid and corn vinasse significantly decreased the abundances of Pseudomonas syringae, Ralstonia solanacearum, and total coliforms (P<0.05). A significant correlation was found between pathogens (Arcobacter, Bacillus cereus, Pantoea agglomerans, and Fecal bacteroidetes) and antibiotic resistance genes (tetA, tetB, tetO, tetQ, sul1, ermB, and ermC). In conclusion, while monitoring pathogens and antibiotic resistance genes in the agricultural environment under reclaimed water irrigation, attention should be paid to the rational application of soil amendments to avoid exacerbating the spread of biological contamination.

[Effects of Selected Biochars Application on the Microbial Community Structures and Diversities in the Rhizosphere of Water Spinach (Ipomoea aquatica Forssk.) Irrigated with Reclaimed Water].

The utilization of reclaimed water is one of the most important ways of alleviating the shortage of water resources for agricultural irrigation. As an effective disposal method for biomass waste, biochar has been widely used in the improvement and remediation of agricultural environments. However, few studies have been performed on the effects of biochar application on microbial community structures and pathogen abundances in rhizosphere soils irrigated with reclaimed water. Based on a pot experiment, high throughput sequencing technology and quantitative polymerase chain reaction (PCR) methods were used to investigate the effects of different biochars on the microbial community structure and diversity and pathogen abundance of rhizosphere soils irrigated with reclaimed water. The results showed that four different types of biochars had different effects on the soil nutrient status. Rice hull-derived biochar and rice straw-derived biochar resulted in significantly increased soil pH with reclaimed water irrigation, while peanut shell-derived biochar, rice straw-derived biochar, and wheat straw-derived biochar significantly increased EC values (P<0.05). The Sobs index, Shannon index, and Chao1 index of bacterial community in the rhizosphere soil significantly increased with rice straw-derived biochar treatment, while the Simpson index significantly decreased by adding peanut shell-derived biochar, rice hull-derived biochar, and wheat straw-derived biochar (P<0.05). There were differences in the relative abundances of bacterial communities in rhizosphere soils under different treatments. The dominant taxonomic groups at the phylum level were Proteobacteria, Actinobacteria, Chloroflexi, Bacteroidetes, and Acidobacteria. The dominant genera included Pseudomonas, Rheinheimera, Arthrobacter, Sphingomonas, and Aeromonas (relative abundance>5%). Redundancy (RDA) and heatmap analyses showed that the diversities and compositions of bacterial communities in rhizosphere soils in different treatments were closely related to soil EC values, organic matter, total nitrogen, and cadmium contents. Biochar application had no significant effect on the abundances of Aeromonas hydrophila and Bacillus cereus. Rice straw-derived biochar and peanut shell-derived biochar could significantly reduce the γ-Proteobacteria, while rice hull-derived biochar and wheat straw-derived biochar could significantly reduce the relative abundance of AOA (P<0.05). In conclusion, there were no obvious negative effects observed from reclaimed water irrigation on soil quality. Biochar application not only significantly improved the physicochemical properties of the soil, but also impacted the bacterial community structure and the abundance of the functional bacteria in the rhizosphere soil, which was closely related to the soil properties.

[Effect of Different Reclaimed Water Irrigation Methods on Bacterial Community Diversity and Pathogen Abundance in the Soil-Pepper Ecosystem].

Reclaimed water is considered to be a reasonable and sustainable alternative water resource to improve water resource layout and mitigate the shortage of traditional water resources. Its use in irrigation will cause changes in the microbial community structure and opportunistic pathogen abundance in soils and crops, but few studies have been conducted on this subject. Peppers were used as the research subjects, and the treatments were direct irrigation of reclaimed water, mixed irrigation with freshwater and reclaimed water, rotated irrigation with freshwater and reclaimed water, with potable water irrigation as the control. The effects of different irrigation methods of reclaimed water on the soil physicochemical properties were analyzed through a pot experiment. Furthermore, changes in bacterial community and opportunistic pathogen abundance in pepper fruit and the rhizosphere under reclaimed water irrigation conditions were investigated based on high-throughput sequencing technology and quantitative PCR methods. The results showed that direct irrigation with reclaimed water increased soil EC and decreased soil pH. 16S rDNA high-throughput sequencing showed that Proteobacteria, Bacteroides, Actinobacteria, and Firmicutes were present in both pepper fruit and the rhizosphere at phylum level, and the most dominant genera (Pantoea, Pseudomonas, Sphingomonas, Sphingopyxis, Luteimonas, and Mariniflexile) were greatly affected by reclaimed water irrigation methods. Quantitative PCR results indicated that the influence of reclaimed water irrigation on the distribution and abundance of pathogenic bacteria in the soil-pepper system was different, and the abundance of Legionella spp. in pepper fruit and Pseudomonas syringae in the rhizosphere increased with reclaimed water irrigation. Our results indicated that the reclaimed water was suitable for agricultural irrigation, but different reclaimed water irrigation methods may introduce different degrees of microbial contamination. In addition, attention must be given to some opportunistic pathogens and phytopathogens.

Amendment soil with biochar to control antibiotic resistance genes under unconventional water resources irrigation: Proceed with caution.

The spread of antibiotic resistance genes (ARGs) has become a cause for serious concern because of its potential risk to public health. The use of unconventional water resources (e.g., reclaimed water or piggery wastewater) in agriculture to relieve groundwater shortages may result in an accumulation of ARGs in soil. Biochar addition has been proven to be a beneficial method to alleviate the pollution of ARGs in manure-amended soil. However, the role of biochar on ARGs in soil-plant systems repeatedly irrigated with unconventional water resources is unknown. Under reclaimed water or piggery wastewater irrigation, rhizobox experiments using maize plants in soil amended with biochar were conducted to investigate the variation of typical ARGs (tet and sul genes) in soil-plant systems during a 60-day cultivation, and ARGs was characterized by high-throughput qPCR with a 48 (assays) × 108 (samples) array. Only piggery wastewater irrigation significantly increased the abundance of ARGs in rhizosphere and bulk soils and root endophytes. Following 30-day cultivation, the abundance of ARGs in soil was significantly lower due to biochar addition. However, by day 60, the abundance of ARGs in soil supplemented with biochar was significantly higher than in the control soils. Antibiotics, bio-available heavy metals, nutrients, bacterial community, and mobile gene elements (MGEs) were detected and analyzed to find factors shaping ARGs dynamics. The behavior of ARGs were associated with antibiotics but not with bio-available heavy metals. The correlation between ARGs and available phosphorus was stronger than that of ARGs with total phosphorus. MGEs had good relationship with ARGs, and MGEs shifts contributed most to ARGs variation in soil and root samples. In summary, this study provides insights into potential options for biochar use in agricultural activities.