Environmentally Relevant-Level CeO NP with Ferrous Amendment Alters Soil Bacterial Community Compositions and Metabolite Profiles in Rice-Planted Soils.

The environmental risks and benefits associated with the introduction of CeO nanoparticle (NP) in agricultural soil must be carefully assessed. The ferrous ion is rich in rhizosphere soil of rice due to the reduction states underground. The aim of this study was to investigate the effects of environmentally relevant-level CeO NP (25 mg·kg) in the absence or presence of ferrous (30 mg·kg) amendment on soil bacterial communities and soil metabolomics in rice-planted soil over 150 days. Results showed that CeO NP exposure changed soil bacterial community compositions and soil metabolomics, and the above changes were further shifted with the ferrous amendment. Several functionally significant bacterial phyla containing Proteobacteria and Bacteroidetes abundances, which were associated with carbon and nitrogen cycling, were promoted after CeO NP exposure with ferrous amendment. However, CeO NP inhibited plant-growth-promoting rhizobacteria containing genera and irrespective of the presence or absence of ferrous. Among rhizosphere soil enzyme activities, cellulose activity was the most sensitive for CeO NP exposure. NP decreased Firmicutes and increased Chloroflexi, Rokubacteria, and Thaumarchaeota abundances at the phylum level, which contributed to reduce soil cellulose activity. Additionally, CeO NP positively or negatively affected soil pH, Ce accumulation in root, and rice physiological properties (root-POD, stem-POD). As a result, the above factors were related to the changes of Chloroflexi, Gemmatimonadetes, Rokubacteria, Thaumarchaeota, and Nitrospirae at the phylum level. After adding CeO NP with ferrous or not, the main metabolic changes were concentrated on fluctuations in starch and sucrose metabolism, nitrogen metabolism, sulfur metabolism, propanoate metabolism, fatty acid metabolism, and urea cycle. The eight changed metabolites containing glycerol monstearate, boric acid, monopalmitin, palmitic acid, alkane, ethanol, dicarboximide, and stearic acid accounted for the separation of different treatments with CeO NP exposure. Activities of soil enzymes (urease, invertase, and cellulose), pH, and soil organic matter affected dominant metabolites containing fatty acids, inorganic acid, and sugar. Network analysis showed that the influence of soil bacterial community on metabolites varied with metabolites and bacteria species. The presence of CeO NP mainly promoted fatty acids (hexanoic acid, nonanoic acid) and amino acid (oxoproline) and amine (diethanolamine) concentrations, which could be from the increased Proteobacteria abundance after CeO NP exposure. Phylum Proteobacteria had the most genus species containing 13 genera affecting soil metabolite profiles. These results provide valuable information for understanding the impact of environmentally relevant-level CeO NP exposure on soil microbial communities and metabolites with or without ferrous, which is needed to understand the ecological risk posed by long-term CeO NP exposure in rice-planted soil with rich ferrous.