Cyanobacterial biofertilizer inoculation has a distinctive effect on the key genes of carbon and nitrogen cycling in paddy rice

Abstract

Cyanobacterial biofertilizers provide soil fertility and productivity gains at varying levels in paddy rice cultivation. The colonization and influences of introduced strains in different soil types with characteristic compositions of native cyanobacteria remain largely unknown. In this work, seven paddy rice soils with the composition of indigenous cyanobacteria described by amplicon sequencing analysis were inoculated with the cyanobacterial biofertilizer. The microbial abundance and the cyanophage concentrations were evaluated under light-dark or continuous dark cycles using quantitative polymerase chain reaction (qPCR) assays. The copies of cyanobacterial-16S rRNA gene markers varied from 5.65 × 10⁶ to 9.22 × 10⁷ g^-1 soil, and their abundance increased significantly in the inoculated soils. The cyanophage concentrations, quantified using the capsid assembly protein gene g20 in the soils tested, ranged from 3.04 × 10⁸ to 9.24× 10⁸ g^-1 soil on 30 days after incubation. There were significant increases in the abundance of the nifH gene copies, about 1.54×10⁵ to 1.35×10⁶ g^-1, in the inoculated soils, albeit with soil type-specific responses. The gene markers of C and N cycling (i.e., cbbL and nifH, respectively), taxonomic markers (of archaea, bacteria, and cyanobacteria), and cyanophage-specific gene copies showed strong and positive correlation with the cyanobacterial biofertilizer inoculation. However, the genes related to nitrification (bacterial and archaeal amoA) and denitrification (nirS, nirK, narG, and nosZ) were clustered together in the uninoculated soils. The rice rhizospheres in three representative paddy soil types, using metatranscriptomics analysis, showed distinctive colonization by cyanobacteria, with several members yet to be described. These results indicate the potential for improving cyanobacterial biofertilizers for their contributions to plant growth and fertility gains in a soil-specific way.