Objectives The effect of green manure on soil phosphorus-cycle(P-cycle) has gained great attention, but the involved microbial mechanism is largely unknown. In order to clarify the microbial mechanism of how green manure impacts on soil P-cycle, we comprehensively analyzed the effects of long-term green manure on soil microbial P-cycle genes by Metagenomics.
Methods The study was based on a long-term green manure field experiment in Qiyang, Hunan, China. The rice rhizosphere soil was collected in the four treatments, including with or without green manure returning in combination with or without rice straw returning. The shortgun metagenomic sequencing was performed on an Illumina NovaSeq platform with a PE150 strategy. The read_qc module in MetaWRAP was used for quality control, and the assembly module megahit method was used for assembly. The assembled long sequences (> 1000 bp) were aligned with SWISS-PROT databases using Diamond BLASTX, and then screened according to 64 P-cycle genes seven functional groups of the three major categories including P activation (phosphate mineralization, phosphonate mineralization, inorganic P solubilization), P uptake (phosphonate transport, organic phosphate transport, inorganic P transport) and P-starvation inducible response and regulation. Then, the R software was used to analyze the relative abundance of P-cycle genes, and to analyze the relationship between the relative abundance of genes and soil properties.
Results Nine soil physicochemical and biological indexes including pH showed no significant differences between the treatments of green manure and non-green manure under both conditions of straw and non-straw amendment. Among the 64 functional genes related to P cycle, the relative abundances of 11 genes were significantly affected by green manure returning under condition of without straw amendment. These 11 genes were widely distributed in five groups except organic phosphate transport and inorganic phosphate solubilizing genes, and the relative abundance of 3 genes, i.e. phnA, phnN, phnV, were increased, and those of the other 8 genes such as phnI, phnL, glpB, glpO, pitA, phoA, phoP and phoU, were decreased. However, only the relative abundance of phnPP gene was significantly decreased by green manure in straw-amended plots. Correlation analysis showed that soil pH was significantly and negatively correlated with acid phosphatase activity and relative abundance of inorganic phosphate-soluble genes pqqB, pqqC, pqqCD, pqqE and pqqF significantly (P < 0.05). The relative abundance of four genes pqqB, pqqC, pqqCD and pqqE in the functional group of inorganic P solubilization were positively correlated with available P content, phosphatase activity and organic N content (P < 0.05). Redundancy analysis (RDA) showed that pH, available P content and phosphatase activity were the three important factors significantly correlated with the effect of green manure treatment on P-cycle genes.
Conclusions Green manure could impact multiple processes of the P cycling in rice rhizosphere under straw amendment. Straw amendment under green manure returning does not have that significant effect like without straw amendment on the P-cycle related genes, or even weaken the effect possibly due to competition among microorganisms and P-functional genes involved in straw and green manure decomposition.
DownLoad: