Relationships Between Abundance of Microbial Functional Genes and the Status and Fluxes of Carbon and Nitrogen in Rice Rhizosphere and Bulk Soils

Rapid nitrogen (N) transformations and losses occur in the rice rhizosphere through root uptake and microbial activities. However, the relationships between rice roots and rhizosphere microbes for N utilization are still unclear. We analyzed different N forms (NH4+,NO, and dissolved organic N), microbial biomass N and C, dissolved organic C, CH4 and N(2)0 emissions, and abundance of microbial functional genes in both rhizosphere and bulk soils after 37-d rice growth in a greenhouse pot experiment. Results showed that the dissolved organic C was significantly higher in the rhizosphere soil than in the non-rhizosphere bulk soil, but microbial biomass C showed no significant difference. The concentrations of NH4+, dissolved organic N, and microbial biomass N in the rhizosphere soil were significantly lower than those of the bulk soil, whereas NO3- in the rhizosphere soil was comparable to that in the bulk soil. The CH4 and N20 fluxes from the rhizosphere soil were much higher than those from the bulk soil. Real-time polymerase chain reaction analysis showed that the abundance of seven selected genes, bacterial and archaeal 16S rRNA genes, amoA genes of ammonia-oxidizing archaea and ammonia-oxidizing bacteria, nosZ gene, mcrA gene, and pmoA gene, was lower in the rhizosphere soil than in the bulk soil, which is contrary to the results of previous studies. The lower concentration of N in the rhizosphere soil indicated that the competition for N in the rhizosphere soil was very strong, thus having a negative effect on the numbers of microbes. We concluded that when N was limiting, the growth of rhizosphere microorganisms depended on their competitive abilities with rice roots for N.