| description |
The metabolic trait of ammonia-oxidizing archaea (AOA) and bacteria (AOB) cells at micrometer level determines the nitrogen cycle at global scale. Ureolytic metabolism has been proposed as a mechanism for AOB in acid soil to cope with the substrate paucity due to the ionization of ammonia to ammonium, but it remains unclear whether urea hydrolysis could afford AOA greater ecological advantages. By combining DNA-based stable isotope probing (SIP) and high-throughput pyrosequencing, here we show that autotrophic ammonia oxidation in two acid soils was primarily driven by AOA that contain urease ureC genes. In urea-amended SIP microcosms of forest soil (pH=5.40) and tea orchard soil (pH=3.75), nitrification activity was stimulated significantly by urea hydrolysis, when compared to water-amended soils in which nitrification resulted solely from the oxidation of ammonia generated through mineralization of soil organic nitrogen. The stimulation of nitrification activity by urea fertilization was well paralleled by abundance and composition changes of archaeal amoA genes. Time-course incubations indicated that archaeal amoA genes were increasingly labeled by 13CO2 in both water- and urea-amended soil microcosms. Pyrosequencing of the total 16S rRNA genes at the whole communities level revealed that archaeal populations were highly enriched in the 13C-labeled DNA up to four times greater in soil microcosms amended with urea than water. Furthermore, archaeal ureC gene encoding α subunit of urease was successfully amplified in the 13C-DNA, while bacterial amoA and 16S rRNA genes of AOB were not detected. The 13C-labeling of archaeal amoA, 16S rRNA and ureC genes was abolished when nitrification activity was blocked by acetylene, indicating chemolithoautotrophy of soil AOA. Phylogenetic analysis suggested that autotrophic ammonia oxidation in two acid soils was dominated by urease gene-containing AOA within the marine Group 1.1a-associated lineage. The findings implied that ureolytic metabolism of AOA may be important to global nitrogen cycle. |