Long-term chemical fertilization-driving changes in soil autotrophic microbial community depresses soil CO₂ fixation...

Soil is the largest C pool in the terrestrial ecosystem. Numerous studies have been devoted to the decomposition of soil organic C as influenced by agricultural management. However, little is known about the effect of fertilization on the microbial CO₂ fixation potential. Here, we examined the atmospheric CO₂ fixation rates and structure of autotrophic cbbL-containing bacterial communities and accA-containing archaeal communities in response to 38 years of chemical and/or organic fertilizer application in a Mollisol. The autotrophic microbial abundance and community composition were analyzed by quantitative polymerase chain reaction and high throughput sequencing, respectively. Our results showed that chemical fertilization additions significantly decreased CO₂ fixation rates by 57%, but organic manure use resulted in no notable differences compared to no fertilizer regimes (0.38 mg CO₂ kg^-1 soil d^-1) through stable isotope methods. The declining soil pH and increasing Olsen-phosphorus in soils with chemical fertilization dramatically reduced the cbbL gene diversity and accA gene abundances and altered both the autotrophic bacterial and archaeal community compositions. The changes in CO₂-fixation rate were more greatly attributed to the shifts in autotrophic bacterial community composition than to the diversity and abundance. The C fixation potentials were positively correlated with the relative abundances of Acidiphilium and Methylibium but were negatively related to those of Azospirillum and Nitrosospira. Both composition and abundance of the autotrophic archaeal community contributed together to the CO₂ fixation activities. Our finding suggests that long-term chemical fertilization has a strong impact on the soil microbial CO₂ fixation activity and autotrophic microorganisms in upland soils and highlight the important roles of the CO₂ fixing process in soil organic carbon sequestration.