Impacts of short-term increased CO2 levels on the composition of amoA-type nitrifying and nirS-type denitrifying bacterial communities in soybean rhizosphere in Mollisols

Abstract

Increased atmospheric carbon dioxide (CO2) concentrations cause climate change, affecting the stability of agricultural ecosystems. Soil nitrification and denitrification, which are fundamental N cycling processes, determine soil N availability for plants, nitrous oxide (N2O) emissions, and N flow in ecosystems. However, the responses of specific nitrifying and denitrifying soil microorganisms to increased CO2 levels remain unknown. In this study, four soybean [Glycine max (L.) Merr.] varieties (Xiaohuangjin [XHJ], Suinong 14 [SN14], Mufeng 5 [MF5], and Dongsheng 1 [DS1]) were planted in open-top chambers containing Mollisols under ambient CO2 (aCO2, 410 ppm) and elevated CO2 (eCO2, 550 ppm) conditions. The abundance, composition, and potential activity of ammonia mono‑oxygenase subunit A gene (amoA) of ammonia oxidise bacteria (AOB) and nitrite reductase S gene (nirS)-type denitrifiers under increased atmospheric CO2 levels were investigated. The results showed that eCO2 decreased the nitrification potential in XHJ and MF5 rhizospheres, whereas increasing it in the SN14 rhizosphere. Denitrification potential decreased in the rhizosphere of all soybean cultivars, except XHJ, under eCO2 conditions. The decreased nitrification and denitrification potentials imply that eCO2 conditions may slow N2O emission. Principal co-ordinate analysis revealed that the composition of amoA and nirS communities was not altered by eCO2, but rather by the presence of different soybean cultivars. The eCO2-induced variations in the nitrification and denitrification potentials of microbes in the rhizosphere among different cultivars can affect the N cycle and should be considered in future soybean breeding programmes.