Active microbial taxa preferentially assimilate inorganic nitrogen in acidic upland soils using a 15N-DNA-SIP approach

Soil endogenous nitrogen is an essential source of crop nitrogen supply. Nitrogen assimilation is a vital transformation process that maintains nitrogen availability and promotes the mineralization of endogenous organic nitrogen in acidic soils. However, the dominant microbial populations that assimilate NH4+ and NO3- in acidic upland soils were unknown, and their response to long -term fertilization remained unclear. In this study, 15N- labeled NH4+ and NO3- were added to acidic upland soils characterized by long -term (32 years) differentiation in organic matter inputs. Glucose was added to facilitate nitrogen assimilation, and a nitrification inhibitor (dicyandiamide, DCD) was added to distinguish the contribution of NH4+ and NO3- assimilation. Results showed that soil inorganic N assimilation mainly occurred in the first week after inorganic N addition. Soil net inorganic N assimilation rates were 0.006-0.42 mg N kg -1 d-1 in the NH4+, NO3- and NH4++DCD treatments. The analysis of 15N -DNA -based stable isotope probing (SIP) found that the majority of inorganic N assimilation in our soils was conducted by a limited number of active microbial taxa, which accounted for 1.4%-2.0% of the entire bacterial community and 15.7%-28.8% of the fungal community. The dominant bacteria governing inorganic N assimilation in acidic upland soils included Bacillus, Burkholderia-Caballeronia-Paraburkholderia and Micrococcaceae. The primary fungi included Penicillium, Chaetomium, Aspergillus, and Fusarium. No obvious bias was detected in nitrogen assimilation rates and the dominant microbial taxa for NH4+-N and NO3- -N. Compared with unfertilized soil, long -term organic matter input decreased the dominating N assimilating microbial taxa. These findings demonstrate the feasibility of 15N -DNA -SIP technology in soil nitrogen cycling research and suggest the importance of active microbial taxa in acidic agricultural soil nitrogen retention and loss.