Utilisation and transformation of organic and inorganic nitrogen by soil microorganisms and its regulation by excessive carbon and nitrogen availability

The process of nitrogen (N) transformation after microbial utilisation of organic and inorganic N is unclear. 15N-glycine (Gly), 15NH4+ and 15NO3− were used to investigate the uptake, release and reutilisation of N by microorganisms over 9 days. In addition, high amounts of unlabelled carbon (C) or N were added to explore how C or N availability affects the cycling of inorganic and organic N by microorganisms. Within 15 min, 67% of the added 15N-Gly was taken up by soil microorganisms; within 1 h, 8% was released as NH4+. The released 15NH4+ was reutilised by the microorganisms within hours. Microorganisms took up 50% of the added 15NH4+ (15 min) and released 13% of the taken up NH4+ (1 h). Microorganisms prefer to take up Gly rather than NH4+ because they can directly acquire C from Gly for maintaining its growth and synthesising more complex compounds. NO3− was taken up by microorganisms within minutes but not released into the soil. NO3− was likely stored in the cytoplasm, to be used as an N source to face future N-deficient environments. When high concentrations of C or N were added, the assimilation of Gly and NH4+ increased, whereas N mineralisation and nitrification rates decreased, and the uptake of NO3− remained stable. Overall, Gly and NH4+ were taken up, released and re-taken up by microorganisms and were preferentially utilised under excess C or N sources, while NO3− was stored in the microbiome. These findings provide new insights into N uptake by microorganisms in short-term.