Abstract
The process of nitrogen (N) transformation after microbial utilisation of organic and inorganic N is unclear. N-15-glycine (Gly), (NH4+)-N-15 and (NO3-)-N-15 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 N-15-Gly was taken up by soil microorganisms; within 1 h, 8% was released as NH4+. The released (NH4+)-N-15 was reutilised by the microorganisms within hours. Microorganisms took up 50% of the added (NH4+)-N-15 (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.
