High biological N fixation potential dominated by heterotrophic diazotrophs in alpine permafrost rivers on the Qinghai-Tibet Plateau

Abstract Background Biological nitrogen (N) fixation is a pivotal N source in N-deficient ecosystems. The alpine permafrost rivers on the Qinghai‒Tibet Plateau (QTP), which are recognized as N-limited and sub-anoxic environments, provide a suitable habitat for diazotrophs. However, the diversity, structure, and ecological functions of diazotrophic communities in these alpine permafrost QTP rivers remain poorly understood. Here, we examined diazotrophic communities in QTP rivers via the nitrogenase ( nifH ) gene sequencing and assessed their N-fixing activities via a 15 N isotope incubation assay. We also investigated the regulating mechanism of altitude and other geochemical factors on the diazotrophic communities and their associated N-fixing activities in QTP rivers. Results Strikingly, anaerobic heterotrophic diazotrophs, such as sulfate- and iron-reducing bacteria, had emerged as dominant N fixers, overshadowing the contributions of well-known phototrophic cyanobacterial diazotrophs. Remarkably, the nifH gene abundance and N fixation rates increased with altitude, and the average nifH gene abundance (2.57 ± 2.60 × 10 8 copies g − 1 ) and N fixation rate (2.29 ± 3.36 nmol N g − 1 d − 1 ) surpassed that documented in most aquatic environments. Such distinctive heterotrophic diazotrophic communities and high N fixation potential in QTP rivers were associated with low-nitrogen, abundant organic carbon and special C:N:P stoichiometries. Additionally, the significant presence of psychrophilic bacteria within the diazotrophic communities, along with the enhanced stability and complexity of the diazotrophic networks at higher altitudes, clearly demonstrate the adaptability of diazotrophic communities to extreme cold and high-altitude conditions in QTP rivers. We further determined that altitude, coupled with organic carbon and C:N:P stoichiometries, was the predominant driver shaping diazotrophic communities and their N-fixing activities. Based on the N fixation rates obtained in this study, the biological N fixation was estimated to be 2.79 ± 4.10 Gg N yr − 1 in QTP rivers. Conclusions The alpine permafrost rivers on the Qinghai-Tibet Plateau (QTP) possess distinctive heterotrophic diazotrophic communities and high biological N fixation potential, which are associated with altitude, abundant organic carbon, and special C:N:P stoichiometries. The estimated nitrogen input into the QTP rivers by biological N fixation would largely compensate for the nitrogen loss by denitrification. Overall, our study provides novel insights into nitrogen dynamics in alpine permafrost rivers.