Nitrogen (N) uptake is a key process in stream ecosystems that is mediated mainly by benthic microorganisms (biofilms on different substrata) and has implications for the biogeochemical fluxes at catchment scale and beyond. Here, we focused on the drivers of assimilatory N uptake, especially the effects of hydromorphology and other environmental constraints, across three spatial scales: micro, meso and reach. In two seasons (summer and spring), we performed whole-reach ¹⁵N-labelled ammonium injection experiments in two montane, gravel-bed stream reaches with riffle–pool sequences. N uptake was highest in epilithic biofilms, thallophytes and roots (min–max range 0.2–545.2 mg N m⁻² day⁻¹) and lowest in leaves, wood and fine benthic organic matter (0.05–209.2 mg N m⁻² day⁻¹). At the microscale, N uptake of all primary uptake compartments except wood was higher in riffles than in pools. At the mesoscale, hydromorphology determined the distribution of primary uptake compartments, with fast-flowing riffles being dominated by biologically more active compartments and pools being dominated by biologically less active compartments. Despite a lower biomass of primary uptake compartments, mesoscale N uptake was 1.7–3.0 times higher in riffles than in pools. At reach scale, N uptake ranged from 79.6 to 334.1 mg N m⁻² day⁻¹. Highest reach-scale N uptake was caused by a bloom of thallopyhtes, mainly filamentous autotrophs, during stable low discharge and high light conditions. Our results reveal the important role of hydromorphologic sorting of primary uptake compartments at mesoscale as a controlling factor for reach-scale N uptake in streams.

氮吸收是河流生态系统中的一个关键过程，主要由底栖微生物（不同基质上的生物膜）介导，并且对流域规模及更大范围的生物地球化学通量具有影响。在这里，我们重点研究了三个空间尺度上同化氮吸收的驱动因素，特别是水形态学和其他环境约束的影响：微观，中观和范围。在两个季节（夏季和春季）中，我们在两个浅水潭，砾石床流和浅滩-池序列中进行了全范围^15次N标记的铵盐注入实验。上层生物膜，盐生植物和根系的氮吸收量最高（最小-最大范围0.2-545.2 mg N m ^-2天^-1），在叶片，木材和精细的底栖有机物中含量最低（0.05-209.2 mg N m ^－2天^-1）。在微观尺度上，除木材以外，所有主要吸收区室的氮吸收率在浅板中均高于池中。在中尺度上，水形态学决定了主要的吸收区室的分布，其中快速流动的浅滩以生物活性较高的区室为主，而池则以生物活性较低的区室为主。尽管主要吸收区的生物量较低，但浅滩中尺度的氮吸收量比池中高1.7-3.0倍。在达到规模时，N的吸收范围为79.6至334.1 mg N m ^-2天^-1。在稳定的低放电和强光条件下，拟南芥大量繁殖（主要是丝状自养生物）引起了最大的N吸收。我们的研究结果揭示了中尺度主要摄取区室的水形态学分选对于河流中氮素吸收水平控制的重要作用。