Dissolved organic carbon (DOC) influences stream nitrogen (N) dynamics by regulating the nutrient demand of heterotrophic microbes and mediating their interactions with nitrifiers. However, DOC supply to streams is dynamic in space and time, which may create variability in N dynamics as a result of shifts between heterotrophic and chemoautotrophic influences. To test this, we measured spatial and temporal variation in concentrations and net uptake of dissolved organic nitrogen (DON), ammonium (NH4+), and nitrate (NO3−) along a 1.4-km boreal stream fed by 4 discrete groundwater inflow zones. We also performed constant rate additions of NH4+, with and without acetate, to test the influence of labile DOC availability on N cycling. Groundwater N supply did not drive spatial patterns in N concentrations. However, we observed high rates of net NH4+ uptake at the sub-reach with the greatest groundwater DOC inputs, whereas net nitrification occurred where such inputs were negligible. At the reach scale, net DON and NH4+ uptake increased with greater groundwater discharge, DOC∶DIN, and ecosystem respiration, whereas net nitrification increased with greater DOC aromaticity. Finally, constant rate additions showed that, under increased DOC availability, NH4+ uptake increased 2×, whereas the proportion of NH4+ nitrified decreased from 42 to 15%. Together, these observations suggest that nitrification rivals heterotrophic uptake when aromatic DOC promotes heterotrophic carbon limitation. Discrete groundwater inflows and periods of elevated discharge can partially alleviate this limitation by supplying labile DOC from riparian soils. Hence, accounting for these land–water connections, over both time and space, is critical for understanding N dynamics in boreal streams.