Denitrification and total nitrate uptake in streams of a tropical landscape

Rapid increases in nitrogen (N) loading are occurring in many tropical watersheds, but the fate of N in tropical streams is not well documented. Rates of nitrate uptake and denitrification were measured in nine tropical low‐order streams with contrasting land use as part of the Lotic Intersite Nitrogen eXperiment II (LINX II) in Puerto Rico using short term (24‐hour) additions of K¹⁵NO₃ and NaBr. Background nitrate concentrations ranged from 105 to 997 μg N/L, and stream nitrate uptake lengths were long, varying from 315 to 8480 m (median of 1200 m). Other indices of nitrate uptake (mass transfer coefficient, Vf [cm/s], and whole‐stream nitrate uptake rate, U [μg N·m⁻²·s⁻¹]) were low in comparison to other regions and were related to chemical, biological, and physical parameters. Denitrification rates were highly variable (0–133 μg N·m⁻²·min⁻¹; median = 15 μg N·m⁻²·min⁻¹), were dominated by the end product N₂ (rather than N₂O), and were best predicted by whole‐stream respiration rates and stream NO₃ concentration. Denitrification accounted for 1–97% of nitrate uptake with five of nine streams having 35% or more of nitrate uptake via denitrification, showing that denitrification is a substantial sink for nitrate in tropical streams. Whole‐stream nitrate uptake and denitrification in our study streams closely followed first‐order uptake kinetics, indicating that NO₃ uptake is limited by delivery of substrate (NO₃) to the organisms involved in uptake or denitrification. In the context of whole‐catchment nitrogen budgets, our finding that in‐stream denitrification results in lower proportional production of N₂O than terrestrial denitrification suggests that small streams can be viewed as the preferred site of denitrification in a watershed in order to minimize greenhouse gas N₂O emissions. Conservation of small streams is thus critical in tropical ecosystem management.