Denitrification and total nitrate uptake in streams of a tropical landscape

• Published in: Ecological applications 2010-12, Vol.20 (8), p.2104-2115
• Main Authors: Potter, J. D, McDowell, W. H, Merriam, J. L, Peterson, B. J, Thomas, S. M
• Format: Article
• Language: English
• Subjects: Denitrification; Ecosystem; Environmental Monitoring; Freshwater ecosystems; Groundwater; Land use; Lotic systems; Marine ecosystems; N loading; N2O emissions; nitrate uptake; Nitrates; Nitrates - chemistry; Nitrates - metabolism; Nitrogen; O emissions; Puerto Rico; Riparian forests; Rivers - chemistry; Streams; Tropical Climate; tropical streams; tropics; Watersheds
• ISSN: 1051-0761; 1939-5582
• DOI: 10.1890/09-1110.1

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 K 15 NO 3 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, V f [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 N 2 (rather than N 2 O), and were best predicted by whole-stream respiration rates and stream NO 3 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 NO 3 uptake is limited by delivery of substrate (NO 3 ) 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 N 2 O 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 N 2 O emissions. Conservation of small streams is thus critical in tropical ecosystem management.