McDowell W.H.

C and N dynamics in the riparian and hyporheic zones of a tropical stream, Luquillo Mountains, Puerto Rico

C and N Dynamics in the Riparian and Hyporheic Zones of a Tropical Stream, Luquillo Mountains, Puerto Rico
Tamara J. Chestnut and William H. McDowell
Journal of the North American Benthological Society
Vol. 19, No. 2 (Jun., 2000), pp. 199-214

Hydrologic and chemical characteristics were determined for both riparian and hyporheic subsurface flow along a 100-m reach of a sandy-bottom tributary of the Rio Icacos in the Luquillo Experimental Forest, Puerto Rico. Hydrologic data (vertical hydraulic gradient and hydraulic conductivity of streambed sediments) and the topographic and morphological features of the watershed indicated diffuse inputs of groundwater from the near-stream riparian zone along this site. Cumulative groundwater discharge, determined by tracer dilution techniques, was ∼1.5 L/s or 10% of the total stream discharge. Spatial heterogeneity in hydrologic and chemical properties of riparian and hyporheic sediments was large. Hydraulic conductivity explained much of the variation in NH<sub>4</sub>-N and dissolved organic carbon (DOC) concentrations, with highest concentrations in sites having low conductivity. A mass-balance approach was used to examine the influence of the near-stream zone on nutrient transport and retention. Outwelling riparian groundwater had the potential to increase stream N concentrations by up to 84% and DOC concentrations by up to 38% along our 100-m reach. Because stream concentrations were constant downstream despite this input, we conclude that significant N and C retention or loss were occurring in the near-stream zone. Lotic ecosystems and their associated riparian groundwater can have a quantitatively significant impact on the nutrient budgets of tropical headwater catchments.

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Nitrous Oxide Fluxes

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Nitrous Oxide Fluxes
William B. Bowden, William H. McDowell, Clyde E. Asbury and Amy M. Finley
Vol. 18, No. 2 (1992), pp. 77-99

Fluxes of N<sub>2</sub>O at the soil surface, dissolved N<sub>2</sub>O in near-surface groundwater, and potential N<sub>2</sub>O production rates were measured across riparian catenas in two rain forest watersheds in Puerto Rico. In the Icacos watershed, mean N<sub>2</sub>O fluxes were highest at topographic breaks in the landscape (∼40-300 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>). At other locations in the riparian zone and hillslope, fluxes were lower (≤ 2 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>). This pattern of surface N<sub>2</sub>O fluxes was persistent. In the Bisley watershed, mean suface N<sub>2</sub>O fluxes were lower (<40 μg N<sub>2</sub>O-N m<sup>-2</sup> h<sup>-1</sup>) and no identifiable spatial or temporal pattern. Although the spatial patterns and intensities of N<sub>2</sub>O emissions differed between the two watersheds, surface soils from both sites had a high potential to reduce NO<sub>3</sub> to N<sub>2</sub>O (and perhaps N<sub>2</sub>). This potential declined sharply with depth as did soil %C, %N, and potential N-mineralization. Simple controls on denitrification (i.e. aeration, nitrate, and carbon) explained characteristics of potential N<sub>2</sub>O production in surface and deep soils from riparian and upslope locations. In the field, spatial patterns in these controlling variables were defined by geomorphological differences between the two watersheds, which then explained the spatial patterns of observed N<sub>2</sub>O flux.
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