nitrification

Characterizing nitrogen dynamics, retention and transport in a tropical rainforest stream using an in situ 15N addition

Merriam, J. L. , McDowell, W. H. , Tank, J. L. , Wollheim, W. M. , Crenshaw, C. L. and Johnson, S. L. (2002), Characterizing nitrogen dynamics, retention and transport in a tropical rainforest stream using an in situ15N addition. Freshwater Biology, 47: 143–160. doi: 10.1046/j.1365-2427.2002.00785.x

Abstract: 
1. This study was part of the Lotic Intersite Nitrogen eXperiment (LINX); a series of identical 15NH4 tracer additions to streams throughout North America. 15NH4Cl was added at tracer levels to a Puerto Rican stream for 42 days. Throughout the addition, and for several weeks afterwards, samples were collected to determine the uptake, retention and transformation pathways of nitrogen in the stream. 2. Ammonium uptake was very rapid. Nitrification was immediate, and was a very significant transformation pathway, accounting for over 50% of total NH4 uptake. The large fraction of NH4 uptake accounted for by nitrification (a process that provides energy to the microbes involved) suggests that energy limitation of net primary production, rather than N limitation, drives N dynamics in this stream. 3. There was a slightly increased 15N label in dissolved organic nitrogen (DON) the day after the 15NH4 addition was stopped. This DO15N was < 0.02% of DON concentration in the stream water at the time, suggesting that nearly all of the DON found in-stream is allochthonous, or that in-stream DON production is very slow. 4. Leptophlebiidae and Atya appear to be selectively feeding or selectively assimilating a very highly labelled fraction of the epilithon, as the label found in the consumers became much higher than the label found in the food source. 5. A large spate (>20-fold increase in discharge) surprisingly removed only 37% of in-stream fine benthic organic matter (FBOM), leaves and epilithon. The fraction that was washed out travelled downstream a long distance (>220 m) or was washed onto the stream banks. 6. While uptake of 15NH4 was very rapid, retention was low. Quebrada Bisley retained only 17.9% of the added 15N after 42 days of 15N addition. Most of this was in FBOM and epilithon. Turnover rates for these pools were about 3 weeks. The short turnover times of the primary retention pools suggest that long-term retention (>1 month) is minimal, and is probably the result of N incorporation into shrimp biomass, which accounted for < 1% of the added 15N.

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
Biogeochemistry
Vol. 18, No. 2 (1992), pp. 77-99

Abstract: 
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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