Distribution of Nitrous Oxide and Regulators of Its Production across a Tropical Rainforest Catena in the Luquillo Experimental Forest, Puerto Rico

MCSWINEY, CLAIRE P.; MCDOWELL, WILLIAM H.; KELLER, MICHAEL 2001. Distribution of nitrous oxide and regulators of its production across a tropical rainforest catena in the Luquillo Experimental Forest, Puerto Rico. Biogeochemistry 56: 265-286.

Understanding of N2O fluxes to the atmosphere is complicated by interactions between chemical and physical controls on both production and movement of the gas. To better understand how N2O production is controlled in the soil, we measured concentrations of N2O and of the proximal controllers on its production in soil water and soil air in a field study in the Rio Icacos basin of the Luquillo Experimental Forest, Puerto Rico. A toposequence (ridge, slope-ridge break, slope, slope-riparian break, riparian, and streambank) was used that has been previously characterized for groundwater chemistry and surface N2O fluxes. The proximal controls on N2O production include NO−3 , NH+4 , DOC, and O2. Nitrous oxide and O2 were measured in soil air and NO−3 , NH+4 , and DO were measured in soil water. Nitrate and DOC disappeared from soil solution at the slope-riparian interface, where soil N2O concentrations increased dramatically. Soil N2O concentrations continued to increase through the flood plain and the streambank. Nitrous oxide concentrations were highest in soil air probes that had intermediate O2 concentrations. Changes in N2O concentrations in groundwater and soil air in different environments along the catena appear to be controlled by O2 concentrations. In general, N processing in the unsaturated and saturated zones differs within each topographic position apparently due to differences in redox status.

The Effects of Natural and Human Disturbances on Soil Nitrogen Dynamics and Trace Gas Fluxes in a Puerto Rican Wet Forest

The Effects of Natural and Human Disturbances on Soil Nitrogen Dynamics and Trace Gas Fluxes in a Puerto Rican Wet Forest
P. A. Steudler, J. M. Melillo, R. D. Bowden, M. S. Castro and A. E. Lugo
Vol. 23, No. 4, Part A. Special Issue: Ecosystem, Plant, and Animal Responses to Hurricanes in the Caribbean (Dec., 1991), pp. 356-363

We examined the effects of two disturbances (Hurricane Hugo and forest clearcutting) on soil nitrogen dynamics and on the exchanges of N20, CO,, and CH, between soils and the atmosphere of a subtropical wet forest in Puerto Rico. The disturbances resulted in prolonged increases in ammonium pools and short-term increases in rates of net N-mineralization and net nitrification. Nitrous oxide emissions increased following both disturbances. The most dramatic increase was observed 4 mo after clearcutting; N 2 0 emissions (109.49 pg N/m2-hr) from the cut plot were about two orders of magnitude higher than emissions from the reference plot (1.71 pg N/m2-hr). Carbon dioxide emissions from both disturbed plots (mean 102.47 mg C/m2-hr) were about 30 percent lower than the reference (mean 15 1.28 mg C/m2-hr). Soils at all sites were generally sinks for CH,. Methane uptake, however, was suppressed by both disturbances. This suppression may be related to disturbance-induced changes in the nitrogen cycle, as we have previously observed in temperate zone forests.

Control of Nitrogen Export from Watersheds by Headwater Streams

Peterson, B.J. et al. 2001. Control of Nitrogen Export from Watersheds by Headwater Streams.
Science 6 April 2001:
Vol. 292 no. 5514 pp. 86-90
DOI: 10.1126/science.1056874

A comparative 15N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.

Can uptake length in strams be determined by nutrient addition experiments? Results from an interbiome comparison study

Mulholland, P. J; Tanks, J. L.; Webster, J. R.; Bowden, W. B.; Dodds, W. K; Gregory, S. V.; Grimm, N. B; Meriam, J. L.; Meyer, J. L.; Peterson, B. J.; Valett, H. M.; Wollheim, W. M. 2002. Can uptake length in strams be determined by nutrient addition experiments? Results from an interbiome comparison study. J. N. Am. Bethol. Soc. 2002, 21(4): 544-560.

Nutrient uptake length is an important parnmeter tor quantifying nutrient cycling in streams. Although nutrient tracer additions are the preierred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more irequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (Sw') and uptake length determined by tracer additions (Sw)predicted that Sw' should be consistently longer than 5,", and that the overestimate of uptake length by Sw( should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data irom an interbiorne study of NH,- uptake length in which 15NH,- tracer and short-term NH,-a ddition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely contirmed the theoretical predictions: sw' was consistently longer than Sw and Sw':Sw ratios were directly related to the level of NH,- addition and to indicatvrs of N limitation. The experimentally derived Sw':Sw, ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed the tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or cliiferent times in the same stream. however, provided that nutrient additions are kept as low as possible and of similar miagnitude.

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

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: Subsurface Solute Patterns

Riparian Nitrogen Dynamics in Two Geomorphologically Distinct Tropical Rain Forest Watersheds: Subsurface Solute Patterns
William H. McDowell, William B. Bowden and Clyde E. Asbury
Vol. 18, No. 2 (1992), pp. 53-75

Nitrate, ammonium, dissolved organic N, and dissolved oxygen were measured in stream water and shallow groundwater in the riparian ones of two tropical watersheds with different soils and geomorphology. At both sites, concentrations of dissolved inorganic N (DIN: NH-4+ and NO-3-N) were low in stream water lt 110 mu-g/L). Markedly different patterns in DIN were observed in groundwater collected at the two sites. At the first site (Icacos watershed), DIN in upslope groundwater was dominated by NO-3-N (550 mu-g/L) and oxygen concentrations were high (5.2 mg/l). As groundwater moved through the floodplain and to the stream, DIN shifted to dominance by NH-4+-N (200-700 mu-g/L) and groundwater was often anoxic. At the second site (Bisley watershed), average concentrations of total dissolved nitrogen were considerably lower (300 mu-g/L) than at Icacos (600 mu-g/L), and the dominant form of nitrogen was DON rather than inorganic N. Concentrations of NH-4+ and NO-3- were similar throughout the riparian zone at Bisley, but concentrations of DON declined from upslope wells to stream water. Differences in speciation and concentration of nitrogen in groundwater collected at the two sites appears to be controlled by differences in redox conditions and accessibility of dissolved N to plant roots, which are themselves the result of geomorphological differences between the two watersheds. At the Icacos site, a deep layer of coarse sand conducts subsurface water to the stream below the rooting zone of riparian vegetation and through zones of strong horizontal redox zonation. At the Bisley site, infiltration is impeded by dense clays and saturated flow passes though the variably oxidized rooting zone. At both sites, hydrologic export of nitrogen is controlled by intense biotic activity in the riparian zones. However, geomorphology appears to strongly modify the importance of specific biotic components.
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