15N

Feeding preferences of West Indian manatees in Florida, Belize, and Puerto Rico as indicated by stable isotope analysis

Alves-Stanley C.D., Worthy G.A.J., Bonde R.K. Feeding preferences of West Indian manatees in Florida, Belize, and Puerto Rico as indicated by stable isotope analysis. Marine Ecology Progress Series, Vol 402. pp. 255-267, 2010.

Evidence for nitrogen fixation associated with macroalgae from a seagrass - mangrove - coral reef system

France, R., J. Holmquist, M. Chandler, and A. Cattaneo. 1998. Delta N-15 evidence for nitrogen fixation associated with macroalgae from a seagrass-mangrove coral reef system RID H-3188-2011. Marine Ecology-Progress Series 167 : 297-9.

Abstract: 
Early studies using acetylene reduction have suggested that macroalgae may contribute to nitrogen enrichment of waters near coral reefs via nitrogen fixation by their epiphytic cynaphytes. Our objectives were to investigate the potential of stable nitrogen isotope analysis for detecting nitrogen fixation in near-reef macroalgae, and to compare these finding with those for different fixing and non-fixing autotrophs from other systems. We made collections of near-reef algae in Puerto Rico, seagrasses and macroalgae from streams in Quebec, Canada. The mean + SD 6 1 5 ~fo r near-reef algea was 0.3 r i.G%, vaiues whlch were significantly lower than those from our other sampling areas and also well below published values for other marine benthic nitrogen fixers (ca 2%) and non-fixers (ca 6%0). Our results provide a useful test of the value of stable nitrogen analysis in detecting nitrogen fixation of near-reef algae, thereby supporting previous non-isotope work in suggesting that this macroalgalcyanophyte complex may provide an important source of fixed nitrogen to reef systems.

Nutrient relations of dwarf Rhizophora mangle L. mangroves on peat in eastern Puerto Rico

Medina E, Cuevas E, Lugo AE (2010) Nutrient relations of
dwarf Rhizophora mangle L. mangroves on peat in eastern
Puerto Rico. Plant Ecol 207:13–24

Abstract: 
Dwarf mangroves on peat substrate growing in eastern Puerto Rico (Los Machos, Ceiba State Forest) were analyzed for element concentration, leaf sap osmolality, and isotopic signatures of C and N in leaves and substrate. Mangrove communities behind the fringe presented poor structural development with maximum height below 1.5 m, lacked a main stem, and produced horizontal stems from which rhizophores developed. This growth form departs from other dwarf mangrove sites in Belize, Panama, and Florida. The dwarf mangroves were not stressed by salinity but by the low P availability reflected in low P concentrations in adult and senescent leaves. Low P availability was associated with reduced remobilization of N and accumulation of K in senescent leaves, contrasting with the behavior of this cation in terrestrial plants. Remobilization of N and P before leaf abscission on a weight basis indicated complete resorption of these nutrients. On an area basis, resorption was complete for P but not for N. Sulfur accumulated markedly with leaf age, reaching values up to 400%, compared with relatively modest accumulation of Na (40%) in the same leaves. This suggests a more effective rejection of Na than sulfate at the root level. Dwarf mangrove leaves had more positive d13C values, which were not related to salinity, but possibly to drought during the dry season due to reduced flooding, and/or reduced hydraulic conductance under P limitation. Negative leaf d15N values were associated with low leaf P concentrations. Comparison with other R. mangle communities showed that P concentration in adult leaves below 13 mmol kg-1 is associated with negative d15N values, whereas leaves with P concentrations above 30 mmol kg-1 in non-polluted environments had positive d15N values.

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils

Dissimilatory Nitrate Reduction to Ammonium in Upland Tropical Forest Soils
Whendee L. Silver, Donald J. Herman and Mary K. Firestone
Ecology
Vol. 82, No. 9 (Sep., 2001), pp. 2410-2416

Abstract: 
The internal transformations of nitrogen in terrestrial ecosystems exert strong controls over nitrogen availability to net primary productivity, nitrate leaching into groundwater, and emissions of nitrogen-based greenhouse gas. Here we report a reductive pathway for nitrogen cycling in upland tropical forest soils that decreases the amount of nitrate susceptible to leaching and denitrification, thus conserving nitrogen in the ecosystem. Using 15N tracers we measured rates of dissimilatory nitrate reduction to ammonium (DNRA) in upland humid tropical forest soils averaging ;0.6 mg·g21·d21. Rates of DNRA were three times greater than the combined N2O and N2 fluxes from nitrification and denitrification and accounted for 75% of the turnover of the nitrate pool. To determine the relative importance of ambient C, O2, and NO3 concentrations on rates of DNRA, we estimated rates of DNRA in laboratory assays using soils from three tropical forests (cloud forest, palm forest, and wet tropical forest) that differed in ambient C and O2 concentrations. Rates of DNRA measured in laboratory assays ranged from 0.5 to 9 mg·g21·d21 in soils from the three different forests and appeared to be primarily limited by the availability of NO3, as opposed to C or O2. Tests of sterile soils indicated that the dominant reductive pathway for both NO2 and NO3 was biotic and not abiotic. Because NH4 is the form of N generally favored for assimilation by plants and microbes, and NO3 is easily lost from the ecosystem, the rapid and direct transformation of NO3 to NH4 via DNRA has the potential to play an important role in ecosystem N conservation.

Nutrient relations of dwarf Rhizophora mangle L. mangroves on peat in eastern Puerto Rico

Medina E, Cuevas E, Lugo AE (2010) Nutrient relations of
dwarf Rhizophora mangle L. mangroves on peat in eastern
Puerto Rico. Plant Ecol 207:13–24

Abstract: 
Dwarf mangroves on peat substrate growing in eastern Puerto Rico (Los Machos, Ceiba State Forest) were analyzed for element concentration, leaf sap osmolality, and isotopic signatures of C and N in leaves and substrate. Mangrove communities behind the fringe presented poor structural development with maximum height below 1.5 m, lacked a main stem, and produced horizontal stems from which rhizophores developed. This growth form departs from other dwarf mangrove sites in Belize, Panama, and Florida. The dwarf mangroves were not stressed by salinity but by the low P availability reflected in low P concentrations in adult and senescent leaves. Low P availability was associated with reduced remobilization of N and accumulation of K in senescent leaves, contrasting with the behavior of this cation in terrestrial plants. Remobilization of N and P before leaf abscission on a weight basis indicated complete resorption of these nutrients. On an area basis, resorption was complete for P but not for N. Sulfur accumulated markedly with leaf age, reaching values up to 400%, compared with relatively modest accumulation of Na (40%) in the same leaves. This suggests a more effective rejection of Na than sulfate at the root level. Dwarf mangrove leaves had more positive d13C values, which were not related to salinity, but possibly to drought during the dry season due to reduced flooding, and/or reduced hydraulic conductance under P limitation. Negative leaf d15N values were associated with low leaf P concentrations. Comparison with other R. mangle communities showed that P concentration in adult leaves below 13 mmol kg-1 is associated with negative d15N values, whereas leaves with P concentrations above 30 mmol kg-1 in non-polluted environments had positive d15N values.

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.
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