Influence of diet on the distribution of nitrogen isotopes in animals

The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

The use of stable isotopes to solve biogeochemical problems in ecosystem analysis is increasing rapidly because stable isotope data can contribute both source-sink (tracer) and process information: The elements C, N, S, H, and all have more than one isotope, and isotopic compositions of natural materials can be measured with great precision with a mass spectrometer. Isotopic compositions change in predictable ways as elements cycle through the biosphere. These changes have been exploited by geochemists to understand the global elemental cycles. Ecologists have not until quite recently employed these techniques. The reasons for this are, first, that most ecologists do not have the background in chemistry and geochemistry to be fully aware of the possibilities for exploiting the natural variations in stable isotopic compositions, and second, that stable isotope ratio measurements require equipment not normally available to ecologists. This is unfortunate because some of the more intractable problems in ecology can be profitably addressed using stable isotope measurements. Stable isotopes are ideally suited to increase our understanding of element cycles in ecosystems. This review is written for ecologists who would like to learn more about how stable isotope analyses have been and can be used in ecosystem studies. We begin with an explanation of isotope terminology and fractionation, then summarize isotopic distributions in the C, N, and S biogeochemical cycles, and conclude with five case studies that show how stable isotope measurements can provide crucial information for ecosystem analysis. We restrict this review to studies of natural variations in C, N, and S isotopic abundances, cxcluding from consideration ~5N enrichment studies and hydrogen and oxygen isotope studies. Our focus on C, N, and S derives in part from our

Stable isotopes in ecosystem studies

This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially dis- connected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N2 by denitrification.

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Nitrogen cycles: past, present, and future

Stepwise enrichment of 15N along food chains: Further evidence and the relation between δ15N and animal age

Natural abundance of 15N in particulate organic matter in the North Pacific Ocean

Simultaneous determinations of nitrogen gas production, ammonia, and particulate organic nitrogen formation in the coastal sediments of Mangoku-Ura, Simoda Bay, and Tokyo Bay were made by using the 15N-label tracer method. The rate of nitrogen gas production in the sediment surface layer was about 10−2 μg atom of N per g per h, irrespective of the location of the sediments examined. [15N]ammonia and -particulate organic nitrogen accounted for 20 to 70% of the three products, and after several hours of incubation, the major fraction of nondenitrified 15N in Mangoku-Ura and Simoda Bay sediments was recovered as ammonia. In Tokyo Bay sediments, particulate organic nitrogen was produced at a greater rate than was ammonia. The reduction rate data suggest that the pathway of nitrate reduction to ammonia is important in coastal sediments.

Denitrification and ammonia formation in anaerobic coastal sediments.

The water column distribution of suspended particulate organic nitrogen (PON) and its natural abundance ratio of 15N: 14N were investigated to a depth of ∼4000 m at 13 stations in the North Pacific, and the South China, Philippine and Bering seas. At two stations in the northern North Pacific, sediment trap experiments were also carried out. The δ15N of PON ranged from −1.5 to 23.3 per mil. The 15N natural abundance of PON increased with depth between 0 and 200 m, while the PON concentration decreased sharply in the same depth range. In the vertical profiles, the PON in the deep water was, on an average, enriched with 15N by approximately 6 per mil as compared with that in the euphotic zone. These findings imply that the vertical transport of organic matter is mediated primarily by rapidly sinking particles, and that most of the decomposition of organic matter takes place in the shallow layer beneath the bottom of the euphotic zone (<200 m) in a similar manner at all locations.

The average 15N abundance of PON in the water column was higher in the eastern tropical and central gyre portions of the Pacific than in the western Pacific, the South China Sea, the Philippine Sea, and the Bering Sea. Year-round stratification, the influence of 15N enriched nitrate produced during denitrification and the lack of significant nitrogen fixation in the surface layer probably caused the 15N enrichment in the eastern tropical Pacific.

Geographical variation of the water column distrubution of suspended particulate organic nitrogen and its 15N natural abundance in the Pacific and its marginal seas

Particulate organic matter (POM) has a central role in the vertical transport of material in the sea1. In the open ocean, POM is produced in the euphotic layer by phytoplankton and degraded in the aphotic layer during sinking to the sea floor. Isotopic abundance of biophilic elements such as C and N in POM is altered by isotopic fractionations associated with biochemical reactions2. Natural abundances of 13C or 15N thus provide useful information on biochemical behaviour of POM in the sea. We report here the first comprehensive data on the vertical distribution of 15N in suspended POM3 collected at a station in the northeastern Indian Ocean. Also, we discuss how nitrogen isotopic analysis could be used for the identification and quantification of the vertical transport processes.

15 N natural abundance in oceanic suspended particulate matter

Denitrification and nitrification in sediments of Tama Estuary and Odawa Bay, Japan, were investigated by the combined use of a continuous-flow sediment-water system and a 15N tracer technique At Odawa Bay, the nitrification rate was comparable to the nitrate reduction rate, and 70% of the N2 evolved originated from nitrogenous oxides (nitrate and nitrite) which were produced by the action of nitrifying bacteria in the sediments At Tama Estuary, the nitrate reduction rate was 11 to 17 times higher than the nitrification rate, and nitrogenous oxides derived from ammonium accounted for only 6 to 9% of the N2 evolution by denitrification

Akihiko Hattori

Papers

Natural abundance of 15N in particulate organic matter in the North Pacific Ocean

Denitrification and ammonia formation in anaerobic coastal sediments.

Geographical variation of the water column distrubution of suspended particulate organic nitrogen and its 15N natural abundance in the Pacific and its marginal seas

15 N natural abundance in oceanic suspended particulate matter

Estimates of Denitrification and Nitrification in Coastal and Estuarine Sediments