Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable N isotopes, 15N and 14N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ15N] deviations from the standard atmospheric N2) depend on isotopic signatures of inputs and outputs, the input–output balance, N transformations and their specific isotope effects, and compartmentation of N within the system. Products along a sequence of reactions, e.g. the N mineralization–N uptake pathway, should, if fractionation factors were equal for the different reactions, become progressively depleted. However, fractionation factors vary. For example, because nitrification discriminates against 15N in the substrate more than does N mineralization, NH4+ can become isotopically heavier than the organic N from which it is derived.Levels of isotopic enrichment depend dynamically on the stoichiometry of reactions, as well as on specific abiotic and biotic conditions. Thus, the δ15N of a specific N pool is not a constant, and δ15N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring δ15N in small and dynamic pools of N in the soil–plant system, and the complexity of the N cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of 15N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ15N might offer the advantage of giving insights into the N cycle without disturbing the system by adding 15N tracer.Such attempts require, however, that the complex factors affecting δ15N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NOx, NH3, N2-fixation), (ii) the depth(s) in soil from which N is taken up, (iii) the form(s) of soil-N used (organic N, NH4+, NO3−), (iv) influences of mycorrhizal symbioses and fractionations during and after N uptake by plants, and (v) interactions between these factors and plant phenology. Because of this complexity, data on δ15N can only be used alone when certain requirements are met, e.g. when a clearly discrete N source in terms of amount and isotopic signature is studied. For example, it is recommended that N in non-N2-fixing species should differ more than 5‰ from N derived by N2-fixation, and that several non-N2-fixing references are used, when data on δ15N are used to estimate N2-fixation in poorly described ecosystems.As well as giving information on N source effects, δ15N can give insights into N cycle rates. For example, high levels of N deposition onto previously N-limited systems leads to increased nitrification, which produces 15N-enriched NH4+ and 15N-depleted NO3−. As many forest plants prefer NH4+ they become enriched in 15N in such circumstances. This change in plant δ15N will subsequently also occur in the soil surface horizon after litter-fall, and might be a useful indicator of N saturation, especially since there is usually an increase in δ15N with depth in soils of N-limited forests.Generally, interpretation of 15N measurements requires additional independent data and modelling, and benefits from a controlled experimental setting. Modelling will be greatly assisted by the development of methods to measure the δ15N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level 15N studies will be necessary to further develop the interpretation of variations in δ15N in soil–plant systems. Another promising approach is to study ratios of 15N[ratio ]14N together with other pairs of stable isotopes, e.g. 13C[ratio ]12C or 18O[ratio ]16O, in the same ion or molecules. This approach can help to tackle the challenge of distinguishing isotopic source effects from fractionations within the system studied.

Tansley Review No. 95 15N natural abundance in soil-plant systems

Publisher Summary This chapter focuses on the uses of isotopes to understand water chemistry.I Isotopic compositions generally cannot be interpreted successfully in the absence of other chemical and hydrologic data. The chapter focusses on uses of isotopes in tracing sources and cycling of nitrogen in the water-component of forested catchment, and on dissolved nitrate in shallow waters, nutrient uptake studies in agricultural areas, large-scale tracer experiments, groundwater contamination studies, food-web investigations, and uses of compound-specific stable isotope techniques. Shallow waters moving along a flowpath through a relatively uniform material and reacting with minerals probably do not achieve equilibrium but gradually approach some steady-state composition. The chapter also discusses the use of isotopic techniques to assess impacts of changes in land-management practices and land use on water quality. The analysis of individual molecular components for isotopic composition has much potential as a method for tracing the source, biogeochemistry, and degradation of organic liquids and gases because different materials have characteristic isotope spectrums or biomarkers.

Tracing nitrogen sources and cycling in catchments

Nitrate (NO3 ) concentrations in public water supplies have risen above acceptable levels in many areas of the world, largely as a result of overuse of fertilizers and contamination by human and animal waste. The World Health Organization and the U.S. Environmental Protection Agency have set a limit of 10 mg L nitrate (as N) for drinking water because nitrate poses a health risk, especially for children, who can contract methemoglobinemia (blue-baby syndrome). Nitrate in lower concentrations is non-toxic, but the risks from long-term exposure are unknown, although nitrate is a suspected carcinogen. High concentrations of nitrate in rivers, lakes, and coastal areas can cause eutrophication, often followed by fi sh-kills, due to oxygen depletion. Increased atmospheric loads of anthropogenic nitric and sulfuric acids have caused many sensitive, low-alkalinity streams in North America and Europe to become acidifi ed. Still more streams that are not yet chronically acidic could undergo acidic episodes in response to large rain storms and/or spring snowmelt, seriously damaging sensitive local ecosystems. Future climate changes may exacerbate the situation by affecting biogeochemical controls on the transport of water, nutrients, and other materials from land to freshwater ecosystems. The development of effective management practices to preserve water quality, and remediation plans for sites that are already polluted, requires the identifi cation of actual N sources and an understanding of the processes affecting local nitrate concentrations. In particular, a better understanding of hydrologic fl owpaths and solute sources is required to determine the potential impact of contaminants on water supplies. Determination of the relation between nitrate concentrations in groundwater and surface water and the quantity of nitrate introduced from a particular source is complicated by:

Tracing Anthropogenic Inputs of Nitrogen to Ecosystems

Pollution of the hydrosphere and atmosphere by compounds of nitrogen is a serious problem. This paper reviews the manner in which studies of natural abundance 15N/14N ratios may be employed in investigating the sources and mechanisms of pollution. Cultivation-induced mineralization of soil nitrogen, fertilizer, and animal or sewage wastes are the three main sources of nitrate pollution in the hydrosphere. In many cases these sources produce nitrate with distinguishable 15N/14N ratios, and on this basic isotopic data for nitrate have been successfully used for identifying the source of pollution in a wide variety of ground- and surface water environments. Distinction between continentally- and marine-derived organic nitrogen in ecologically sensitive coastal waters also appears possible. These differences in 15N/14N ratios, however, are largely the result of kinetic isotope fractionation associated with bacterially-mediated reactions. The unpredictable magnitude of this type of fractionation tends to restrict the use of nitrogen isotope data in the hydrosphere to semi-quantitative interpretations. Observations of the isotopic fractionation between nitrogen compounds in the atmosphere may provide valuable information on whether their physico-chemical reactions are controlled by kinetic or by equilibrium processes. The possibility of using 15N/14N data for distinguishing between anthropogenic and natural sources of NOx gases, potentially a very important application, is as yet unproven.

Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: A review

Nutrient enrichment as a result of anthropogenic activity concentrated along the land-sea margin is increasing eutrophication of near-shore waters across the globe. Management of eutrophication in the coastal zone has been hampered by the lack of a direct method to trace nitrogen sources from land into coastal food webs. Stable isotope data from a series of estuaries receiving nitrogen loads from 2 to 467 kg N ha -1 yr -1 from the Waquoit Bay watershed, Cape Cod, Massachusetts, indicate that producer and consumer 15 N-to- 14 N ratios record increases in wastewater nitrogen inputs. Nitrate from groundwater-borne wastewater introduces a 15 N-enriched tracer to estuaries. This study explicitly links anthropogenically derived nitrogen from watersheds to nitrogen in estuarine plants and animals, and suggests that wastewater nitrogen may be detectable in estuarine biota at relatively low loading rates, before eutrophication leads to major changes in species composition and abundance within estuarine food webs.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1997.42.5.0930

Nitrogen-stable isotope signatures in estuarine food webs : A record of increasing urbanization in coastal watersheds

publication. Several of the Commissioners objected to the inclusion of this vote with the Declaration because they did not consider it official, and this presumably forms the basis for the statement in Mayr et al. (1) concerning the vote. However, the issue was submitted a second time to the Commission, asking if they approved the publication of the vote, and the ballot on this question was 17 to 5 in favor. The comments of the Commissioners as published show a deep schism within the Commission on the validity of the actions taken on this Declaration, and we consider it unfortunate that Mayr and his colleagues have not made this schism clear in their statement because it will be read and acted upon by many who have no access to the Declaration itself. If the opinions of the majority of the Commissioners are to be accepted, one cannot accept the minority opinion expressed by Mayr et al. Mayr et al. state, as if 'a fait accompli, that the Commission cannot repeal Art. 23(b). However, in the history following Declaration 43, it is clear that the legal adviser to the Commission, the Secretary, and some Commissioners do believe that the Commission has the authority to delete or suspend parts of the Code, such as Art. 23(b). The important question for the working taxonomist is, if the Commission does have this authority, did they in fact repeal or suspend Art. 23(b). As defined by Art. 78 of the Code, a Declaration is a provisional amendment to the Code. It is to be issued by the Commission in a case that \"involves a situation that is not properly or completely covered by the Code,\" and this Declaration remains in force until the next succeeding Congress ratifies or rejects it. We believe that the Commission did suspend Art. 23(b) until the next International Congress of Zoology this summer and that zoologists who are seeking to preserve well-established names must apply to the Commission to preserve them under the plenary powers. BRUCE B. COLLETTE DANIEL M. COHEN National Marine Fisheries Service, Systematics Laboratory, National Museum of Natural History, Washington, D. C. 20560 JAMES A. PETERS Division of Amphibians and Reptiles, National Museum of Natural History, Washington, D. C. 20560

http://science.sciencemag.org/content/sci/177/4047/453.full.pdf

F. E. Broadbent

Papers

Use of Variations in Natural Nitrogen Isotope Abundance for Environmental Studies: A Questionable Approach