Tansley Review No. 9515N natural abundance in soil‐plant systems
Authors: HÖGBERG PETER 
Published: March 2003

summary

Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable X isotopes,15N and14N, 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 van. For example, because nitrification discriminates against15N 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, and15N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring15N in small and dynamic pools of N in the soil‐plant system, and the complexity of the X cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of15N 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 adding15N tracer.

Such attempts require, however, that the complex factors affecting15N 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 Na‐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 produces15N‐enriched NH4and N‐depleted NO3. As many forest plants prefer NH4−they become enriched in15N in such circumstances. This change in plant15N 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 in15N with depth in soils of N‐limited forests.

Generally, interpretation of15N 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 the15N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level15N studies will be necessary to further develop the interpretation of variations in15N in soil‐plant systems. Another promising approach is to study ratios of15N:14N together with other pairs of stable isotopes, e.g.13C:12C or18O: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.

CONTENTSSummary179I.Introduction180II.Units, causes of isotope effects, stoichiometry, modelling181III.N dynamics and variations in15N abundance in soil‐plant systems183IV.Applications189V.Conclusions and suggestions for future research197Acknowledgements198References198