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

Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable X isotopes, ¹⁵ N and ¹⁴ N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ¹⁵ N] 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 against ¹⁵ N 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 δ¹⁵ N of a specific N pool is not a constant, and ¹⁵ N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring ¹⁵ N 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 of ¹⁵ N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ¹⁵ N might offer the advantage of giving insights into the N cycle without disturbing the system by adding ¹⁵ N tracer. Such attempts require, however, that the complex factors affecting ¹⁵ N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NO_X , 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 δ¹⁵ N 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 δ¹⁵ N are used to estimate N_a -fixation in poorly described ecosystems. As well as giving information on N source effects, δ¹⁵ N 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 ¹⁵ N-enriched NH₄ and N-depleted NO₃ . As many forest plants prefer NH₄ ^- they become enriched in ¹⁵ N in such circumstances. This change in plant ¹⁵ N 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 ¹⁵ N with depth in soils of N-limited forests. Generally, interpretation of ¹⁵ N 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 ¹⁵ N of small dynamic pools of N in soils. Direct comparisons with parallel low tracer level ¹⁵ N studies will be necessary to further develop the interpretation of variations in ¹⁵ N in soil-plant systems. Another promising approach is to study ratios of ¹⁵ N: ¹⁴ N together with other pairs of stable isotopes, e.g. ¹³ C: ¹² C or ¹⁸ O:¹⁶ O, 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. CONTENTS Summary 179 I. Introduction 180 II. Units, causes of isotope effects, stoichiometry, modelling 181 III. N dynamics and variations in ¹⁵ N abundance in soil-plant systems 183 IV. Applications 189 V. Conclusions and suggestions for future research 197 Acknowledgements 198 References 198.