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

Abstract

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, 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 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.