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

http://utsc.utoronto.ca/%7Ebritto/publications/amtox.pdf

NH4+ toxicity in higher plants: a critical review

Effects of atmospheric ammonia (NH3) on terrestrial vegetation: a review.

THE high incidence of failure when late-successional conifer species are replanted on disturbed forest sites is a considerable problem1–3. Here we advance a hypothesis that might explain many of these reforestation problems on a physiological basis, within the framework of forest succession. It is known that the chemical speciation of inorganic nitrogen in forest soils changes from predominantly ammonium (NH+4) in late-successional (mature forest) soils to mostly nitrate (NO–3) after disturbances such as clearcut harvesting2–6. The capacity of plant roots to take up and use these two sources of nitrogen is therefore very important for species establishment on successionally different sites. We have used kinetic and compartmental-analysis techniques with the radiotracer 13N to compare the efficiency of nitrogen acquisition from NH+4 and NO–3 sources in seedlings of white spruce, an important late-successional conifer. We found that uptake of NH+4 was up to 20 times greater than that of NO–3 from equimolar solution, cytoplasmic concentration of NH+4 was up to 10 times greater than that of NO–3, and physiological processing of NO–3 was much less than that of NH+4. This reduced capacity to use NO–3 is thought to present a critical impediment to seedling establishment on disturbed sites, where species better adapted to NO-3 would have a significant competitive advantage.

Conifer root discrimination against soil nitrate and the ecology of forest succession

Effects of atmospheric ammonia on vegetation—A review

Relationships between root zone temperature, concentrations and uptake rates of NH
 4
 +
 and NO
 3
 −
 were studied in non-mycorrhizal roots of 4-year-old Norway spruce under controlled environmental conditions. Additionally, in a forest stand NH
 4
 +
 and NO
 3
 −
 uptake rates along the root axis and changes in the rhizosphere pH were measured. In the concentration (Cmin) range of 100–150 μM uptake rates of NH
 4
 +
 were 3–4 times higher than those of NO
 3
 −
 The preference for NH
 4
 +
 uptake was also reflected in the minimum concentration (Cmin) values. Supplying NH4NO3, the rate of NO
 3
 −
 uptake was very low until the NH
 4
 +
 concentrations had fallen below about 100 μM. The shift from NH
 4
 +
 to NO
 3
 −
 uptake was correlated with a corresponding shift from net H+ production to net H+ consumption in the external solution. The uptake rates of NH
 4
 +
 were correlated with equimolar net production of H+. With NO
 3
 −
 nutrition net consumption of H+ was approximately twice as high as uptake rates of NO
 3
 −
 In the forest stand the NO
 3
 −
 concentration in the soil solution was more than 10 times higher than the NH
 4
 +
 concentration (<100 μM), and the rhizosphere pH of non-mycorrhizal roots considerably higher than the bulk soil pH. The rhizosphere pH increase was particularly evident in apical root zones where the rates of water and NO
 3
 −
 uptake and nitrate reductase activity were also higher. The results are summarized in a model of water and nutrient transport to, and uptake by, non-mycorrhizal roots of Norway spruce in a forest stand. Model calculations indicate that delivery to the roots by mass flow may meet most of the plant demand of nitrogen and calcium, and that non-mycorrhizal root tips have the potential to take up most of the delivered nitrate and calcium.

Max Häussling

Papers

Ammonium and nitrate uptake rates and rhizosphere pH in non-mycorrhizal roots of Norway spruce [ Picea abies (L.) Karst.]