The use of the ¹⁵N stable isotope technique to improve the management of nitrogen nutrition of fruit trees - a mini review

Vegetative and reproductive organs of fruit trees require an appropriate amount of nitrogen (N) for their growth. An excess of N, however, besides being often detrimental to fruit quality, might lead to N losses into the environment and cause pollution of aquifers and atmosphere. It is therefore important to study and understand how N moves between plants and soil, in and out of the ecosystem. A powerful technique to deepen this knowledge takes advantage of the existence of the two stable isotopes of N, ¹⁴N and ¹⁵N. On average, the ratio between these two isotopes is constant, but it might change in specific compartments of the ecosystem due to the effect of fractionation and mixing. Studying how this ratio changes provides deep and accurate information on how N moves within the ecosystem. Moreover, it is possible to artificially add ¹⁵N in excess respect to its natural abundance to a specific compartment of the ecosystem and to follow its fate in space and/or in time. The first case is known as “natural abundance studies” while the latter as “trace (or enrichment) experiments”. Using these two techniques, several aspects of the N cycle within plants - e.g., the timing of uptake, the reabsorption before leaf abscission and the remobilization in spring - can be studied. This minireview summarizes some of the available literature and includes original results where ¹⁵N techniques have been used to study the uptake, the internal cycling and the N availability in soils and trees.