Elements like carbon, nitrogen and oxygen have more than one stable isotope. For example, most carbon atoms have an atomic weight of 12, while a few have an atomic weight of 13. Fractionation is a change in the ratio that two such isotopes bear to one another. For example, the two carbon isotopes bear a definite ratio to one another in atmospheric carbon dioxide. However, when atmospheric carbon dioxide undergoes chemical reactions, the two isotopes bear a different ratio to one another in the products of the reaction because the two isotopes react at different rates. Physical processes may also result in fractionation, since two isotopes may undergo physical changes at a different rate.
Plants produce food by photosynthesis. They do this in one of two different ways. The C3 pathway produces three-carbon compounds almost at the beginning of a series of reactions, while the C4 pathway starts out with a four-carbon compound. In both cases, fractionation takes place. The percentage of the rare carbon isotope (13C) in the food products is less than its percentage in atmospheric carbon dioxide. However, the C3 pathway reduces the percentage far more than the C4 pathway. For this reason, by analyzing the isotopic carbon composition of a given plant species, it is possible to determine which pathway the species use in its photosynthetic activity. Moreover, by analyzing the isotopic composition of bones, archeologists can determine whether the predominant foods of ancient peoples were C4 plants like corn or C3 plants like rice.
Most nitrogen has an atomic weight of 14. A rare stable isotope has an atomic weight of 15. Atmospheric nitrogen contains a higher percentage of the rare isotope than nitrogen compounds in the soil. For this reason, a nitrogen-fixing plant, which uses atmospheric nitrogen, has a higher percentage of the rare isotope than the nitrogen compounds in the soil. By isotopic analysis of the nitrogen contents of plants, it is possible to determine whether they fix nitrogen and how efficiently they do so.
Isotopes serve as tracers. Substances employing an isotope are fed to people, animals or plants. Then various instruments watch what happens to it. Though radioactive isotopes often serve as tracers, stable isotopes serve the same purpose. A study of the metabolism of glucose may employ glucose containing a stable isotope as a tracer. Stable isotopes also reveal how effectively plants use fertilizer and other nutrients. For example, a study conducted by the Agricultural Chemistry Department of the Universidad Autónoma de Madrid used a stable iron isotope to analyze the effectiveness of foliar applications of a lignosulfonate-iron complex as a means to correct iron deficiency in cucumbers, according to the "Journal of the Science of Food and Agriculture."