Interpreting your results must be done in conjunction with the other knowledge you have of your samples, as there may not be a direct effect and many factors can influence isotopic enrichment. Delta values will tend to be more negative for process that have ocoured in the vapour phase as this tends to favour enrichment of the lighter isotopes, while other process may favour fractionation towards the heavier isotope.
Sometimes absolute levels of enrichment can be used to classify your sample. These absolute 15N and 13C levels can be interpreted directly to identify sources. An example of this would be if your sample had a delta 13C PDB of around -10 per mil, this would indicate that your carbon is from a C4 source such as grasses, as opposed to if your sample had a delta 13C PDB of around -22 per mil which would indicate that the carbon is most likely from a C3 source such as a tree. Another example is if there is distinct partitioning of the isotopic abundances within your system. Delta values will tend to be more negative for processes that have ocoured in the vapour phase as this tends to favour enrichment of the lighter isotopes, while other processes favour the heavier isotope. This may lead to deltas that can be associated directly with certain phases in your system.
In most examinations you will be looking at how the 13C and 15N delta's change within your sample as they undergo treatments. An example of this is a comparison of a stressed plant to a non stressed plant. As the plant becomes stressed the underlying mechanisms for nutrient uptake can change and this will result in a measurable shift in delta values. This can be used to both identify a stressed state and to point to what mechanisms may be involved in the uptake.
Enrichment is a traditional way of tracing pathways in your system. By spiking your system with a known enrichment, it is possible to track the changes in enrichment as they propagate through the system. Where the enrichment is finally expressed will be a function of the pathways that are active at the time of the experiment. Doing this has an advantage of provide a recognizable spike that can be followed through your system and additionally raising the isotope levels to an easily measured level which reduces measurement errors. Sometimes predicting the degree of uptake can be very difficult and may require trial and error experiments.
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