Nitrogen-15 and Nitrogen-14 can be used for various applications that focus on the study of metabolic pathways in plants and nitrogen transformations in soils. The growing use of stable isotopes designed for detecting adulteration in foods has increased in recent years. Furthermore, using stable isotopes in agricultural techniques the fate of applied nitrogen can be determined. Stable isotopes are also used in the differentiation of mass/charge ratios among various isotopically labeled nitrogen compounds also known as isotope ratio analysis. Measuring the biological fixation of atmospheric nitrogen is another application.
Nitrogen-15 adds to the atmospheric fixation in that small amounts of NO are also formed in lightning discharges. Lightning actually breaks the triple bond between the nitrogen molecules allowing its atoms to bind with oxygen in the air forming nitrogen oxides. When it rains, nitrogen oxides dissolve to form nitrates that are carried to the earth. The triple bond in Nitrogen-N2 is so strong, that it slows down its reactivity with other compounds, inhibiting reactions. Yet, in the presence of O2, at excessive temperatures, N2 and O2 forms NO2. NO2 is a source for acid rain and smog in sunshine via aerial oxidation to NO2.
While nitrogen is a vital nutrient to plants, it is also the cause of deficiencies, as it contributes to the reduction of agricultural yields. With the unavailability of molecular nitrogen–N2 to higher plants metabolically, Biological Fixation allows microorganisms to experience what was once metabolically unfeasible. In Biological Nitrogen Fixation, atmospheric nitrogen is converted to ammonia by nitrogenase, yet energy is essential. Nitrogenase is the enzyme responsible for catalyzing the transformation of nitrogen to ammonia. The symbiotic relationship between diazotrophs called rhizobia and legumes (for example, clover and soybean) can provide large amounts of nitrogen to the plant and can have a significant impact on agriculture.
Stable isotopes can be used to determine the authenticity of wines, adulteration of honey, orange juice, fruit drinks, jams and syrups. More specifically, carbon-13 stable isotope methods can detect these food adulterations. Within the last decade new research surrounding the assessment of wine genuiness has evolved, incorporating multi-compound stable isotope abundance investigations. Moreover, a statistical analysis of the patterns of the stable isotopes and the intermolecular and intra-molecular isotopic correlations.
Ammonia (NH3) can be formed under intense pressure and at 600°C, in the presence of a catalyst with atmospheric nitrogen and hydrogen. Typically, the hydrogen is derived from natural gas or petroleum. It is the ammonia that is used as fertilizer with the majority of it being processed to urea and ammonium nitrate (NH4NO3).
To unbind the inert nitrogen molecule (N2), requires a lot of energy that can be obtained through nitrogen fixations. With nitrogen fixations, soil N is of more abundance in 15N versus in the atmosphere. Non-nitrogen fixating plants need all their nitrogen from the soil whereas nitrogen-fixing plants have an alternative N source in the form of air, which is isotopically lighter. It is expected that nitrogen fixation and non-fixation plants will differ in their 15N values. The lighter, meaning a more negative delta, the plant material is found to be with respect to soil N, the better its nitrogen fixing ability. This difference forms the basis for the 15N natural abundance technique of estimating symbiotic N. Nitrogen fixation can also be quantified using tracer methods, as tracer techniques are more popular for examining Nitrogen fixation in crop plants.
Nitrogen-15N labeled fertilizers such as urea and ammonium nitrate can be used in tracing studies. They are used to investigate the nitrogen cycling in crop plants. The tracer yields allows for quantification of the fate of the added fertilizer 15N. This occur while the fertilizer passes through the portion taken up by the plants, portions remaining in the soil N pool, portions lost by denitrification into the atmosphere, as well as portions leached into runoff waters. Waters, in addition to 15N levels in soil can be an indication of the origin of the N.