Nitrogen is fundamental to all life, forming the building blocks of proteins and DNA. It exists in multiple forms called isotopes, specifically the stable isotopes Nitrogen-14 ($^{14}$N) and Nitrogen-15 ($^{15}$N). Analyzing the subtle variations in the ratio of these two forms provides scientists with a powerful, naturally occurring tracer to track biological and environmental processes, from the diets of ancient humans to the sources of modern water pollution.
The Stable Isotopes of Nitrogen
The vast majority of nitrogen in nature is the lighter isotope, Nitrogen-14 ($^{14}$N), while the remaining fraction is the heavier isotope, Nitrogen-15 ($^{15}$N). These isotopes are stable because their atomic nuclei do not decay over time, making them permanent records of chemical and physical reactions. Scientists measure the relative abundance of these isotopes using delta notation, or $\delta^{15}$N, expressed in parts per thousand (per mil, ‰). This value represents the ratio of the heavy isotope ($^{15}$N) to the light isotope ($^{14}$N) in a sample, compared against atmospheric nitrogen gas. A positive $\delta^{15}$N value indicates the sample is enriched with the heavier $^{15}$N, while a negative value indicates depletion.
Fractionation and Nitrogen Cycling
Isotopic tracing relies on isotopic fractionation, the natural tendency for chemical and physical reactions to preferentially select the lighter isotope. Bonds involving the lighter $^{14}$N isotope react or move faster than those involving the heavier $^{15}$N isotope. This effect is evident throughout the nitrogen cycle, particularly in biological processes. When an organism consumes nitrogen, it preferentially excretes the lighter $^{14}$N, causing its tissues to become enriched with $^{15}$N. This process creates a predictable increase in the $\delta^{15}$N value as nitrogen moves up the food web, known as trophic enrichment. Consumers typically show a $\delta^{15}$N increase of approximately +3‰ to +5‰ relative to their diet, meaning apex predators possess higher $\delta^{15}$N values than primary producers.
Tracing Ancient Diets and Migration
Trophic enrichment allows researchers to reconstruct the diets of past populations by analyzing $\delta^{15}$N values preserved in ancient remains. Bone collagen, a protein that slowly remodels over a person’s lifetime, incorporates the isotopic signature of consumed protein. Higher $\delta^{15}$N values in human remains are associated with higher trophic levels, such as meat consumption or resources from longer food chains. For instance, marine protein sources result in $\delta^{15}$N values that are several per mil higher than those derived from land-based foods, as marine ecosystems feature more trophic levels. The baseline $\delta^{15}$N values of the local environment also vary geographically, allowing researchers to compare an individual’s signature with the regional baseline to infer if they were a non-local person.
Mapping Modern Environmental Processes
Nitrogen isotopes are routinely used to distinguish between different sources of nitrogen pollution in water systems. This is possible because different nitrogen sources undergo distinct fractionation processes, resulting in unique $\delta^{15}$N signatures. Synthetic inorganic fertilizers, which are produced chemically, have $\delta^{15}$N values that are near-zero or slightly negative, ranging from approximately -1.4‰ to +2.6‰. In contrast, organic sources such as animal manure or human sewage have been processed biologically, leading to higher $\delta^{15}$N values, sometimes ranging from +3.5‰ to over +16‰. By measuring the isotopic signature of nitrate pollution, scientists can fingerprint the source, determining whether contamination is primarily from agricultural runoff or septic system discharge. This technique is also applied in food ecology to map complex food webs and in forensic science to trace the geographic origin and authenticity of food products.