Nitrogen-15 tracing

Nitrogen-15 tracing, also known as ¹⁵N tracing or the δ¹⁵N method, is a technique used to follow how nitrogen enters an ecosystem or organism. There are two common isotopes of nitrogen: ¹⁴N (also called Nitrogen-14), which makes up 99.6% of naturally occurring nitrogen, and the heavier ¹⁵N (Nitrogen-15), which makes up the rest. Since ¹⁵N is so much more rare in nature, scientists can introduce ¹⁵N into an ecosystem in order to follow how nitrogen is absorbed by organisms. For example, if a fertilizer containing a very high amount of ¹⁵N is used to grow a plant, then a high level of ¹⁵N in that plant most likely means that it absorbed nitrogen from the fertilizer.

Overview

δ¹⁵N is related to how much more ¹⁵N something contains than is normal

To measure whether an ecosystem or organism contains a high level of the rare ¹⁵N (Nitrogen-15), scientists need to establish a normal level of ¹⁵N. Often, this is the amount of ¹⁵N found in the atmosphere, or how much is commonly found in whatever organism is being studied. Scientists measure δ¹⁵N, which is similar to how much more ¹⁵N is measured than is normal. If scientists introduce ¹⁵N to an ecosystem, a high δ¹⁵N in an organism probably means that the organism absorbed this introduced ¹⁵N.

Applications

Nitrogen is necessary to life, so nitrogen flows through an ecosystem in a process called the nitrogen cycle. Scientists can follow this flow of nitrogen in an ecosystem or follow how nitrogen moves in a single organism with ¹⁵N tracing. For example, if insects containing a high amount of ¹⁵N come into contact with a plant, scientists can measure whether the plant absorbed nutrients from those insects by measuring δ¹⁵N in the plant, or how much the plant's ¹⁵N is higher than normal. In 2020, scientists used this method to test whether Triantha occidentalis is a carnivorous plant, meaning it absorbs nutrients from organisms it traps. They tested whether T. occidentalis is carnivorous by answering the question, "does T. occidentalis absorb nitrogen from insects it traps?" Scientists taped flies containing large amounts of ¹⁵N to three plants: Erigeron peregrinus, a control plant which is not carnivorous, Drosera rotundifolia, a plant which is known to be carnivorous, and T. occidentalis. After two weeks, they measured the δ¹⁵N in each. The δ¹⁵N of E. peregrinus was about 0, meaning it absorbed no nitrogen from the flies, but the δ¹⁵N of D. rotundifolia and T. occidentalis were very high, meaning they both absorbed nitrogen from the flies. By tracing Nitrogen-15, the scientists had verified that T. occidentalis absorbed nitrogen from flies, meaning that they discovered a new carnivorous plant.