Arctic provides insights to understanding nitrogen cycle

Jenny Schlauch

UT researchers have discovered a close relationship between the living and nonliving elements in the Arctic that regulate the nitrogen cycle.

The study, published in Nature Communications, found that the Arctic plays a significant role in the planet’s denitrification process through sediment core analysis of biotic and abiotic factors.

Amber Hardison, a UT doctorate candidate in chemistry and marine scientist who researches nutrient cycling in coastal waters, chose to focus her research on the Arctic’s Chukchi Sea.

“Scientists are still trying to figure out exactly how much denitrification is occurring in marine systems globally,” Hardison said. “[We] study the nitrogen cycle to better understand the specific processes that produce, consume and transport nitrogen on Earth.”

Nitrogen systems can mirror organism dynamics and ecology — too much nitrogen can cause a number of global problems such as algal blooms and “dead zones” in gulfs, according to Wayne Gardner, UT professor emeritus of marine science.

The nitrogen cycle has multiple steps that regulate how much nitrogen is present in an ecosystem and in what forms nitrogen is found. For example, one process called denitrification converts nitrate, a chemical found naturally and in fertilizers, into dinitrogen gas. Once converted into dinitrogen gas, it is relatively inaccessible by organisms, unlike nitrogen compounds such as nitrate or ammonium. The researchers studied how the cycle takes place on continental shelves, which are shallow areas between a land mass and the deep ocean. The Arctic contains 18 percent of the world’s continental shelves, which makes the region a potentially huge player in global nitrogen budget.

Using a tool invented by Gardner that allows sediment collection without disturbing the sea floor surface, the researchers collected and incubated samples from various areas in the Chukchi Sea.

“We measured three of the processes at the same time in the same samples to understand the nitrogen budget,” said Nathan McTigue, Ph.D. marine scientist from UT. “[This shows] of the nitrogen entering the system, how much is being recycled or how much is leaving.”

Out of the three nitrogen-cycling steps, the researchers discovered that denitrification was by far the dominant process in Arctic continental shelves, Hardison said. 

Biogeochemical processes such as denitrification are carried out by bacteria. In temperate regions where seasons change, such as the Gulf of Mexico, the rate of nitrogen processing rises and falls with temperatures, according to McTigue. 

The seasonal temperature changes on the Arctic sea floor are much smaller than in temperate latitudes and are always cold, ranging between negative-2 and positive-2 degrees Celsius year-round. But the researchers found that the Arctic denitrification process occurs just as quickly as in temperate latitudes, although rates did vary greatly among samples. McTigue said this could mean that temperature isn’t the only factor contributing to the speed of processes in the nitrogen cycle.

McTigue, who has researched the microbes and organisms present in Arctic sediment, turned to living factors that might explain the geochemical findings.

He focused on an effect called bioturbation, or how much an animal disturbs the soil sediment by burrowing or feeding. This value can be quantified into a bioturbation index and compared across a region. 

Through computing the bioturbation index over the areas in which samples were taken, the researchers found a very close correlation between areas with a high bioturbation index and areas with a high rate of denitrification.

“This shows that biogeochemical processes are tightly mediated by the organisms that live in the system,” McTigue said.

Organism populations in Arctic sediment are relatively well-studied. Using already present data, the researchers constructed an Arctic-wide nitrogen budget and found that the Arctic shelves play a significant role in the nitrogen cycle for the entire planet, according to Hardison.

Due to the interplay between living and nonliving factors, the Arctic is able to participate in denitrification much more than expected, Gardner said. This research can help further our understanding of how nitrogen dynamics work globally, he added.

The findings were significant because they showed how closely related the biology and chemistry of a system can be, McTigue said.

“[A major part of this project was] taking these data that scientists have and combining them to produce novel findings,” he said. “When different fields work together, that’s where the interesting results come in.”