Bedrock Holds Unexpected Source of Global Nitrogen

The amount of nitrogen derived from rock weathering rivals contributions from the atmosphere to the terrestrial nitrogen budget. | photos_by_clark / CC BY-NC 2.0

While nitrogen within terrestrial soils and vegetation is largely thought to come from the atmosphere, a new study in the April 6 issue of Science points to a previously underestimated source: weathered bedrock.

Since the availability of nitrogen within ecosystems is essential for plant growth, which in turn modulates the amount of planet-warming atmospheric carbon that can be stored in these ecosystems, the study authors note that their findings could have important implications for global climate change.

The research team led by Benjamin Houlton of University of California, Davis suggests that weathering of rock contributes 6% to 17% of the total terrestrial nitrogen budget, or 11 to 18 teragrams of nitrogen annually, an amount that rivals the level of nitrogen contributed from the atmosphere.

Nitrogen plays a critical role in supporting life, acting as a building block for DNA, amino acids, proteins and cell walls. "It's widespread in the biophysical fabric of our world," explained Houlton. "Nitrogen is very mobile in the environment, however, meaning that it must be replenished to maintain life processes on Earth."

Humans have known the importance of nitrogen for plant growth for quite some time. During the early 1800s, farmers were interested in finding ways to boost crop yields and searched for ways to better nurture their plants with nitrogen. People looked to the sky, using chemistry to turn atmospheric nitrogen into ammonia, which was applied to crops along with manure — but the idea of nitrogen coming from the ground remained largely unexplored.

Houlton and his colleagues used three different approaches to explore the amount of nitrogen being released from the Earth's interior, and their results reveal that the amount is much more substantial than previously thought.

The team looked at the overall abundance of nitrogen across various Earth processes, finding that a substantial amount must be brought up to the surface of the Earth as its interior churns and tectonic plates shift. They also analyzed a wide swath of geochemical data, including quantifying how much nitrogen is released from various types of rocks over time, arriving at a value that aligns well with their findings from tectonic activity. Finally, they used modeling to estimate how much nitrogen is released from rocks globally.

"The biggest surprise to us was how large the flux of rock nitrogen was globally and especially that rocks bested atmospheric sources of nitrogen in many high latitude and mountainous ecosystems," said Houlton. "Our past work had indicated this possibility, but to see that all three basic approaches were given consistently large numbers, despite different data sets and assumptions, was surprising."

Houlton noted that these findings are just the beginning. "Our understanding of rock nitrogen is still crude at this point."

His team is beginning to link their nitrogen model to global carbon forecasts. These next steps will help scientists better understand how the nitrogen and carbon cycles will react to human-caused climate change.