At a glance, nitrogen seems to be an extremely abundant element, comprising nearly 80% of Earth’s atmosphere. But for organisms to actually access nitrogen–an essential building block for proteins, DNA molecules, and more–the element must be “fixed” by bacteria that convert atmospheric nitrogen into a usable form through a process known as nitrogen fixation. This makes nitrogen a primary limiting nutrient in many ecosystems, largely affecting the growth of plants along with ecosystem diversity, productivity, and resilience.

A recent study led by EESA Postdoc Kelsey Crutchfield-Peters took to the field to explore nitrogen cycling in an deep old growth forest rhizosphere–a region of the subsurface composed of plant roots and their microbiomes–where nutrient uptake has been largely unexplored. The study, published in the Proceedings of the National Academy of Sciences, found that dissolved nitrogen in deep weathered bedrock was an order of magnitude higher than in upper-layer soils, where most nutrient uptake is assumed to occur. The team’s findings from this forest subsurface in particular, with roots extending meters into underlying weathered bedrock beyond thin soils, can help to expand our understanding of where plants get nutrients, and suggests that ecosystem models and nitrogen budgets may be missing a major piece of the nitrogen story. 

Exploring a deeply rooted rhizosphere

The rhizosphere is a hot-spot for nutrient cycling because of the sugars, amino acids, and other important substances released from plant roots that attract microbes, and is where most plant nutrient and water uptake occurs. 

Traditionally, studies exploring nutrient cycling have focused on soil, particularly upper-layer soils, as the primary location of nutrient cycling and plant uptake. This is because high nutrient input from leaf litter and warmer temperatures make soils a hot-spot for nutrient cycling. However, plant and tree roots can extend meters into the subsurface, and in many environments may grow into bedrock, leaving how plants cycle and acquire nutrients from weathered bedrock horizons largely unknown.

“When you’re walking past a tree or forest, think of where the roots are going,” Crutchfield-Peters said. “Sometimes, with little soil, plants are basically growing in rock fractures. It’s time to think beyond soil as the sole source of nutrients–and our study can help us do that.”

Old-growth forest where the study took place, in which roots extend deep into the subsurface and in bedrock environments. Photo Credit: Kelsey Crutchfield-Peters

The researchers studied rhizosphere nitrogen cycling in an old-growth forest located in Northern California. With roots extending several meters into bedrock, this environment in particular allowed the team a closer look at nitrogen uptake in deeply rooted forests.

The team measured dissolved inorganic nitrogen, a form of nitrogen-containing ions in water able to be taken up by roots, like ammonium and nitrate.  

They also measured dissolved organic nitrogen, a form of nitrogen incorporated into carbon-rich molecules also in water and able to be used by plants. They took these measurements every 1.5 meters for 16 meters (over 50 feet!) deep into the subsurface for two years to better understand the cycling of nitrogen and carbon, along with how these molecules were being used by trees.

An unexpected but significant nutrient pool

The results of the study showed that the amount of total dissolved nitrogen increased with depth, was primarily organic, and was significantly higher in bedrock compared to the first 30 centimeters of soil. This nitrogen likely came from a combination of soil leaching, inputs from plants and microbes, and rock weathering.

“Our findings challenge the traditional view that plant-driven nitrogen cycling happens exclusively in soil,” Crutchfield-Peters said. “Nitrogen dynamics in the weathered bedrock beneath soil demonstrated deep rhizosphere nitrogen cycling in this old-growth forest, where roots extend to about seven meters. This suggests that the rhizosphere is fully functioning meters into the weathered bedrock, playing a bigger role in nitrogen cycling than we previously understood.”

Previous EESA research has also found dissolved nitrogen deep in weathered bedrock–but mostly inorganic nitrogen derived from rocks or minerals. This study found 90% of the nitrogen in the sampled subsurface was organic nitrogen, which comes from carbon-containing organic material like plants, bacteria, and dead organisms.

The team analyzed the carbon molecules throughout the soil depth to understand the source of organic matter, which facilitates microbial activity and nutrient cycling. Using carbon isotopes–different forms of carbon atoms that differ in mass and can be used to track a materials origin–the researchers were able to identify the source of carbon molecules in the weathered bedrock compared to soil, concluding that most of the dissolved organic matter containing nitrogen likely came from plants and their associated microbes. This indicates that the deep rhizospheres significantly contribute to carbon inputs and play a large role in facilitating nutrient cycling, even meters below the surface.

Towards a fuller picture of nutrient cycling

With more organic nitrogen in deep weathered bedrock compared to top-layer soils, this study emphasizes the role of a deep rhizosphere in nutrient cycling and the importance of considering bedrock rhizospheres for future studies on nutrient cycling. As ecosystems face changing environmental conditions and severe disturbances, a holistic understanding of where and how forests access nutrients can inform the prediction of ecosystem function and resilience.