Phantom nutrient: The secret force that fuels plant growth

Conventional wisdom has long held that iron oxides merely trap phosphorus in the environment. Yet new research has uncovered a more dynamic role for these minerals in driving the conversion of organic phosphorus into the inorganic form that plants require for growth. 

A new study led by Northwestern University reveals that this activity can occur at rates on par with biological enzymes, overturning assumptions about how phosphorus becomes available in soils and sediments.

The fate of phosphorus

Study senior author Ludmilla Aristilde is an associate professor of environmental engineering at Northwestern. 

“Phosphorus is essential to all forms of life. The backbone of DNA contains phosphate. So, all life on Earth, including humans, depends on phosphorus to thrive,” said Aristilde.

“That’s why we need fertilizers to increase phosphorus in soils. Otherwise, the crops we need to feed our planet will not grow. There is a profound interest in understanding the fate of phosphorus in the environment.”

Availability of inorganic phosphorus

Most soil phosphorus exists in organic form, bound to carbon atoms in plant, animal, or microbial remains. Plants, however, need inorganic phosphorus – the sort found in common fertilizers – for growth and survival. 

Traditional thinking held that microbial and plant enzymes alone could break down organic phosphorus, releasing the inorganic portion.

Prior work by Aristilde’s group demonstrated that iron oxides, which are abundant in soils and sediments, can likewise facilitate this transformation.

In the new study, lead author and PhD student Jade Basinski, along with other members of Aristilde’s lab, sought to determine just how much phosphorus these minerals could release and how efficiently they function in various scenarios.

The role of iron oxides

To investigate, the researchers examined three frequently occurring iron oxides: goethite, hematite, and ferrihydrite. They exposed these minerals to different ribonucleotides, the building blocks of RNA, which contain phosphorus in organic bonds. 

By measuring the appearance of inorganic phosphorus both in the surrounding solution and on the mineral surfaces, the team assessed the extent of the reaction and how quickly it proceeded.

The researchers also varied reaction times and the concentration of ribonucleotides to observe how these conditions influenced the process.

The findings showed that iron oxides are capable of driving the conversion of organic phosphorus to inorganic phosphorus at rates comparable to enzymes. 

Iron oxides as ‘catalytic traps’

Moreover, the researchers uncovered a particular twist: some minerals can hold on to the newly released inorganic phosphorus, effectively sequestering it on their surfaces. 

“We concluded that iron oxides are ‘catalytic traps’ because they catalyze the reaction to remove phosphate from organic compounds but trap the phosphate product on the mineral surface,” Aristilde said. 

“Enzymes don’t trap the product; they make everything available. We found goethite was the only mineral that didn’t trap all the phosphorus after the reaction.”

While ferrihydrite and hematite tended to keep the cleaved phosphorus locked in place, goethite proved more inclined to release it back into the surrounding medium. 

Hematite was notably effective at processing ribonucleotides containing one phosphorus, whereas goethite showed heightened performance with those containing three.

These differences may reflect each mineral’s specific surface chemistry or structural characteristics, though the precise reasons remain unknown.

Implications for crops and plant growth

This discovery has far-reaching implications for agricultural practices. Farmers worldwide have long relied on adding phosphorus to fields to promote plant growth, yet phosphorus is a finite resource sourced primarily from phosphate rock in places such as the United States, Morocco, and China. 

As supply concerns grow, identifying alternative means to free up naturally occurring organic phosphorus could help maintain affordable food production.

Aristilde’s group believes that a deeper understanding of why goethite more readily releases inorganic phosphorus might guide the design of synthetic catalysts or soil amendments. 

“Our work is providing a steppingstone for designing and engineering a synthetic catalyst as a way to recycle phosphorus,” Aristilde said. 

“We uncovered a reaction that’s happening naturally. The dream will be to leverage our findings as a way to make catalysts to contribute to the production of fertilizers for our food security.”

Future research directions

The next step will be to discover precisely how different mineral compositions drive these catalytic reactions.

Researchers initially hypothesized that the shape or structure of the phosphorus compounds themselves might determine the release rates, but the data did not support that idea. 

Now, attention is turning to the internal chemistry of each mineral, with the hope that clarifying these fine-scale details may point to new pathways for managing soil phosphorus.

In revealing that iron oxides can act as powerful catalysts for phosphorus release, the Northwestern study challenges the notion of these minerals as inert storage sites. Instead, they emerge as dynamic participants in a process vital to crop growth and, ultimately, the global food supply. 

Understanding this process better could help scientists and farmers find sustainable solutions for using phosphorus more wisely, ensuring this critical nutrient remains within reach for future generations.

The study is published in the journal Environmental Science & Technology. 

Image Credit: Ludmilla Aristilde/Northwestern University

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