Study Bolsters Theory Of Plant Nutrients Trapped In Soil

The work is part of an ongoing study to determine whether large molecules of decomposed plant matter called “humic acid” are able to envelop smaller molecules and trap them for years, or even indefinitely. Over time, humic acid may trap and store molecules that replenish soil nutrients, effectively keeping them out of reach of living plants that need those nutrients to grow.

In his previous work, Patrick G. Hatcher, professor of chemistry at Ohio State, proved that molecules of humic acid can trap environmental contaminants. This study proved that humic acid can also trap nitrogen-containing proteins, vital substances in soil nutrient replenishment.

Hatcher and Ohio State chemistry graduate student Xu Zang presented their results at the recent national meeting of the American Chemical Society in Anaheim, California.

Hatcher said researchers want to understand how humic acid traps molecules so they can figure out whether the process can be reversed.

“Think of a molecule of humic acid as a shell that is very strong, but if you hammer on it long enough, you can crack it and release what’s inside,” Hatcher said.

If so, researchers could one day release nutrient-generating substances trapped in soil from depleted farmland.

They could also know whether contaminants trapped in humic acid will remain there and do no harm to the environment.

“That’s a very real possibility,” said Hatcher, “one that certainly deserves further investigation. There is enough reason to believe that the natural environment may be able to stop the action of pollutants very readily by such a process.”

Hatcher and Zang fed a rare isotope of nitrogen to algae in the laboratory, and then exposed protein extracts of the algae to a sample of humic acid from peat that Hatcher gathered in the Florida Everglades.

They used this particular isotope, nitrogen-15, because it is highly visible in nuclear magnetic resonance (NMR) data. The more common isotope of nitrogen in the air and soil is nitrogen-14, Hatcher explained, but nitrogen-15 was an appropriate substitute because plants can absorb it in the same way.

An NMR scan of the humic acid revealed that it had trapped the nitrogen-15 enriched proteins.

To test Hatcher’s idea that the humic acid had stored the nitrogen-15 enriched proteins within tough molecular shells, he and Zang treated the humic acid with a strong acid -- hydrochloric acid.

Subsequent NMR images revealed that the nitrogen-15 enriched proteins still remained trapped inside the humic acid.

Hatcher said that if hydrochloric acid didn’t significantly attack the nitrogen-15 enriched proteins in the laboratory, weaker forces of decomposition in the environment such as bacteria would be unlikely to release the nitrogen in nature.

He added that this process may be the same one that can cause farmland to lose its fertility over time.

Farmers regularly fertilize with nitrogen to replace nutrients that crops take from the soil. They also till crops back into soil so that bacteria can break down the protein-rich plant remains and release the nutrients.

If humic acid is responsible for trapping nitrogen-bearing proteins in farmland, Hatcher says, the process would happen most often in watery areas such as rice farms. That would explain why rice paddies in southeast Asia have yielded less rice over the past few decades, despite the fact that farmers add nitrogen-rich fertilizer.

Hatcher said little of the nitrogen fertilizer ever reaches the rice plants. The fertilizer promotes the growth of algae, he explained, which absorb the nitrogen and eventually settle into the underwater sediment along with other decomposed plant matter. Humic acid can then absorb the proteins in the dead algae, trapping the nitrogen.

“A similar process may be at work on land, but we haven’t investigated it yet,” said Hatcher. “We’re basically at the tip of the iceberg.”

Next Hatcher and Zang will try the same experiment with humic acid from other locations, and see whether it can trap other molecules.

“Then we’ll have a better understanding of what’s really going on here, whether we’re looking at a chemical reaction or just a physical one,” said Zang.

This work was funded by the National Science Foundation. In a related future project, Hatcher plans to study whether humic acid traps nitrogen in forests.