Soil carbon, nitrogen, soil fertility, climate change tightly linked

University Of Massachusetts Amherst (WeatherFarm) – Soil carbon determines whether mineralized nitrogen is available in the soil as ammonium, or further transformed into either nitrates that are easily lost to runoff and contribute to toxic algal blooms or nitrous oxide, according to new research from a group of scientists led by Ashley Keiser of the University Of Massachusetts Amherst’s Stockbridge School of Agriculture

“All living organisms need nutrients in specific ratios,” said Keiser. “Just think of your daily vitamins. If you take too much vitamin C, for example, your body excretes it so that you can stay healthy.”

The same holds true for the soil’s microbial communities. There are billions of microbes in every teaspoon of soil. They are the predominant form of life on Earth and they depend upon both carbon and nitrogen for their survival. Just like with humans and vitamin C, microbes take the carbon and nitrogen they need into their bodies, and process and transform the rest, eventually passing it back into the soil.

Microbes break down organic matter — think of dead leaves, rotting wood and the uncountable microbe carcasses. This process is called “mineralization,” and it pulls the carbon out of the dead matter and makes it available for the microbe to burn as energy.

Microbes also mineralize the nitrogen in that dead matter into ammonium, which plants love. But microbes can further transform the nitrogen into nitrates, which are easily dissolved in water. When nitrates are abundant in the soil, they can wash into streams and rivers, where they eventually feed vast algal blooms toxic to many aquatic plants and animals. Furthermore, nitrification can also produce nitrous oxide, a far more potent greenhouse gas than carbon dioxide.

What Keiser’s team discovered is that the amount of carbon in the soil drives how microbes process nitrogen.

“When soil carbon stocks are high, microbes need much more nitrogen for themselves, because, just like us, they need a balanced diet,” Keiser stated.

While increased microbial nitrogen usage means that less is transformed into nitrates, it also means that there’s less ammonium available for nourishing plants. The opposite holds true, too. When there’s less carbon in the soil, microbes pass more of the nitrogen through their systems, turning it into ammonium, nitrates, and, possibly, nitrous oxide.

The key is the carbon, an insight that Keiser and Williams College biology professor Allison Gill discovered with the help of the National Science Foundation’s Long Term Ecological Research Network.

“To test our hypotheses, we leveraged databases associated with 14 terrestrial LTER sites across the U.S.,” said Gill. “These sites encompassed diverse ecosystems including tundra, boreal forests, deserts, and grasslands. The large dataset, which included measurements collected over a 40-year period, provided a powerful tool for evaluating how soil carbon concentration interacts with and constrains nitrogen cycling.”

“[We] were surprised to see that carbon’s role as a ‘gatekeeper’ holds across such different ecosystems. We don’t see patterns like this in ecology very often,” Keiser said.

The discovery of this relationship has implications for everything from agriculture to climate change mitigation solutions that rely on storing carbon in the soil.

“Now that we know this link exists, we can ask new questions that peel back more layers on these fundamental processes which much of Earth’s life depends upon,” she added.