Microbes Play Important Role in Soil’s Nitrogen Cycle

By Susan V. Fisk

Under our feet, in the soil, is a wealth of microbial activity. Just like humans have different metabolisms and food choices, so do those microbes. In fact, microbes play an important role in making nutrients available to plants.

A recent review paper from Xinda Lu and his team looks at different roles that various soil microbes have in soil’s nitrogen cycle. Lu is a researcher at Massachusetts Institute of Technology.

According to Lu, “Soil microbes catalyze most of the transformations of soil nitrogen into plant-usable forms. Diverse microbes use different processes – and sometimes work together. Knowing the various styles of soil microbes, and linking microbes to specific soil processes, can be important knowledge for farmers.”

Modern nitrogen fertilizers are applied in the form of ammonium. Through a biological process called nitrification, soil microbes convert ammonium to nitrates that plants can absorb. In order to be efficient at this process, microbes need oxygen. Researchers are studying nitrification because it can be linked to greenhouse gases and loss of fertilizer.

Although microbiologists have been studying the nitrogen cycle for over a century, not all steps were well understood. New microorganisms have recently been identified. A type of prokaryote (single-celled organism) called archaea has also been playing a role in nitrification.

Archaea are not technically soil bacteria, due to their structure. There are many more archaea that contribute to nitrification in some soils than there are bacteria responsible for the same activity. Including the role of archaea in nitrification has broadened the understanding of scientists and researchers.

Researchers reviewed various studies of soil nitrification. This included the abundance of microbes in soil in relation to various environmental factors. Soil pH, temperature and the ratio of soil carbon to soil nitrogen were all compared to the number of microbes in each soil sample. Soil depth and other factors also influence microbe abundance.

Previous studies have shown, for example, that nitrification archaea are more abundant than bacteria in warmer temperatures. Other microbes thrive in lower temperatures.

Soil pH also influences how active soil microbes are in the nitrification process. Soil bacteria Nitrospira were more dominant in acidic soils, including forests and farm fields.