In last month’s blog ‘The Hidden World Beneath Your Feet,’ we looked at some of the microscopic inhabitants of the soil that influence soil health and crop productivity. In this article, we will take a closer look at how soil microbes participate in soil nitrogen cycling, and ultimately the movement of nitrogen from the atmosphere into forms that enable life on earth.

The air you breathe – that may seem like an odd way to begin a discussion about soil biology, but have you ever paused to consider where most of the nitrogen present in our environment is located? Although you and I, and all aerobic creatures for that matter, need air to receive life-giving oxygen, air is comprised of a mixture of gases including oxygen, nitrogen, carbon dioxide, and trace amounts of other elements like argon. Nitrogen makes up the greatest proportion of air, approximately 78 percent, and it has been calculated that an air column an acre in size at its footprint and extending up into the atmosphere contains approximately 35,000 tons of nitrogen present as N₂ gas! To put that quantity in perspective, an acre of an undisturbed prairie soil might only contain two to three tons of nitrogen. Would it surprise you if I told you that nitrogen present in the soil was at some point part of that vast quantity of nitrogen in the atmosphere? How did nitrogen gas in the air you breathe become part of the soil nitrogen pool? Let’s find out!

Understanding diazotrophs

Diazotroph – remember this word for your next game of Scrabble. Diazotroph is scientific jargon for microbial organisms such as bacteria and archaea (like bacteria but more unique) that have the necessary biochemical tools to fix atmospheric nitrogen into forms that plants can use. Plants cannot use atmospheric N₂ gas directly. Nitrogen gas (N₂) is comprised of two nitrogen atoms that are connected by a triple bond. Triple bonds are incredibly strong and contain a considerable amount of energy. Plants do not have the necessary equipment on their own to break a triple bond, but certain microbes do. Through the process of fixation, N₂ gas is converted by microbes into more usable forms of nitrogen, primarily ammonium. Diazotrophs can be found as free-living organisms that occur in the soil, in association (but not symbiotic) with plants, or as symbiotic bacteria living in the nodules present on roots of legume plants like soybean or alfalfa. Prior to the development of the Haber-Bosch process, through which anhydrous ammonia is synthesized from atmospheric nitrogen using natural gas (to contribute hydrogen) and a combination of intense pressure, heat, and a catalyst, diazotrophs were the primary route through which atmospheric nitrogen was fixed and moved into the soil.

Where does nitrogen go after fixation?

Fixation results in the conversion of N₂ to ammonium. Ok, got it. So now what? In some cases, like that of fixation occurring in a nodule on a soybean root, ammonium will be converted into other nitrogen containing compounds that the plant will use directly as it grows. If ammonium is being produced (or supplied as fertilizer) in the soil, it will often be converted to another form of nitrogen known as nitrate. This is a process called nitrification and is facilitated by other soil bacteria. A bacterium known as Nitrosomonas first converts ammonium to nitrite (NO₂), and then Nitrobacter takes over to finish the conversion of nitrite to nitrate (NO₃). There are likely many other soil bacteria that contribute to the nitrification process, but Nitrosomonas and Nitrobacter are the two groups that are usually referenced. In agricultural soils, most of the inorganic nitrogen (nitrogen not part of soil organic matter) is found in the form of nitrate. Before we move on from nitrate, let’s also consider the process of denitrification. Denitrification refers to the microbial process through which nitrate is converted back to a gas that rejoins the atmospheric pool of nitrogen. Denitrification occurs when soils are saturated and when aerobic bacteria in those soils are looking for oxygen for respiration. The oxygen atoms in nitrate are stripped away from nitrogen, releasing nitrogen back to the atmosphere as nitrous oxide.

How is organic nitrogen recycled?

It has been said that there are two certainties in life: death and taxes. While soil microbes escape the annoyance of paying income tax, they, like all other living organisms, will eventually die naturally or be eaten by predators in the soil food chain. While we often think about soil organic matter as bits and pieces of dead plant material, much of the soil organic matter is comprised of dead soil microbes. In the process of their death and eventual decomposition by living microbes, the nitrogen locked in their cellular structures is released for use by soil biology and plants. This the process that is referred to as mineralization – the conversion of nutrients (nitrogen in this example) from organic forms to plant available mineral forms.

Why is this topic important?

It is indisputable that nitrogen is a fundamental building block of life. For soybean growers, diazotrophs in the form of Bradyrhizobium bacteria provide a significant quantity of nitrogen directly to the soybean crop, while other soil microbes involved in fixing, converting, or mineralizing nitrogen play a role in providing the nitrogen nutrition not directly supplied through symbiotic fixation. In the bigger picture, as agriculture continues to change and focus on sustainability, carbon farming, and soil health, understanding and benefiting from the marvels of soil biology and its impact on key nutrient cycling processes will be the foundation for continued productivity, quality, and profitability.