Nitrogen Cycling: The Role of Soil Microbes

Nitrogen: A Most Wanted Nutrient

Nitrogen is the nutrient that keeps farmers up at night. It’s the most commonly deficient nutrient in soils, and the most commonly supplied by fertiliser. Without nitrogen, plants cannot build proteins or produce chlorophyll, which is essential for growth and energy production. Yet right above our heads is an endless supply: 78% of the Earth’s atmosphere is nitrogen gas. The catch? Plants can’t access it directly. Unlocking this nitrogen and keeping it in the soil depends heavily on the soil microbial community.

How Plants Access Nitrogen

Plants primarily absorb nitrogen in ionic forms: nitrate (NO₃^–) and ammonium (NH₄⁺). Nitrate is highly soluble in water, making it readily available to plants, but it is also easily washed away. Ammonium, on the other hand, tends to bind to soil particles, providing a more stable source, but it is also less accessible to plants. Other forms of nitrogen, such as atmospheric nitrogen (N₂), must first be converted into ionic forms like ammonium (NH₄⁺) or nitrate (NO₃^–) by biological processes to become accessible to most plants. Organic nitrogen, present in organic matter like plant residues and animal waste, must first be decomposed by soil microbes. This decomposition process breaks down organic matter into simpler organic compounds (such as amino acids) or transforms it into inorganic ionic forms of nitrogen.

Nitrogen Fixation Collaboration

This is where nitrogen-fixing bacteria, such as Rhizobia, play a crucial role. These bacteria form a symbiotic relationship with legumes, infecting their roots and forming nodules where they convert nitrogen gas (N₂) into ammonium (NH₄^–). In return, the plants provide the bacteria with sugars, nutrients and a protective environment. Plants aren’t picky when it comes to forming partnerships for nitrogen – bacteria other than Rhizobia also come to help. For example, actinomycetes of the genus Frankia are soil-dwelling microbes that team up with certain non-legume plants, forming root nodules where they convert nitrogen gas into ammonium, just like their legume-friendly cousins. Then there’s cyanobacteria, a group of microbes that can fix nitrogen in specialised structures called heterocysts. By collaborating with nitrogen-fixing bacteria, plants gain access to the bioavailable forms of nitrogen they need to thrive.

Soybean root nodules

Root nodules are specialised structures that house nitrogen-fixing bacteria. They provide these bacteria with a low-oxygen environment that is ideal for their activity. This environment is perfect for nitrogenase, the enzyme that converts atmospheric nitrogen into ammonium.

Unlocking Organic Nitrogen

Unfortunately, most crops don’t have the luxury of nitrogen-fixing partnerships. For them, organic matter becomes the main source of nitrogen. Organic matter in the soil is made up of plant and animal residues. Microorganisms break it down and convert organic nitrogen into ammonium through a process called mineralisation. Ammonium can then be taken up by plants or further transformed into nitrate through nitrification. It’s a dynamic system where microbes do the heavy lifting, and plants reap the benefits. But plants don’t just wait passively. They attract microbes by releasing root exudates – sugars, amino acids, and signalling compounds that create a hospitable environment around their roots.

To stretch their reach even further, plants also team up with mycorrhizal fungi. These fungi spread their hyphal networks, which transport nutrients, including nitrogen, from decomposing matter. In exchange, they receive sugars, creating a nutrient sharing system that improves soil fertility.

Plant and Microbes Compete for Nitrogen (and Why That's Good)

Plants and soil microbes often compete for nitrogen, especially when the supply in the soil is limited. Bacteria, with their rapid growth, are remarkably efficient at absorbing and storing nitrogen by immobilising it in their cells. This temporarily keeps nitrogen out of the reach of plants. While this may seem unfair, it’s actually a brilliant system. When these microbes eventually die and decompose, they release their stored nitrogen back into the soil in plant-friendly forms such as ammonium and nitrate. This gradual release acts as a natural slow-release fertiliser, ensuring that plants have a steady supply of nitrogen over time.

Keeping Nitrogen in The Soil

The challenge isn’t just about how to get nitrogen but also how to retain it effectively. Nitrogen can be lost through leaching, where nitrate (NO₃^–) and ammonium (NH₄⁺) are washed out of the soil and into the ground water, or by volatilisation, where ammonia (NH₃) is lost as a gas to the atmosphere. Denitrification, a microbial process under low-oxygen conditions, can also release nitrogen as nitrous oxide (N₂O), a potent greenhouse gas. Thankfully, microbes and organic matter act as a nitrogen protection. Microbes “lock up” nitrogen in their cells, preventing it from leaching or volatilising until they decompose. Organic matter, on the other hand, acts like a sponge, binding nitrogen in a stable form and providing a slow-release nutrient reservoir. Together they help the soil retain its nitrogen, reducing losses and keeping this valuable nutrient available to plants.

To see how these processes work in harmony and the roles regenerative agriculture plays in improving nitrogen retention, check out this video by Jimi Sol on nitrogen cycling in the soil.

Building Resilient Soil Ecosystems

An active and diverse microbial community ensures steady nitrogen availability, improving soil health, organic matter buildup, and nutrient efficiency. Rather than relying on excessive fertilisation, farmers can promote nitrogen fixation and retention through sustainable practices such as adding organic matter (compost, composted manure), using diverse cover crops, enhancing microbial activity and balanced plant nutrition.

Take The Next Step to Healthy Soils

After all, in the world of soil microbes, teamwork really does make the dream work – and BIOTREX analysis is here to reveal the dynamics at play. Whether you’re advancing innovative solutions or promoting resilient agricultural systems, BIOTREX equips you with the insights needed to uncover the role of microbial community in building healthier soils and ecosystems.

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