Series 17 - Nutrient Cycling III - Ammonicfication & Denitrification

Summary

The video provides an in-depth exploration of the nitrogen cycle, breaking it into pools, sources, losses, and fluxes while focusing on microbial interactions and environmental controls. Key processes include **ammonification** (converting organic nitrogen into ammonium), **nitrification** (converting ammonium to nitrate), and **denitrification** (reducing nitrate to nitrogen gas). Environmental factors like **oxygen availability, water saturation**, and **cation exchange sites** significantly influence these processes. The speaker highlights interactions among microorganisms (aerobic and anaerobic bacteria) and the soil’s role in nutrient exchange. Understanding these components is crucial for comprehending nutrient cycling, soil health, and sustainable agricultural practices.

Takeaways

• 😀 The nitrogen cycle consists of three main questions: pools, sources and losses, and fluxes, each with specific controls on their components.
• 😀 Key nitrogen pools in the cycle include plants, soil organic material, and the atmosphere, each contributing to nitrogen availability in ecosystems.
• 😀 Ammonification converts organic nitrogen into ammonium (NH₄⁺), which can be taken up by plants or microbes during decomposition.
• 😀 Nitrification is the conversion of ammonium to nitrite (NO₂⁻) and then to nitrate (NO₃⁻), a process that requires oxygen and is vital for plant nutrient uptake.
• 😀 Denitrification converts nitrate into nitrogen gas (N₂), a process that occurs in oxygen-poor, anaerobic environments.
• 😀 Nitrogen in the form of ammonium can be lost to the atmosphere if not absorbed by plants, while nitrate can be leached into water systems.
• 😀 Soil plays a crucial role in the nitrogen cycle by exchanging nitrogen ions, with ammonium being more likely to bind to soil particles compared to nitrate.
• 😀 Oxygen availability regulates whether nitrification or denitrification occurs; nitrification requires oxygen, while denitrification occurs in anaerobic conditions.
• 😀 The microbial populations responsible for nitrification (Nitrosomonas and Nitrobacter) are highly sensitive to the accumulation of nitrites, which can be toxic at high levels.
• 😀 Water content in soil directly impacts microbial activity; as soil water increases, oxygen decreases, promoting denitrification over other processes.
• 😀 The nitrogen cycle is tightly regulated by environmental factors like water and oxygen levels, as well as the type of microbial populations present in the soil.

Q & A

What are the main components of the nitrogen cycle that are discussed in the transcript?

The main components discussed in the transcript are pools (plants, soil organic material, atmosphere), fluxes (ammonification, nitrification, denitrification), and losses (ammonium escaping as gas, nitrate leaching or denitrification).

What is ammonification, and how does it contribute to the nitrogen cycle?

Ammonification is the process by which organic nitrogen (such as amino nitrogen in plants and other organisms) is decomposed into ammonium (NH₄⁺) by microorganisms. This process makes nitrogen available in an inorganic form that can be used by plants and other organisms.

How does nitrification differ from ammonification?

Nitrification is the two-step conversion of ammonium (NH₄⁺) into nitrate (NO₃⁻). The first step, ammonium oxidation to nitrite, is performed by Nitrosomonas bacteria, and the second step, conversion of nitrite to nitrate, is done by Nitrobacter. In contrast, ammonification is the conversion of organic nitrogen into ammonium.

What are the roles of Nitrosomonas and Nitrobacter in the nitrogen cycle?

Nitrosomonas oxidizes ammonium (NH₄⁺) to nitrite (NO₂⁻), while Nitrobacter converts nitrite to nitrate (NO₃⁻). Both are crucial for nitrification, which makes nitrogen available in forms plants can absorb.

What is denitrification, and why is it important for the nitrogen cycle?

Denitrification is the microbial process where nitrate (NO₃⁻) is converted back into nitrogen gas (N₂) in anaerobic conditions. This process is important because it returns nitrogen to the atmosphere, completing the cycle and preventing excess nitrate buildup in the environment.

How does oxygen availability affect the nitrogen cycle, specifically nitrification and denitrification?

Nitrification requires oxygen, as the nitrifying bacteria involved (Nitrosomonas and Nitrobacter) use oxygen for their metabolic processes. Denitrification, on the other hand, occurs under low oxygen (anaerobic) conditions, as denitrifying bacteria use nitrate instead of oxygen for respiration.

How does water content in soil influence microbial activity in the nitrogen cycle?

As soil water content increases, oxygen availability decreases. This affects microbial activity; for instance, high water content favors denitrification (anaerobic process), while it reduces ammonification and nitrification (aerobic processes), especially when water fills over 60-70% of soil pores.

Why can ammonium potentially be lost from the system, and how does this affect the nitrogen cycle?

Ammonium can escape from the soil as a gas, particularly if it is not captured by plant roots or if the soil is not moist enough. This loss reduces the amount of nitrogen available to plants and disrupts the cycle, especially in areas with high microbial activity or soil disturbances.

What is the relationship between microbial respiration and nitrogen cycling processes like denitrification?

Microbial respiration plays a key role in nitrogen cycling. In denitrification, microbes use nitrate (NO₃⁻) instead of oxygen for respiration in anaerobic conditions, converting nitrate to nitrogen gas. This is crucial for completing the nitrogen cycle and regulating nitrogen availability in ecosystems.

How does the concept of cation exchange relate to ammonium in the nitrogen cycle?

Ammonium (NH₄⁺) is a positively charged ion, meaning it can interact with the cation exchange sites in soil. This allows ammonium to be temporarily fixed in the soil, where it can be absorbed by plants or microbes, influencing nitrogen availability and retention in the soil.