Series 17 - Nutrient Cycling II - Nitrogen Cycle Intro

Summary

This lecture from Cornell University delves into the nitrogen cycle, explaining how nitrogen moves between various pools in the environment, including the atmosphere, biosphere, lithosphere, and hydrosphere. Key nitrogen sources such as biological fixation, industrial fertilizers, lightning, and emissions are discussed. The lecture highlights the impact of human activities like fertilizer use and coal-burning emissions, which not only contribute to nitrogen fertilization but also cause environmental acidification. It emphasizes the importance of nitrogen cycling for soil fertility and the broader ecological consequences of nitrogen management.

Takeaways

• 😀 The nitrogen cycle is a complex system involving nitrogen movement through various environmental pools and fluxes.
• 😀 The main nitrogen pools include the biosphere, lithosphere, hydrosphere, and atmosphere, with the largest pool being in the lithosphere (rocks).
• 😀 The second-largest nitrogen pool is the atmosphere, primarily consisting of nitrogen gas (N2).
• 😀 Biological nitrogen fixation involves microorganisms converting nitrogen gas (N2) into organic forms that plants can use, making it a key part of the nitrogen cycle.
• 😀 Fertilizer use, both industrial and from emissions, significantly contributes to nitrogen availability in agriculture.
• 😀 Lightning is a natural process that converts atmospheric nitrogen into forms that can be used by plants, contributing about 5% of natural nitrogen sources.
• 😀 Offsite organic matter additions and nitrogen in dust are also sources of nitrogen, moved by wind to different locations.
• 😀 The Green Revolution greatly increased fertilizer use, leading to higher nitrogen fixation and more productive crops.
• 😀 Emissions from coal plants contribute nitrogen to the atmosphere, which can result in both acid rain and unintended fertilization of the environment.
• 😀 Nitrogen fixation by free-living or symbiotic microorganisms in plants like legumes provides a crucial nitrogen source, benefiting both plants and soil microorganisms.

Q & A

What is the nitrogen cycle, and how is it represented in the diagram?

The nitrogen cycle represents the movement of nitrogen through different pools (reservoirs) in the Earth system, including the atmosphere, biosphere, hydrosphere, and lithosphere. The diagram uses boxes to represent the nitrogen pools and arrows to indicate the fluxes, or the movement of nitrogen between these pools.

What are the major nitrogen pools on Earth?

The largest nitrogen pool is in the lithosphere (rocks), followed by the atmosphere, which holds nitrogen gas (N2). Smaller nitrogen pools are found in the biosphere (organisms), hydrosphere (water), and terrestrial soils.

What are the primary sources of nitrogen in the biosphere?

The primary sources of nitrogen in the biosphere include biological nitrogen fixation by certain bacteria, fertilizer use (both industrial and emissions), lightning, organic matter additions from wind, and nitrogen in dust particles.

How does biological nitrogen fixation work, and which organisms are involved?

Biological nitrogen fixation is the process by which certain bacteria convert nitrogen gas (N2) from the atmosphere into organic forms of nitrogen, like ammonia (NH3), that plants can use. This process can occur in free-living organisms or in symbiotic relationships with plants, such as legumes.

What role do fertilizers play in the nitrogen cycle?

Fertilizers contribute nitrogen to the biosphere, often through industrial nitrogen fixation. Fertilizers boost plant productivity by providing nitrogen in a form that plants can readily absorb, leading to greener, more productive crops. However, excessive use can result in environmental problems like nutrient runoff.

How does lightning contribute to nitrogen fixation?

Lightning causes a natural form of nitrogen fixation by breaking the bonds of nitrogen gas (N2) in the atmosphere. The energy from the lightning reacts with nitrogen and oxygen to form nitrates, which can then be absorbed by plants in the soil.

What is the significance of nitrogen emissions from coal-fired power plants?

Nitrogen emissions from coal-fired power plants release nitrogen compounds into the atmosphere, which can later combine with water to form nitric acid. This acidifies the environment and also leads to nitrogen fertilization in soils, resulting in a dual environmental impact: acidification and nutrient enrichment.

What are the environmental consequences of nitrogen fertilizer use?

Excessive use of nitrogen fertilizers can lead to environmental issues such as nutrient runoff, which can pollute water bodies, causing eutrophication. This leads to algal blooms and oxygen depletion, harming aquatic life. Additionally, excess nitrogen can cause soil acidification.

What are symbiotic relationships in the context of nitrogen fixation?

Symbiotic relationships in nitrogen fixation refer to the mutualistic partnership between certain plants (like legumes) and nitrogen-fixing bacteria. The bacteria fix nitrogen gas from the atmosphere and provide the plant with usable nitrogen, while the plant provides carbon and energy to the bacteria.

Why is nitrogen fixation by certain organisms considered a significant advantage?

Nitrogen fixation by organisms is a significant advantage because it allows them to access nitrogen in a form that is otherwise hard to obtain, as atmospheric nitrogen (N2) has a strong triple bond. Organisms capable of nitrogen fixation can thrive in nitrogen-limited environments where other forms of nitrogen are unavailable.