Problem 3 - Biology

Question

Which of the following statements about the nitrogen cycle is false? a. Nitrogen fixing bacteria exist on the root nodules of legumes and in the soil. b. Denitrifying bacteria convert nitrates (NO3) into nitrogen gas (N2). c. Ammonification is the process by which ammonium ion \(\left(\mathrm{NH}_{4}^{+}\right)\) is released from decomposing organic compounds. d. Nitrification is the process by which nitrites \(\left(\mathrm{NO}_{2}^{-}\right)\) are converted to ammonium ion \(\left(\mathrm{NH}_{4}^{+}\right)\)

Step-by-Step Solution

Answer

d. Nitrification is the process by which nitrites (NO2) are converted to ammonium ion (NH4+).

Step 1 - Understand the Nitrogen Cycle

The nitrogen cycle is a process by which nitrogen is converted between its various chemical forms. This transformation can be carried out through both biological and physical processes.

Step 2 - Review Nitrogen Fixation

Nitrogen fixation is the process of converting nitrogen gas (N2) from the atmosphere into ammonia (NH3) or related nitrogenous compounds in soil. This process often involves nitrogen-fixing bacteria found on the root nodules of legumes and in the soil.

Step 3 - Review Denitrification

Denitrification is the reduction of nitrates (NO3) back into the largely inert nitrogen gas (N2), making it unavailable to plants. This process is carried out by denitrifying bacteria.

Step 4 - Review Ammonification

Ammonification is the process by which the organic nitrogen of dead organisms and waste products is converted to ammonium ions (NH4+). This happens through the decomposition process.

Step 5 - Review Nitrification

Nitrification is a step in the nitrogen cycle where ammonia (NH4+) is oxidized to nitrites (NO2-) and then to nitrates (NO3-). This process is carried out by nitrifying bacteria.

Step 6 - Identify the False Statement

Using the information reviewed, determine which statement does not correctly describe a process within the nitrogen cycle. Statement d claims that 'Nitrification is the process by which nitrites (NO2) are converted to ammonium ion (NH4+).' However, this is incorrect because nitrification involves the conversion of ammonium ions (NH4+) to nitrites (NO2-) and then to nitrates (NO3-).

Key Concepts

Nitrogen Fixation

Nitrogen fixation is a critical component of the nitrogen cycle. During this process, nitrogen gas (N2) from the atmosphere is converted into ammonia (NH3) or related compounds that plants can use. This transformation is essential because most plants cannot use atmospheric nitrogen directly.

There are two primary ways nitrogen fixation occurs:

• Biological nitrogen fixation: This is mostly carried out by symbiotic bacteria, such as those found in the root nodules of legumes (e.g., beans, peas, and clovers). These bacteria possess the enzyme nitrogenase, which facilitates the conversion of nitrogen gas to ammonia.


• Industrial nitrogen fixation: The Haber-Bosch process is an example, where nitrogen gas is converted to ammonia under high pressure and temperature.

Overall, nitrogen fixation makes nitrogen available to living organisms and is a key initial step in the nitrogen cycle.

Denitrification

Denitrification is the process of converting nitrates (NO3-) back into nitrogen gas (N2), which is then released into the atmosphere. This process helps close the nitrogen cycle loop as it reduces the amount of fixed nitrogen in the soil.

Denitrification is carried out by denitrifying bacteria, typically under anaerobic (oxygen-free) conditions found in waterlogged soils or deep sediment layers. These bacteria utilize nitrates as an alternative to oxygen for respiration.

Key points about denitrification:

• Reduces soil nitrate levels.
• Occurs in anaerobic environments.
• Produces nitrogen gas and sometimes nitrous oxide (N2O) as a by-product.

Denitrification is vital for preventing the excessive buildup of nitrates, which can lead to issues such as groundwater contamination and eutrophication in aquatic environments.

Ammonification

Ammonification is the decomposition process where microorganisms convert organic nitrogen from dead organisms and waste products into ammonium ions (NH4+). This step is critical for recycling nitrogen within ecosystems.

Decomposing organisms, like fungi and bacteria, break down proteins and nucleic acids from dead matter, releasing ammonium. This ammonium can then be used directly by plants or further processed by other microbes through nitrification.

Main features of ammonification:

• Breaks down organic matter into inorganic ammonium.


• Recycles nitrogen within the ecosystem.
• Performed by decomposing bacteria and fungi.

Without ammonification, the nitrogen stored in dead plants and animals would remain locked in organic form and be unavailable to living organisms.

Nitrification

Nitrification is a two-step process in the nitrogen cycle where ammonium ions (NH4+) are converted into nitrites (NO2-) and then into nitrates (NO3-). This process is essential for making nitrogen available in a form that plants can absorb and use for growth.

Nitrification is performed by two groups of specialized bacteria:

• Ammonia-oxidizing bacteria (AOB): These bacteria convert ammonium ions (NH4+) to nitrites (NO2-).


• Nitrite-oxidizing bacteria (NOB): These bacteria convert nitrites (NO2-) to nitrates (NO3-).

Since nitrates are highly soluble and can easily be taken up by plants, nitrification plays a crucial role in plant nutrition.

Important points about nitrification:

• Transforms ammonium into usable forms of nitrogen for plants.
• Happens in well-aerated soils.
• Carried out by two specific types of bacteria.

Understanding nitrification helps explain how nitrogen passes through different forms in the soil, ultimately becoming accessible to plants, thereby supporting growth and development.