Question

Which of the following statements about the nitrogen cycle is false? a. Ammonification converts organic nitrogenous matter from living organisms into ammonium \(\left(\mathrm{NH}_{4}^{+}\right)\) b. Denitrification by bacteria converts nitrates \(\left(\mathrm{NO}_{3}^{-}\right)\) to nitrogen gas \(\left(\mathrm{N}_{2}\right)\) c. Nitrification by bacteria converts nitrates ( \(\mathrm{NO}_{3}\) ) to nitrites \(\left(\mathrm{NO}_{2}^{\circ}\right)\). d. Nitrogen fixing bacteria convert nitrogen gas (N \(_{2}\) ) into organic compounds.

Short answer

The false statement is c. Nitrification by bacteria converts nitrates (NO$_{3}$) to nitrites (NO$_{2}$∘). Nitrification actually involves the conversion of ammonium (NH$_{4}^{+}$) into nitrite (NO$_{2}^{-}$) and then into nitrate (NO$_{3}^{-}$), not the conversion of nitrate (NO$_{3}$) to nitrite (NO$_{2}$∘).

Step-by-step solution

1. Understand ammonification

Ammonification is the process where organic nitrogenous matter from living organisms is converted into ammonium ions (NH4+). This process is carried out by decomposers, such as bacteria and fungi, which break down organic materials into simpler compounds, returning nutrients to the soil.

2. Understand denitrification

Denitrification is the process by which bacteria convert nitrates (NO3-) into nitrogen gas (N2). This occurs under oxygen-poor conditions, such as waterlogged soils. Nitrogen gas is released back into the atmosphere in this process.

3. Understand nitrification

Nitrification is a two-step process that occurs in the soil. First, ammonia-oxidizing bacteria convert ammonium (NH4+) into nitrite ions (NO2-). Then, nitrite-oxidizing bacteria convert nitrite ions (NO2-) into nitrate ions (NO3-). The nitrates can then be taken up by plants and assimilated into organic nitrogen compounds.

4. Understand nitrogen fixation

Nitrogen fixation is the process by which nitrogen-fixing bacteria convert nitrogen gas (N2) into organic compounds, such as ammonia (NH3) or nitrate (NO3-), that can be used by plants. This process is essential for the availability of nitrogen in the soil, as nitrogen gas is inert and cannot be used directly by most organisms.

5. Analyze the four statements

Now let's analyze the four statements given: a. "Ammonification converts organic nitrogenous matter from living organisms into ammonium (NH4+)." This statement is true based on our understanding of ammonification in Step 1. b. "Denitrification by bacteria converts nitrates (NO3-) to nitrogen gas (N2)." This statement is true based on our understanding of denitrification in Step 2. c. "Nitrification by bacteria converts nitrates (NO3) to nitrites (NO2∘)." This statement is false based on our understanding of nitrification in Step 3. Nitrification actually involves the conversion of ammonium (NH4+) into nitrite (NO2-) and then into nitrate (NO3-), not the conversion of nitrate (NO3-) to nitrite (NO2-). d. "Nitrogen fixing bacteria convert nitrogen gas (N2) into organic compounds." This statement is true based on our understanding of nitrogen fixation in Step 4.

6. Conclusion

The statement "c. Nitrification by bacteria converts nitrates (NO3) to nitrites (NO2∘)" is false.

Key concepts

Ammonification

Imagine the circle of life where plants and animals grow, live, and eventually pass on. In this cycle, ammonification plays a crucial role in recycling nitrogen. It happens when microorganisms like bacteria and fungi decompose organic nitrogenous matter—like dead plants, animals, and waste—into ammonium (h{NH_4^+}). This process makes nitrogen available in a form that plants can absorb and use to grow.

During ammonification, complex organic compounds like proteins and nucleic acids break down into simpler substances. So when you see fallen leaves decomposing on the forest floor, that's ammonification in action, making the nitrogen within them accessible again to the ecosystem.

Denitrification

In contrast to other processes that add nitrogen to the soil, denitrification takes nitrogen away. Certain specialized bacteria play a unique role under oxygen-poor conditions—often found in water-logged soils or deep within the earth—where they transform nitrate (h{NO_3^-}) into nitrogen gas (h{N_2}). This gas then drifts up and returns to the atmosphere.

Denitrification is crucial as it prevents the excessive accumulation of nitrates in the ecosystem, which could lead to problems like algal blooms in water bodies. It's nature's way of balancing the nitrogen levels and ensuring that the cycle can continue without detrimental build-ups.

Nitrification

As seedlings seek sustenance from the soil, nitrification ensures they have the right form of nitrogen to thrive. This two-step bacterial process starts with ammonia-oxidizing bacteria that convert ammonium (h{NH_4^+}) into nitrite (h{NO_2^-}). Then, nitrite-oxidizing bacteria join the dance, turning nitrite (h{NO_2^-}) into nitrate (h{NO_3^-}).

Why are both steps important?

Plants predominantly absorb nitrogen in the form of nitrate. Without nitrification, the ammonia from decomposition would reach toxic levels, harming plant growth. This elegant bacterial conversion process ensures plants get their much-needed nitrogen 'food' safely.

Nitrogen Fixation

While nitrogen is abundant as a gas in our atmosphere, most life can't use it in that form. Enter the superheroes of the soil: nitrogen-fixing bacteria. These remarkable microorganisms, often living in a symbiotic relationship with plants (like legumes), convert nitrogen gas (h{N_2}) into a biological form, such as ammonia (h{NH_3}) or organic nitrogen compounds.

This process is akin to a factory that takes raw materials and assembles a product plants can 'purchase'—using energy from the plant to power their 'machinery.' Through this partnership, plants get their essential nitrogen, essential for making proteins and DNA, and the bacteria get a home and energy. It's a win-win for everyone involved and is a foundation for plant-based ecosystems on land.