In the nitrogen cycle, nitrification is the biological process that primarily converts which nitrogen form into which other form in its first step?

Introduction / Context:
The nitrogen cycle describes how nitrogen moves through the atmosphere, soil, water and living organisms. Several key biological processes are involved, including nitrogen fixation, nitrification, assimilation and denitrification. Nitrification is especially important in soil chemistry and wastewater treatment because it transforms ammonia into more oxidised forms that plants can use and that affect water quality. This question asks you to identify the main conversion that happens in the first step of nitrification, focusing on which nitrogen form is converted into which other form by nitrifying bacteria.

Given Data / Assumptions:

• Nitrification is carried out by specific aerobic bacteria in soil and water.
• It involves oxidation reactions that increase the oxidation state of nitrogen.
• Ammonia is often produced from the decomposition of organic nitrogen compounds or from fertilizers.
• In many texts, nitrification is described as a two step process: ammonia to nitrite and nitrite to nitrate.
• The question emphasises the first main biological conversion.

Concept / Approach:
In nitrification, ammonia (NH3) or ammonium ions (NH4 plus) are first oxidised to nitrite ions (NO2 minus) by bacteria such as Nitrosomonas. This is the first step. In the second step, nitrite is further oxidised to nitrate ions (NO3 minus) by bacteria such as Nitrobacter. Nitrification does not involve direct conversion of nitrogen gas to nitrate and is different from nitrogen fixation. It also does not convert ammonia back to nitrogen gas; that is denitrification. The correct approach is to select the option that describes ammonia being converted into nitrite, which represents the initial nitrification step.

Step-by-Step Solution:
Step 1: Recall that nitrification is an oxidation process that converts reduced nitrogen species into more oxidised ones in the soil. Step 2: The starting nitrogen form for nitrification is usually ammonia or ammonium, produced by decay of organic matter or added as fertilizer. Step 3: In the first step, bacteria such as Nitrosomonas oxidise ammonia (NH3 or NH4 plus) to nitrite ions (NO2 minus). Step 4: In a subsequent second step, other bacteria oxidise nitrite to nitrate (NO3 minus), but that second step is beyond the scope of this particular question. Step 5: Nitrogen gas (N2) is not directly involved in nitrification; processes involving N2 are nitrogen fixation and denitrification. Step 6: Therefore, the correct description of nitrification in its first step is the conversion of ammonia into nitrite ions.

Verification / Alternative check:
Standard environmental science and microbiology textbooks describe nitrification as a two stage process: Stage 1: NH3 plus 1.5 O2 gives NO2 minus plus H plus plus H2O. Stage 2: NO2 minus plus 0.5 O2 gives NO3 minus. These reactions show clearly that ammonia is converted first to nitrite, then to nitrate. Field studies of soil nitrogen confirm that when ammonia based fertilizers are applied to soil, they are gradually converted into nitrite and then nitrate by nitrifying bacteria. Nitrogen gas appears in other parts of the cycle, such as nitrogen fixation (N2 to ammonia) and denitrification (nitrate to N2), not in nitrification. This evidence supports option C as correct.

Why Other Options Are Wrong:
Option A describes nitrogen gas being turned directly into nitrate, which would require nitrogen fixation plus additional oxidation, not nitrification alone. Option B suggests N2 to nitrite, which again is not the standard nitrification step and does not match the biological processes carried out by nitrifying bacteria. Option D claims ammonia is converted directly into nitrogen gas, which would be denitrification in reverse and is not observed in nitrification. Option E describes nitrate being converted into ammonia, which is the opposite of oxidation and is sometimes called nitrate reduction, not nitrification. Only option C correctly states that the first step of nitrification is the conversion of ammonia into nitrite ions.

Common Pitfalls:
Learners often confuse the different nitrogen transformations and mix up terms like nitrogen fixation, nitrification and denitrification. Another pitfall is to forget that nitrification is specifically an oxidation process and therefore must move nitrogen from a more reduced state (ammonia) to more oxidised states (nitrite and nitrate). To keep them straight, remember the sequence: fixation brings nitrogen into the system by converting N2 to ammonia, nitrification oxidises ammonia to nitrate via nitrite, and denitrification returns nitrogen to the atmosphere by reducing nitrate back to N2. This mental map helps to answer nitrogen cycle questions accurately.

Final Answer:
In the first step of nitrification, bacteria convert Ammonia (NH3 or NH4 plus) into nitrite ions (NO2 minus), which is then further oxidised to nitrate in the second step.