Topic 1.5 The Nitrogen Cycle: describe the steps of the nitrogen cycle and explain how nitrogen fixation, the role of bacteria and human activities move nitrogen between reservoirs.

What this topic is asking

The College Board (Topic 1.5) wants you to describe the nitrogen cycle step by step and explain why bacteria are central to it. The key insight is that nitrogen is abundant in the atmosphere but almost entirely unusable until it is fixed, so the availability of usable nitrogen often limits how much life an ecosystem can support. You must also explain how human use of fertilizer disrupts the cycle.

Why nitrogen must be fixed

The steps of the cycle

The cycle is a sequence of conversions, most carried out by bacteria:

• Nitrogen fixation: N2 is converted to ammonia/ammonium by nitrogen-fixing bacteria (free-living, or living in root nodules of legumes) and, to a lesser extent, by lightning.
• Nitrification: nitrifying bacteria convert ammonium to nitrite (NO2-) and then nitrate (NO3-), the form most easily taken up by plants.
• Assimilation: plants absorb ammonium or nitrate and build it into proteins and nucleic acids; animals get nitrogen by eating plants or other animals.
• Ammonification: decomposers convert nitrogen in dead organisms and waste back into ammonium.
• Denitrification: denitrifying bacteria convert nitrate back into N2 gas, returning nitrogen to the atmosphere.

The central role of bacteria

Almost every transformation in the nitrogen cycle depends on a specific group of microbes. This makes the cycle biologically driven, unlike the partly geological carbon and phosphorus cycles, and it is why disturbing soil microbial communities can disrupt nitrogen availability.

Human disruption

Humans now fix more nitrogen than all natural processes combined, mainly through the industrial Haber-Bosch process that makes synthetic fertilizer. This has boosted food production but flooded ecosystems with reactive nitrogen. Excess fertilizer washes off fields into rivers, lakes and coastal waters, where it acts as a nutrient and triggers eutrophication: rapid algal growth, followed by the death and decomposition of the algae, which consumes dissolved oxygen and creates dead zones where fish and other aquatic life cannot survive. Burning fossil fuels also releases nitrogen oxides that contribute to acid deposition and smog. The lesson of Topic 1.5 is that nitrogen, normally a limiting nutrient that ecosystems must work hard to capture, becomes a pollutant when humans supply it in excess.

Try this

Q1. Identify the form of nitrogen most readily absorbed by plant roots. [1 point]

• Cue. Nitrate (NO3-), produced by nitrification.

Q2. Explain why legume crops can improve soil nitrogen. [2 points]

• Cue. Legumes host nitrogen-fixing bacteria in their root nodules, which convert atmospheric N2 into usable forms that enrich the soil.