Common Misconceptions
Part of The Nitrogen Cycle · GCSE GCSE Biology revision
This common misconceptions covers Common Misconceptions within The Nitrogen Cycle for GCSE Biology. The nitrogen cycle: nitrogen-fixing, nitrifying, denitrifying bacteria, ammonification, and the role of legumes It is section 9 of 14 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 9 of 14
Practice
15 questions
Recall
15 flashcards
⚠️ Common Misconceptions
Misconception 1: "Plants absorb nitrogen gas from the air"
This is one of the most common mistakes in the nitrogen cycle. Plants cannot absorb atmospheric nitrogen (N₂) — even though it makes up 78% of the air, the triple bond in N₂ is too strong for plants to break. Plants can only absorb nitrogen in the form of nitrates (NO₃⁻) dissolved in soil water, taken up through root hair cells via active transport. Before plants can use atmospheric nitrogen, bacteria must first "fix" it — converting N₂ to ammonia, and then nitrifying bacteria convert that ammonia to nitrates. In other words, plants are entirely dependent on bacteria for their nitrogen supply from the atmosphere.
Misconception 2: "All bacteria in the nitrogen cycle are helpful to farmers"
Not true. While nitrogen-fixing bacteria and nitrifying bacteria increase the amount of plant-available nitrates in the soil (which is good for farmers), denitrifying bacteria actively reduce soil fertility by converting nitrates back into nitrogen gas. They thrive in waterlogged, anaerobic soils — which is why waterlogged fields have poor crop growth. Farmers try to prevent waterlogging (by drainage and ploughing) to limit denitrification and preserve soil nitrogen levels.
Misconception 3: "The nitrogen cycle and carbon cycle are the same process"
They are completely different cycles involving different elements, different organisms, and different processes. The carbon cycle is driven primarily by photosynthesis (removing CO₂) and respiration/combustion/decomposition (returning CO₂). Bacteria play only a minor role. The nitrogen cycle, by contrast, is almost entirely dependent on specific bacteria at every stage — nitrogen fixation, nitrification, ammonification, and denitrification. Without bacteria, the nitrogen cycle would collapse entirely. The two cycles do overlap (decomposers appear in both), but they are not the same process.
Quick Check: A field is flooded for two weeks following heavy rainfall. After the flood recedes, the farmer notices the crop is growing poorly despite having added fertiliser before the flood. Suggest why the crop is performing so badly.
Flooding creates waterlogged, anaerobic conditions in the soil. Under these conditions, denitrifying bacteria thrive — they convert nitrates in the soil into nitrogen gas, which is released into the atmosphere. This means the nitrates from the farmer's fertiliser have been removed from the soil by denitrifying bacteria during the flood period. With low nitrate levels, the crop cannot absorb enough nitrogen to make amino acids and proteins, so growth is poor. Additionally, in anaerobic conditions, nitrifying bacteria (which need oxygen) cannot function, so any ammonia produced by decomposers is not being converted to nitrates. The combined effect is a dramatic reduction in plant-available nitrogen.