错误做法:

Forgetting to include decomposition as a process that returns $C{O}_2$ to the atmosphere in the carbon cycle.

原因:

Many students only list respiration and combustion, missing that decomposers respire as they break down dead matter, which is a required mark point.

正确做法:

Always explicitly name photosynthesis, respiration, decomposition, and combustion as the four key carbon cycle processes.

错误做法:

Stating that plants can absorb atmospheric nitrogen gas directly.

原因:

Nitrogen gas is unreactive, so plants cannot use it without first being converted into nitrates or ammonium by bacteria.

正确做法:

Specify that plants only absorb nitrogen in the form of nitrates from soil via their roots.

错误做法:

Naming specific bacteria species (e.g., Rhizobium) in nitrogen cycle answers.

原因:

The Edexcel specification explicitly states specific bacterial names are not required, and including them wastes time and risks misspelling and losing marks.

正确做法:

Refer only to the functional groups: nitrogen-fixing, nitrifying, denitrifying, and decomposer bacteria.

错误做法:

Confusing the roles of nitrifying and denitrifying bacteria.

原因:

Students often mix up which bacteria convert ammonium to nitrates and which convert nitrates back to nitrogen gas.

正确做法:

Use the mnemonic: nitrifying bacteria make nitrates, denitrifying bacteria remove nitrates from soil.

错误做法:

Omitting feeding as a way carbon and nitrogen move along food chains.

原因:

Many students focus only on abiotic transfers, forgetting that both elements are passed between trophic levels when organisms eat other organisms.

正确做法:

Add a line stating carbon/nitrogen is passed up food chains via feeding in longer answer questions.