Explain how the chemical composition of plant litter affects the rate of decomposition in a terrestrial ecosystem.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a temperate forest, oak leaves with a high lignin and tannin content decompose significantly more slowly than alder leaves, which have lower lignin levels and higher nitrogen content.
Model answer (4 marks)
Lignin is a complex, recalcitrant polymer that resists enzymatic degradation, so litter high in lignin (oak) decomposes slowly.
A high carbon‑to‑nitrogen (C:N) ratio limits decomposer growth because nitrogen is required for enzyme and protein synthesis; oak leaves have a high C:N ratio.
Tannins and other polyphenols bind to extracellular enzymes or litter proteins, inhibiting hydrolysis and further slowing decomposition.
In contrast, alder leaves have low lignin and a lower C:N ratio, providing an easier substrate for microbes and fungi, which increases their biomass and enzyme production and therefore speeds up decomposition and nutrient cycling.
A high carbon‑to‑nitrogen (C:N) ratio limits decomposer growth because nitrogen is required for enzyme and protein synthesis; oak leaves have a high C:N ratio.
Tannins and other polyphenols bind to extracellular enzymes or litter proteins, inhibiting hydrolysis and further slowing decomposition.
In contrast, alder leaves have low lignin and a lower C:N ratio, providing an easier substrate for microbes and fungi, which increases their biomass and enzyme production and therefore speeds up decomposition and nutrient cycling.
Examiner tips
- Use the exact terms: lignin, C:N ratio, tannins, decomposer enzymes. Explain the causal link: high lignin/low N → slower enzyme action → slower decomposition. Keep each point brief and separate with a line break or numbering.
Common mistakes
- Mixing up lignin with cellulose; forgetting that lignin is recalcitrant. Failing to mention the C:N ratio as a limiting factor. Over‑generalising tannins as simply ‘bad’ without explaining enzyme inhibition.
Mark scheme (4 marks)
- Lignin is a complex, recalcitrant organic polymer that is resistant to enzymatic breakdown by decomposers, slowing decomposition.
- A high carbon-to-nitrogen (C:N) ratio in litter limits decomposer growth because nitrogen is a limiting nutrient required for synthesis of decomposer enzymes and proteins.
- Tannins and other polyphenols inhibit decomposer enzyme activity by binding to extracellular enzymes or to the proteins in litter, reducing hydrolysis.
- Litter with low lignin and high nitrogen (e.g. alder leaves) provides a favourable substrate for decomposer growth, increasing microbial / fungal biomass and enzyme secretion, thereby accelerating decomposition and nutrient cycling.
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Transfers of energy and matter questions
- Explain how detritivores and decomposers contribute to the cycling of matter in …
- Explain why the efficiency of energy transfer between trophic levels in a food c…
- Explain why biomass tends to decrease at successive trophic levels in most food …
- Explain why the net primary productivity (NPP) of an ecosystem is always less th…