Decomposition and C:N Ratio

What decomposition does

Decomposition is driven by microbes consuming: - Carbon as an energy source - Nitrogen to build proteins

If a material is carbon-rich, microbes may temporarily tie up nitrogen in their biomass. This is called nitrogen immobilisation.

C:N ratio

C:N describes how carbon-rich a material is relative to nitrogen.

General behaviour: - <20:1: net nitrogen release - 20–30:1: roughly balanced - >30:1: nitrogen immobilisation likely

Practical implications in substrates

High-carbon materials can cause early nitrogen deficiency symptoms even when feeding appears adequate. This is most likely with fresh or poorly composted bark, wood fibre, or green waste.

Composting reduces C:N and stabilises materials, reducing immobilisation risk.