5.1.14—Soil carbon role

Compare carbon inputs and outputs: inputs greater than outputs make soil a sink, outputs greater make it a source, and equality is balance.

Compare inputs from litter and roots with losses through decomposition, respiration and erosion over a stated depth and period. Tropical forest soils often store little carbon because warm, moist conditions speed decomposition; tundra and waterlogged wetlands accumulate carbon because cold or low-oxygen conditions slow decay, while temperate grasslands add substantial carbon through dense roots.

A soil is a sink while inputs exceed outputs, a source while outputs exceed inputs, and a store describes the carbon stock present; the same soil can hold a large store yet currently be a source.

Classify sink or source from the net rate of stock change, not from dark colour or the absolute size of the carbon store.

A sink is defined by net rate, not by having a large carbon store.