QCE Geography Practice Questions

Tropical rainforest in the Amazon Basin functions as a vast terrestrial carbon store, holding carbon in two principal reservoirs: living vegetation biomass (above-ground and below-ground) and soil organic matter. When forest is cleared for cattle pasture, these stores are disrupted through several interconnected mechanisms.

During the initial clearing phase, large amounts of vegetation biomass — estimated at 150–200 tonnes of carbon per hectare in mature Amazonian rainforest — are either burned or left to decompose. Combustion of felled timber and undergrowth releases stored carbon directly into the atmosphere as CO₂ and particulate carbon almost instantaneously, representing a rapid, large-magnitude carbon flux from the biosphere to the atmosphere. This process also releases methane (CH₄) from incomplete combustion, further intensifying the greenhouse effect.

Where cleared material is left to decompose rather than burned, microbial decomposition of dead organic matter releases CO₂ gradually over subsequent years. Importantly, conversion to pasture dramatically reduces the rate of net primary productivity (NPP): grassland cover fixes far less carbon through photosynthesis than the rainforest it replaces, meaning the land transitions from a net carbon sink to a net carbon source or near-neutral state.

The disruption extends below ground. Forest clearance exposes soil to elevated temperatures and UV radiation, accelerating the oxidation of soil organic carbon (SOC). Compaction from cattle hooves also reduces soil structure, limiting the capacity for new organic matter to accumulate. Over time, SOC stocks decline substantially, releasing additional CO₂ to the atmosphere through aerobic microbial respiration.

The cumulative effect is a net positive carbon flux from the terrestrial biosphere to the atmosphere, reducing the Amazon's role as a planetary-scale carbon sink and contributing to elevated atmospheric CO₂ concentrations and the enhanced greenhouse effect.