19.3 Nutrient cycles

Carbon cycle limited to six Cambridge processes: photosynthesis (CO₂ removed by plants), respiration (CO₂ released by all organisms), feeding (carbon transferred between organisms), decomposition (dead matter broken down by bacteria/fungi releasing CO₂), formation of fossil fuels (dead organisms compressed over 100M+ years), and combustion (burning fossil fuels releases stored CO₂). HL extension: nitrogen cycle with four named microorganism roles — decomposition (protein → NH₄⁺), nitrification (NH₄⁺ → NO₃⁻), nitrogen fixation (N₂ → NH₄⁺), and denitrification (NO₃⁻ → N₂). Plants need nitrogen for proteins and DNA but cannot use atmospheric N₂ directly because the triple bond is too strong; bacteria do the work.