2.3.3 Terrestrial carbon cycle

The uptake of carbon through photosynthesis by land plants is larger than the corresponding uptake by phytoplankton, in particular in spring because of the greening of forest at mid and high latitudes and of the growth of herbaceous plants (Fig. 2.26). About half of this primary production is directly transferred back to the atmosphere by the respiration of the land plants themselves, the remaining part being incorporated into leaves, woods and roots (this fraction is defined as the net primary production, NPP)

The large majority of the carbon fixed by NPP returns to the atmosphere through decomposition in soil, the respiration of herbivores and carnivores, and fire. However, the time needed for this transfer back to the atmosphere is related to a large number of relatively complex processes. For instance, the carbon is stored for much longer in trees than in leaves or grasses. When plants die or leaves fall on the ground and enter the pool of organic matter in the soil, the decomposition rate depends strongly on the chemical composition of the organic material, the temperature and humidity of the soil, etc. Some of the organic matter in the soil is quickly remineralised, but much of it remains in the soil pool for decades or longer.