Carbon Flows in Ecosystems— Ecosystem Processes

Gross primary productivity (GPP) is the annual photosynthetic carbonuptake of all leaves over an area of land. Integrated terrestrial GPP andoceanic CO2 exchange account for the two largest fluxes of carbon between Earth and the atmosphere. GPP and its partitioning between plant auto trophic respiration (Ra) and net primary productivity (NPP) are key measures of the linkages among solar energy, atmospheric CO2, and the terrestrial biosphere (see Fig. 3.1. Terrestrial Carbon Uptake and Storage, this page, and Fig. 3.2. Terrestrial Photosynthetic Carbon Cycle, p. 28). CO2 uptake during photosynthesis is only temporary—Ra returns about half of the captured carbon to the atmosphere almost immediately. The rest is incorporated into biomass, comprising NPP—the total amount of organic matter created annually. Additional partitioning and processing distribute this organic matter to heterotrophs, and its subsequent assimilation and respiration eventually return most of the remaining carbon to the atmosphere. The organic carbon left after respiration by plants, heterotrophs, and decomposers is defined as net ecosystem productivity (NEP). The resultant aboveground biomass, woody plants, and soil organic matter (SOM) can persist for millennia. Another measure of carbon flow within an ecosystem is net biome productivit