Net primary productivity is the excess of gross_primary_producivity (rate of synthesis of biomass per unit area from inorganic precursors by autotrophs, or "producers", especially by photosynthesising plants using sunlight for energy) over the rate at which they themselves respire some of this biomass (plant_respiration, assumingall producers to be plants). "Productivity of carbon" refers to the productionof biomass expressed as the mass of carbon which it contains. "Productivity" means production per unit area.