For any ecosystem, a constant input of energy is the basic requirement for its functioning and sustenance. The productivity of an ecosystem is calculated as the rate of biomass production expressed as gram/square meter/year (g. m⁻². yr⁻¹). The rate of production of organic matter during photosynthesis is called gross primary productivity (GPP). A considerable portion of GPP is consumed and utilized by plants during respiration. Therefore, the biomass available for heterotrophs is known as net primary productivity (NPP). The rate of formation of new organic matter by consumers is known as secondary productivity (SP).
Primary productivity depends on the plant species inhabiting a particular area, availability of nutrients, photosynthetic capacity of plants, and other environmental factors. The annual NPP of whole biosphere is approximately 170 billion tons, out of which, productivity of oceans is around 55 billion tons. In soil, the dead organic matter is converted to humus through humification. Humus undergoes slow degradation and is a reservoir of soil nutrients. Over a period of time, humus is further degraded by microbes and release nutrients by a process called mineralization.