Primary productivity is the amount of biomass or organic matter produced per unit area over a time period.
However, a good fraction of gross primary productivity is utilized in respiration of green plants. The amount of energy-bound organic matter created per unit area and time that is left after respiration is net primary productivity (NPP).
Net productivity of energy = Gross productivity – Energy lostin respiration.
The rates at which the heterotrophic organisms resynthesise the energy-yielding substances are called secondary productivity. Here, the net primary productivity (NPP) results in the accumulation of plant biomass, which serves the food of herbibores and decomposers.
Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants.