Net primary productivity (NPP) in ecosystems is calculated by determining the difference between gross primary productivity (GPP) and plant respiration.

NPP is a fundamental concept in ecosystem ecology, as it quantifies the amount of energy that is available to consumers, including herbivores and decomposers, within an ecosystem. Specifically, NPP represents the rate at which plants produce net useful chemical energy. It can be expressed mathematically as:

where $R$ denotes the energy utilized by the plants for respiration.

Gross Primary Productivity (GPP) refers to the total energy captured through photosynthesis. However, not all of this energy is accessible to other organisms in the ecosystem, as a portion is consumed by the plants themselves for vital metabolic processes such as growth and reproduction. This energy expenditure is accounted for as plant respiration.

To measure NPP, scientists commonly employ two primary methods: the oxygen method and the carbon method.

1. Oxygen Method: This approach involves measuring the volume of oxygen produced by a plant during photosynthesis and then subtracting the volume of oxygen consumed during respiration. The resulting net oxygen production can be converted into energy units, typically expressed in kilocalories or kilojoules, based on the known energetic content of oxygen.

2. Carbon Method: In this method, scientists measure the amount of carbon dioxide absorbed by a plant during photosynthesis, then subtract the amount of carbon dioxide released during respiration. The net carbon measurement can also be converted into energy units using the established energetic content of carbon.

In terrestrial ecosystems, NPP is often estimated by assessing the increase in plant biomass, which refers to the total mass of all plants over a specified period. In contrast, in aquatic ecosystems, NPP is frequently estimated by monitoring changes in the concentrations of dissolved oxygen or carbon dioxide in the water.

Understanding NPP is essential for grasping energy flow within ecosystems. It also provides valuable insights into the effects of environmental changes, such as climate change and deforestation, on ecosystem dynamics and functioning.