Definitions of net primary productivity on the Web:

• Primary production is the production of organic compounds from atmospheric or aquatic carbon dioxide, principally through the process of ...
  en.wikipedia.org/wiki/Net_primary_productivity

• the amount of organic material available for consumption in a given area. GPP minus losses due to plant respiration.
  www.tuition.com.hk/geography/n.htm

• The rate at which organic matter is incorporated into plant bodies so as to produce growth. See also gross primary productivity.
  www.environment.nelson.com/0176169040/glossary.html

• NPP is a measure of plant growth obtained by calculating the amount of carbon absorbed and stored by plants. ...
  cfs.nrcan.gc.ca/subsite/ecoleap/definitions

• The total amount of energy fixed per unit of time minus the amount of energy expended by the metabolic activities of the photosynthetic organisms ...
  www.econguru.com/fundamentals_of_ecology/glossary.html