How is net primary productivity calculated?

1. Plants pull carbon dioxide out of the air and fix its carbon into organic matter. 2. That total fixed each year is the gross figure. 3. The plants then burn some of that organic matter themselves in cellular respiration. 4. Carbon oxidized in respiration is no longer available as food for anything else. 5. So the net figure is the carbon fixed per year minus the carbon oxidized in respiration. 6. What remains is an estimate of all the organic matter available as food. 7. Adding the two instead of subtracting would count respired carbon as if it were still available. 8. Reversing the subtraction would give a negative value for every healthy ecosystem. _Source: OpenStax Biology 1e (CC BY 4.0), Chs 44 and 46, section 44.2 Biogeography_