Energy and nutrient cycling
Primary producers incorporate carbon into organic molecules using an energy source. Net primary productivity is the rate of biomass energy storage after producers meet their own respiratory costs. Consumers obtain only some production from a lower trophic level because not all biomass is eaten or assimilated and organisms respire. Transfer efficiency varies; ten percent is an approximation, not a law. Decomposers return nutrients to accessible pools while also respiring. Nitrogen fixation converts atmospheric N₂ into forms that can enter biological pathways.
If gross primary production is 1,200 energy units and producer respiration is 500, net primary production is 700. Those 700 units are not a promise that the next trophic level receives all 700.
Energy is not recycled by decomposers. Nutrient atoms can cycle, while organisms continually need an energy input.