Dear Ihab,

Generally, NEE is also the net productivity of the ecosystem. Typically, an ecosystem has heterotrophic organisms and even autotrophic organisms (roots and certain bacteria) all given off carbon from respiration activities whiles the autotrophs equally remove carbon through photosynthesis for primary production which ultimately releases oxygen. Thus the NEE is the difference between the amount of carbon produced by heterotrophic respiration and that from the net primary production for that ecosystem. Thus as suggested by my earlier colleagues refer to the document where you obtained the equation and I believe these parameters will be well-defined and the values may be obtained from literature (or by parameterizing other equation with data) for that particular environment. I believe this helps. Best wishes

Your question is not very clear. If you measured CO2 exchange and PPFD, you need to use a statistical program (or Solver module of MS Excel) to fit your data with this function (corrected by Ihab) by changing the Amax and alpha parameters.

thank you so much for all