Biomes can be associated with specific temperature and precipitation regimes. In many cases, the supply of N determines RUBP availability. Hence the N content of leaves control RUBP and the rate of fixation. P could be limiting in some instance whereas Mg and Mn are seldom in short supply. In fertile soils, NUE is low and in nutrient deficient soils, NUE is high.
Respiration arises from metabolic activity of mitochondria and largely occurs in roots and stems.
Note that often more than half of the productivity is below ground . Root growth difficult to study. This measure does not care what subsequently happen to the plant tissue. NEP - clearly refers to the net ecosystem accumulation rate of C taking into account, not just plant respiration but also return flux through herbivores and decomposition. Below 30 years, bulk of NEP arises from aggradation of living biomass.