Nutrient addition experiments primary production

It is now evident that marine N2 fixation can be limited by either P or Fe, although the relative importance of these two nutrients is stiU highly debatable. There may be an interaction between iron supply and P limitation of primary production through N2 fixation. Recently, Mills et al. (2004) used a factorial nutrient addition experiment to show that Fe and P can co-limit N2 fixation in the eastern tropical North Atlantic. They used trace-metal clean techniques with the addition of N, P, Fe, or Saharan dust individually or in combination to investigate which nutrient limits N2 fixation. Their experiments demonstrated that at two out of three stations, N2 fixation was enhanced by 2-3 times by the addition of P and Fe together, but the addition of either P or Fe alone had less effect (Fig. 38.14). Added dust also enhanced N2 fixation, which they suggested could be due to mineral dissolution of both Fe and P. 

---