Biological nitrogen fixation is a fundamental source of nitrogen input to terrestrial ecosystems. Although molybdenum and phosphorus are both crucial elements in nitrogenase which catalyzes the conversion of di-nitrogen to ammonia, we have a poor understanding of how and when limitation by these nutrients arises in nature. Here we demonstrate in Panamanian tropical forest soils a mechanism for molybdenum and phosphorus limitation on asymbiotic nitrogen fixation.  

Results/Conclusions

We find that the organic soil horizon serves as a biogeochemical niche where asymbiotic nitrogen fixers are favored and where the availability of phosphorus is promoted relative to that of molybdenum. In organic matter, molybdenum is strongly complexed while phosphorus is held by weaker ester bonds, resulting in exceedingly high ratios of bioavailable phosphorus to molybdenum. Soil chemistry therefore favors the limitation of molybdenum over phosphorus in organic soils, however asymbiotic nitrogen fixers appear to integrate this local control on nutrient availability with landscape gradients of soil nutrients. Examination of forests along a steep gradient of soil phosphorus revealed that fixation was limited by molybdenum in phosphorus-rich soils, but co-limited by both molybdenum and phosphorus in phosphorus-poor soils. Our findings indicate that asymbiotic nitrogen fixation is constrained by the relative availability and dynamical interaction of phosphorus and molybdenum.