Geologically-derived nutrients such as phosphorus (P) and molybdenum (Mo) play an important role in tropical forests. Nutrients may be critical in supporting tropical forest productivity and carbon sinks as well as their future stability in the face of rapid global change. In addition, nutrient availability may affect important biogeochemical processes such as nitrogen (N2) fixation. However, our understanding of the mechanisms that control nutrient accessibility of Mo and P to plants is limited. In this study we evaluate several mechanisms that may control the availability: i) weathering rate and soil age, which control total concentrations of Mo and P; ii) soil properties such as clay and iron-rich minerals, which affect the local availability and ecosystem loss of Mo and P; and iii) potential differences between Mo and P binding to organic carbon. We measured P and Mo concentrations (total and bio-available) in soil profile samples from 23 sites across the Amazon Basin that varied in soil development and soil properties (clay, organic carbon, and pH). To see how soil nutrient availability scales up to the forest community, we measured foliar P and Mo as well as Mo isotopes of leaf tissue from N2-fixing and non-N2 fixing trees at each site.
Results/Conclusions
We found that weathering over time controlled the total P concentrations but did not explain total Mo concentrations. Soil total Mo concentrations did correspond with soil properties. Soils with higher soil clay content and soil organic matter had higher total Mo concentrations but lower bio-available Mo concentrations, supporting the hypothesis that soil carbon plays an important role in controlling ecosystem Mo. High soil pH also was positively correlated with higher Mo availability. We also found that foliar Mo concentrations significantly differed across soil type and correlated with soil available Mo concentrations. Finally, we will present some of the first soil and foliar Mo isotopic measurements from the Amazon Basin. These findings may help us understand how soil nutrient constraints are translated into forest nutrient limitation and help us to identify areas of the Amazon where N2-fixers are most likely to experience P and Mo limitation in the future.