It is typically assumed that shortages of the element phosphorus (P) limit plant growth in tropical forests. This is thought to occur because while P is relatively rare in such soils, it is a critical constituent of many organic compounds, particularly those responsible for making nitrogen (N) available to plants. Global biogeochemical analyses and models frequently incorporate this assumption.

A second element is necessary for this N-availability step, however: molybdenum (Mo). In this study Wurzberger et al., building on previous work, test the hypothesis that landscape-scale variations in P and Mo availability can limit N fixation. They do so by sampling and manipulating soils from a naturally-occurring gradient of P availability.

The authors found that when P was abundant, Mo alone limited N fixation; conversely, in P-poor soils, both P and Mo were limiting, and N fixation did not respond to either single element. Mo was particularly scarce in the upper soil horizons where N-fixing organisms are most active. Why does this scarcity occur? Wurzberger et al. performed additional experiments suggesting that interactions of Mo with soil organic matter reduce its availability. This occurs, they suggest, because Mo is strongly bound by certain organic compounds (polyphenols), whereas plants can access P much more readily.

A final unusual aspect of this study is that the authors suggest a model linking landscape-level forest dynamics with molecular biochemistry (i.e., Mo and P availability and effects). This model contextualizes their results, provides testable hypotheses for future research, and explains how the influence of Mo can extend across a wide range of soil P levels.

Soil fertility provides a strong check on forest response to changing climate, and this elegant study by Wurzberger et al. will force scientists to re-think how fertility, nitrogen, and other trace elements interact in tropical—and potentially global—forests. There are clearly significant surprises still in store for us.