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Storage and bioavailability of molybdenum in soils increased by organic matter complexation

Abstract

The micronutrient molybdenum is a necessary component of the nitrogen-fixing enzyme nitrogenase1,2. Molybdenum is very rare in soils, and is usually present in a highly soluble form, making it susceptible to leaching3,4. However, it is generally thought that molybdenum attaches to mineral surfaces in acidic soils; this would prevent its escape into the groundwater, but would also impede uptake by microbes3. Here we use X-ray spectroscopy to examine the chemical speciation of molybdenum in soil samples from forests in Arizona and New Jersey. We show that in the leaf litter layer, most of the molybdenum forms strong complexes with plant-derived tannins and tannin-like compounds; molybdenum binds to these organic ligands across a wide pH range. In deeper soils, molybdenum binds to both iron oxides and natural organic matter. We suggest that the molybdenum bound to organic matter can be captured by small complexing agents that are released by nitrogen-fixing bacteria; the molybdenum can then be incorporated into nitrogenase. We conclude that the binding of molybdenum to natural organic matter helps prevent leaching of molybdenum, and is thus a critical step in securing new nitrogen in terrestrial ecosystems.

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Figure 1: Mo distribution and modes of binding in unamended Arizona soils.
Figure 2: Modes of Mo binding in natural samples.
Figure 3: Bioavailability of Mo bound to tannic acid.
Figure 4: Terrestrial cycle of Mo.

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Acknowledgements

We are grateful to F. M. M. Morel for useful discussions and support throughout this work. We are indebted to P. E. Bishop for providing the mutant strain of A. vinelandii and to M. Chaffee (USGS) for sharing his maps of soil Mo concentrations in Arizona. We also thank S. Khalid and T. Lanzirotti at the National Synchrotron Light Source (Brookhaven) and J. Bargar at the Stanford Synchrotron Radiation Light (Menlo Park) for their help in X-ray measurements. The tree cartoon in Fig. 4 is a public domain artwork (www.cksinfo.com). This work was supported from the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry to T.W. B.M. was supported by NSF funded Stanford Environmental Molecular Science Institute.

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Correspondence to Bhoopesh Mishra.

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Wichard, T., Mishra, B., Myneni, S. et al. Storage and bioavailability of molybdenum in soils increased by organic matter complexation. Nature Geosci 2, 625–629 (2009). https://doi.org/10.1038/ngeo589

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