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

The micronutrient molybdenum is a necessary component of the nitrogen-fixing enzyme nitrogenase(1,2). Molybdenum is very rare in soils, and is usually present in a highly soluble form, making it susceptible to leaching(3,4). 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 microbes(3). 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.