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An Elegant Cycle: Molybdenum’s Availability in Soil Deciphered

Researchers from Princeton University working at NSLS have solved the mystery of how molybdenum, one of the elements needed to prepare nitrogen for plant consumption, is available in soil.

Nitrogen is key to cellular growth and the functioning of proteins and nucleic acids in plants and other organisms. Pure nitrogen molecules are plentiful in the atmosphere, but their atmospheric form is not usable by most organisms. Nature’s solution is a process called nitrogen fixation, in which nitrogen is combined with hydrogen or oxygen to create a new molecule that can be used by various organisms.

Molybdenum is one of the elements needed to jump-start nitrogen fixation. Until now, scientists haven’t entirely understood how molybdenum is present and available in the soil to become a catalyst.

To solve this mystery, Princeton scientists collected samples from molybdenum-rich Arizona soil and a temperate hardwood forest in New Jersey.

After some initial troubleshooting at the Stanford Synchrotron Radiation Lightsource, they brought their samples to NSLS to map out, on the micro-scale, how molybedenum is distributed and in what form, as well as how it adheres to other minerals.

Consistent with past observations, molybdenum was found to stick to the surfaces of iron oxides. But the scientists also found strong evidence of molybdenum binding to natural organic matter, particularly in the upper soil layers, rich in leaf-litter and free-living nitrogen-fixing bacteria.

This finding suggests a molybdenum cycle: molybdenum in the soil is taken up by tree roots and deposited in leaves. These leaves fall to the ground and decompose, releasing molybdenum, which binds to tannins and tannin compounds in soil.

To understand how molybdenum transforms into a catalyst for nitrogen fixation, the researchers studied the nitrogen-fixing bacteria in soil with tannin-bound molybdenum. They found that the bacteria excrete a compound that strongly attracts iron and other metals. Molybdenum, held in the upper soil by tannin, binds to this compound, which can outcompete the tannin in attracting molybdenum. This new molybdenum compound is one that nitrogen-fixing bacteria can access to create the enzyme responsible for nitrogen fixation.

In turn, fixed nitrogen will penetrate the soil, be taken up by the root system of plants and promote plant growth.

T. Wichard, B. Mishra, S.C. Myneni, J-P Bellenger, A.M.L. Kraepiel, “Storage and Bioavailability of Molybdenum in Soils Increased by Organic Matter Complexation,” Nature Geoscience, 2, 625-629 (2009).

Top: The molybdenum (Mo) cycle in soil: Mo present in deep soil is extracted by the root network of trees and incorporated into leaves. These leaves fall to the ground and decompose, providing a Mo-enriched environment for nitrogen-fixing bacteria living in the upper soil. Binding of Mo to leaf organic matter reduces Mo leaching rates from the soil, keeping Mo in the soil environment where it can be used by the bacteria. In turn, the new nitrogen fixed by the bacteria fertilizes tree growth, resulting in a classic mutualistic relationship.

Bottom: Corresponding author Bhoopesh Mishra (now at Argonne National Laboratory), who conducted this study as a post-doc at Princeton University

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