Molybdenum Bioavailability and Asymbiotic Nitrogen Fixation in Soils are Raised by Iron (Oxyhydr)oxide-Mediated Free Radical Production

Nitrogen (N) fixation in soils is closely linked to microbially mediated molybdenum (Mo) cycling. Therefore, elucidating the mechanisms and factors that affect Mo bioavailability is crucial for understanding N fixation. Here, we demonstrate that long-term (26 years) manure fertilization increased microbial diversity and content of short-range ordered iron (oxyhydr)oxides that raised Mo bioavailability (by 2.8 times) and storage (by ∼30%) and increased the abundance of nifH genes (by ∼14%) and nitrogenase activity (by ∼60%). Nanosized iron (oxyhydr)oxides (ferrihydrite, goethite, and hematite nanoparticles) play a dual role in soil Mo cycling: (i) in concert with microorganisms, they raise Mo bioavailability by catalyzing hydroxyl radical (HO^•) production via the Fenton reactions and (ii) they increase Mo retention by association with the nanosized iron (oxyhydr)oxides. In summary, long-term manure fertilization raised the stock and bioavailability of Mo (and probably also of other micronutrients) by increasing iron (oxyhydr)oxide reactivity and intensified asymbiotic N fixation through an increased abundance of nifH genes and nitrogenase activity. This work provides a strategy for increasing biological N fixation in agricultural ecosystems.