Molybdenum bioavailability in agricultural soils with implications to asymbiotic nitrogen fixation

Guanghui Yu^a,b, and F.S. Sun^a,b

^a Institute of Surface-Earth System Science, Tianjin University,Tianjin 300072, China

^b College of Resources & Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China

yuguanghui@tju.edu.cn

Interactions between microbes and minerals have the potential to contribute significantly to the global cycles of various elements, and serve as a link between the geosphere and life. In particular, the microbially mediated cycle of molybdenum (Mo) within soils is closely tied to the nitrogen cycle. Here, soil samples under four fertilization treatments (Control, no fertilization; NPK, chemical nitrogen, phosphorus and potassium fertilization; NPKM, NPK plus manure; and M, manure alone) from a 25-year long-term field experiment sites in China were examined. Both two-dimensional correlation spectroscopy (2DCOS) and synchrotron-radiation-based micro X-ray fluorescence (µ-XRF) spectromicroscopy were used to explore the binding site of Mo in soils. The results showed that both NPKM and M increased bioavailable Mo over 1.7-fold when compared to Control and NPK treatments. In the soil profile (0-80 cm), both NPKM and M also had a higher bioavailable Mo than Control and NPK treatments. Furthermore, bioavailable Mo was positively correlated with δ¹⁵ N and wheat yield, sugggesting a potential contribution of bioavailable Mo to nitrogen uptake by wheat. Quantification of nifH genes in response to long-term fertilization treatments indicated that both NPKM and M improved the abundance of nifH genes when compared to NPK treatment.DithioniteCitrate-Bicarbonate (DCB) and acid ammonium oxalate extractions suggested a significant increase of reactive iron. Both 2DCOS and µ-XRF showed that iron oxides played an important role in sequestering bioavailable Mo. Together, these results indicate that manure amendments improve the abailability of Mo, which may be retained by reactive iron in agricultural soils.