Background

The transition metal molybdenum (Mo) is an essential micronutrient for many organisms (Williams and Frausto de Silva 2002). Molybdenum serves as a cofactor of the nitrogenase enzymes responsible for nitrogen (N) fixation in bacteria and of the nitrate reductase enzymes responsible for nitrate assimilation in plants (Galloway et al. 2004). Molybdenum-dependent N fixation therefore represents a principal natural input of N to ecosystems. Molybdenum limitation is common and has been shown to constrain the ability of some forests to secure new N, thereby limiting ecosystem productivity (Wurzberger et al. 2012). Molybdenum is present at varying levels in the Earth’s crust, in the soils formed as a result of mineral weathering of that crust, and in the communities of organisms that interact with those soils. Molybdenum is inaccessible to plants when bound up in its various mineral forms and becomes available as it is released to the soil solution through chemical weathering processes (Gupta 1997). At soil pHs above 4.23, Mo speciation is dominated by the molybdate anion (MoO4 2-), the ionic form most readily available for plant uptake but also most susceptible to soil leaching due to its high solubility (Reddy and Gloss 1993). Partitioning of Mo among various biotic and abiotic constituents in terrestrial forest systems, and mechanisms of Mo transport within these environments, have been widely studied (Gupta 1997, Wichard et al. 2009). Nutrient dynamics within any particular system are greatly influenced by the chemical characteristics of local substrates and the specific compositions of plant, animal, and microbial communities present (Reddy et al. 1997, Smith et al. 1997), and large between-site variability often exists with respect to nutrient availability. Previous work (Barron et al. 2009, Wichard e