Subsurface settings in aquatic ecosystems display strong coupling between hydrologic conditions and the cycling of carbon, nitrogen, and other major elements as well as trace element micronutrients and contaminants. In many such systems, including wetland soils and the hyporheic zone of streambeds, anaerobic processes are responsible for a number of biogeochemical processes with substantial environmental impacts. These include servings as major sources of the greenhouse gases methane (CH₄) and nitrous oxide (N₂O) as well as being key locations of Hg methylation. The biogeochemistry of subsurface zones of aquatic systems has been widely explored from the perspective of redox conditions, substrate availability, and thermodynamic controls on metabolic processes. However, an additional yet under-examined constraint is the availability of trace element micronutrients. An array of metalloenzymes that contain trace elements in cofactors are essential to many metabolic pathways that underlie important biogeochemical processes. Pure culture studies have demonstrated inhibition of methanogenesis, denitrification, and Hg methylation when essential trace elements have low availability. However, similar studies of natural systems are rare, and it is unclear if trace element availability limits biogeochemical processes in the environment. We have investigated possible trace element limitations during methanogenesis in anaerobic peat and muck horizons from wetland soils from two U.S. states, Missouri and Florida. 40-day incubations of wetland soil from the Missouri site showed a 10-fold increase in CH₄ production following addition of Ni, which is required in all enzymatic pathways to CH₄. In contrast, soil from the Florida site showed no change in CH₄ production following the addition of Ni or other trace elements involved in the enzymatic pathways for methanogenesis. Surprisingly, the Florida wetland showed lower native dissolved and solid-phase trace element concentrations than the Missouri site yet produced more CH₄ in unamended systems. X-ray absorption spectroscopy (XAS) revealed that Ni added to the Missouri soil bound to reduced sulfur, possibly as a mixture of NiS and thiol-bound Ni, but formed carboxyl-bound species in the Florida site. This is consistent with the 100-times greater S content of the Missouri soil as well as a larger reduced S fraction. Speciation of the native Ni present at both sites appeared to be similar, occurring as mixtures of sulfur- and carboxyl-bound species, but XAS provided limited information because of the low solid-phase Ni contents (6-25 ppm) in the soils. X-ray microfluorescence imaging indicated that Ni was associated with both organic and inorganic sulfur species in the Missouri soil, but did not correlate with sulfur in the Florida soil. Together, these observations suggest that sulfur speciation and abundance strongly impact trace element availability and associated limitations on other biogeochemical processes in wetlands. This work highlights the critical need to understand the speciation of trace elements in soil and aquatic systems in order to predict their impact on their environment, a key theme of Prof. Spark's long research career. It also demonstrates the challenges posed to determining trace element speciation in natural materials at low concentrations, necessitating novel approaches and further advances in analytical instrumentation.