Laura T. Rea

Ice thaw and enhanced bedrock weathering are increasing sulfate export in alpine streams, which may change sulfur (S) and other biogeochemical cycles in adjacent wetlands. We compared S and carbon (C) concentrations and sulfate reduction rates (SRRs) across three wetland types in the Colorado Rocky Mountains, USA: snowmelt‐fed wetlands (SFWs), periglacial solifluction lobes (PSLs), and subalpine wetlands (SAWs). We found that each wetland type had unique biogeochemical characteristics. Subalpine wetlands had the highest soil C (37.2 ± 8.7%C) and SRRs (29.3 ± 21 nmol mL⁻¹ soil day ⁻¹) compared with SFWs and PSLs, which had lower %C and moderate to low SRRs, respectively. Subalpine wetlands accumulated little sulfate, whereas PSLs had high concentrations (0.04 ± 0.04 vs. 0.6 ± 1.4 mg S g⁻¹ dry soil respectively); SFWs had low sulfate concentrations (0.02 ± 0.01 mg S g⁻¹ dry soil). Sulfate‐S stable isotope data suggest different sources of S in the SFWs and PSLs: atmospheric and geologic, respectively. The data indicate that high C supports high SRRs in SAWs, whereas SRRs may be C‐limited and co‐limited by C and S in PSLs and SFWs, respectively. With climate warming, SAWs have the greatest potential to release more C to the atmosphere, SFWs will likely decrease in size and experience changes in plant community composition, and PSLs may be sources of acid rock drainage. These data demonstrate different biogeochemical fates of S and C in three wetland types present across alpine landscapes, and notable consequences for biogeochemical cycling as warming continues.

Trace metal ion speciation in natural waters is often under kinetic control due to slow exchange reactions involving multidentate ligands (both natural and anthropogenic) and constituent ions (e.g. calcium). Incomplete understanding of the kinetic behavior (rates, rate laws, and mechanisms) of multidentate ligand exchange reactions hinders prediction of metal ion bioavailability and mobility in these systems. Here, we aim to improve understanding (1) by examining the mechanism by which calcium suppresses ligand exchange rates and (2) by developing conceptual tools for predicting the kinetic behavior in environmental systems. Using capillary electrophoresis, we examined the effect of calcium concentration on the kinetics of a model disjunctive multidentate ligand exchange reaction between nickel(II)-nitrilotriacetate (NiNTA) and trans-1,2-cyclohexylenedinitrilotetraacetate (CDTA) at constant temperature (25 °C) and ionic strength (10 mM). Initial NiCDTA formation rates under varying reactant and calcium concentrations were fit with a single disjunctive ligand exchange kinetic model. While the overall reaction pathway is disjunctive under all conditions studied, the presence of calcium causes a non-linear decrease in reaction rates and alters the overall reaction order with respect to free multidentate ligand concentration. The overall rate law is affected by the relative calcium affinity of exchanging ligands, which controls the balance of product formation and reactant reformation rates from the free metal ion intermediate. The log ratio of these rates is proposed as a metric for determining the appropriate simplified form of the rate law for disjunctive ligand exchange reactions. The implications for in situ ligand lability assays and metal ion bioavailability are discussed.