Modeling organic solutes in peatland soils using acid analogs

Dissolved organic carbon (DOC) dominates the chemistry of porewaters in many peatland soils. Yet modeling its contribution to acid-base chemistry and to anionic charge has not been attempted for peatland porewaters. Using 3 yr of field data, we investigated the effectiveness of four analog organic acid models in describing the acid-base chemistry of DOC in porewaters from two acidic (mean pH ≃ 4.0-4.2), high-DOC (mean DOC ≃ 4.2-5.4 mmol (C L^-1) peatlands in southern Michigan. Although developed for dilute lake waters, we applied these models to porewaters of peatland soils. All sites exhibited a large deficit in anionic charge (mean = 229 μmol(c) L^-1), which was strongly correlated with DOC (r² = 0.82) and accounted for 70 to 80% of the total anionic charge. We, therefore, used charge balance to evaluate the models. The four models varied in effectiveness; variations in model organic anion accounted for between 50 and 84% of the variation in anion deficit. Representing organic acid anions with a triprotic acid analog model yielded the best results (r² = 0.84, slope = 0.96), while a monoprotic acid model was least effective (r² = 0.50, slope = 0.91). Mean charge densities of the DOC pool (44-52 μmol(c) mmol^-1 DOC) varied little with differences in site characteristics (i.e., water levels and emergent plant communities) and were consistent with other studies. The triprotic acid model appears to be an effective analog for the acid-base and ionic charge-contributions of DOC in porewaters of acidic peatland soils.