Lifting the Humic Veil: A Novel Approach to Quantitating Occluded Iron in Peat Porewater

Anaerobic conditions common in peatlands lead microbes to reduce alternative electron acceptors for respiration in the order of their energetic yield: nitrate, manganese, ferric iron (Fe[III]), sulfate, and CO2. Many studies of peatland respiration report high CO2 fluxes that cannot be completely explained by measured pathways of microbial metabolism: it appears that an unquantified pool of electron acceptors is driving respiration and the efflux of CO2. The notion that undetected iron reduction may be an important contributing factor to these fluxes has been proposed, but has yet to be addressed. Among the possible causes for this anomaly is the widespread use of colorimetric assays for iron quantitation and speciation. Peat porewater contains high concentrations of dissolved humic and fulvic acids that occlude Fe(III), blocking colorimetric complex formation, potentially leading to an underestimate of Fe(III) in these challenging analytical matrices. We evaluated a novel ionic liquid extraction method in a variety of temperate peatland porewater samples and compared these results to data acquired simultaneously using colorimetric methods. We found a >50% average improvement in accuracy for total iron quantitation using ionic liquid extraction. Moreover, ionic liquid extraction of peat porewater consistently recovered Fe(III) whereas colorimetric methods detected 50-100% less, suggesting that DOM bound Fe(III) is not reliably detected using colorimetric assays in these systems and may represent a previously unidentified pool of electron acceptors in peat porewater.