ALBERT

Description: Soil organic matter (SOM), including humic substances (HS), is redox-active, can be microbially reduced, and transfers electrons in an abiotic reaction to Fe(III) minerals thus serving as electron shuttle. The standard procedure to extract HS from soil and separate it into humic acids (HA) and fulvic acids (FA) involves alkaline and acidic solutions potentially leading to unwanted changes in SOM chemical and redox properties. To determine the effects of extraction conditions on the redox and electron shuttling properties of SOM extracts, we prepared HS and SOM extracts from a forest soil applying either a combination of 1 M NaOH and 6 M HCl, or water (pH 7). Both chemical extractions (NaOH/HCl) and water extractions were done in separate setups under either oxic or anoxic conditions. Furthermore, we applied to a subsample of the water-extracted-SOM the NaOH/HCl treatment. We found that soil extraction with NaOH lead to ca. 100 times more extracted C and the extracted HS had 2-3 times higher electron exchange capacities (EEC) than SOM extracted by water. For water-extracted SOM, anoxic extraction conditions lead to about 7 times more extracted C and 1.5 times higher EEC than under oxic extraction conditions. This difference was probably due to the occurrence of microbial reduction and dissolution of Fe(III) minerals in the soil during the water extraction at neutral pH and the concomitant release of Fe(III) mineral-bound organic matter. NaOH/HCl treatment of the water-extracted SOM lead to 2 times higher EEC values in the HA isolated from the SOM compared to the water-extracted SOM itself, suggesting the chemical treatment with NaOH and HCl caused changes of redox-active functional groups of the extracted organic compounds. Higher EEC of extracts in turn resulted in a higher stimulation of microbial Fe(III) mineral reduction by electron shuttling, i.e. faster initial Fe(III) reduction rates, and in most cases also in higher reduction extents. Our findings suggest that SOM extracted with water at neutral pH should be used to better reflect environmental SOM redox processes in lab experiments and that potential artefacts of the chemical extraction method and anoxic extraction condition need to be considered when evaluating and comparing abiotic and microbial SOM redox processes.