Extraction efficiency and molecular characterization of organic matter from soils and sediments using high resolution mass spectrometry

Soil organic matter (SOM) represents a key reservoir for carbon (C) and is comprised of a heterogeneous mixture of plant, animal and microbial detritus. To study SOM and gain a better understanding of microbial communities, rhizosphere interactions, and biogeochemical processes, protocols have been developed using different solvents to extract this OM. Most of these extraction protocols use an aqueous solvent to extract the water soluble OM from the soil. In this study, we calculate the extraction efficiency and compare the molecular composition of the soil after repeated extractions with water. Three soil types; peat, Alaska permafrost, and river sediment, with varying percentages of organic C content were extracted sequentially with water up to 15 times. The organic matter in the extracts was characterized at the molecular level using high resolution mass spectrometry. We found that repeated extractions with smaller volumes of water extracts a higher percentage of dissolved organic carbon (DOC) compared to using one large volume of water. The DOC concentration measured in soil water extracts decreased with extraction number, regardless of soil type. Interestingly, the repeated sequential extractions were able to extract only 4% of total SOM regardless of soil type. These results are important as they suggest that water extractable organic matter (WEOM) accounts for only a small proportion of the total organic matter in the soil. However, even with this small percentage of WEOM, it is very diverse in chemical composition, with thousands of compounds resolved by high resolution mass spectrometry.