Characterization of soil-derived dissolved organic matter from permafrost in northern Alaska

Abstract:

The fate and transport of soil organic carbon accumulated in Arctic terrestrial ecosystems have received increasing attention due to recent significant changes in climate and the environment. However, end-member soil organic carbon from permafrost with different organic carbon contents, C-14 ages, and degradation states remain poorly characterized, especially for soil-derived dissolved organic matter (DOM) which is the active component in carbon cycling. Soil samples collected from permafrost in northern Alaska were used to determine the yield of dissolved organic carbon (DOC) from the bulk soil organic carbon (SOC) pool. The soil-derived DOM was further characterized for organic composition using fluorescence excitation-emission matrices (EEMs) coupled with PARAFAC analysis and for DOM size distribution using flow field-flow fractionation techniques. Degradation of soil-derived DOM was also carried out to examine the degradation rate and changes in DOM composition, size spectra and other optical properties. The soil DOC yield was generally low, ranging from ~1-5% of the total SOC, and was correlated to bulk SOC contents and C-14 age, with higher DOC yields in older soil. High SUVA (at 254 nm) values were measured for soil DOM samples, ranging from 8 to 10 mg-C/L/m. Major fluorescent DOM components include terrestrial fulvic-like, protein-like, and humic-like DOM with their Ex/Em at 245/444, <240/376, and 275/508 nm, respectively. Humic-like DOM was partitioned mostly in the <3-6 nm size range, while protein-like DOM partitioned in a much wider size spectrum. However, protein-like DOM in old soil became mostly in the small colloidal size range (<5-8 nm), indicating the preferential degradation of large MW protein compounds. Humic components seem to degrade preferentially compared to protein-like DOM components, resulting in a relatively higher percentage of protein-like DOM in old soil samples. Overall, different soil DOM components may have their distinct fate and degradation pathways during their interaction with aquatic environments in a changing climate.