Yields and Characterization of Dissolved Organic Matter From Different Aged Soils in Northern Alaska

Soil organic matter (SOM) in northern high-latitude regions is a major component of the global carbon cycle. However, the yield of soil-dissolved organic matter (DOM) during soil-water interactions and its chemical characteristics and reactivities remain poorly understood. We report here elemental composition and isotopic signatures of bulk-SOM from northern Alaska, and yields of water-leachable soil-DOM, including dissolved organic carbon, dissolved organic nitrogen, and dissolved organic phosphorus, and nutrients (dissolved inorganic nitrogen and phosphate). The bulk-SOM contained 5,400–373,900 μg-C/g-soil, 500–12,610 μg-N/g-soil, and 38.6–384 μg-P/g-soil, with a C/N ratio of 13–37 and a C/P ratio of 80–3,911. The water-extractable dissolved organic carbon, dissolved organic nitrogen, and dissolved organic phosphorus only accounted for 0.58–2.5%, 0.91–3.4%, and 1.2–3.0% of their corresponding total soil-OC, N, and OP, respectively. While SOM-δ¹³C was less variable (−27.08‰ to −26.49‰) showing the same overall C source, ¹⁴C-ages varied widely from 1,170 ± 20 to 16,200 ± 55 years before present, indicating different deposition-conditions and subsequent-processes among the soil samples. The specific-UV-absorbance at 254 nm (or aromaticity) of soil-DOM differed considerably and was negatively correlated to spectral slope values. A humic-like (C1), a low-molecular-weight fulvic-like (C2), and a protein-like component (C3) were identified as the major fluorescent-DOM components. The C3/C2 and C3/C1 ratios generally increased with SOM-¹⁴C-age, suggesting a preferential degradation/transformation of humic-like and fulvic-like components or a transformation/production of protein-like components within permafrost. Both biological and humification indices increased with ¹⁴C-age, excluding the oldest soil, which pointed to a preferential preservation or transformation/production of protein-like DOM within permafrost, implying that soil-DOM derived from old permafrost can be highly vulnerable and readily decomposed upon permafrost thaw.