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

Abstract

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-δ 13 C was less variable (À27.08‰ to À26.49‰) showing the same overall C source, 14 C-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-14 C-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 14 C-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.Plain Language Summary The yields of soil dissolved organic matter (DOM) and nutrients (N and P) from different aged soils in northern Alaska were quantified in the laboratory mimicking soil-water interactions upon permafrost thaw. The overall fluxes of soil-DOM and nutrients can significantly influence carbon and nutrient cycle and the ecosystem as a whole in Arctic waterways and coastal environment although the yields were generally within 4% of their corresponding total mass in the bulk soil. Humic-like substances with high optical reactivity were the major organic components preserved in permafrost, but the relative percentage of protein-like components having high biological activity increased with increasing soil C-14 age. This implies that soil-DOM released from old-permafrost thaw can be highly vulnerable and readily decomposed, causing a positive feedback to climate change.

Aerobic release and biodegradation of dissolved organic matter from frozen peat: Effects of temperature and heterotrophic bacteria

“…It is thus possible that the frozen peat used in our study releases only labile OC at 4 and 25°C and that the excess of DOC observed at 45°C corresponds to the recalcitrant OM of peat. It should be further noted that in our study, the substrate-normalized DOC yields of frozen peat (~5 mg g − 1 peat) were substantially higher than the values obtained for organic soil samples of Alaska (0.14-2.2 mg g − 1 of soil, Gao et al, 2018).…”

Humic surface waters of frozen peat bogs (permafrost zone) are highly resistant to bio- and photodegradation

“…Mechanistic reasons for the extremely low bioavailability of DOC from studied peatlands remain unclear and require an in-depth analysis of the DOM molecular structure and stoichiometry as well as high-resolution microbial approaches. It is known that the DOM released from frozen soils contains high proportions of biologically labile proteinlike and photochemically reactive aromatic substances (Gao et al, 2018). Following the pioneering study of Ward et al (2017), we hypothesize that, similar to DOC from the organic (non-permafrost) layer, the concentration of highly labile, aliphatic-like DOC in surface waters of frozen peatlands is too low to sustain microbial populations, or that this aquatic DOC, remaining after the microbial processing of soil pore-water DOC, is of low lability for microbes capable of degrading aliphatic-like DOC and inhabiting the aerobic zone of permafrost surface waters.…”

Humic surface waters of frozen peat bogs (permafrost zone) are highly resistant to bio- and photodegradation

“…Mechanistic reasons for extremely low bioavailability of DOC from studied peatlands remain unclear and require in-depth analysis of DOM molecular structure and stoichiometry as well as high resolution microbial approaches. It is known that the DOM released from frozen soils contains high proportions of biologically labile protein-like and photochemically reactive aromatic substances (Gao et al, 2018). Following the pioneering study of Ward et al 2017 associated with an increase in SUVA by up to 7.4 %, which also implies an increase in the proportion of aromatic compounds (Hulatt et al, 2014).…”