Chemical composition of dissolved organic matter draining permafrost soils

Abstract

Please cite this article as: Ward, C.P., Cory, R.M., Chemical composition of dissolved organic matter draining permafrost soils, Geochimica et Cosmochimica Acta (2015), doi: http://dx. AbstractNorthern circumpolar permafrost soils contain roughly twice the amount of carbon stored in the atmosphere today, but the majority of this soil organic carbon is perennially frozen.Climate warming in the Arctic is thawing permafrost soils and mobilizing previously frozen dissolved organic matter (DOM) from deeper soil layers to nearby surface waters. Previous studies have reported that ancient DOM draining deeper layers of permafrost soils was more susceptible to degradation by aquatic bacteria compared to modern DOM draining the shallow active layer of permafrost soils, and have suggested that DOM chemical composition may be an important control for the lability of DOM to bacterial degradation. However, the compositional features that distinguish DOM drained from different depths in permafrost soils are poorly characterized. Thus, the objective of this study was to characterize the chemical composition of DOM drained from different depths in permafrost soils, and relate these compositional differences to its susceptibility to biological degradation. DOM was leached from the shallow organic mat and the deeper permafrost layer of soils within the Imnavait Creek watershed on the North Slope of Alaska. DOM draining both soil layers was characterized in triplicate by coupling ultra-high resolution mass spectrometry, 13 C solid-state NMR, and optical spectroscopy methods with multi-variate statistical analyses. Reproducibility of replicate mass spectra was high, and compositional differences resulting from interfering species or isolation effects were significantly smaller than differences between DOM drained from each soil layer. All analyses indicated that DOM leached from the shallower organic mat contained higher molecular weight, more oxidized, and more unsaturated aromatic species compared to DOM leached from the deeper permafrost layer. Bacterial production rates and bacterial efficiencies were significantly higher for permafrost compared to organic mat DOM; however, respiration rates were similar 3 between DOM sources. Increased release of permafrost DOM from arctic soils to surface waters will change the chemical composition of DOM and its lability to bacteria, but this study suggests that these shifts in DOM composition and lability may not increase the carbon dioxide produced by bacterial respiration of permafrost DOM exported to arctic surface waters compared to DOM currently draining the shallow active layer.4

Dissolved organic carbon and nitrogen release from boreal Holocene permafrost and seasonally frozen soils of Alaska

“…The one study we identified measured greater concentrations of nitrate, ammonia, and DOC in leachates of permafrost soils compared to active-layer soils in cores collected near Barrow, Alaska (Reyes and Lougheed 2015), similar to our results. Differences in DOM optical properties between activelayer and permafrost soils collected from sites in arctic Alaska (Ward and Cory 2015) and northern Finland (Selvam et al 2017) are consistent with our observations of greater proportions of LMW DOM, a more 'microbial' FI signature, and lower DOM humification in permafrost soils. While our experiment focused on initial release upon thaw, boreal soils with high C and N content continue to release DOC and DON with multiple leaching events over long periods Hooper 2002, Kim et al 2014), suggesting that our short-term yields are conservative.…”

“…Studies of soil leachates in circumpolar regions have focused primarily on potential DOC release from active-layer soils (Neff and Hooper 2002, Michaelson and Ping 2003, Guo et al 2007, Vonk et al 2015, O'Donnell et al 2016 and from deep Pleistocene yedoma soils (Dutta et al 2006, Drake et al 2015, Ewing et al 2015. Only a small number of studies have compared solute leachate chemistry and/or biodegradability from paired active-layer and shallow permafrost soils (Ward and Cory 2015, Reyes and Lougheed 2015, Selvam et al 2017 and those studies are limited to soils collected from a single location.…”

Dissolved Organic Matter Chemistry and Transport Along an Arctic Tundra Hillslope

“…On 8 and 12 August 2016, we collected pore waters for detailed chemical analysis into sterile 60-ml amber borosilicate bottles, placed each bottle on ice in the field, froze them within 2 hr of collection and transported them to Colorado State University for analysis (described below). We feel our EEMS results are robust because (1) SUVA 254 values (<3.0 L·mg-C −1 ·m −1 ) were below the threshold where significant alterations have previously been found , (2) DOM chemical composition and aromatic characteristics are similar to previously reported values for unfrozen samples collected from the same sampling location (Imnavait watershed, Ward & Cory, 2015) and from other active layer soil horizons (low Arctic, Waldrop et al, 2010), (3) DOC concentrations are within range of groundwater samples collected the year prior from the same watershed (1,500-2,000 μM vs. 300-2,100 μM, respectively, Neilson et al, 2018) and DOC concentrations in unfrozen soil-water samples (Waldrop et al, 2010;Ward & Cory, 2015), (4) our NMR results support those found using EEMS, and (5) we observed no evidence of flocculation or production of brown particles in thawed samples, which is characteristic of abiotic particle formation and OM precipitation Giesy & Briese, 1978). More specifically, freeze-thaw dynamics may alter fluorescent DOM composition and concentration (>10%) when sample aromaticity is high (SUVA 254 > 3.5 L·mg-C −1 ·m −1 ; Fellman, D'Amore, & Hood, 2008; Pokrovsky et al, 2018), although evidence is mixed (Coble et al, 2014;Otero et al, 2007;Spencer et al, 2010).…”

Dissolved organic carbon and major and trace elements in peat porewater of sporadic, discontinuous, and continuous permafrost zones of western Siberia

“…The change of SUVA from 2.4 to 3.4 in hollows demonstrates a significant shift in the composition of the DOM and may have a pronounced effect upon the biogeochemical processing of DOM upon export, as has been recently shown in eastern Siberia . This contradicts the conclusion reached in recent studies of surface waters and soil leachates that the DOM leached from the permafrost soil layer has a consistently lower concentration of aromatic carbon (i.e., lower SUVA 254 values; Mann et al, 2012;Cory et al, 2013Cory et al, , 2014Abbott et al, 2014;Ward and Cory, 2015), compared to DOM draining from the active, organic surface layer. However, the majority of previous studies dealt with non-peat permafrost environment.…”