Importance of lateral flux and its percolation depth on organic carbon export in Arctic tundra soil: Implications from a soil leaching experiment

Abstract

Temperature rise in the Arctic is causing deepening of active layers and resulting in the mobilization of deep permafrost dissolved organic matter (DOM). However, the mechanisms of DOM mobilization from Arctic soils, especially upper soil horizons which are drained most frequently through a year, are poorly understood. Here we conducted a short‐term leaching experiment on surface and deep organic active layer soils, from the Yukon River basin, to examine the effects of DOM transport on bulk and molecular characteristics. Our data showed a net release of DOM from surface soils equal to an average of 5% of soil carbon. Conversely, deep soils percolated with surface leachates retained up to 27% of bulk DOM while releasing fluorescent components (up to 107%), indicating selective release of aromatic components (e.g., lignin and tannin), while retaining nonchromophoric components, as supported by spectrofluorometric and ultrahigh‐resolution mass spectroscopic techniques. Our findings highlight the importance of the lateral flux of DOM on ecosystem carbon balance as well as processing of DOM transport through organic active layer soils en route to rivers and streams. This work also suggests the potential role of leachate export as an important mechanism of C losses from Arctic soils, in comparison with the more traditional pathway from soil to atmosphere in a warming Arctic.

Dissolved Organic Matter Chemistry and Transport Along an Arctic Tundra Hillslope

“…During microbial decomposition, plant-derived molecules are broken into smaller compounds, increasing the relative proportion of polar and ionizable groups (Lehmann & Kleber, 2015). We also observed greater contributions of NMR region I-hydrophobic, aliphatic species primarily related to methylene or methyl-bearing protons-in the organic horizon, and high concentrations of chromophoric DOM (cDOM) at depth, suggesting these exchange mechanisms directly fractionate DOM across the soil profile (Zhang et al, 2017). As a result, the distribution of DOM at depth may be partially independent of preferential flow paths and contribute to incomplete pore water mixing (Neilson et al, 2018).…”

“…As a result, the distribution of DOM at depth may be partially independent of preferential flow paths and contribute to incomplete pore water mixing (Neilson et al, 2018). We also observed greater contributions of NMR region I-hydrophobic, aliphatic species primarily related to methylene or methyl-bearing protons-in the organic horizon, and high concentrations of chromophoric DOM (cDOM) at depth, suggesting these exchange mechanisms directly fractionate DOM across the soil profile (Zhang et al, 2017). The soil profile may thus act as a dynamic chromatograph (Kaiser & Kalbitz, 2012), where compounds are selectively retained or leached depending on their solubility, association with the soil mineral matrix, and chemical reactivity (Fellman, Amore, Hood, & Boone, 2008;Lehmann & Kleber, 2015).…”

“…During microbial decomposition, plant-derived molecules are broken into smaller compounds, increasing the relative proportion of polar and ionizable groups (Lehmann & Kleber, 2015). These fragments are more soluble in water and can be preferentially leached from organic to mineral soil horizons (Zhang et al, 2017), where they may accumulate at depth if isolated within the mineral matrix. As a result, the distribution of DOM at depth may be partially independent of preferential flow paths and contribute to incomplete pore water mixing (Neilson et al, 2018).…”

Permafrost Organic Carbon Mobilization From the Watershed to the Colville River Delta: Evidence From 14C Ramped Pyrolysis and Lignin Biomarkers

“…Numerous studies have focused on the dynamics of permafrost degradation in Arctic soils (Payette et al, 2004;Schädel et al, 2016;Schuur et al, 2009;Zimov et al, 2006). As for the transport of permafrost-derived OC by river systems, most research has focused on the dissolved organic carbon (DOC) pool (Frey & McClelland, 2009;Spencer et al, 2015;Striegl et al, 2007;Zhang et al, 2017). Because of the high lability of permafrost-derived DOC, much of the DOC in river headwaters that is derived from watershed soils is consumed by microbes and photodegradation before reaching the coast (Cory et al, 2014;Mann et al, 2015;Vonk et al, 2013).…”

Optical and Molecular Signatures of Dissolved Organic Matter Reflect Anthropogenic Influence in a Coastal River, Northeast China

“…However, these methods can only detect chromophoric and fluorescent DOM (Coble, 1996; Stedmon et al, 2003), whereas a large portion of DOM remains poorly characterized due to its extreme complexity and the limitation of optical techniques for unraveling the molecular information in DOM. In recent years, the electron spray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT‐ICR MS) has been applied to give a better understanding of the molecular composition of DOM in various environments such as rivers, lakes, soils, and oceans (Sleighter and Hatcher, 2008; Hansell and Carlson, 2014; Stubbins et al, 2014; Kellerman et al, 2015; Wagner et al, 2015b; Zhang et al, 2017). Molecular composition is an essential factor for assessing sources of DOM and its biological and photochemical lability (Hedges et al, 1997; Nebbioso and Piccolo, 2013; Hansell and Carlson, 2014; D'Andrilli et al, 2015).…”