Dynamics of deep soil carbon – insights from <sup>14</sup>C time series across a climatic gradient

Abstract

Quantitative constraints on soil organic matter (SOM) dynamics are essential for comprehensive understanding of the terrestrial carbon cycle. Deep soil carbon is of particular interest as it represents large stocks and its turnover times remain highly uncertain. In this study, SOM dynamics in both the top and deep soil across a climatic (average temperature ∼ 1-9 • C) gradient are determined using time-series (∼ 20 years) 14 C data from bulk soil and waterextractable organic carbon (WEOC). Analytical measurements reveal enrichment of bomb-derived radiocarbon in the deep soil layers on the bulk level during the last 2 decades. The WEOC pool is strongly enriched in bomb-derived carbon, indicating that it is a dynamic pool. Turnover time estimates of both the bulk and WEOC pool show that the latter cycles up to a magnitude faster than the former. The presence of bomb-derived carbon in the deep soil, as well as the rapidly turning WEOC pool across the climatic gradient, implies that there likely is a dynamic component of carbon in the deep soil. Precipitation and bedrock type appear to exert a stronger influence on soil C turnover time and stocks as compared to temperature.

Climatic and Geochemical Controls on Soil Carbon at the Continental Scale: Interactions and Thresholds

“…We also found that the relative importance of MAP‐PET in predicting SOC remained relatively constant with depth (Figure 6a), consistent with the finding of van der Voort et al. (2019). The relative importance of MAT as a predictor of SOC even increased with depth, contrasting with previous studies that did not include geochemical variables (Gray et al., 2016; Jobbágy & Jackson, 2000).…”

“…In addition, SOC at depth may be protected from biological attack because of spatial isolation from decomposers, decreased labile C supply, and/or differences in temperature or moisture relative to surface horizons (Schmidt et al., 2011). The relative importance of precipitation for predicting SOC may remain consistent (van der Voort et al., 2019) or decrease (Jobbágy & Jackson, 2000) with depth, while the importance of temperature often decreases with depth (Gray et al., 2016; Jobbágy & Jackson, 2000). However, geographically comprehensive research on controls over deep SOC versus surface SOC remains limited.…”

Millennial-age glycerol dialkyl glycerol tetraethers (GDGTs) in forested mineral soils: <sup>14</sup>C-based evidence for stabilization of microbial necromass

“…Long-term persistence of GDGTs could arise from their stabilization by soil minerals at greater soil depths. The amphiphilic nature of lipids, such as GDGTs with both polar and hydrophobic components, promotes the association with mineral surfaces and therefore may afford physical protection from degradation (Jandl et al, 2004;Kleber et al, 2007;von Lützow et al, 2008;Van der Voort et al, 2017). By comparison, in surface soils with high organic matter contents and less availability of reactive mineral surfaces, GDGTs are continuously produced and degraded, which results in a younger mean radiocarbon age and evidence for turnover on decadal timescales (Weijers et al, 2010).…”

Shale as a Source of Organic Carbon in Floodplain Sediments of a Mountainous Watershed

“…There are two primary mechanisms by which low Δ 14 C values could be observed in HF in our study: (1) presence of shale-derived OC and (2) unusually long aging of plant or microbial-derived OC through chemical alteration and mineral protection. Rock-derived OC has been implicated in low Δ 14 C values observed in areas where OC-rich (e.g., shale) bedrock is present, including in soils (Hemingway et al, 2018;Longbottom & Hockaday, 2019;van der Voort et al, 2019), lake sediments (Blattmann et al, 2019;Vonk et al, 2016), and riverine suspended particulate matter (Kao & Liu, 1996;Vonk et al, 2016). Gurwick et al (2008) observed 14 C ages of 10,650-17,050 years B.P.…”