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

Abstract

<p><strong>Abstract.</strong> 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 rates 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) <sup>14</sup>C data from bulk soil and water-extractable organic carbon (WEOC). Analytical measurements reveal enrichment of bomb-derived radiocarbon in the deep soil layers on the bulk level during the last two decades. The WEOC pool is strongly enriched in bomb-derived carbon, indicating that it is a dynamic pool. We used a numerical model to determine turnover time of the bulk, slow and dynamic pool as well as the size of the dynamic pool. The presence of bomb-derived carbon in the deep soil, as well as the rapidly turning over WEOC pool and sizeable dynamic fraction at depth across the climatic gradient implies that there likely is a dynamic component of carbon in the deep soil. Precipitation appears to exert a stronger influence on soil C dynamics than temperature. Overall, geology seems to impact the carbon cycling in three key ways: (1) bedrock-derived (petrogenic) carbon can comprise an important component of the soil carbon pool even at relatively shallow depths (< 1 m). (2) Bedrock type influences water logging either by its porosity or by determining texture, and (3) rock and soil mineralogy controls C stabilization.</p>

14C characteristics of dissolved lignin along a forest soil profile

“…The ∆ 14 C values of bulk SOC showed clear evidence for the presence of 'bomb 14 C' (Trumbore, 2009), peaking at the bottom of the organic layer (~167‰ at -5 cm) and decreasing to -173‰ at 60 cm ( Fig. 2a), reflecting rapid accumulation of organic carbon at this site (van der Voort et al, 2018). In contrast to SOC, DOC displayed relatively constant ∆ 14 C values throughout the profile during the sampling period (74~108‰; Fig.…”

Geogenic organic carbon in terrestrial sediments and its contribution to total soil carbon

“…So far there is not much literature about sediments with only low amounts of OC. There are estimations that assume sandstones to be GOC free (van der Voort et al, 2018) or, in contrast, a storage model that assumes generally high GOC amounts of 2.4 g kg -1 for all sandy deposits (Copard et al, 2007). Therefore, more information about the amounts of OC in sediments is needed.…”