Tephra deposits and carbon dynamics in peatlands of a volcanic region: Lessons from the Hekla 4 eruption

Abstract

Interactions between tephra deposits from volcanic eruptions and peatland carbon (C) dynamics are poorly understood despite the significant extent of peatlands influenced by tephra worldwide. Tephra falls can affect peat accumulation within a radius of at least 1,000 km. In Iceland, volcanic activity is amongst the highest in the world and it might even increase due to pressure release on subglacial volcanoes. A potent combination of sparse vegetation, low cohesion of Andosols, and frequent strong winds, facilitates the regular input of mineral aeolian material from eroded areas into peatlands in Iceland, in addition to tephra deposits. We present results of a study on the impact of a major tephra deposit, the Hekla 4 tephra, on C dynamics in Icelandic peatlands. We investigated three sites at differing distances from the active volcanic zones and major erosion areas in the interior of the country. A combination of variables was applied, including laboratory C mineralization at 5, 15 and 25°C and C structure obtained by solid‐state 13C NMR spectroscopy. Tephra deposits seem to affect C dynamics. Differences in C mineralization can only be partially explained by C structure. The C mineralization from soils with conspicuously similar C structure differs by a factor of up to 2.7. Temperature sensitivity of decomposition after the tephra deposition increases by a factor of up to 2.2. Changes in inorganic matter and the ratio of soil organic matter to soil organic carbon by a factor of up to 1.3 and 1.4, respectively, were also observed.

Andic Soil Properties and Tephra Layers Hamper C Turnover in Icelandic Peatlands

“…Particularly at Hrafnabjörg and Tindar, lower A:O/N ratios, facilitated by a relative decrease of the more recalcitrant alkyl C and accumulation of the more labile O/N-alkyl C, resemble much younger and less decomposed soil layers, that is, the upper 0-10 cm at Hrafnabjörg and the upper 0-40 cm at Tindar (Figures 3a and 3c). Although an initial trend of increased decomposition with depth and a reversal to decreased decomposition at greater depth is consistent with findings of previous studies (e.g., Möckel et al, 2021;Preston et al, 1987), it is conspicuous that the reversal coincides with the Hekla 4 tephra layer. Here, other factors than stage of decomposition may contribute to the observed variations in C structure and the stability of the stored C, for example, the characteristics of the local vegetation serving as parent material of the Histosols.…”

“…The positive relation of Fe p and ferrihydrite predominantly with O/N‐alkyl C of 110–60 ppm indicates a preferential complexation of active Fe with O/N‐alkyl C, which is in agreement with previous studies (e.g., Miltner & Zech, 1998; Schöning et al., 2005). This, in turn, would support the hypothesis that stabilization of C is related to its association with mineral soil components rather than its chemical recalcitrance (e.g., Kögel‐Knabner & Kleber, 2011), which is also interesting in light of previous studies at the same peatlands (see Möckel et al., 2021). There, C stability as indicated by soil C respiration was greater at Hrafnabjörg than Tindar despite even slightly higher content of carbohydrates at Hrafnabjörg.…”

“…Through the greater part of the profiles, decomposition unsurprisingly increases with depth (see Malmer & Holm, 1984; Tfaily et al., 2014), with more recalcitrant C forms increasing in quantities toward deeper layers (e.g., Leifeld et al., 2012; Tfaily et al., 2014). This is indicated by a relative accumulation of alkyl C, a depletion of O/N‐alkyl C (particularly 110–60 ppm), and, hence, by increasing A:O/N ratios (Baldock et al., 1997; Möckel et al., 2021; Preston et al., 1989), and by overall decreasing (70–75)/(52–55) ratios (Bonanomi et al., 2013; Möckel et al., 2021). Beneath the Hekla 4 tephra deposits, this development is interrupted or even reversed (Figure 2).…”

“…A recent study by Möckel et al. (2021) provides evidence for the impact of major tephra layers on C mineralization of Icelandic Histosols and its temperature sensitivity without inducing a major shift in 13 C NMR spectra. This indicates that the stability of SOC of Histosols in volcanic regions is governed not only by the chemical structure of the SOC and hydrology (e.g., Kögel‐Knabner & Amelung, 2021) but also by other factors of the soil environment, not least soil mineral constituents and tephra deposits.…”

High-resolution stalagmite stratigraphy supports the Late Holocene tephrochronology of southernmost Patagonia

“…In order to identify such volcanic signals, we need to consider the direct primary elemental deposition from tephra, volcanic volatiles and leached components. In addition, there may be a secondary response to tephra deposition on peatland soils that potentially affects nutrient cycling 33 , chemical leaching 15, 16 , and soil C stocks 34, 35 . Pristine peatland ecosystems such as those of southern Patagonia may be particularly sensitive to such volcanic impacts with high tephra-loading and variable climate conditions 1, 36 .…”