Synthesizing the Use of Carbon Isotope (14C and 13C) Approaches to Understand Rates and Pathways for Permafrost C Mobilization and Mineralization

Abstract:

To better understand the permafrost carbon (C) feedback it is important to synthesize our current knowledge, and knowledge gaps, of how permafrost thaw can cause in situ mineralization or downstream mobilization of aged soil organic carbon (SOC) and the rate of this release. This potential loss of old SOC may occur via gaseous flux of CO₂ and CH₄ exchanged between soil and the atmosphere and via waterborne flux as DOC, POC (and their subsequent decomposition and release to the atmosphere). Carbon isotope (¹⁴C and ¹³C) approaches have been used to estimate both rates and pathways for permafrost C mobilization and mineralization. Radiocarbon (¹⁴C) has been used to estimate the contribution of aged C to overall respiration or waterborne C export. We aim to contrast results from radiocarbon studies, in order to assess differences between ecosystems (contrasting wet and dry ecosystems), thaw histories (active layer deepening or thermokarst landforms), greenhouse gas considered (CO₂ and CH₄) and seasons. We propose to also contrast methodologies used for assessing the contribution of aged C to overall C balance, and include studies using ¹³C data. Biological fractionation of ¹³C during both uptake and decomposition has been taken advantage of both in order to aid the interpretation of ¹⁴C data and on its own to assess sources and mineralization pathways. For example, ¹³C data has been used to differentiate between CH₄ production pathways, and the relative contribution of anaerobic CO₂ production to overall respiration. Overall, carbon isotope research is proving highly valuable for our understanding of permafrost C dynamics following thaw, and there is a current need to synthesize the available literature.