Sensitivity of soil carbon fractions and their specific stabilization mechanisms to extreme soil warming in a subarctic grassland

• Published in: Global Change Biology
• Main Authors: Poeplau, Christopher, Kätterer, Thomas, Leblans, Niki, Sigurdsson, Bjarni D.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Chemistry; Biology; Iceland; Subarctic
• Online Access: https://hdl.handle.net/10067/1404240151162165141; https://repository.uantwerpen.be/docman/irua/8e24a1/140424_2017_09_16.pdf

Abstract: Terrestrial carbon cycle feedbacks to global warming are major uncertainties in climate models. For in-depth understanding of changes in soil organic carbon (SOC) after soil warming, long-term responses of SOC stabilization mechanisms such as aggregation, organo-mineral interactions and chemical recalcitrance need to be addressed. This study investigated the effect of 6 years of geothermal soil warming on different SOC fractions in an unmanaged grassland in Iceland. Along an extreme warming gradient of +0 to ~+40 °C, we isolated five fractions of SOC that varied conceptually in turnover rate from active to passive in the following order: particulate organic matter (POM), dissolved organic carbon (DOC), SOC in sand and stable aggregates (SA), SOC in silt and clay (SC-rSOC) and resistant SOC (rSOC). Soil warming of 0.6 °C increased bulk SOC by 22 ± 43% (010 cm soil layer) and 27 ± 54% (2030 cm), while further warming led to exponential SOC depletion of up to 79 ± 14% (010 cm) and 74 ± 8% (2030) in the most warmed plots (~+40 °C). Only the SA fraction was more sensitive than the bulk soil, with 93 ± 6% (010 cm) and 86 ± 13% (2030 cm) SOC losses and the highest relative enrichment in 13C as an indicator for the degree of decomposition (+1.6 ± 1.5 in 010 cm and +1.3 ± 0.8 in 2030 cm). The SA fraction mass also declined along the warming gradient, while the SC fraction mass increased. This was explained by deactivation of aggregate-binding mechanisms. There was no difference between the responses of SC-rSOC (slow-cycling) and rSOC (passive) to warming, and 13C enrichment in rSOC was equal to that in bulk soil. We concluded that the sensitivity of SOC to warming was not a function of age or chemical recalcitrance, but triggered by changes in biophysical stabilization mechanisms, such as aggregation.