title: Sensitivity of soil carbon fractions and their specific stabilization mechanisms to extreme soil warming in a subarctic grassland.
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% (0-10 cm soil layer) and 27 ± 54% (20-30 cm), while further warming led to exponential SOC depletion of up to 79 ± 14% (0-10 cm) and 74 ± 8% (20-30) in the most warmed plots (~+40 °C). Only the SA fraction was more sensitive than the bulk soil, with 93 ± 6% (0-10 cm) and 86 ± 13% (20-30 cm) SOC losses and the highest relative enrichment in13C as an indicator for the degree of decomposition (+1.6 ± 1.5‰ in 0-10 cm and +1.3 ± 0.8‰ in 20-30 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, and13C 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.
authors: Thuenen Institute of Climate-Smart Agriculture, Bundesallee 50, 38116, Braunschweig, Germany.|Department of Ecology, Swedish University of Agricultural Sciences (SLU), S-75 007, Uppsala, Sweden.
authors: Christopher
authors: C
authors: Poeplau
authors: Department of Ecology, Swedish University of Agricultural Sciences (SLU), S-75 007, Uppsala, Sweden.
authors: Thomas
authors: T
authors: Kätterer
authors: Department of Biology, University of Antwerp, 2610, Wilrijk, Belgium.|Agricultural University of Iceland, Hvanneyri, 311, Borgarnes, Iceland.
authors: Niki I W
authors: NI
authors: Leblans
authors: Agricultural University of Iceland, Hvanneyri, 311, Borgarnes, Iceland.
authors: Bjarni D
authors: BD
authors: Sigurdsson
mesh_terms: D002244:Carbon; D057232:Global Warming; D065948:Grassland; D007055:Iceland; D012987:Soil
chemical_list: D012987:Soil; D002244:Carbon
keywords: global change; soil carbon fractionation; soil organic matter; soil warming; temperature manipulation; temperature sensitivity; trophic fractionation; δ13C