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

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~c 43% (0–10 cm soil layer) and 27 ~c 54% (20–30 cm), while further warming led to exponential SOC depletion of up to 79 ~c 14% (0–10 cm) and 74 ~c 8% (20–30) in the most warmed plots (~+40 °C). Only the SA fraction was more sensitive than the bulk soil, with 93 ~c 6% (0–10 cm) and 86 ~c 13% (20– 30 cm) SOC losses and the highest relative enrichment in 13C as an indicator for the degree of decomposition (+1.6~c 1.5ppt in 0–10 cm and +1.3 ~c 0.8ppt 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, 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.