Current and future contributions of older C pools to soil respiration from High Arctic tundra in NW Greenland - first 14C-based assessments

High Arctic tundra soils contain significant amounts of organic carbon (C) that may become accessible to microbial decomposition with climate warming and provide a positive climate feedback. To estimate if older C pools are currently contributing to net C emissions from High Arctic tundra of NW Greenland we studied the seasonal pattern and radiocarbon (14C) signature of CO2 within the soil and emitted as soil respiration over the course of two growing seasons. To estimate if climate change may increase the loss of older C pools, we monitored the 14C content of soil respiration from a warming and warming + water addition experiment over the course of one growing season. Measurements are challenged by low CO2 fluxes and very high spatial variability in the distribution and age of soil C stocks. Preliminary data suggests that very old (630-5400 yr) C pools are present in the topsoil. Before leaf-out, older (pre-1950) C pools contribute 20-63% to soil respiration. Old C decomposition rates are highest in mid-summer, but after leaf-out, large C fluxes from roots and, or microorganisms decomposing recently fixed C mask losses of older C.