Plant cover changes drive soil carbon pool responses in High Arctic dry heath exposed to decades of experimentally increased summer rain and nutrient addition

With accelerating climate change, higher summer rainfall and warmer soils are expected for High Arctic ecosystems. Yet, how increased rainfall and soil nutrient availability will affect plant composition and ecosystem carbon (C) storage in these arid, low-productivity ecosystems remains unclear. We utilised a long-term experiment in dry shrub heath tundra in Zackenberg, NE Greenland, in which N and P availability was increased and precipitation doubled experimentally every summer for 25 years. We determined soil and vegetation C pools, plant cover and leaf chemistry, and ecosystem CO2 fluxes in peak growing season. Watering increased the cover of graminoids and all plants by 78% and 18%, respectively, which likely drove a moderate 6% increase in upper soil C stocks. Soil respiration was consistently stimulated in watered plots, confirming high sensitivity of soil microbes to moisture in dry tundra, but also stimulation of microbial activity by increased plant inputs. We suggest that belowground processes linked to root growth, root exudation and/or microbial turnover of organic matter are important in driving the C pool changes. Our results show that increased summer rainfall can lead to greening and enhanced soil C pool in High Arctic dry heaths, potentially providing moderate negative feedback to climate change.