Abstract
Rapid warming in the Arctic threatens to amplify climate change by releasing the region’s vast stocks of soil carbon to the atmosphere. Increased nutrient availability may exacerbate soil carbon losses by stimulating microbial decomposition or offset them by increasing primary productivity. The outcome of these competing feedbacks remains unclear. Here we present results from a long-term nutrient addition experiment in northern Alaska, United States, coupled with a mechanistic isotope-tracing experiment. We found that soil carbon losses observed during the first 20 years of fertilization were caused by microbial priming and were completely reversed in the subsequent 15 years by shrub expansion which promoted an increasingly efficient carbon–nitrogen economy. Incorporating long-term stoichiometric responses in Earth system models will improve predictions of the magnitude, direction and timing of the Arctic carbon–climate feedback.