Simulated mechanisms of soil N feedback on the forest CO2 response

An improved understanding of the response of forest ecosystems to elevated levels of CO₂ in the atmosphere is crucial because atmospheric CO₂ concentration continues to increase at an accelerating rate and forests are an important sink in the global carbon cycle. Several CO₂‐enrichment experiments have now been running for more than 10 years, with highly variable short‐term results after the first decade. Responses to rising CO₂] over the next few decades will depend on several plant and ecosystem feedbacks that are inadequately understood. In this study, we conduct a sensitivity analysis, within the context of the simulated CO₂ response, using a new version of the G'DAY ecosystem model, with an improved decomposition submodel, applied to a nitrogen‐limited Norway spruce forest site in the north of Sweden. The new decomposition model incorporates important modifications to soil processes, including some that constitute negative feedbacks on an ecosystem's growth response to elevated CO₂]. The sensitivity analysis reveals key parameters and processes that are important for the simulated CO₂ response on the short term and others that are more important on the long term. A process that has a strong impact on the short‐term response is a change in decomposer composition, potentially in response to altered litter quality. Parameters that become increasingly important in the long term are carbon allocation to root exudates that are directly or indirectly associated with atmospheric N₂ fixation, and the rate of humification of soil organic matter. We identify factors intrinsic to species and site (microbes and resources) and ecosystem nutrient supply that determine the duration of the enhanced simulated growth response to elevated CO₂].