Assessment of the impacts of biological nitrogen fixation structural uncertainty in CMIP6 earth system models

[摘要] Biological nitrogen fixation is the main source of newnitrogen into natural terrestrial ecosystems and consequently in thenitrogen cycle in many earth system models. Representation of biologicalnitrogen fixation varies, and because of the tight coupling between thecarbon and nitrogen cycles, previous studies have shown that this affectsprojected changes in net primary productivity. Here we present the firstassessment of the performance of biological nitrogen fixation in modelscontributing to CMIP6 compared to observed and observation-constrainedestimates of biological nitrogen fixation. We find that 9 out of 10 modelsrepresent global total biological nitrogen fixation within the uncertaintyin recent global estimates. However, 6 out of 10 models overestimate the amount of fixation in the tropics and therefore the extent of the latitudinalgradient in the global distribution. For the SSP3-7.0 scenario of futureclimate change, models project increases in fixation over the 21st century of up to 80 %. However, while the historical range of biologicalnitrogen fixation amongst models is large (up to 140 kg N ha −1  yr −1 at the grid cell level and 43–208 Tg N yr −1 globally) this does not have explanatory power for variations within the model ensemble ofnet primary productivity or the coupled nitrogen–carbon cycle. Models withshared structures can have significant variations in both biologicalnitrogen fixation and other parts of the nitrogen cycle without differing intheir net primary productivity. This points to systematic challenges in therepresentation of carbon–nitrogen model structures and the severelimitations of models using net primary productivity or evapotranspirationto project the biological nitrogen fixation response to elevated atmosphericcarbon dioxide or other environmental changes.

[学科分类] 大气科学