Nitrogen isotopes improve predictions of nitrogen losses and climate change
A key issue in climate change research concerns the role of nitrogen (N) in radiative forcing. Nitrous oxide produced via denitrifying bacteria can directly warm the climate, and NOx catalyzes the formation of ground level ozone, another warming effect. At the same time, N affects the magnitude of photosynthesis, and thus the biosphere's ability to reduce the concentration of atmospheric carbon dioxide via the global carbon terrestrial sink. Yet, only two of the Earth System Models in the 2014 Intergovernmental Panel of Climate Change (IPCC) report (CESM1-BGC, NorESM1-ME) include the N cycle; both of these models are based on the community land model CLM4.0. Thus, the accuracy of the IPCC’s assessment of carbon-nutrient-climate feedback have so far hinged on the accuracy of the N cycle in CLM4.0.
Here, we quantitatively and independently validate the terrestrial N cycle of CLM4.0 and the next generation model, CLM4.5, using isotope mixing modeling and mass balance of soil N isotopes at 1º resolution globally. Specifically, we calculated the fraction of gaseous N losses compared to leaching losses, and the soil N isotope values predicted by CLM. We compared our results with measurements of soil N isotopes and global budgets based on mass-balance.
Our results show that the dominant N losses in CLM4.0 are gaseous – composing over 99% of terrestrial N losses worldwide. Estimates for N gaseous vs. N leaching losses based on mass balance and isotope modeling varied from 0% to 69% gas loss, with a global mean of 35% of total N losses. The results of CLM 4.5 more closely match the range of values from the isotope model, with N gaseous losses ranging from 0 to 82%, with a global mean of 34%. However, the spatial distribution of results from both CLM 4.0 and CLM 4.5 are strongly at odds with measurements and global budgets based on mass-balance.
By underestimating N leaching fluxes, CLM4.0 and the coupled climate-carbon models will underestimate eutrophication and increase the indirect effects of N on climate change. While CLM4.5 improves results compared to CLM4.0, the spatial distribution of gas vs. leaching fluxes does not match isotope-model based predictions. We propose to improve CLM by using an isotopic mixing model to partition N losses between these two fluxes. This change will improve predictions of climate change and other important ecological issues including biodiversity loss, ozone depletion, and eutrophication.