Measurements of nitrogen isotopes in global soils indicate deficiencies in the nitrogen cycles of Earth System Models

Earth System Models which are used to assessed the human impact on the Earth Systems predict nitrogen fluxes which deviate from fluxes derived from measurements of nitrogen isotopes. As a consequence of this deficiency these models are likely underestimating the availability of soil nitrogen a major plant nutrient controlling the response of ecosystem to increasing carbon dioxide in the atmosphere. These findings have been published in Nature Communication. 

How isotope are used to quantify nitrogen fluxes

The weight of nitrogen atoms naturally occurring in soils varies depending on the number of atoms in their nuclei. Information on the relative abundance of heavier and lighter atoms (isotopes) can be used to quantify rate at which biological and abiotic processes transform nitrogen in ecosystems.  

This is because biological processes prefer lighter nitrogen atoms, and thus are more likely to transform them compared to heavier ones. For example, the emissions of nitrogen in gaseous form from soils by soil microbes which leaves heavier nitrogen atoms in soils.  In contrast, abiotic processes like leaching of nitrogen dissolved in soil water shows no preference for a certain isotope and thus not affect the relative abundance of isotopes in soils.

Global mapping of nitrogen loss pathways

This fact was used in a new study to quantify the rate at which soil microbes emit nitrogen  from soils using a collection of thousands of soil measurements of nitrogen isotopes in combination with machine learning. The international research team lead by Sushi Peng and Maoyuan Feng from the Peking University produced  global maps of isotopic composition of soils and of soil nitrogen emissions. These maps allow to detect and quantify the pathways by nitrogen is lost from soils.

Confronting Earth System Models with estimates of nitrogen losses

The  insights gained from this analysis were confronted with predictions of Earth System Models to evaluate the models' capacity to simulate nitrogen losses and the availability of the plant nutrient nitrogen to support plant growth.  The authors found that these models overestimate gaseous nitrogen emissions and are likely underestimating the availability of nitrogen for plant growth and carbon sequestration. This has far reaching consequences of predictions of the response of ecosystems to global change drivers.

For more information: Feng, M., Peng, S., Wang, Y. et al. Overestimated nitrogen loss from denitrification for natural terrestrial ecosystems in CMIP6 Earth System Models. Nat Commun 14, 3065 (2023). https://doi.org/10.1038/s41467-023-38803-z