Representing the Dynamic Response of Vegetation to Nitrogen Limitation
in the CLASSIC Land Model
Abstract
Despite its pivotal feedback to carbon cycling, representing the dynamic
response of vegetation to nitrogen limitation is a key challenge for
simulating the terrestrial carbon sink in land models. Here, we explore
a representation of this dynamic response of vegetation to nitrogen
limitation with a novel representation of biological nitrogen fixation
and nitrogen cycling in the Canadian Land Surface Scheme Including
Biogeochemical Cycles (CLASSIC) model. First, we assess how
incorporating this dynamic response of vegetation to nitrogen limitation
via biological nitrogen fixation influences carbon sequestration for
CO2 and nitrogen fertilisation experiments, comparing
simulations against observation-based estimates from meta-analyses. This
evaluates whether underlying mechanisms are realistically represented.
Second, we assess how incorporating the dynamic response of vegetation
to nitrogen limitation via biological nitrogen fixation affects carbon
sequestration over the late 20th and early
21st century, examining the effects of global change
drivers (CO2, nitrogen deposition, climate, and land use
change) acting both individually and concurrently. Including nitrogen
cycling reduces the terrestrial carbon sink driven by elevated
atmospheric CO2 concentration over the historical
period. Representing the dynamic response of vegetation to nitrogen
limitation via biological nitrogen fixation increases the present-day
terrestrial carbon sink by 0.2 Pg C yr-1 because the
upregulation of biological nitrogen fixation driven by stronger nitrogen
limitation under elevated atmospheric CO2 concentration
alleviates nitrogen limitation. Our results highlight the importance of
the dynamic response of vegetation to nitrogen limitation for
realistically projecting the future terrestrial carbon sink under global
change with land models.