Nitrification and denitrification in the Community Land Model compared to observations at Hubbard Brook Forest

Models of terrestrial system dynamics often include nitrogen (N) cycles to
better represent N limitation of terrestrial carbon (C) uptake but
simulating the fate of N in ecosystems has proven challenging. Here, key
soil N fluxes and flux ratios from the Community Land Model version 5.0
(CLM5.0) are compared to an extensive set of observations from the Hubbard
Brook Forest Long-Term Ecological Research (LTER) site in New Hampshire.
Simulated fluxes include microbial immobilization and plant uptake, which
compete with nitrification and denitrification, respectively, for
available soil ammonium (NH4+) and nitrate (NO3-). In its default
configuration, CLM5.0 predicts that both plant uptake and immobilization
are strongly dominated by NH4+ over NO3-, and that the model ratio of
nitrification:denitrification is approximately 1:1. In contrast, Hubbard
Brook observations suggest that NO3- plays a more significant role in
plant uptake and that nitrification could exceed denitrification by an
order of magnitude. Modifications to the standard CLM5.0 at Hubbard Brook
indicate that a simultaneous increase in the competitiveness of nitrifying
microbes for NH4+ and reduction in the competitiveness of denitrifying
bacteria for NO3- are needed to bring soil N flux ratios into better
agreement with observations. Such adjustments, combined with evaluation
against observations, may help improve confidence in present and future
simulations of N limitation on the C cycle, although C fluxes such as
gross primary productivity (GPP) and net primary productivity (NPP) are
less sensitive to the model modifications than soil N fluxes.