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

Models of terrestrial system dynamics often include nitrogen (N) cycles to better represent N limitations on 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 with an extensive set of observations from the Hubbard Brook Forest Long-Term Ecological Research 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 ~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 to improve confidence in present and future simulations of N limitation on the C cycle, although C fluxes, such as gross primary productivity and net primary productivity, are less sensitive to the model modifications than soil N fluxes.