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Dryad

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

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Apr 08, 2025 version files 507 B

Abstract

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.