Nitrification and denitrification in the Community Land Model compared to observations at Hubbard Brook Forest
Data files
Apr 08, 2025 version files 507 B
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README.md
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Abstract
Dataset DOI: 10.5061/dryad.gmsbcc2n2
Description of the data and file structure
All files for this dataset are available through Zenodo at https://doi.org/10.5281/zenodo.4641366
Modifications to CLM5.0 at Hubbard Brook LTER
The Community Land Model version 5.0 (CLM5.0) was modified at a single grid cell corresponding to the Hubbard Brook Experimental Forest, a northern hardwood forest site in the White Mountain National Forest in New Hampshire USA (43°56´N, 71°45´W). The purpose was to test alternative parameterizations for nitrification and denitrification in CLM5.0. The CLM5.0 simulations use site-level present day meteorological (from GSWP3 v. 1) and N deposition inputs, created by extracting the single grid cell values from the global gridded forcing data for CLM5.0 [Lawrence et al., 2019]. Atmospheric CO2 concentration (= 367 ppm), land use and N deposition (= 0.7 gN/m2/yr) were fixed at year 2000 conditions throughout the simulations, while meteorological forcings were cycled over 1991-2010. The plant functional type of the grid cell was prescribed as 100% broadleaf deciduous temperate forest. Spin-up for each simulation was run in accelerated decomposition mode for 400 years, followed by a final spin-up for 200 years, of which the last 20 years were sampled for the results archived here. The N fluxes varied interannually but displayed no obvious drift or trends over these 20 years.
We tested a variety of new parameterizations, described in detail in the text, in which model nitrification and/or denitrification was revised based on observed empirical relationships.
1a) Increased nitrification (Parton)
We added an NH4+ mineralization-based term to the CLM5.0 formula for potential nitrification in accord with the Parton et al. [2001] equation, from which the formula is derived.
2a) Increased nitrification (Zhang)
In an alternative approach to boosting nitrifier competitiveness, we implemented a parameterization in which we parameterized potential nitrification as a direct linear function of gross mineralization multiplied by a scalar computed as (pH-4)6, reflecting the empirical linear relationship found by Zhang et al. [2018]. Since CLM5.0 has a uniform default pH of 6.5, this scalar was effectively 0.42
1b and 2b) Reduced denitrification (Reduced Denit)
We reduced the [NO3-]-limited and CO2 respiration-limited equations for potential denitrification by a factor of 100 and 10, respectively. We ran two reduced denitrification modifications: 1b) Reduced Denitrification with Parton nitrification scheme (from modification 1a) and 2b) Reduced Denitrification with Zhang nitrification scheme (from modification 2a).
1c) Denitrification scaled to Nitrification (Denit=Nitrif/10)
We tested an alternative parameterization, building off modification 1a), to reduce the rate of denitrification. In this alternative approach, we bypassed the Del Grosso et al. [2000] algorithm altogether and instead set potential denitrification equal to potential nitrification divided by 10.
1bx) No N2 fixation
We turned off N2 fixation (beginning from year 1 in the spin-up phase) due to concern that CLM adds an excessive amount of N to northern temperate ecosystems such as Hubbard Brook that lack symbiotic N2 fixers and where heterotrophic N fixation rates are low. N deposition was left turned on in this experiment. Modification 1bx was performed with the 1b) modifications (Parton increased nitrification and Reduced Denitrification adjustments) also turned on.
3) Swap NO3-
The order of competition for mineral N between plants and soil microbes was switched such that they competed first for NO3- and second for NH4+. This was a swap in the sense that the default CLM5.0 competition occurs in the opposite order, i.e., first for NH4+ then for NO3-. Aside from reversing that order, we made no other adjustments to the algorithms for potential nitrification and denitrification in modification 3.