Reactive nitrogen gas from soil (NO_x, N₂O)

Jinmu Luo; Peter Hess; Danica Lombardozzi; Steven Hall;

Julius Vira; Maria Val Martin

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Special thanks to the

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CTSM team: Will Wieder, Sam Levis, Keith Oleson, Erik Kluzek, and Sam Rabin

CESM forum

Why study reactive nitrogen gas from soil (NH₃, NO_x, N₂O)

Most ESMs don’t simulate NH₃, NO_x, and N₂O from soil

• NH₃ is a precursor gas of Sulfate-Nitrate-Ammonia aerosol
• NO_x forms OH, O₃, influence CH₄ sink rate
• N₂O, a greenhouse gas, is important to the stratospheric O₃ layer

Mesocosm observations set up (IOWA)

What it controlled

• Fertilizer usage

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What it measured

• NO_x, N₂O emissions
• Harvest N
• Inorganic N leaching
• NO₃^-, NH₄⁺ in soils
• Soil properties

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First simulation on the MESOCOSM site

Single column

CLM5.0*-FANv3

Use local meteorological forcing

Modify Soil properties (e.g. pH, bulk density, organic content)

For either Clarion or Webster soil types

Modify the fertilization amount, time and location

Change the planting date

CLM5.0*-FANv3 under mesocosm

NO_x emission in CLM default driven by flux tower meteorology 1 km away from the MESOCOSM site

Envinronmental scalars (one site)

Only liquid water

• Soil temperature is ok, but soil moisture is overestimated in the model
• In the model, we have replaced the soil bulk density, soil organic matter (only for water flow simulation), soil pH, clay, and sand content.

Using scale factors to constraint the soil water

    params_inst%sand_pf = 25.0_r8

    params_inst%clay_pf = -19.0_r8

    params_inst%bsw_sf = 0.4_r8

    params_inst%hksat_sf = 2.3_r8

New leaching mechanism (PR#2992)

More details will be available on the (PR #3518)

Leaching

before

after ctsm5.3.072

Soil inorganic nitrogen concentrations

Q1: Too much ammonium after fertilization season

S1: Higher the nitrification threshold from 10%per day to 50% per day

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Q2: low nitrate concentration

S2:Find alternative denitrification water limited functions.

 Luo et al., submitted, 2025 

N leaching and N in crop

• N content in crop can be adjusted by changing the initial seed density (3gC/m²)

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• Leaching fluxes can be improved by considering a more physical way of NO₃^- movement in water

Gas emission

• Soil properties; Environmental conditions (temperature, soil moisture, drainage); Nitrate leaching; Plant uptake; Soil nitrogen concentrations; are all consistent with measurements;

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• Some scale factors are used to scale the gas down.

 Luo et al., submitted, 2025 

Nitrogen flows

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1. The emission factor for NO and N₂O is 0.9% and 1.7%, respectively, supported by many observations in the agricultural field.

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• Leaching: denitrification ratio increase from 0.014 to 1.7. isotopic benchmarking gives a 1.4 at natural land.

 Luo et al., submitted, 2025 

Emission factors (NO_x/Fertilizer)

89 studies, 229 observations

Model: 2000-2009

• Direct flux can change over 10 times, but EF-NO is more stable (EF-NO = 1% suggested by IPCC)
• Emission fluxes are controlled by fertilizer input. EF is less sensitive to fertilization but more to environmental conditions

Luo et al ., in preparation

Emission factor comparison

Why model have less variability? Most of the soil processes, like soil chemistry, soil moisture, are under column level, measurements on pfts.

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Luo et al , in preparation

Soil NO_x emission

Luo et al , in preparation

Un-equalibrium issue

(1) EF>100% is impossible; it means the soil is losing C-N, and part of it is emitted as NO_x and N₂O.

(2) Fast nitrification reduces the carbon, after spin-up

Luo et al , in preparation

Losing carbon creates the high emissions

Losing carbon causes the high soil NO_x emissions from forest regions

One point test on the natural land

co2_ppmv = 367.0

co2_type = 'constant’

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10 years of CRU-J reanalysis dataset

N input is stable

CAM-Chem simulated O₃ difference

Luo et al , in preparation

• Enhanced O₃ concentration from soil NO_x is around 1-10 ppb
• 8 hours averaged O₃ > 70 ppb is considered to cause health problems.
• Boulder (20 - 50 ppb)

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Soil NO_x simulated by CLM

Soil NO_x = 0

Left - middle

O₃ difference, zonal

Luo et al , in preparation

Soil NO_x simulated by CLM

Soil NO_x = 0

Left - middle

• Most of the enhancements are not only on the surface, but also at high altitude.
• The long-range transport of soil NO_x is a significant phenomenon.

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Some questions

CESM3_beta06 (FCnudged compset with 0.9x1.25), “2 degree is too coarse to analyze the surface ozone”– -CAM-Chem group

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How to spin up the CTSM (my account is overdraft)

1. Two degrees for the AD, pAD, and 1 degree for HIST
2. Two degrees for AD, pAD, and HIST, and use 2 degrees to restart the CESM simulation

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