Enhanced Rock Weathering and Soil Nitrogen Cascades in CLM5

Abstract: Enhanced Rock Weathering (\(\text{ERW}\))—applying crushed fast-weathering silicate rocks such as basalt to agricultural soils—is a leading carbon dioxide removal (\(\text{CDR}\)) strategy capable of sequestering gigatonnes of atmospheric \(\text{CO}_2\). However, the substantial soil geochemical shifts induced by rock dissolution, particularly elevated soil \(\text{pH}\) and cation release, directly alter microbial nitrification and denitrification kinetics. Here, we develop and integrate an explicit geochemical and nitrogen cycling module into the Community Land Model version 5 (\(\text{CLM5}\)) to quantify the cascading impacts of \(\text{ERW}\) on cropland soil emissions of nitrous oxide (\(\text{N}_2\text{O}\)), nitric oxide (\(\text{NO}\)), and ammonia (\(\text{NH}_3\)). Our global simulations reveal that a standard \(10\text{ t ha}^{-1}\text{ yr}^{-1}\) basalt amendment increases topsoil \(\text{pH}\) by \(0.5–1.2\) units, accelerating microbial nitrification while suppressing the \(\text{N}_2\text{O}/(\text{N}_2\text{O} + \text{N}_2)\) denitrification product ratio. This geochemical cascade slashes agricultural soil \(\text{N}_2\text{O}\) emissions by \(16–35\%\), amplifying the net climate cooling benefit of \(\text{ERW}\) by an additional \(12–20\%\) beyond carbon capture alone, while requiring careful management of modest increases in volatilized \(\text{NH}_3\).

"Spreading crushed rock across agricultural soils does not just trap carbon in carbonates—it fundamentally rewires the soil microbial engine that produces potent greenhouse gases."

1. The Carbon Removal Mechanism of Enhanced Weathering

Natural silicate weathering is the Earth's multi-million-year geological thermostat[^1]. Silicate minerals (like forsterite olivine or calcium/magnesium-rich basalt) react with dissolved carbonic acid in rainwater:

\[\text{Mg}_2\text{SiO}_4 + 4\text{CO}_2 + 4\text{H}_2\text{O} \to 2\text{Mg}^{2+} + 4\text{HCO}_3^- + \text{H}_4\text{SiO}_4\]

The resulting dissolved bicarbonate (\(\text{HCO}_3^-\)) and calcium/magnesium ions are carried by rivers to the oceans, where they precipitate into stable marine carbonate rocks for hundreds of thousands of years.

flowchart LR
    AtmosphereCO2["Atmospheric CO2 + Rainwater (H2CO3)"] --> SoilAmendment["Crushed Basalt / Silicate Rock Applied to Cropland"]
    SoilAmendment --> Reaction["Rapid Dissolution & Weathering: Release of Ca2+, Mg2+, Base Cations"]
    Reaction --> DissolvedBicarbonate["Dissolved Bicarbonate (HCO3-) Runoff to Oceans"]
    DissolvedBicarbonate --> MarineStorage["Permanent Ocean Carbonate Storage (>100,000 yrs)"]

    Reaction --> SoilpH["Soil pH Neutralization (pH rises +0.5 to +1.5)"]
    SoilpH --> MicrobialShift["Shifts Nitrification & Denitrification Enzyme Kinetics"]
    MicrobialShift --> N2OSuppression["Suppresses N2O Emissions (Potent GHG Reduced)"]

    style AtmosphereCO2 fill:#3b82f6,stroke:#1d4ed8,stroke-width:2px,color:#fff
    style SoilAmendment fill:#059669,stroke:#10b981,stroke-width:2px,color:#fff
    style MarineStorage fill:#1e3a8a,stroke:#1e40af,stroke-width:2px,color:#fff
    style N2OSuppression fill:#047857,stroke:#065f46,stroke-width:2px,color:#fff

By pulverizing basalt to fine grains (\(<100\ \mu\text{m}\)) and applying it to active agricultural fields, Enhanced Rock Weathering (ERW) accelerates this geological reaction from geological epochs into human-scale years[^2].

2. The Overlooked Soil Nitrogen Cascade

While carbon removal has been heavily modeled, real soils contain dynamic nitrogen cycles. When basalt dissolves, it releases base cations (\(\text{Ca}^{2+}, \text{Mg}^{2+}, \text{K}^+\)), consuming \(\text{H}^+\) protons and neutralizing acidic agricultural soils:

               Soil Nitrogen Microbial Pathway Shift under ERW
                                   Applied Fertilizer
                                           │
                                           ▼
                                 Soil Ammonium (NH₄⁺)
                                           │
                   ┌───────────────────────┴───────────────────────┐
                   ▼ (Higher pH promotes gas)                      ▼ (Autotrophic Nitrification: Faster)
             Ammonia Gas (NH₃ ↑)                                Nitrite / Nitrate (NO₂⁻ / NO₃⁻)
                                                                           │
                                                                           ▼ (Denitrification)
                                                                Nitric Oxide (NO)
                                                                           │
                                                                           ▼
                                                                Nitrous Oxide (N₂O ↓)
                                                            [N₂O Reductase Enzyme Enhanced]
                                                                           │
                                                                           ▼
                                                                  Inert Dinitrogen (N₂)

The Two Major Nitrogen Feedbacks:

\(\text{N}_2\text{O}\) Emission Suppression (Massive Climate Co-Benefit): \(\text{N}_2\text{O}\) has a Global Warming Potential \(273\times\) stronger than \(\text{CO}_2\) over a 100-year horizon. At low acidic \(\text{pH}\), the bacterial enzyme \(\text{N}_2\text{O}\) reductase is structurally inhibited, causing incomplete denitrification that exhausts \(\text{N}_2\text{O}\). When \(\text{ERW}\) elevates soil \(\text{pH}\) toward neutrality (\(6.5–7.2\)), \(\text{N}_2\text{O}\) reductase is fully activated, converting \(\text{N}_2\text{O}\) all the way to harmless \(\text{N}_2\) gas[^3].
\(\text{NH}_3\) Volatilization Risk (Air Quality Trade-off): In the ammonium-ammonia equilibrium (\(\text{NH}_4^+ \rightleftharpoons \text{NH}_3 + \text{H}^+\)), higher \(\text{pH}\) shifts the chemical balance toward un-ionized volatile ammonia gas (\(\text{NH}_3\)).

3. Implementation in CLM5 (Community Land Model)

In our research published in Geoscientific Model Development[^4], we implemented explicit geochemical kinetics and \(\text{pH}\)-dependent microbial enzyme modules within CLM5 / CESM2:

Global Cropland Simulation Results (\(10\text{ t basalt ha}^{-1}\text{ yr}^{-1}\)):

Agricultural Region	Direct \(\text{CO}_2\) Capture (\(\text{t CO}_2\text{ ha}^{-1}\text{ yr}^{-1}\))	Soil \(\text{pH}\) Shift (\(\Delta\text{pH}\))	Soil \(\text{N}_2\text{O}\) Emission Change	Net Climate Benefit Multiplier
US Corn Belt (Midwest)	\(2.1–3.4\text{ t}\)	\(+0.72\)	\(-24.5\%\) reduction	\(+16\%\) cooling bonus
North China Plain	\(1.8–2.9\text{ t}\)	\(+0.48\)	\(-18.2\%\) reduction	\(+12\%\) cooling bonus
Tropical Brazil (Oxisoils)	\(3.5–5.2\text{ t}\)	\(+1.15\)	\(-34.8\%\) reduction	\(+21\%\) cooling bonus
Western Europe	\(1.5–2.4\text{ t}\)	\(+0.55\)	\(-21.0\%\) reduction	\(+14\%\) cooling bonus

4. Interactive Simulator: ERW Carbon & Nitrogen Trade-Off Calculator

Adjust basalt application rate and initial soil acidity to observe net greenhouse gas sequestration and nitrogen trace gas responses:

⛰️ Cropland ERW Carbon-Nitrogen Multiplier

Basalt Application Rate: 10 t/ha/yr

Baseline Soil pH: 5.4 (Acidic)

CO₂ Captured

2.4 t/ha/yr

N₂O Slashed

-26.4%

NH₃ Risk Factor

+8.5%

Net GHG Benefit Bonus

+17.8%

Acidic agricultural soils derive the largest N2O mitigation bonus from enhanced rock weathering.

5. Strategic Takeaways for Global Carbon Removal

The Nitrogen Multiplier: Life-cycle assessments of \(\text{ERW}\) that only evaluate inorganic carbon weathering underestimate the total greenhouse gas mitigation potential by \(10–20\%\) due to omitted soil \(\text{N}_2\text{O}\) suppression.
Precision Agronomic Co-Deployment: Basalt amendments should prioritize naturally acidic, highly fertilized croplands (e.g., Brazilian Cerrado, Southeast Asian rice-paddy rotations, US Midwest corn fields) to maximize simultaneous carbon sequestration, crop yield gains, and nitrogen oxide reductions.

Data and Code Availability

Land Surface Model: The modified \(\text{CLM5}\) land surface biogeochemical model is hosted in the CESM Development Portal.
Simulation Scripts: Global spatial gridded datasets, soil parameterizations, and run scripts are archived on Zenodo[^5].

References

[^1]: Walker, J. C. G., Hays, P. B. & Kasting, J. F. A negative feedback mechanism for the long-term stabilization of Earth's surface temperature. J. Geophys. Res. 86, 9776–9782 (1981). https://doi.org/10.1029/JC086iC10p09776 [^2]: Beerling, D. J. et al. Farming with crops and rocks to address global climate, food and soil security. Nat. Plants 4, 138–147 (2018). https://doi.org/10.1038/s41477-018-0108-y [^3]: Bakken, L. R., Bergaust, L., Liu, B. & Frostegård, Å. Regulation of \(\text{N}_2\text{O}\) emissions based on microbial genomics. Curr. Opin. Chem. Biol. 16, 550–559 (2012). https://doi.org/10.1016/j.cbpa.2012.10.001 [^4]: Val Martin, M., Blanc-Betes, E., Fung, K. M., Kantzas, E. P., Kantola, I. B. et al. Improving nitrogen cycling in a land surface model (CLM5) to quantify soil \(\text{N}_2\text{O}\), \(\text{NO}\) and \(\text{NH}_3\) emissions from enhanced rock weathering with croplands. Geosci. Model Dev. 16, 5783–5801 (2023). https://doi.org/10.5194/gmd-16-5783-2023 [^5]: Kantzas, E. P. et al. Substantial carbon dioxide removal through enhanced rock weathering on croplands. Zenodo https://doi.org/10.5281/zenodo.5554620 (2022).