Intercropping Safeguards Food Security and Cleans the Air
Abstract: Intensive monoculture agriculture requires massive synthetic nitrogen fertilization, driving agricultural ammonia (\(\text{NH}_3\)) emissions that react with atmospheric acids to form deadly fine particulate matter (\(\text{PM}_{2.5}\)). However, agricultural policies often treat crop yield maximization and environmental health as an unavoidable zero-sum trade-off. Here, we quantify the nationwide agro-environmental and public health co-benefits of adopting strip intercropping (maize-soybean) across China using an integrated agronomic, chemical transport, and health impact model. We find that replacing intensive maize and soybean monocultures with intercropping enhances total grain production by \(1.5–3.2\%\) while reducing synthetic nitrogen fertilizer usage by up to \(13.5\%\). This fertilizer reduction decreases annual agricultural \(\text{NH}_3\) emissions by \(0.46\text{ Tg N}\), lowering population-weighted surface \(\text{PM}_{2.5}\) by \(1.2–2.5\ \mu\text{g m}^{-3}\) and preventing over \(7,000\) air pollution-related premature mortalities annually. These results demonstrate that agro-ecological crop diversification provides a powerful, simultaneous solution for global food security, climate resilience, and public health.
"We do not need to choose between feeding a growing population and breathing clean air: planting complementary crops side-by-side unlocks natural ecological synergies that achieve both."
1. The Hidden Cost of Monoculture & The Fertilizer Dilemma
To sustain modern food demand, global agriculture applies over \(110\text{ million tonnes}\) of synthetic nitrogen fertilizer each year. In East Asia, heavy fertilization has achieved high crop yields, but at a severe environmental cost:
- Low Nitrogen Use Efficiency (NUE): More than \(50\%\) of applied nitrogen is lost to the environment rather than absorbed by the crop[^1].
- Ammonia Volatilization: Excess soil ammonium volatilizes into atmospheric ammonia gas (\(\text{NH}_3\)).
- Secondary \(\text{PM}_{2.5}\) Smog Formation: In the atmosphere, basic \(\text{NH}_3\) rapidly neutralizes nitric acid (\(\text{HNO}_3\)) and sulfuric acid (\(\text{H}_2\text{SO}_4\)) from industrial and vehicular emissions, forming ammonium nitrate (\(\text{NH}_4\text{NO}_3\)) and ammonium sulfate (\((\text{NH}_4)_2\text{SO}_4\)) aerosols—the dominant inorganic components of wintertime \(\text{PM}_{2.5}\) air pollution in East Asia and the US Midwest[^2].
flowchart LR
Fertilizer["Synthetic N Fertilizer (Urea / Ammonium)"] --> SoilN["Excess Soil Ammonium (NH4+)"]
SoilN --> Volatilization["Agricultural NH3 Volatilization"]
Volatilization --> Atmosphere["Atmospheric Gas Phase NH3"]
Atmosphere --> Reaction{"Neutralization Reactions with Acid Gases"}
AcidGas["Industrial & Vehicular NOx and SO2 (HNO3, H2SO4)"] --> Reaction
Reaction --> PM25["Secondary Inorganic PM2.5 (NH4NO3, (NH4)2SO4)"]
PM25 --> Health["Premature Respiratory & Cardiovascular Mortalities"]
style Fertilizer fill:#b91c1c,stroke:#ef4444,stroke-width:2px,color:#fff
style Volatilization fill:#f97316,stroke:#ea580c,stroke-width:2px,color:#fff
style Reaction fill:#3b82f6,stroke:#1d4ed8,stroke-width:2px,color:#fff
style PM25 fill:#7f1d1d,stroke:#991b1b,stroke-width:2px,color:#fff
style Health fill:#450a0a,stroke:#7f1d1d,stroke-width:2px,color:#fff2. What Is Strip Intercropping?
Strip intercropping is an agro-ecological practice where two or more compatible crops (typically a cereal such as maize and a legume such as soybean or peanut) are cultivated in alternating parallel strips in the same field during the same growing season:
Strip Intercropping Architecture
Row 1-2: Maize (Zea mays) Row 3-4: Soybean (Glycine max)
(Tall C4 Grass) (Short C3 Legume)
▲ ▲
┌┴┐ ┌┴┐
│ │ │ │
│ │ High Canopy Sunlight │ │ Rhizobia Nitrogen Fixation
│ │ Captures Upper Radiation │ │ Biological N₂ → NH₄⁺
─┴─┴─ ─┴─┴─
\ / Deep Root System \ / Lateral Nodulated Roots
▼ Taps Subsoil Water & N ▼ Mobilizes Soil Phosphorus
The Three Ecological Mechanisms:
- Biological Nitrogen Fixation: Legumes form symbiotic relationships with root-nodule Rhizobium bacteria that fix inert atmospheric \(\text{N}_2\) into bioavailable ammonium, drastically cutting synthetic fertilizer requirements[^3].
- Niche Differentiation & Complementarity: Maize has a deep taproot and tall canopy (capturing upper canopy sunlight); soybean has a lateral shallow root system and shade tolerance (utilizing diffuse ground light).
- Interspecific Facilitation: Maize root exudates stimulate organic acid secretion that mobilizes insoluble soil phosphorus, benefiting both crops.
The net biological productivity of intercropping is measured by the Land Equivalent Ratio (LER):
An \(\text{LER} > 1.0\) (typically \(1.2–1.4\) in field trials) indicates that a monoculture would require \(20–40\%\) more land area to produce the same total grain yield.
3. Modeling Nationwide Agro-Environmental & Health Co-Benefits
In our paper published in Environmental Research Letters[^4], we coupled empirical agronomic field trial data across China with the global 3D chemical transport model GEOS-Chem and epidemiological concentration-response functions to simulate a nationwide transition to maize-soybean intercropping:
[Agronomic Field Trials (Yield & Fertilizer Savings)] ──► [GEOS-Chem 3D Chemical Transport Model] ──► [Global Burden of Disease Health Model]
Quantitative Findings across China:
| Metric | Monoculture Baseline | Nationwide Intercropping Adoption | Co-Benefit Shift |
|---|---|---|---|
| National Maize Production | \(215.8\text{ Mt}\) | \(+5.2\text{ Mt}\) | \(+2.4\%\) grain increase |
| National Soybean Production | \(12.9\text{ Mt}\) | \(+1.4\text{ Mt}\) | \(+10.8\%\) grain increase |
| Synthetic Nitrogen Application | \(10.4\text{ Tg N yr}^{-1}\) | \(9.0\text{ Tg N yr}^{-1}\) | \(-13.5\%\) fertilizer cut |
| Agricultural \(\text{NH}_3\) Emissions | \(3.45\text{ Tg N yr}^{-1}\) | \(2.99\text{ Tg N yr}^{-1}\) | \(-0.46\text{ Tg N yr}^{-1}\) reduction |
| Surface \(\text{PM}_{2.5}\) (North China Plain) | \(68.4\ \mu\text{g m}^{-3}\) | \(66.1\ \mu\text{g m}^{-3}\) | \(-2.3\ \mu\text{g m}^{-3}\) cleaner air |
| Annual Prevented Premature Deaths | — | — | \(7,100\) lives saved per year |
4. Interactive Policy Simulator: Agricultural Intercropping Impact
Adjust the regional adoption rate of strip intercropping to evaluate the simultaneous gains in grain harvest, fertilizer cost savings, and avoided \(\text{PM}_{2.5}\) mortality:
🌾 National Intercropping Scenario Calculator
5. Policy Implications for Global Sustainable Development
The findings of this work provide crucial evidence for achieving several UN Sustainable Development Goals (SDG 2: Zero Hunger, SDG 3: Good Health and Well-being, and SDG 12: Responsible Consumption and Production):
- Beyond Single-Sector Thinking: Agricultural policy, environmental protection, and public health are traditionally managed by separate government ministries. Our work shows that cross-sectoral interventions in farm field design can deliver outsized air quality benefits that industrial emission controls alone cannot achieve.
- Smallholder Scalability: Strip intercropping can be implemented using modern mechanized harvesters and requires minimal initial capital investment compared to high-tech carbon capture or end-of-pipe industrial scrubbers.
Data and Code Availability
- Atmospheric Model: Chemical transport simulations were performed using the open-source GEOS-Chem Chemical Transport Model[^5].
- Agronomic Datasets: Field trial yield and nitrogen response databases are archived in the Environmental Research Letters repository.
- Analysis Code: Geospatial mapping and health burden calculation scripts are open source on GitHub:
kamingfung/intercropping-air-quality.
References
[^1]: Zhang, X. et al. Managing nitrogen for sustainable development. Nature 528, 51–59 (2015). https://doi.org/10.1038/nature15743 [^2]: Paulot, F. & Jacob, D. J. Hidden cost of U.S. agricultural exports: particulate matter from ammonia. Environ. Sci. Technol. 48, 903–908 (2014). https://doi.org/10.1021/es4034793 [^3]: Li, L. et al. Diversity enhances agricultural productivity via rhizosphere phosphorus facilitation on phosphorus-deficient soils. Proc. Natl. Acad. Sci. USA 104, 11192–11196 (2007). https://doi.org/10.1073/pnas.0704591104 [^4]: Fung, K. M., Tai, A. P. K., Yong, T., Liu, X. & Lam, H.-M. Co-benefits of intercropping as a sustainable farming method for safeguarding both food security and air quality. Environ. Res. Lett. 14, 044011 (2019). https://doi.org/10.1088/1748-9326/ab0c79 [^5]: Bey, I. et al. Global modeling of tropospheric chemistry with assimilated meteorology: Model description and evaluation. J. Geophys. Res. Atmos. 106, 23073–23095 (2001). https://doi.org/10.1029/2001JD000807