Introduction

Maize (Zea mays) is a global staple crucial for food, feed, and biofuel. However, continuous monoculture leads to soil degradation and environmental issues. Crop rotation, especially with legumes, is a key strategy to enhance soil health and system sustainability. While crops like alfalfa, rye, wheat, canola, chickpea, soybean, flax, and sunflower are proposed for rotation with maize, comprehensive studies comparing their full environmental footprints are limited. This study aims to fill this gap by conducting a comparative Life Cycle Assessment (LCA) of these eight rotation crops to identify those with the lowest environmental burdens, thereby informing the design of sustainable maize-based cropping systems.

Material and Methods

The study followed the ISO 14040 and 14044 standards for LCA. The modeling was performed using SimaPro software (v9.5) with the Ecoinvent database (v3.9, cut-off model), providing globally averaged inventory data for inputs (seeds, fertilizers, pesticides, fuel) and yields for each crop. The functional unit was the Material Product Transport (mpt) unit, encompassing all input, product, and transport flows. The life cycle impact assessment was conducted using the ReCiPe 2016 Endpoint (v1.08) method with its default weighting set. This method aggregates impacts into three endpoint categories: damage to Human Health (measured in DALY), damage to Ecosystems (measured in species.year), and resource scarcity (Resources, measured in USD). Results for each crop in each category were calculated in absolute mpt values and normalized to a percentage scale (Effect%) relative to the highest-impacting crop within that specific category to facilitate visual comparison. No further weighting was applied to maintain objectivity.



Results and Discussion

The LCA revealed that all eight crops impacted 22 midpoint categories. Flax (Linum usitatissimum) consistently demonstrated the highest negative environmental impact across all three endpoint categories. In the normalized results (Effect%), Flax scored 100% in Human Health, Ecosystems, and Resources. Its mpt values were also the highest (e.g., 503 for Human Health, 70 for Ecosystems, 6 for Resources). In stark contrast, Alfalfa (Medicago sativa) exhibited the lowest environmental burden, with normalized scores of only 8% for Human Health, 9% for Ecosystems, and 12% for Resources, and the lowest corresponding mpt values. Wheat, Rye, and Soybean showed relatively high negative impacts, particularly on Human Health and Ecosystems. Canola had significant effects on Ecosystems and Resources but a lower impact on Human Health. Sunflower and Chickpea presented intermediate impacts. A notable exception was the negative characterization factor for Rye concerning water consumption and terrestrial ecosystems (-1.29E-10), indicating a net positive environmental effect in this specific area due to avoided burdens, likely from its role as a cover crop in reducing nitrate leaching and soil erosion. The poor environmental performance of Flax, Wheat, and Rye can be attributed to their high reliance on external inputs, particularly nitrogen fertilizers. The production and application of synthetic nitrogen fertilizers are major contributors to greenhouse gas emissions (e.g., nitrous oxide), aquatic eutrophication, and acidification, directly impacting human health and ecosystems. Furthermore, their pest management may require more pesticide applications. Conversely, Alfalfa's superior profile stems from its inherent biological nitrogen fixation capability, drastically reducing the need for synthetic N fertilizers. Its deep, perennial root system also improves soil structure and prevents erosion, providing additional ecosystem benefits. These findings align with previous studies highlighting high-input cereals as significant environmental burdens and praising alfalfa for enhancing agro-ecosystem sustainability.

Conclusion

From a purely environmental perspective, Alfalfa is the most favorable crop for inclusion in a rotation system with maize, offering the lowest impact across human health, ecosystems, and resource use. Flax is the least favorable option due to its substantially higher environmental footprint. Crops like Wheat, Rye, and Soybean also carry considerable burdens and should be chosen with caution if environmental impact is a primary concern. Canola, Chickpea, and Sunflower represent intermediate choices. The positive net effect of Rye on water and terrestrial ecosystems underscores the value of cover crops in specific impact categories. It is crucial to note that these results are based on global average data; local conditions, management practices, and economic factors will influence final decision-making. Therefore, while this LCA provides a robust scientific basis for environmentally-informed choices, optimal rotation design must integrate these findings with agronomic suitability and economic feasibility. Future studies using region-specific inventory data are recommended for localized applications.