In a recent article published in Land, researchers comprehensively examined the effectiveness of visible and near-infrared (vis-NIR) spectroscopy in monitoring copper (Cu) levels in urban soils. They investigated how different types of sample similarities impact the accuracy of Cu estimation models developed with vis-NIR spectroscopy.
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Background
Soil is essential for human survival and development, serving as a critical component of Earth's biosphere. However, rapid urbanization and industrialization, particularly in developing countries, have led to widespread soil contamination by heavy metals, posing serious health and environmental risks. Cu, a common heavy metal in urban soils, is particularly concerning due to its toxicity, persistence, and tendency to accumulate over time.
Traditional methods for determining Cu levels in soil, such as the diethylenetriamine penta-acetic acid (DTPA) method, rely on lab-based chemical analysis, which is often time-consuming, expensive, and harmful to the environment.
As an alternative, vis-NIR spectroscopy offers a rapid, cost-effective, and environmentally friendly approach to soil analysis. This technique uses the interaction of light with soil components to create unique spectral signatures that correlate with specific soil properties, including Cu content.
About the Research
In this study, the authors compared how three types of sample similarities—compositional, spectral, and spatial—affect the accuracy of Cu measurement models developed using vis-NIR spectroscopy.
The research was conducted in Shenzhen City, China, which is known for its high level of heavy metal contamination. The researchers collected 250 topsoil samples (between 0 to 20 cm depth) from various locations across the city. These samples were analyzed using a vis-NIR spectrometer, covering the spectral range of 350-2500 nm. Traditional laboratory methods were also employed to measure the Cu concentrations in the collected samples.
To assess the impact of different similarities, samples were divided into five groups based on each type of similarity. For spectral similarity, samples were grouped by their reflectance values across the vis-NIR spectrum, indicating similar soil compositions. For compositional similarity, samples were grouped by Cu content, representing different Cu concentration ranges. Lastly, for spatial similarity, samples were grouped by geographic location, aiming to capture spatial trends in Cu levels.
The authors developed separate Cu measurement models using partial least-squares regression (PLSR), a commonly used statistical method for creating predictive models from spectroscopic data. The performance of each model was evaluated using standard metrics like the coefficient of determination in prediction (Rp²), root mean square error of prediction (RMSEP), and residual predictive deviation (RPD).
Key Findings
The study revealed significant differences in Cu measurement model performance based on the type of similarity used. The model based on compositional similarity, which grouped samples by Cu content, demonstrated the highest accuracy, with an Rp² of 0.92 and an RPD of 3.57. This model outperformed those based on spatial similarity (Rp² = 0.73, RPD = 1.88) and spectral similarity (Rp² = 0.71, RPD = 1.85).
These results highlighted the crucial role of compositional similarity in accurate Cu estimation using vis-NIR spectroscopy, suggesting that the soil's chemical composition, particularly Cu content, had a greater influence on spectral signatures than its physical characteristics or geographic location.
The authors also observed a complex interplay between these similarity types. While compositional similarity produced the most accurate models, achieving higher similarity across all three types was challenging.
In complex urban environments, soil properties vary significantly due to human activities, making it difficult to maintain high similarity across all types. This underscored the importance of considering specific research objectives and the limitations of each similarity type when using vis-NIR spectroscopy for soil analysis.
Applications
This research has important implications for applying vis-NIR spectroscopy to monitor Cu levels in urban soils. By understanding the impact of different sample similarities, researchers and practitioners can improve sampling strategies and model development, leading to more accurate Cu estimation. This can enhance soil management practices to reduce Cu contamination and promote sustainable land use.
The study also provides valuable insights for future research and applications of vis-NIR spectroscopy in soil monitoring. While focused on Cu, the approach can be extended to other soil properties, such as organic matter and other heavy metals, in urban and rural areas. This opens up broader applications of vis-NIR spectroscopy in soil analysis, potentially contributing to more effective soil management and environmental monitoring.
Conclusion
This research provided insights into the performance differences of Cu measurement models based on various sample similarities when using vis-NIR spectroscopy. It demonstrated that compositional similarity, specifically based on Cu content, was key to developing the most accurate Cu estimation models. The study also highlighted the importance of carefully considering sample similarity, particularly in complex urban environments where achieving high similarity across multiple factors is challenging.
The outcomes suggest that as technology advances, vis-NIR spectroscopy could play a crucial role in monitoring soil health, managing environmental risks, and promoting sustainable land use practices. This technology has the potential to contribute significantly to effective soil management and reducing heavy metal contamination risks in urban areas.
Journal Reference
Liu, Y., et al. (2024). Monitoring the Soil Copper of Urban Land with Visible and Near-Infrared Spectroscopy: Comparing Spectral, Compositional, and Spatial Similarities. Land. DOI: 10.3390/land13081279, https://www.mdpi.com/2073-445X/13/8/1279
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