Engineered Biochars Enhances Carbon Dioxide Adsorption while Augmenting Soil Carbon Storage and Crop Yields in Semi-Arid Nigeria

Technical Summary: Engineered Biochars for Carbon Sequestration and Crop Yield in Semi-Arid Nigeria

Problem Statement
Global warming, driven by anthropogenic greenhouse gas emissions, necessitates scalable carbon management strategies. In semi-arid regions like the Sudan Savannah of Nigeria, intensive cropping and unsustainable farming practices have depleted soil organic carbon, creating a "carbon debt" that threatens soil health and crop productivity. While biochar has emerged as a promising tool for soil carbon sequestration, unprocessed biochar often possesses limited capacity to adsorb carbon. Furthermore, existing research on modified biochars has largely been restricted to laboratory-scale CO₂ adsorption from the atmosphere, lacking field-scale validation regarding soil carbon sequestration and the reduction of soil CO₂ efflux. There is a critical gap in understanding how metal-engineered biochars perform in semi-arid agricultural soils, specifically regarding their ability to enhance carbon storage, reduce soil CO₂ emissions, and improve crop yields, while simultaneously assessing the environmental safety of metal modifications.

Methodology
This study investigated the production, metal modification, and field application of biochars derived from rice husk and sawdust in a semi-arid environment in Kano, Nigeria.

• Biochar Production: Feedstocks were subjected to slow pyrolysis at 750–800 °C for 2 hours under anaerobic conditions.
• Engineering: The resulting biochars were impregnated with Magnesium (Mg) and Aluminum (Al) using nitrate salts to achieve a 5% metal loading by weight. The process involved slurry mixing, pH adjustment to 10, and drying.
• Characterization: Biochars were analyzed for elemental composition, proximate analysis, surface area, porosity (via BET), and mineralogical structure (XRD, FTIR, SEM).
• Adsorption Testing: CO₂ adsorption capacity was measured using Thermogravimetric Analysis (TGA) at 30, 40, and 50 °C. Adsorption kinetics were modeled using pseudo-first-order, pseudo-second-order, and Avrami models. Regenerability was tested over five adsorption–desorption cycles.
• Field Trials: A split-plot design with three replications was conducted over two growing seasons using groundnut (Arachis hypogaea) as the test crop. Treatments included unmodified biochar, Mg-modified biochar, and Al-modified biochar applied at 10 t ha⁻¹.
• Soil and Flux Measurements: Soil samples were collected at pre-planting, post-harvest year 1, and post-harvest year 2 to measure carbon stock changes. Soil CO₂ flux was monitored using an infrared gas analysis system (CIRAS-2).

Key Results

• Material Properties: Metal impregnation significantly increased the surface area and microporosity of the biochars. XRD and FTIR analyses confirmed the successful incorporation of MgO and Al₂O₃. Mg-modified biochars exhibited distinct carbonate bands, indicating strong chemisorption sites, whereas Al-modified biochars showed Al–O bands.
• CO₂ Adsorption: Engineered biochars demonstrated significantly higher CO₂ adsorption capacities than unmodified controls. Mg-modified biochar achieved the highest capacity (83 mg g⁻¹ at 30 °C), compared to 63 mg g⁻¹ for unmodified biochar. Adsorption capacity decreased as temperature increased.
• Kinetics and Stability: The Avrami kinetic model provided the best fit (R² > 0.99), indicating mixed physisorption–chemisorption mechanisms. Engineered biochars maintained high adsorption capacity over five regeneration cycles with less than a 5% performance decline.
• Soil Carbon Sequestration: Field trials showed substantial increases in total soil carbon. Mg-modified biochar consistently outperformed Al-modified and control treatments. The application rate of 10 t ha⁻¹ resulted in immediate carbon gains driven by direct pyrogenic carbon addition and secondary inorganic carbonate formation. The highest sequestration rates occurred during the first growing season.
• Soil CO₂ Efflux: While an initial priming effect was observed, engineered biochars, particularly Mg-modified types, led to a net reduction in soil CO₂ efflux over the growing seasons compared to controls.
• Agronomic Impact: Groundnut yields increased by 33–47% with biochar application, attributed to improved soil structure, water retention, and nutrient cycling.
• Environmental Safety: A critical safety assessment revealed that Al-modified biochars pose potential biotoxicity risks in acidic or semi-arid soils due to the potential mobilization of Al³⁺ ions, which can inhibit root growth and disrupt microbial communities. In contrast, Mg-modified biochars offered a liming effect and presented a safer ecological profile.

Significance and Claims
The paper claims that Mg-engineered biochar represents a scalable, dual-benefit strategy for climate change mitigation and sustainable agriculture in semi-arid regions. The study highlights that:

1. Enhanced Mechanisms: Metal modification, specifically with Magnesium, transforms biochar into a highly effective adsorbent through the creation of basic sites for CO₂ chemisorption, surpassing the capabilities of unmodified biochar.
2. Field Validation: The research bridges the gap between laboratory adsorption studies and field application, demonstrating that engineered biochars can significantly augment the soil carbon pool and reduce soil CO₂ efflux in real-world agricultural settings.
3. Safety Trade-offs: The study explicitly identifies a critical trade-off in biochar engineering. While both Mg and Al modifications improve performance, the potential for aluminum toxicity in semi-arid soils makes Mg-modification the preferred and only ecologically safe agronomic strategy for scaling these applications.
4. Synergistic Benefits: The approach offers co-benefits for soil fertility and food security, evidenced by significant yield increases in groundnut crops, positioning engineered biochar as a viable climate-smart agricultural practice.

The authors conclude that while the observed carbon gains are substantial, they result from a combination of direct carbon addition, native soil organic matter stabilization, and inorganic carbonate formation. They caution that future recommendations for metal-modified biochars must prioritize environmental safety, specifically avoiding aluminum in vulnerable soil ecosystems.