Tiger nut (*Cyperus esculentus* L.), a tuber crop rich in dietary fiber, carbohydrates, protein, and micronutrients, is underutilized due to a lack of information on its cultivation and potential. In Nigeria, tiger nut is primarily grown in regions with Alfisols, Inceptisols, and Ultisols, which are often characterized by low fertility and susceptibility to degradation. This low soil fertility limits crop yields significantly. Sustainable soil improvement strategies are therefore crucial for enhancing tiger nut production. Organic amendments, including biochar and green manure, have emerged as potential solutions for improving soil health and crop productivity. Biochar, a stable carbon-rich material produced through pyrolysis, offers several benefits: increased water retention, enhanced nutrient availability, improved soil structure, and increased microbial activity. The type of feedstock used in biochar production affects its properties, with animal manure biochar offering higher nutrient content than plant-derived biochar. Green manure, such as *Tithonia diversifolia*, contributes to improved soil fertility by increasing nutrient content and organic matter, improving soil structure and reducing nutrient leaching. However, the application of biochar alone may not be sufficient to provide adequate nutrients due to its slow degradation. This study aimed to investigate the effects of single and combined applications of wood and cow dung biochar with green manure (*Tithonia diversifolia*) on soil properties, growth, yield, and tuber quality of tiger nut grown on a tropical savanna Alfisol. The research questions addressed were: (1) How do single and combined applications of wood and cow dung biochar and green manure affect soil properties (pH, moisture content, organic carbon, nutrients, and microbial populations)? (2) How do these treatments impact tiger nut growth parameters (plant height, number of leaves, number of tubers, and tuber weight)? (3) How do the treatments affect the proximate composition of tiger nut tubers (moisture, ash, fiber, protein, lipid, and carbohydrate content)? Understanding the effects of these amendments is critical for developing sustainable agricultural practices for enhancing tiger nut productivity in low-fertility soils.

Several studies have highlighted the benefits of biochar and green manure in improving soil health and crop yields. Biochar's properties, including its high porosity, surface area, and cation exchange capacity, contribute to improved water retention and nutrient availability (Liang et al., 2006; Lehmann & Joseph, 2009). The type of feedstock influences biochar properties; animal manure-derived biochar tends to have higher nutrient content than plant-derived biochar (Cantrell et al., 2012). Green manures, particularly *Tithonia diversifolia*, are known for their capacity to enhance soil fertility through nutrient enrichment and improved organic matter content (Sonke, 1997; Adekiya, 2019). However, the optimal combination of biochar and green manure requires investigation. Previous research has shown that combining biochar with organic amendments can lead to improved soil properties and crop yields compared to the sole application of biochar (Adekiya et al., 2019). This study builds on this existing research by comparing the effects of different biochar sources (wood and cow dung) in combination with green manure on a specific crop (tiger nut) and soil type (Alfisol).

Two field experiments were conducted concurrently at Landmark University's Teaching and Research Farm in Omu-Aran, Kwara State, Nigeria, during the 2019 cropping season. The experimental site is located in the derived savanna ecological zone, characterized by a bimodal rainfall pattern and a mean annual temperature of 32 °C. The soil type is Alfisol. Six treatments were employed in a randomized complete block design with three replications: (1) Cow dung biochar (CDB) at 10 t ha⁻¹, (2) Wood biochar (WB) at 10 t ha⁻¹, (3) Green manure (*Tithonia diversifolia*) (GM) at 10 t ha⁻¹, (4) CDB + GM (5 t ha⁻¹ each), (5) WB + GM (5 t ha⁻¹ each), and (6) Control (no amendment). Biochar was produced via slow pyrolysis at 400 °C for 4 hours. Soil samples were collected before and after the experiment to determine various parameters including textural class, bulk density, organic carbon, total nitrogen, available phosphorus, exchangeable potassium, calcium, and magnesium, pH, cation exchange capacity (CEC), and microbial populations (bacteria, actinomycetes, fungi). Tiger nut tubers were planted with appropriate spacing. Growth parameters (plant height, number of leaves) were measured at 72 days after planting (flowering stage). At harvest (approximately 3 months after planting), tubers were counted, weighed, and subjected to proximate analysis (moisture, ash, crude fiber, crude protein, lipid, and carbohydrate content). Data were analyzed using one-way analysis of variance (ANOVA) with Duncan's multiple range test (DMRT) at p=0.05.

Pre-experiment soil analysis revealed that the Alfisol was low in organic matter and several essential nutrients. All treatments incorporating CDB, WB, or GM, either singly or in combination, resulted in significant increases in soil pH, organic carbon, total nitrogen, available phosphorus, potassium, calcium, magnesium, and cation exchange capacity compared to the control. CDB alone and the CDB+GM treatment showed the highest improvements in soil properties. Soil microbial populations (bacteria, actinomycetes, and fungi) were significantly higher in amended soils than in the control. CDB, WB, and GM, as well as their combined applications, significantly enhanced tiger nut growth parameters (plant height, number of leaves, number of tubers, and tuber weight). CDB and CDB+GM exhibited the highest growth and yield, surpassing WB and WB+GM. GM alone produced better growth and yield compared to biochar treatments alone. The combined applications of CDB+GM and WB+GM showed significantly higher growth and yield compared to their respective sole applications, suggesting a synergistic effect. Proximate analysis of tiger nut tubers showed that the treatments (sole or combined) increased moisture, ash, fiber, and protein content compared to the control while decreasing lipid and carbohydrate content. CDB+GM produced the highest values for moisture, ash, fiber, and protein content. Specifically, CDB + GM increased tiger nut weight by 36.1% compared with CDB alone and by 24.5% compared to GM alone. Similarly, WB+GM increased tiger nut weight by 47.5% compared with WB alone and by 14.0% compared with GM alone.

The significant improvements in soil properties observed in this study directly correlated with the enhanced growth and yield of tiger nut. The superior performance of CDB compared to WB can be attributed to its higher nutrient content and greater cation exchange capacity, which facilitate better nutrient retention and availability. The synergistic effect of combining biochar with green manure is likely due to the biochar's capacity to retain nutrients released from the rapidly decomposing *Tithonia* within the rooting zone, leading to more efficient nutrient uptake by the tiger nut plants. The lower C:N ratio of *Tithonia* compared to biochar likely contributed to the faster nutrient release and uptake. The increase in microbial populations in amended soils further supports the overall improvement in soil health and plant nutrient availability. The changes in the proximate composition of tiger nut tubers suggest that the treatments influenced the allocation of resources within the plant, possibly leading to increased protein production at the expense of lipid content. These findings provide valuable insights into the effective use of biochar and green manure for improving soil fertility and enhancing tiger nut production in low-fertility savanna Alfisols.

This research demonstrates the effectiveness of applying cow dung biochar, wood biochar, and green manure (Tithonia diversifolia), either individually or in combination, in enhancing soil fertility and tiger nut yield on a savanna Alfisol. Cow dung biochar, due to its higher nutrient content and cation exchange capacity, proved particularly beneficial. Combining biochar with green manure resulted in synergistic effects, likely due to biochar’s ability to retain nutrients released by the rapidly decomposing green manure. These findings suggest that integrating a fast-releasing nutrient source, such as green manure, with biochar offers a promising strategy for sustainable agriculture in low-fertility environments. Future research could focus on optimizing the application rates of biochar and green manure, exploring other green manure species, and investigating the long-term impacts of these amendments on soil health and tiger nut production.

The study was conducted over a single growing season, limiting the assessment of long-term effects of the amendments. The research was confined to a specific geographic location and soil type, limiting the generalizability of the findings. The study did not address the economic feasibility of implementing the recommended practices. Further investigations incorporating these aspects would provide a more comprehensive understanding of the practical implications of the findings.