Effects of cow dung and wood biochars and green manure on soil fertility and tiger nut (*Cyperus esculentus* L.) performance on a savanna Alfisol

Tiger nut (Cyperus esculentus L.) produces edible, nutrient-dense tubers but remains underexploited. In Nigeria it is mainly grown in the middle belt and northern regions on savanna soils (Alfisols, Inceptisols, Ultisols) that are typically acidic, sandy, low in organic matter and nutrients, and susceptible to degradation, leading to low yields. Organic amendments, notably biochar and green manure, can improve soil organic matter, structure, nutrient availability, and microbial activity. Biochar properties depend strongly on feedstock: plant-derived biochars tend to be more aromatic and stable, whereas manure-derived biochars are richer in ash and nutrients. Green manures can rapidly supply nutrients but may leach if not retained in the rooting zone. Tithonia diversifolia was selected as a GM due to high N, P, K contents and rapid decomposition. The study aimed to test single and combined applications of wood biochar and cow dung biochar with Tithonia green manure on soil properties, tiger nut growth, yield, and tuber quality on a tropical savanna Alfisol.

Prior studies indicate biochar can increase soil organic matter, improve soil structure, reduce nutrient leaching, raise pH, and increase cation exchange capacity (CEC). Feedstock influences biochar performance: plant-based biochars are lignin/cellulose-rich and stable; manure-based biochars have higher ash and nutrient content and often higher CEC and pH. Green manures improve fertility via added organic matter and nutrients with rapid mineralization, but synchronization of nutrient release with crop demand is a challenge. Combining biochar with organic amendments (e.g., composts or green manures) has been reported to enhance soil properties and crop yield compared with biochar alone by retaining rapidly released nutrients. Tithonia decomposes quickly and releases a large proportion of its nutrients within the first week, suggesting potential synergy with nutrient-retentive biochar.

Two concurrent field experiments (Sites A and B) were conducted in 2019 at Landmark University Teaching and Research Farm, Omu-Aran, Kwara State, Nigeria (derived savanna; ~1300 mm annual rainfall; mean temperature ~32 °C; Alfisol: Oxic Haplustalf/Luvisol). Treatments at each site: (1) cow dung biochar (CDB) 10 t ha−1, (2) wood biochar (WB) 10 t ha−1, (3) green manure (GM, Tithonia diversifolia leaves and tender stems) 10 t ha−1, (4) CDB 5 t ha−1 + GM 5 t ha−1, (5) WB 5 t ha−1 + GM 5 t ha−1, (6) control (no amendment). Randomized complete block design with three replications; plot size 3 × 2 m; blocks 1 m apart; plots 0.5 m apart. Biochar production: Cow dung from the farm and Parkia biglobosa wood were air-dried; slow pyrolysis at 400 °C for 4 h in a box-type furnace; cooled 12 h; crushed and sieved to 2 mm. Amendments were incorporated to 10 cm depth; plots rested 3 weeks pre-planting. Tithonia biomass harvested locally; leaves and tender stems chopped and incorporated to 10 cm. Planting: Market-sourced tiger nut tubers selected for uniformity; non-viable (floating) tubers discarded; tubers soaked 24 h pre-planting. Planting in May 2019 at 0.2 m intra-row × 0.6 m inter-row spacing (50 plants per plot; ~83,333 plants ha−1). Weekly manual weeding. Soil sampling and analyses: Pre-experiment composite topsoil (0–15 cm) samples collected; bulk density determined from undisturbed cores (0–15 cm). Post-harvest soil sampled by plot. Analyses: texture (hydrometer method), pH (1:2 soil:water), organic C (Walkley–Black), total N (micro-Kjeldahl), available P (Bray-1, molybdenum blue), exchangeable K and Na (NH4OAc, flame photometer), Ca and Mg (NH4OAc, AAS), CEC (BaCl2 compulsive exchange), gravimetric moisture (105 °C overnight). Soil microbiology: viable counts for bacteria, actinomycetes, fungi using standard media; incubation at 28 °C (bacteria 1 day; fungi 5 days; actinomycetes 7 days). Biochar and GM characterization: Organic C, total N, P, K, Ca, Mg (AOAC methods); CEC by NH4+ replacement; Tithonia leaf tissue analyzed per Tel and Hagarty. Tiger nut measurements: Growth (plant height and leaf number) at 72 days after planting (flowering); harvest ~3 months after planting; tubers washed, air-dried, counted, and weighed per plant. Proximate composition of tubers: moisture, ash, crude fiber, crude protein (micro-Kjeldahl), lipid (Soxhlet, petroleum ether), carbohydrate by difference (AOAC/Muller & Tobin methods). Statistics: One-way ANOVA (SPSS 17.0); means separated by Duncan’s multiple range test at p = 0.05.

- Baseline soils at both sites were sandy loam, acidic (pH ≈ 5.6), low in organic matter and macronutrients (except P around the critical level), with microbial abundance order: bacteria > actinomycetes > fungi. - Biochar and GM properties (Table 2): CDB had higher pH (8.31) and CEC (5.7 cmol kg−1) than WB (pH 7.31; CEC 3.1). CDB had higher Mg (1.36%), Na (1.88%), and P (1.54%) than WB; GM had the highest N (3.88%), K (4.41%), Ca (3.42%), and the lowest C:N (7.16). WB had the highest organic C (56.7%) and highest C:N (63.71). - Soil chemical properties and moisture (Table 3): All amendments (CDB, WB, GM, and their combinations) significantly increased soil pH, organic C, total N, available P, exchangeable K, Ca, Mg, CEC, and moisture content versus control at both sites. Combined treatments outperformed sole applications. Example (Site A → Site B similar): • pH: Control 5.51 → CDB+GM 6.60–6.71; WB+GM 6.59–6.64. • Organic C (%): Control ~0.92 → CDB+GM ~1.95; WB+GM ~1.90. • Total N (%): Control ~0.16 → CDB+GM 0.29; WB+GM 0.25. • Available P (mg kg−1): Control ~9.4–9.5 → CDB+GM 20.6–21.4. • K (cmol kg−1): Control 0.12 → CDB+GM 0.41; WB+GM 0.40. • Ca (cmol kg−1): Control 1.55 → CDB+GM 2.46; WB+GM 2.40. • Mg (cmol kg−1): Control 0.30 → CDB+GM 0.88; WB+GM 0.78. • CEC (cmol kg−1): Control 4.81 → CDB+GM 5.96; WB+GM 5.69. • Moisture (%): Control 10.6–10.9 → CDB+GM 14.6–14.7; WB+GM 14.3–14.4. - Soil microbiology (Table 4): All amendments increased culturable bacteria, actinomycetes, and fungi relative to control; combinations were highest. Bacteria increased from ~1.36–1.39 ×10^5 CFU g−1 (control) to ~10.4–10.85 ×10^5 (CDB+GM); fungi from ~1.60–2.43 ×10^5 to ~4.66–4.86 ×10^5 (CDB+GM). - Growth and yield (Table 5): All amendments improved plant height, leaf number, tuber number, and tuber weight per plant relative to control. Among sole amendments: GM > CDB > WB. Combinations further increased performance, with CDB+GM highest. Using the mean of both sites, CDB+GM increased tuber weight by 36.1% and 24.5% compared with CDB and GM, respectively; WB+GM increased tuber weight by 47.5% and 14.0% compared with WB and GM, respectively. - Tuber proximate composition (Fig. 1): Amendments (sole or combined) increased moisture, ash, fiber, and protein, and decreased lipid and carbohydrate contents versus control. GM generally produced higher proximate values than sole biochars, while CDB+GM gave the best overall nutritional profile (highest moisture, ash, fiber, protein).

The savanna Alfisol at the study site was nutrient-poor and acidic, consistent with regional reports. Amendment with CDB, WB, and GM improved soil chemistry by supplying nutrients (especially ash-derived bases: K, Ca, Mg, Na) and by increasing pH and CEC, which enhance nutrient retention. Biochar’s porous structure and carboxylate functional groups contribute to adsorption and retention of ammonium, nitrate, phosphate, and other ions, elevating CEC and reducing leaching. GM raised soil nutrients through rapid mineralization, reflecting its low C:N ratio and high N, P, K content. Soil moisture increased with biochar due to enhanced water holding in biochar micro- and mesopores; GM likely improved aggregation and biopores, further aiding moisture retention. The improved pH, moisture, and nutrient availability supported higher microbial abundance; positive correlations were observed between microbial counts and soil pH, moisture, and CEC at both sites. Yield gains mirrored soil improvements. CDB and especially CDB+GM outperformed WB and WB+GM, likely because CDB had higher pH, CEC, and nutrient content than WB, enabling better nutrient retention and supply. GM alone outperformed sole biochars for growth and yield due to faster nutrient release (low C:N). Combining biochar with GM created synergy: biochar retained nutrients rapidly released from decomposing Tithonia within the rooting zone, synchronizing supply with plant demand and acting as a slow-release reservoir. This synergy translated into the highest growth and yield in CDB+GM plots. Proximate composition changes (higher protein, fiber, ash, moisture; lower lipid and carbohydrate) align with improved N availability driving protein synthesis and possible trade-offs with oil accumulation. Overall, integrating nutrient-rich manure-derived biochar with fast-decomposing green manure optimized soil fertility, microbial activity, plant growth, and tuber nutritional quality.

Applying cow dung biochar (CDB), wood biochar (WB), and Tithonia green manure (GM), either singly or in combination, improved soil pH, organic carbon, nutrients, CEC, moisture, and microbial abundance, and enhanced tiger nut growth, yield, and tuber nutritional quality compared with the control. CDB exhibited higher N, P, K, Ca, Mg, Na, pH, and CEC than WB. GM alone outperformed sole biochars, while combining biochar with GM further improved outcomes, with CDB+GM producing the greatest growth and yield, attributed to biochar retention of nutrients released rapidly from GM. For improved soil fertility and tiger nut yield on savanna Alfisols, pairing biochar with a fast-releasing nutrient source is recommended. Future work should assess multi-season/long-term effects, diverse biochar feedstocks and pyrolysis conditions, rates and timing strategies, and economic and environmental trade-offs.

- The experiments were conducted in a single year (2019) at one location (two concurrent sites within the same farm), which may limit temporal and spatial generalizability. - Only one pyrolysis temperature and duration (400 °C, 4 h) and specific feedstocks (cow dung and Parkia wood) were tested; results may differ with other biochars. - A single green manure species (Tithonia diversifolia) and fixed application rates were used; rate-response and alternative green manures were not evaluated. - Short-term outcomes were measured; longer-term residual effects of biochar-GM combinations on soil properties and yields were not assessed. - No mineral fertilizer comparison treatment was included to benchmark against conventional fertilization regimes.