Effects of bio-slurry and chemical fertilizer on soil fertility and productivity of bread wheat (Triticum aestivum L.) in Northwestern Ethiopia

BACKGROUND: Declining soil fertility and rising mineral fertilizer costs severely constrain wheat production in Ethiopia, threatening food security. Bio-slurry from biogas digesters offers a potential low-cost organic amendment, but limited field evidence exists on its integrated use with chemical fertilizers. METHODS: A two-year field experiment (2023/2024-2024/2025) was conducted at the University of Gondar Shinta Research Farm, northwestern Ethiopia. Six treatments: control (no amendment), 100% chemical fertilizer (CF: 100 kg N ha⁻¹ as urea + 100 kg NPS ha⁻¹), 75% CF + 25% bio-slurry (BS), 50% CF + 50% BS, 25% CF + 75% BS, and 100% BS (10 t ha⁻¹ dry weight) were arranged in a randomized complete block design with three replications. Wheat agronomic parameters, grain yield, soil physicochemical properties, and heavy metal concentrations (Pb, Cd, Cr) were analyzed using standard methods. RESULTS: Integrated BS and CF applications significantly improved all agronomic parameters compared to sole CF or control (p < 0.05). The 25% CF + 75% BS treatment produced the highest aboveground dry biomass (9.5-11.1 t ha⁻¹) and grain yield (2.8-4.4 t ha⁻¹), representing 40-60% increases over sole CF. Post-harvest soil analysis revealed significant increases (p < 0.05) in pH (from 5.6 to 8.0), organic carbon (from 1.8% to 15.8%), organic matter (from 3.2% to 27.5%), total nitrogen (from 1.0% to 2.2%), and available phosphorus (from 1.4% to 1.9%) under BS-amended treatments. Heavy metals (Pb, Cd, Cr) in both soil and wheat grain remained below detection limits, confirming food safety. CONCLUSION: Partial substitution of chemical fertilizer with bio-slurry at 25% CF + 75% BS enhances wheat productivity and soil fertility while reducing mineral fertilizer dependence by 75%. This integrated nutrient management strategy offers a sustainable, economically viable approach for smallholder farmers in Ethiopian highlands, supporting circular economy principles through organic waste recycling.