Biochar Co-composted Products as Biochar-based Organic Fertilizer Improved Spinach Quality and Soil Properties

Objective The study was to prepare biochar-based organic fertilizer by adding exogenous straw biochar to conventional compost, and to evaluate its effects on spinach yield, quality and soil properties. The aims were to address the role of biochar as a compost amendment and the potential of this co-compost product as a biochar-based organic fertilizer to enhance yield and efficiency in vegetable soil.

Method Using straw and pig manure as composting materials, exogenous biochar was added at ratios of 0%, 5%, 15%, and 25% for co-composting experiments. By analyzing the total carbon and nutrient content of the compost product, the study evaluated whether the co-compost product could be identified as biochar-based organic fertilizer. Subsequently, the biochar-based organic fertilizer was combined with chemical fertilizers to cultivate spinach, investigating its effects on spinach quality and soil properties.

Result The co-compost product exhibited total carbon content of 387.4 - 420.4 g kg⁻¹, pH of 8.3 - 8.4, and total nutrient (N + P₂O₅ + K₂O) content of 6.9% - 11.3%, meeting the main criteria for biochar-based organic fertilizer (NY/T 3618-2020). Compared to conventional compost without biochar, the addition of 15% and 25% biochar increased the total carbon content of the co-compost product by 7.6% - 12.3%, and the humification degree by 47.0% - 101.4%, while reducing total nutrient content by 16.4% - 37.3%. When the identified biochar-based organic fertilizer was for spinach cultivation, compared to conventional compost without biochar, spinach yield increased by 16.4% - 22.2% (P < 0.05), vitamin C content increased by 40.7% - 96.7%, and soluble sugar content increased by 31.1% - 72.6%. Soil organic carbon content increased by 7.4% - 11.1%, while available phosphorus decreased by 11.0% - 32.1%.

Conclusion Considering compost product quality, vegetable yield, quality, and soil organic carbon improvement, the optimal addition ratio for biochar is 15% during co-composting, with a recommended application rate of 5 t hm⁻² as a base fertilizer for the biochar-based organic fertilizer.