Agronomic Response and Environmental Effects of Different Waste Fine Coal-Based Fertilizers on Cauliflower Crop

Coal mining generates considerable amounts of fine coal processing waste, usually disposed of due to its low market value and high treatment costs. This material is rich in humic substances and could enhance soil fertility, due to both biostimulant effects and synergistic interactions when combined with other fertilizers. Therefore, in a two-year cauliflower-zucchini crop rotation a pre-treated coal mining waste, co-composted with compost from municipal wastes plus urea (T1) or plus lawn mowing residues (T2) was applied, in order to evaluate agronomic and environmental performance. These treatments were compared with a not composted mixture of the two substrates (T3) and with on-farm compost made from crop residues (T4). A split-plot experimental design was set up with crimson clover intercropped with cauliflower and terminated before zucchini transplanting, either as green manure (GM) or using the roller crimper technique (RC). In the first year the cauliflower yield showed no significant difference among fertilizer treatments, while in the second one the T3 reached the lowest yield with the highest yield reduction (- 15%) compared to the first season. A significative reduction of yield in the second year was observed in CT (- 22%), while the treatments with cover crops did not show any significant reduction. Among other factors, the fertilization had the greatest impact on both energy consumption and greenhouse gas emissions. On average, T3 showed the highest energy consumption (6750 MJ ha(-)1 yr(-)1) and the lowest energy output, resulting in the worst net energy. Among treatments, T4 performed the best in terms of the net energy index (the difference between energy saved in crop yield and energy consumed), followed by T1 and T2. Considering the net CO2 eq. (emissions minus stocks), T4 also showed the best results, followed by T1, T3, and T2. Our study highlights the importance of composting to increase the value of different industrial/agricultural organic wastes and confirms the positive effect of cover crop introduction in organic crop rotations as a sustainable agronomic strategy.