Valorizing lignite waste into engineered nitro-humic fertilizer: Advancing resource efficiency in the era of a circular economy

This study investigates using lignite waste as a carrier matrix for creating engineered nitro-humic fertilizer (NHF). The NHF is synthesized through an innovative ozone oxidation method and nitrogen enrichment. Agronomic efficiency was evaluated through maize growth responses, soil incubation experiments, and fertilizer performance, comparing NHF to commercial urea and NPK fertilizers. Nitrogen release patterns and kinetic models were studied to understand the nitrogen release mechanism of NHF. The engineered structure of NHF displayed globular-like, microporous, and heterogeneous properties, with a negative charge density of 48.85 mV and lower thermal stability of aliphatic carbons. The NHF exhibited a remarkable capability to increase water-holding capacity by up to 48% and extend the water-retention period by 57% during 30 d of soil application. NHF significantly reduced soil urease enzyme activity by more than two-fold. It exhibited a longer nitrogen release period (77 d) with a slow-release pattern, releasing 71.75% of total nitrogen compared to 88.5% in 14 d for urea. The simple Elovich model accurately predicted NHF's nitrogen release kinetics (R² = 0.9943), identifying Fickian diffusion-based controlled release. Applying NHF improved multiple aspects of maize growth: height, stem diameter, biomass, root weight, leaf area, SPAD value, and leaf nitrogen concentration. Total chlorophyll, carotenoid, and nitrogen uptake also greatly increased compared to the control sample. NHF exhibited superior nitrogen use efficiency (41.5%) and agronomic fertilizer efficiency (36.22%) compared to urea and even surpassed commercial NPK fertilizers. Generally, the produced NHF is an eco-friendly alternative to slow-release fertilizers, offering the advantages of humic substances and nitrogen fertilizers.