Nutrient transfer and transformation mechanism of biochar-based infiltrated urea fertilizer from different raw materials

Objective To study and compare the fertilizer formation and nutrient release characteristics of biochar-based infiltrated urea fertilizer (BIUF) from different raw materials, explore the effects of biomass type, pyrolysis temperature and surface functional group content on the release mechanism of fertilizer granules in fertilizer production by percolation method, and provide a theoretical guidance for further optimization of the slow release performance of BIUF.

Method BIUF was prepared by the infiltration method using rice husk bichar (HB), pine nut biochar (PNB), oil-tea biochar (OTB), peanut biochar (PB) and urea pellets as raw materials, and its molding characteristics and nutrient migration mechanism were investigated using a universal pressure tester, scanning electron microscope, thermogravimetric analyzer and Fourier infrared spectrometer.

Result The apparent morphology and pore wall thickness of biochar particles influenced the physical properties of BIUF. Compared with fragile and flaky biochar particles, the structure of honeycomb-shaped biochar particles was more conducive to improve the physical and mechanical properties of BIUF, and the resistance to fragmentation increased by 1.7−2.3 times. The content of functional groups on the surface of the biochar affects the slow release performance of BIUF. Functional groups such as —COOH, —C=O and —OH on the charcoal surface produced a solid chemical state urea with urea, while —OH on chain fatty alcohols reacted in an alcoholysis reaction with urea, resulting in reduced fertility of BIUF and a 40% to 50% shorter release cycle. Due to the complex pore structure of the honeycomb husk biochar, the abundant oxygen-containing functional groups and the low content of chain fatty alcohols, the prepared peanut BIUF possessed excellent fragmentation resistance, while the nutrient release cycle was prolonged by more than 90%, with good release performance.

Conclusion The unique physicochemical properties of different biochars have good nutrient adsorption properties. By increasing the pyrolysis temperature and extending the pyrolysis time, the charring degree of biomass materials can be enhanced, and the prepared BIUF has excellent physical properties and certain slow-release properties, which have a positive impact on the sustainable development of agriculture.