Unveiling biochar-urea interactions in slow-release nitrogen fertilizers: a multinuclear solid state NMR study

Composites constituted by urea and biochar are slow-release fertilizers expected to be particularly beneficial for the soil. The interaction between urea and biochar is a key aspect for the successful use of these composites, as it affects urea retention and subsequent release, and its understanding at a molecular level is a challenging task. In this work, multinuclear Solid State NMR (SSNMR) spectroscopy, also combined with the use of 13C and 15N isotopically enriched urea, was employed to get insight into urea-biochar interactions. Composites were prepared according to two different impregnation methods, one based on saturated aqueous urea solution and the other on molten urea, resulting in the BUs and BUm composites, respectively. 13C and 15N SSNMR experiments carried out on the composites before and after release in water at different times revealed that urea predominantly exists in the composites in the paracrystalline form on the surface of biochar or physically entrapped within biochar pores. In BUm, products of thermal degradation of urea were also detected, mainly in the form of biuret. Release experiments showed 78% and 64% loss of urea, for BUs and BUm, respectively, after 24 hours of immersion in water. Notably, residual urea is mainly physically confined in the biochar pores. In the case of BUm, species arising from the thermal degradation of urea are also partially retained. The adopted comprehensive characterization methodology elucidated the impact of the preparation method on the composition and the release behavior of the composites and offered an unprecedented view of the interactions between urea and biochar at the molecular level.