Impact of Urea-Intercalated-Biochar on N-Release in Soil and Humified Soil Organic Matter

This research investigated the impact of urea-intercalated-biochar (ICB) on nitrogen release as well as on humified soil organic matter (SOM) distribution in highly weathered soil. Incubation studies were performed with a typical dystrophic red Ultisol and with 10 treatments- soil without amendment (control, CON), soil with three doses of biochar (BC75, BC145, and BC 290), and soil with ICB doses (ICB75, ICB145 and ICB 290). BC and ICB were characterized by 13C Nuclear Magnetic Resonance 13C-NMR CP/MAS spectroscopy and elemental analyses. N-extractable forms were periodically determined by Kjeldahl method and humic substances (HS) distribution was analyzed on the 28th day after incubation begins (DAI). Intercalation of urea in BC increased sample N content from 2.7 to 22 %, whereas that of C decreased from 40 % to 28 %. As a consequence, C- N decreased from 14.9 to 1.3. The 13C NMR spectra of BC and ICB were similar but relative abundance of major organic functional groups differs, for example, ICB presented a greater proportion of C=O groups due to the carbonyl group of urea and a greater proportion of N/O alkyl C, O-alkyl C, and anomeric C groups due to the addition of starch. There was a significant increase in the concentration of extractable N-NH4+ by addition of urea from the onset of incubation down late period and a significant difference (P > 0.05) was obtained between the ICB and BC over time. This could be as a result of the less accessible form of N in the biochar materials due to the pyrolysis process. There was no evidence of extractable N-NO3- ion until 14 DAI, significant difference in N-NH4+ concentrations at 28 DAI between the ICB and control was observed over time and the highest value recorded from ICB treated pots at 145 and 290 kg N ha-1.