Mineralisation of urea and urea derivatives in anaerobic soils

Summary The rates of mineralisation of urea and urea derivatives were studied in a laboratory anaerobic incubation experiment. Urea, urea phosphate and sulphur coated urea were hydrolysed rapidly and, even at the highest level of application, had disappeared in just over 8 days. The presence of the phosphate anion depressed pH in the early stages. Hydrolysis of the less soluble organic derivatives of urea, isobutylidine diurea, ureaform and glycoluril was very much slower and in the case of glycoluril a lag period of 8 to 16 days occurred before hydrolysis began.In the initial stages the system was anaerobic but between days 8 and 16 a change to partial aerobic conditions occurred. At this stage nitrification commenced and at day 16 nitrite was detected. Reduction of Fe(III) increased with time, reaching a maximum at day 32. More Fe(II) was produced in the presence of organic derivatives of urea than with the other fertilizers, possibly due to stabilisation by organic ligands. From day 16 nitrification, denitrification and reduction of Fe(III) proceeded together even through E_h values indicated that oxidation of Fe(II) would be expected. This did not occur until after day 32. Once nitrification began denitrification quickly followed so that for all six fertilisers, except at the highest level of application, virtually all the mineralised-N had been lost by denitrification at the end of the experiment.