Nitrogen and carbon mineralisation of different Meliaceae derivatives

Among Meliaceae derivatives, neem cake is usually used as a fertilizer; however its origin and industrial processing are often unknown, so that its effect on soil fertility is not predictable. In this study, the effect of soil incorporation of 6 commercial neem cakes and leaves of Melia azedarach L. on nitrogen (N) and carbon (C) dynamics was investigated in a 118-day laboratory incubation experiment. Neem cake at a rate of 8 g/kg of soil and melia leaves at 16 g/kg were incorporated into the soil and their net N and C mineralisation were evaluated 2 h after application and at day 1, 2, 6, 12, 26, 54 and 118, by analysing a 50-g soil sample placed in 250 glass jars. The apparent net N mineralisation was well predicted by N concentration and C/N ratio of derivatives. The derivatives with a C/N ratio < 24 caused a net N mineralisation, whereas those with a C/N ratio ≥ 24 caused net N immobilisation. C mineralisation ranged between 15% and 25% and was not related to chemical composition of the derivative. Neem cake with a C/N ratio < 24 can be used to add N, while neem cake with a C/N ratio > 24 can be used to reduce soil mineral N.