Abstract
A field study was conducted to assess the effect of neem seed cake (NSC) on the kinetics of net nitrogen mineralization and parameter estimates. The experiment consisted of sorghum plots to which milled NSC (with %N > 3) and urea (inorganic N source) were applied. Ammonium-nitrogen and nitrate-nitrogen were determined after 0, 2, 4, 6, 8, 12, 19 and 34 weeks from soils covered with PVC tubes inserted into the sixteen treatment combination plots. Inorganic N concentration in the amended plots and the control were analysed using five mathematical N mineralization models. N0, Ne, h and k were estimated from the fit of each mathematical model. The treatments were highly variable in their respective N mineralization characteristics, but the peaks for mineralized N content were observed at early incubation periods, i. e., 10 -28 days. Potential respective mineralizable N (N0) content of the first order rate, consecutive (h ‚ k), consecutive (h = k), Gompertz and mixed order rate constant was -927E-22, 430.90, 1.9E-21, 568.40 and 427.20 mg kg-1 soil and the respretively rate constant (k) was -5E+21, 0.2269, 2E+23, 0.0954 and 0.8116.respretively Net N mineralization was best described by a mixed order rate model (R2 = 0.992) and residual mean square error (RMSE)was = 28.41. A Gompertz function also fitted the data closely (R2 = 0.999; RMSE = 52.47). The two models recognize that N mineralization is not just a function of substrate N concentration but also of the capacity of the microbial community to adjust to substrate quality and composition affecting their growth and activity. Key words : mineralization model, neem seed cake, N mineralization, parameter estimates, PVC tubes
Highlights
Good management of agricultural production systems such as soil resources is a response to challenges posed by world food crisis
The cumulative N mineralization with time showed an initial rapid rate in the first six weeks after incubation followed by slower release.later The N mineralization pattern showed that the NO3- -N fraction in the soil far exceeded the NH4+-N fraction, the formeraccounting for 64% of the N mineralized from neem seed cake.( NSC) The high nitrification rates recorded could be attributed to the retardation effect of neem residues on nitrification which is due to their active ingredient azadirachtin and other di- and triterpenoidal compounds which are toxic to nitrifying bacteria (Singh et al, 1991)
The mean amounts of N mineralized was lower in the control than in the other treatments except in the 1⁄4 recommended rates (RR) of NPK fertilizer for sorghum with a mean total inorganic N of 56.76 mg kg-1 (Table 3)
Summary
Good management of agricultural production systems such as soil resources is a response to challenges posed by world food crisis. The cumulative N mineralization with time showed an initial rapid rate in the first six weeks after incubation followed by slower release.later The N mineralization pattern showed that the NO3- -N fraction in the soil far exceeded the NH4+-N fraction, the formeraccounting for 64% of the N mineralized from neem seed cake.( NSC) The high nitrification rates recorded could be attributed to the retardation effect of neem residues on nitrification which is due to their active ingredient azadirachtin and other di- and triterpenoidal compounds which are toxic to nitrifying bacteria (Singh et al, 1991).
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