Release Characteristics of Different N Forms in an Uncoated Slow/Controlled Release Compound Fertilizer

This study examined the release characteristics of different N forms in an uncoated slow/controlled-release compound fertilizer (UCRF) and the N uptake and N-use efficiency by rice plants. Water dissolution, soil leaching, and pot experiments were employed. The dynamics of N release from the UCRF could be quantitatively described by three equations: the first-order kinetics equation [N_t=N_o(1-e-^kt)], Elovich equation (N_t = a + blnt), and parabola equation (N_t = a + +br^0.5), with the best fitting by the first-order kinetics equation for different N (r = 0.9569^** - 0.9999^**). The release potentials (N_o values estimated by the first-order kinetics equation) of different N in the UCRF decreased in the order of total N > DON > urea-N > NH₄+-N > NO₃-N in water, and total N > DON > NH₄+-N > NO₃-N in soil, respectively, being in accordance with cumulative amounts of N release. The constants of N release rate (k values and b values) for different N forms were in decreasing order of total N > DON > NH₄+-N > NO₃-N in water, whereas the k values were urea-N >DON > NH₄+-N > total N > NO₃–N, and the b values were total N > NH4+-N > DON > NO3–N > urea-N in soil. Compared with a common compound fertilizer, the N-use efficiency, N-agronomy efficiency, and N-physiological efficiency of the UCRF were increased by 11.4%, 8.32 kg kg⁻¹, and 5.17 kg kg⁻¹, respectively. The ratios of different N to total N in the UCRF showed significant correlation with N uptake by rice plants. The findings showed that the first-order kinetics equation [N_t = N_o (l-e-h)] could be used to describe the release characteristics of different N forms in the fertilizer. The UCRF containing different N forms was more effective in facilitating N uptake by rice compared with the common compound fertilizer containing single urea-N form.