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We study the influence of defect states (shallow donors and deep acceptors) on the carrier relaxation dynamics of gallium nitride in the picosecond regime for different excitation intensities and different lattice temperatures. Time-resolved luminescence, degenerate, and nondegenerate four-wave mixing experiments show a saturation threshold in the blue and yellow spectral region, which is found to disappear for lattice temperatures below 200 K. When analyzing all these results in the frame of a rate-equation model, we give a relaxation scenario for the carriers, the lifetimes of the population of the different states, and identify radiative and nonradiative transitions. After filling defect states by an optical excitation, the ambipolar diffusion coefficient of GaN is measured through degenerate four-wave mixing experiments. A law value of 0.16 cm(2)/s at room temperature is determined, indicating that defect states still influence the diffusion. Nondegenerate four-wave mixing experiments exhibit a competition between an electronical and a thermal contribution to the nonlinear susceptibility in GaN. [S0163-1829(99)00340-9].