Surface reconstructions and adatom kinetics of silicon on GaN(0001) and (0001) surfaces are studied by scanning tunneling microscopy, electron diffraction, and first-principles calculations. For silicon coverage near 0.5 monolayer, a 2 X 2 structure is observed, and is interpreted in terms of a model consisting of a Ga adatom on a monolayer of 3 Ga+ 1Si and a Si-Ga atom in the third layer. For higher silicon coverage, disordered 2 X 2 domains and "1 X 1" domains are found to coexist. After annealing above 300 degreesC the "1 X 1" regions become dominant and a 4 X 4 structure is seen near step edges. It is concluded that the silicon adatoms tend to reside in subsurface sites on the Ga-polar surface. Surface morphology in the presence of Si is smooth for the (0001) surface but rough for the (0001) surface. This difference is attributed to the presence of multiple Ga surface layers in the former case, which enhance surface diffusivities.