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We present an ab initio density functional theory study of the dominant defects in hydrogenated amorphous silicon nitrides covering different stoichiometries, the influence of hydrogen, and the influence of the annealing history. Whereas nitrogen (N) lone pair states dominate the valence band edge in stoichiometric a-Si${}_{3}$N${}_{4}$, we find that K defects, threefold coordinated silicon (Si) atoms, and Si-Si bond-related states dominate electronic defect contributions in the gap for N-deficient a-Si${}_{3}$N${}_{4\ensuremath{-}x}$. Hydrogen saturates the dangling Si bonds, significantly reducing the number of electronic defects related to undercoordinated Si atoms.