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Dissipative and dispersive optomechanical couplings are experimentally observed in a photonic crystal split-beam nanocavity optimized for detecting nanoscale sources of torque. Dissipative coupling of up to approximately 500 MHz/nm and dispersive coupling of 2 GHz/nm enable measurements of sub-pg torsional and cantileverlike mechanical resonances with a thermally limited torque detection sensitivity of 1.2×10^{−20}  Nm/sqrt[Hz] in ambient conditions and 1.3×10^{−21}  Nm/sqrt[Hz] in low vacuum. Interference between optomechanical coupling mechanisms is observed to enhance detection sensitivity and generate a mechanical-mode-dependent optomechanical wavelength response.