  We probe the relaxation dynamics of the full three-level spin system of
near-surface nitrogen-vacancy (NV) centers in diamond to define a $T_{1}$
relaxation time that helps resolve the $T_{2} \leq 2T_{1}$ coherence limit of
the NV's subset qubit superpositions. We find that double-quantum spin
relaxation via electric field noise dominates $T_{1}$ of near-surface NVs at
low applied magnetic fields. Furthermore, we differentiate $1/f^{\alpha}$
spectra of electric and magnetic field noise using a novel noise-spectroscopy
technique, with broad applications in probing surface-induced decoherence at
material interfaces.
