We have carried out a wavelength-dependent, near-infrared pump-probe study of micelle-suspended single-walled carbon nanotubes whose linear absorption spectra show a number of chirality-dependent peaks. Two distinct relaxation regimes are observed: fast (0.3-1.2 ps) and slow (5-20 ps). The slow component, which has not been observed previously, is resonantly enhanced whenever the pump photon energy resonates with an interband absorption peak, and we attribute it to radiative carrier recombination. The slow component also exhibited a drastic decrease in intensity with decreasing pH (or increasing H$^+$ density) in the solution, especially in large-diameter tubes, which indicates an important role played by H$^+$ ions surrounding the nanotubes (as ``acceptors'').