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We investigate nonlinear interaction of nitrogen molecules with a two-color laser field composed by an intense 800nm laser pulse and a weak 400nm laser pulse.It is demonstrated that the spectrum of 400nm pulses is dramatically broadened when the two beams temporally overlap.In comparison,the spectral broadening in argon is less pronounced,although argon atoms and nitrogen molecules have comparable ionization potentials.We reveal that the dramatic spectral broadening originates from the greatly enhanced nonlinear optical effects in the near-resonant condition of interaction between the 400nm pulses and the nitrogen molecular ions.