This paper deals with the problem of audio source separation using multichannel observation. Utilizing the sparseness of sound signals in the time-frequency domain is a successful approach to source separation that enables us to perform separation based on spatial features obtained from a microphone array. On the other hand, nonnegative matrix factorization (NMF) is also a promising approach for audio source separation, which performs separation based on spectral features. This paper incorporates the idea of NMF into sparseness-based source separation and proposes a novel approach to multichannel source separation based on both spatial and spectral features. Experimental results reveal that our proposed method improves the signal-to-distortion ratio (SDR) by 0.26 dB and the signal-to-interference ratio (SIR) by 1.96 dB compared with a conventional sparseness-based approach. In addition, our proposed model eliminates the need for a number of matrix inversions thanks to the sparseness assumption, and thereby requires a much lower computational cost than a previously-proposed multichannel NMF approach, which also utilizes spectral and spatial features.