From ImageMagick's command line options:
-sampling-factor
This option specifies the sampling factors to be used by the JPEG encoder for chroma downsampling. If this option is omitted, the JPEG library will use its own default values.
Here is a good thread.. on Knowing which chroma-subsampling was used in a JPEG
Chroma Subsampling in JPG Compression
I hope this helps :)