There a couple ways to do this. I'm guessing you need the landscape without the cube so you can add color later, so render that alone. Then, apply a Shadow Catcher material to the landscape. Set this material to Environment and For Compositing. You might also need a Corona Compositing tag on the sky set to Not Seen By Camera. Set your Render Setting so save Alpha. Render out as a PNG or any format that support an alpha channel. Boom.
If i missed any step, someone please correct me.
Edit: If you're only adding color to the landscape you'll need a mask for it, or render it separately.