We're doing "lasagna bed" gardening (a form of sheet composting) with tomatoes planted in it. The top layer is maybe a foot of soybean hay that acts as a mulch. This hay contained a bunch of dried soybean pods, and those soybeans have sprouted and are growing in the mulch.

Since the roots are only in the mulch, it wouldn't be hard to pick the seedlings. But, I'm wondering, would it be better to leave them? I've read that soybeans can capture nitrogen, is it worthwhile to just let them be as they might help the tomatoes out, or will they compete against them? Thanks.