soil algae and nematodes versus bacteria????

I assume that this is because nematodes feed on organic material, helping with decomposition, and most organic material is found on the top layers of soil.

Generally there is more pore space at or close to the soil surface, and (generally) pores became smaller with increasing depth. Nematodes (which are not unicellular organisms like bacteria) require pore space to move around. Hence, abundance of organic matter & presence of larger pores at or close to the soil surface enhance larger number of nematodes at soil surface relative to the deeper depths.

As for why are soil nematodes smaller than nematodes that live in aquatic systems, the explanation is the same as for algae.