In the summertime the water level is low due to evaporation and low rainfall. The Riccia falls to the bottom and grows in the mud along with the Eleocharis sp. entangled. (Lower the water level in the aquarium to 5cm or so, place the Riccia where needed and then plant Eleocharis sp. in the substrate in the clumps of Riccia, make sure the substrate is relatively fine grained and give it good light. The Riccia does not have to be buried, let it do what it does...).
In the autumn and winter, rainfall increases and the water level rises. The Riccia remains on the bottom amid the Eleocharis. Of course, some naturally floats to the top. (When the Riccia and Eleocharis sp. have established in the aquarium substrate, raise the water level slowly. If need be, use small stones to hold any recalcitrant Riccia down).
Karen Randall, in another post on this board, reminded us that there are several different types of Riccia. Some might be more appropriate for this type of substrate dwelling than others.