As per title the power to run the sensor can be generate using Biomass and Bacterial colony.

There are several species of bacteria that can produce Bio-gas to generate power for sensors.

To produce biogas or power, these bacterial colony will be depend on  liquid of leaves of tree, grass. The liquid can be obtained by just crushing the leaves and grass. As per sketch just  infuse the liquid gained from tree leaves or grass into the Vial filled with bacterial colony as a food for bacterial colony and the colony will work to provide power in the form of gas. This gas can be used directly to powering the sensor or as a fuel cell. I know there might be some challenges to implement this Idea but can be tackle easily. The important point is the the bacterial colony can produce the power using nearby biomass. Even it can be designed in this way that it can feed itself.