Question

what organisms are responsible for making nitrogen usable for plants?
bacteria
fungi
producers
decomposers
why are food chains usually short?
energy is lost at each step
producers run out of space
predators are too large
prey migrate away

Explanation:

Nitrogen - fixing bacteria convert atmospheric nitrogen into forms usable by plants. In food - chains, energy is lost as heat and for metabolic processes at each trophic level, limiting the length of food - chains.

Answer:

1. A. Bacteria
2. A. Energy is lost at each step