What is nitrogen fixation and why are bacteria crucial to this cycle of life?

Nitrogen fixation- atmospheric nitrogen is assimilated into organic compounds especially by certain microorganisms as part of the nitrogen cycle.

By water evaporation from the leaves is created a negative pressure at the top of the tree that promotes the ascension  of the water through channels that are called xylem. there are also two molecule forces that are involved: the intra-water molecules cohesion (water molecules stick to each other) and the adhesion when the water molecules stick to the channels they are running in. this is what enables water to go from the roots to the top of a tall tree.