Nonpolar amino acid residues are typically found in the interior of globular proteins like chymotrypsin. which chemical force is most directly responsible?

The chemical force most directly responsible in the given case is hydrophobic interactions.  

The hydrophobic interactions illustrate the associations between the hydrophobes, that is, low water-soluble molecules, and water. Hydrophobes are nonpolar molecules and generally exhibit a lengthened chain of carbons, which do not associate with the molecules of water.  

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.