Plant roots absorb nitrogen-containing nutrients in the soil through the process of

The given blank can be filled with nitrogen assimilation.

Explanation:

The production of organic nitrogen constituents like amino acids from inorganic nitrogen constituents found in the environment is known as nitrogen assimilation.  

The species like fungi, plants, and some kinds of bacteria that are not capable of fixing nitrogen gas relies upon the tendency to assimilate ammonia or nitrate for their requirements. The plants captivate nitrogen from the soil in the form of NO₃⁻ (nitrate) and NH₄⁺ (ammonium).