How do plants and animals get nitrogen if not from the atmosphere?

Through soil using its roots. Also could be from groundwater because nitrates can enter it.

Plants can absorb nitrogen throu their roots from the soil , some plants are better at it than others , humans and animals get the nitrogen from eating the plants .

Elemental nitrogen (N2) has a strong triple bond, which prevents plants and animals from converting it into useful forms, even while N2 is readily available in the atmosphere comprising about 78% of air. In breathing, N2 that is inhaled is again exhaled as N2. To be available to plants and animals, the nitrogen in the atmosphere has to be converted by a difficult process called fixation. This conversion can be made by certain bacteria that exist at the roots of a few specific types of plants and trees, and this process is known as symbiotic fixation since the relationship of the bacteria with the plants is symbiotic (mutually beneficial). Fixation can also happen through the intense heat of lightning after which molecular nitrogen reaches the soil by precipitation (eg. rainfall); this process is called non-symbiotic fixation. Nitrogen (N) which has been fixed is converted to ammonia (NH3) by bacteria which can be assimilated by bacteria or plants; these plants may be consumed by animals and bacteria and plants may die leaving organic nitrogen in the soil. A process of mineralization (or ammonification) by bacteria and fungi converts dead organic nitrogen to ammonia (NH3) and ammonium (NH4) which can be assimilated by plants. Nitrification is an aerobic process (a process requiring oxygen) which converts ammonia and ammonium to amines (NH2OH), nitrite (NO2) and then to nitrate (NO3). Note that oxygen molecules are added to the nitrogen molecules in this process. Nitrate moves well through soils and may be assimilated by many types of plants and bacteria. Under conditions where soil is oxygen poor, a denitrification process allows plants to use NO3 as an oxidant to convert organic carbon to CO2, converting the NO3 back to N2. Denitrification is an anaerobic process converting NO2 to NO, N2O, and N2. Note the oxygen molecules are removed from the nitrogen molecules in this process. During the processes of mineralization, nitrification and denitrification, gases may be released into the atmosphere (N2, NH3, N2O, NO). Overall, plants will take in nitrogen by roots through the soil, and decay of plant material will return nitrogen to the soils; animals will eat the plants that are rich in nitrogen, and return nitrogen to the soil when they die. Specialized bacteria which exist in symbiosis with specialized plants are critical to nitrogen fixation, and lightning also contributes to nitrogen fixation, producing nitrogen in forms that can be utilized by plants and animals that otherwise cannot assimilate the nitrogen which is so abundant in the air.