Question

how plants fulfill their demand of nitrogen

Answer 1

for some plants like pea bacteria like rhyzobium helps......
and insectivorous plants fulfill their nitrogen demands by digesting insects..........and some plants take nitrogen through soil with the hlp of water........

Answer 2

there is a cycle of nitrogen which is known as nitrogen cycle you can see it in the pic by this cycle the nitrogen fixes in the ground and by dissolving in water plants take it. but some plants eat animals like Nepenthes lowii (see in the picture) is a carnivorous plant basically they eat insects these insects eat plants who get nutrients from soil by eating the plants they get nitrogen stored in plants and then again carnivorous plant eat that insects and gets that nitrogen from eating him.
I THINK THIS IS ENOUGH FOR YOU 
BEST OF LUCK FOR FUTURE