It is done by some bacteria.............. 
in some plants it is done by digesting insects..............

1.As organic matter decomposes, nitrogen is slowly converted to ammonium, which is absorbed by plant roots. 
2.insectivorous plants like venus flytrap and sundew depend on insects to fulfill .
3.some plants like pea plant depend on bacteria like rhyzobium.
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the plants full fill their nitrogen requirements through many process. There is about 72 percentage of nitrogen in our environment but the plants cannot directly use it they need some ways or even some organisms which can help them to fulfill their nitrogen like organisms such as the rhyzobium  which help in the nitrogen fixation for the plants. the plants use some amount of the nitrogen and other would be saved by it and when the plant would die then the stored nitrogen is again in the atmosphere or into the soil and this process goes on all time.