Living organisms can incorporate nitrates and ammonia into their metabolism. Nitrates (NO3-) can be reduced to nitrites (NO2-) and then to ammonia (NH3) through processes such as assimilation in plants and microbial processes. Nitrogen gas (N2) can be incorporated into metabolism by certain bacteria through nitrogen fixation, but most organisms cannot directly utilize it without the involvement of these specialized microbes.
In summary, the primary forms that organisms directly incorporate into their metabolism are nitrates and ammonia.